Title of Invention	A METHOD OF ESTIMATION OF TORQUE FROM DIRECT ONLINE STARTING AT FREE RUNNING CONDITION OF AN INDUCTION MOTOR
Abstract	This invention related to a simplified measurement of the electromagnetic torque produced by an induction motor from measurements carried out while motor is started with the application of direct-online voltage to the motor terminals. The motor is supposed to be in standstill condition when the voltage is applied to the motor terminals and the three phase currents and speed are measured using a high speed recording equipment. The motor attains its rated speed in a few seconds while the recording is continuously made during the starting period. Later the data recorded on the transient recorder is transferred to a computer and the data is processed using a computer program offline to get an estimate of the electromagnetic torque produced by the motor during the no-load direct online starting. This computer program accepts the data recorded during the transient starting condition and the motor parameters measured from the no- load and block rotor test conducted on the motor as a routine test. The computation of torque is carried out from the measurement data based on the dq-model of induction motor. This invention proposes a simple method for measuring torque speed characteristics of large or medium size induction motors in a short time without incurring expensive equipment or consuming large amount of power. The test procedure is completed in a few minutes and the accuracy of measurement is within acceptable limits.

Title of Invention

A METHOD OF ESTIMATION OF TORQUE FROM DIRECT ONLINE STARTING AT FREE RUNNING CONDITION OF AN INDUCTION MOTOR

Abstract

This invention related to a simplified measurement of the electromagnetic torque produced by an induction motor from measurements carried out while motor is started with the application of direct-online voltage to the motor terminals. The motor is supposed to be in standstill condition when the voltage is applied to the motor terminals and the three phase currents and speed are measured using a high speed recording equipment. The motor attains its rated speed in a few seconds while the recording is continuously made during the starting period. Later the data recorded on the transient recorder is transferred to a computer and the data is processed using a computer program offline to get an estimate of the electromagnetic torque produced by the motor during the no-load direct online starting. This computer program accepts the data recorded during the transient starting condition and the motor parameters measured from the no- load and block rotor test conducted on the motor as a routine test. The computation of torque is carried out from the measurement data based on the dq-model of induction motor. This invention proposes a simple method for measuring torque speed characteristics of large or medium size induction motors in a short time without incurring expensive equipment or consuming large amount of power. The test procedure is completed in a few minutes and the accuracy of measurement is within acceptable limits.

Full Text	FIELD OF THE INVENTION The present invention relates to a method of estimation of torque from direct online starting at free running condition of an induction motor. More particularly the subject invention is related to measurement of electro magnetic torque produced by an induction motor at any speed in the range from standstill to its full speed. BACKGROUND OF THE INVENTION A motor which is an electromagnetic device that converts electrical energy to mechanical energy. The electrical energy conversion device related to an induction motor works on the principle of electromagnetic energy conversion and while accepting electrical energy it converts it into a mechanical rotational energy and involves the principle of electromagnetic induction. The equipment in question is a large size induction motor which has a three phase winding on the stationary part and a short circuited cage winding in the rotating part. The stationary part is known commonly as stator and the rotating part as rotor. Since there are various sizes of induction motors used for industrial and domestic purposes it is helpful to categories it into range of capacity as small, medium and large. While a small capacity motor may only deliver a few watts to a few kilowatt of power, a medium capacity motor delivers a few hundred kilowatts of power to about thousand kilowatts and the large capacity motors would deliver the range from a thousand kilowatt and above. This invention is concerned with medium and large size induction motors. The stator comprises of an iron part which consists of magnetic steel stamping sheets and windings. The stampings are punched to form a toothed surface on the inner side for the purpose of locating the windings. The function of the magnetic sheets is to carry the magnetic flux while the windings carry the electrical currents. When the windings are connected to an electrical power source the currents flow through these and a magnetic flux is created which permeates everywhere. The shape of the stator is cylindrical with a hollow bore. The rotor is also solid cylindrical in construction and resides within the stator and thus the magnetic flux created by the stator also engulfs the rotor. Similar to the stator the rotor is also made of magnetic steel stampings and windings. A small gap separates the stator from rotor which is commonly known as air-gap, and it facilitates the smooth rotation of the machine. The configuration of the structure of motor described in this invention is a commonly adopted one for large induction motors in industry. There are other configurations also such as axial motor or spherical motors which are mostly for special purpose and are of smaller capacity. When the stator windings are connected to an electrical power source, while the motor is in a stationary condition, heavy currents flow through the winding which brings the rotor into a rotational state. Within a few seconds the motor attains a high speed, which is very close to its synchronous speed, and is a function of the winding design. The time taken to start a motor is short and during this short interval the motor accelerates to attain a higher speed. The electromagnetic torque produced in the motor helps the motor to accelerate and the amount of torque produced is a function of the speed. The torque produced at any speed is an important feature of the machine. It is the endeavour of the machine manufacturers and the user to measure the torque characteristics. The torque versus speed characteristics of an induction motor is also estimated from the measured rotor speed, knowing the inertia of the rotating mass. Since this method is not so accurate the torque speed characteristics is directly measured using a dynamometer, which is a loading device, and amounts to loading of the motor to its rated operating power or more. Since the later method of loading requires expensive tests facility and consumes large power, it is limited to only small capacity motors. The proposed invention addressed to this difficulty of measurement of torque-speed characteristics of an induction motor with the direct-online starting of a motor in free running condition in order to control generation of torque to secure accurate torque speed characteristics according to specification. The direct-online starting method of any motor is a severe condition as heavy currents are drawn from the power source. However, during this starting duration the motor traces the complete characteristic as it starts from standstill condition and attains the full speed. In the free-running condition the full speed is very close to its synchronous speed. DESCRIPTION OF THE PRIOR ART In a number of patents methods have been proposed which are basically for obtaining the diagnostics of an induction motor while the motor is under operation. The U.S. Patent No. 6,262,550 by Kliman, et.al. relates to electrical motor monitoring system and method. In this patent a sensorless torque monitoring system is described. In this method of torque calculation phasors representing the current and voltage of the motor output are utilized. The voltages and currents are acquired from the sampled data stored in the database. The best approximation to a fundamental phasor is extracted by discrete Fourier transform (DFT) or similar means very quickly for each voltage and current. Using the current and voltage phasors, the torque calculation is done algebraically in terms of the phasors. Since the voltage excitation is nearly a pure sine wave, nearly all of the information needed for torque calculations is carried by the current data. Little information is lost (but much efficiency is gained) by extracting the best fit of the voltage fundamental phasor calculation, which may then be analytically integrated to produce the flux linkage phasor. The flux linkage phasor is used to produce a time function that is combined with the current waveform to yield the time varying torque. Moreover, this calculation is done without numerical integration and, thus, is accomplished without the substantial processing resources used with prior torque calculations. In particular, instantaneous torque is computed by multiplying several factors times the "cross" product of flux and current. The method relies on measurement of voltage and current phasors, whereas in the proposed invention only currents are measured and the torque speed characteristics is estimated from the parameters of the induction motor. The parameters of the motor are in turn measured from standard routine test methods detailed in standards IS-325, IEC- 60034 Part 12, 2002 and IEEE Standard 112, 1996. The proposed invention relies on the measurement of the currents in the three phase windings at a fast sampling rate and accurate measurement system. The recording process is on-line, but the torque calculation is an off-line process which is described in the body of the specification of this invention. DESCRIPTION OF THE INVENTION One object of the invention is measurement of the electromagnetic torque produced by an induction motor at any speed in the range from standstill to its full speed. Another object of the invention is determination of torque of an induction motor from measurement yielding the torque versus speed characteristics of the motor which is vital for its operation in industry and field. A yet another object of the invention is to make it possible to carry out the measurement process in a fairly inexpensive manner by test setup which is proposed to utilize only the normally used test facilitates, and the complete test procedure is conducted in a short time, within a few minutes. A further objective of the invention is to measure torque speed characteristics under the transient condition by simple means, adopting known testing procedure. A still another objective of the invention is to compute torque of an induction motor by using four variables the three currents flowing in the stator windings, the speed of the rotor and using measured parameters namely the winding resistance, winding leakage resistance, magnetizing reactance and inertia of rotor, obtaining change of variables of dq model machine by transforming them dq-axis reference frame, reducing the relationship to four equations based on voltages and currents flowing in the coil circuits, further reducing to two sets of unknown rotor currents in dqr-reference frame, further simplifying flux linkages in terms of the currents, finally representing the induction motor by two coupled differential equations, solving the equations with measured currents and speed of the motor resulting flux linkages and estimating torque on calculating product of these two flux linkages form the reduced simplified two differential equation. The present invention relates to the estimation of torque speed characteristics of an induction motor from the measured transient currents and speed of the motor during free running and direct online starting of the motor. The following parameters are measured on a transient recorder with a sampling time of 0.0002 second, an accuracy of +/-0.5% on full scale, with 12 bit resolution and frequency response of DC to 3 MHz (-3 dB point): 1. R-phase current 2. Y-phase current 3. B-phase current 4. Speed of motor The currents and speed are recorded for a duration which allows the induction motor to start from a standstill condition and reach its final steady operating speed. Later the data recorded on the transient recorder is transferred to a computer and the data is processed using a computer program offline to get an estimate of the electromagnetic torque produced by the motor during the no-load direct-on-line starting. This computer program accepts the data recorded during the transient starting condition and the motor parameters measured from the no-load and block rotor test conducted on the motor as a routine test which are given below: 1. rs - stator winding resistance 2. xs - stator winding leakage reactance 3. rr - rotor winding resistance referred to stator winding 4. xr - rotor winding leakage reactance referred to stator winding 5. xm - magnetizing reactance 6. J - polar moment of inertia of rotor. The parameters K1 and J1 for the deep-bar effect are also required by the computer program (refer to Alger's book for deep bar effect and parameters K1 and J1). The computation of torque is carried out from the measurement data based on the dq-model of induction motor. The voltage-current relationships of the windings in the motor are functions of machine inductances which change with the rotor position, consequently the coefficients of the differential equations describing the voltage-current relationships vary with rotor position and time. In the proposed invention change of variables is used to reduce the complexity in the relationship. The change in variable is obtained by transforming them to a reference frame with two perpendicular axes which are called direct and quadrature axis and are commonly known as dq-axis. The machine model based on dq-axis is called dq-model. In this method all variables such as voltages, currents, flux etc. and all parameters such as inductances are transformed to the dq-frame. The subsequent relationships become simpler and are easier for implementation in computer. The voltages applied to the stator winding and the resulting currents flowing in the winding of the motor are transformed to the dq-reference frame. In the three-phase squirrel-cage inducting motor there are three set of windings made form interconnection of several coils. Because of the symmetry of these windings and the balanced nature of the voltages and currents flowing in these circuits and by adopting the transformation to the dq-frame it is possible to reduce the relationships to four numbers. Among these four relationships two are for the stator windings and two for rotor windings. Further since the squirrel-cage winding of the rotor is shorted, hence the voltage across these are zero. In addition it should be noted that the stator winding voltages are known and the stator winding currents are measured during the test. Consequently, there are only two set of unknowns which are the rotor currents in the dq-reference frame. A further simplification is carried out by representing the flux-linkage in terms of the currents. This helps in finally representing the induction motor by two coupled differential equations. Solution of these equations with the measured currents and speed of the motor will result in flux linkages and a product of these flux linkages results in the torque. In this procedure for estimation of the torque, four variables are used namely the three phase currents flowing in the stator windings, the speed of the rotor. The parameters used are the winding resistances, winding leakage reactances, the magnetizing reactance, and the inertia of the rotor. These parameters are calculated from standard test procedures which are covered in the standard IS-325, IEC-60034 Part 12, 2002 and IEEE Standard 112,1996. Therefore, the value of torque from these measurements results in a close estimation to the real value of torque. Further since the value of currents and speed change every instant the estimated torque also changes from instant to instant. For the currents which are alternating at a frequency of 50 cycles per second the time duration of one cycle is 0.02 second, therefore, the current samples are collected after a time duration of 0.0002 seconds which gives 100 samples during every cycle. In order to record the samples of the currents and speed at a fast rate a high resolution transient recorder is used. The accuracy of transient recorder used is 0.05% of the full scale, and the resolution is 12-bit. In other words while a current of 100 amperes can be measured with an accuracy ranging from 99.95 to 100.05 amperes the resolution will be 1 in 4096 levels. Such transient recorders are available in the market. The Induction motor is started by direct application of the voltages to the motor windings when it is in standstill condition. The motor starts rotating and eventually reaches its full speed which is close to its full speed which is close to its synchronous speed. The time taken for the motor to reach its full speed from standstill is the starting time of the motor. The starting time of induction motors generally ranges from 1 to 20 seconds in case of motors in the range of 100 to 10,000 kW, and 1000 to 3000 rpm motors. This is only to indicate the order of starting time for a motor and the amount of data which is measured and has to be stored on the recorder. The data is of the order of 5 mega bytes from a test record. A computer is used for the estimation of torque from the measured data. Since the recording is carried out online and has to be done at a fast speed the computer connected to the recorder waits till the test is completed. Thereafter the data which is recorded in the memory of the transient recorder is transferred to the computer and the torque is estimated by the computer in an off-line manner. However, the complete procedure of data transfer and computation of torque is finished within a minute. DESCRIPTION OF THE INVENTION WITH ACCOMPANYING DRAWINGS Figure 1 represents existing torque speed characteristics of 520 KW 4 pole induction motor at different voltages. Figure 2 represents schematically the test set up according to the invention. Figure 3 represents measurement record of the speed versus time characteristics according to the invention. Figure 4 represents measurement of record of the three phase currents versus time characteristics according to the invention. Figure 5 represents estimated torque from the measured currents according to the invention. Figure 1 shows a typical torque-speed characteristics of an induction motor. In the torque-speed characteristics there are two important values which are of more interest to any user, namely, the starting torque and the maximum torque. The starting torque occurs in standstill condition and the maximum torque at a speed close to the synchronous speed. The test setup shown in the Figure 2 is a schematic diagram of the motor under test and the connection of the measurement equipment for recording the data during the starting transient. The description of the items numbered from 1 to 13 in the Figure 2 are given below: Item No. Description 1. Induction motor 2. Speed sensor / tachogenerator and transducer 3. Current sensor and transducers 4. Transient recorder 5. Contactor for application of the power to motor winding 6. Protection device a numeric-relay 7. Current and Voltage transformers (CTS & PTS) 8. Control desk for switching ON and monitoring functions 9. High voltage cables inter-connecting the motor to the power source 10. Shielded cables interconnecting the transducers with transient recorder 11. Serial cable interconnecting transient recorder to micro- computer 12. Printer 13. Tachogenerator coupling 14. Computer DETAILED DESCRIPTION OF EACH ITEM IS GIVEN BELOW: 1. Induction motor: The induction motor is the test motor for which the measurement is being carried out. In the figure it is shown in an uncoupled condition, which signifies its free running state. It is a three phase induction motor and has a provision for connection of the speed sensor, which is a tachogenerator, on the non-drive end of the shaft. The induction motor on which the test was carried out was of capacity 1100 kW, 6.6 kV, 3000 rpm, flame proof squirrel cage induction motor with sleeve bearings. 2. Speed sensor / tachogenerator and transducer: In measurement of any quantity two aspects are involved, one is the conversion of the physical quantity into a measurable electrical signal and the second aspect is converting this electrical signal into a voltage / current signal which may be transmitted over cables to a recorder which is located at a distance from the test setup. The device which converts the signal from the sensor into a signal for transmission is called a transducer. Usually the sensor and the transducer are two separate devices. However, in this test setup a DC tachogenerator is used which is commonly available in a testing laboratory. The DC tachogenerator performs both the functions of sensor and transducer and produces a voltage signal which is powerful enough to be transmitted over cables. The DC tachogenerator used for the testing described here was of capacity 4000 rpm 200 V DC. 3. Current sensor and transducer: A hall current sensor is used for converting the current signal into a measurable electrical voltage signal and a transducer is also built into the same device which derives its power from the alternating magnetic flux emanating from the current carrying high voltage cable. The device is a clamp type equipment and produces at its terminals a milli-volts signal which can be communicated over shielded cable to the transient recorder. The current sensor capacity is 400 A rms, frequency 20 Hz to 20 kHz and the transducer output is 10 mili-volt per ampere. The accuracy of the transducer is +/- 0.5 %. 4. Transient recorder: The transient recorder is a recording instrument of the following specification. The device performs the function of digitising the analogue signals, namely the speed and currents, at a fast sampling rate of 0.0002 seconds and store it in its memory. The recorder also displays the recorder speed and currents on its screen. Since the recording time is short and the signals have to be recorded at a fast rate the recording function is done online along with the test when the motor is started by application of the direct-online voltages. Subsequently the recorded signals are transmitted to a micro- computer over a serial communication link on an off-line condition. It is important to note here that the transient recorder has an inbuilt timer with is accurate and provides the sample time. This time-record is used in the computation of torque. THE SPECIFICATIONS OF TRANSIENT RECORDER IS AS FOLLOWS: DESCRIPTION: A transient recorder measures transient voltages and currents in transient as well as in steady state conditions. The equipment is capable of recording quantities from DC up to a frequency of 100 kHz storing and displaying the waveforms. It posses capability to measure / compute functions such as phase angle, time period, frequency, FFT etc. Inputs to be measured: Voltages and currents. Direct voltage input up to 250 Volts rms, currents and higher voltage inputs through Clamps Type probes / sensors. Accuracy of measurement : +/- 0.5 % of full scale No. of channels : 8 channels Transient measurement : Sampling rate of 1 Mega Samples per second, and storage of data for all the 8 channels. Analog / Digital (A/D): 12 bit resolution Converter Display : Display screen type TFT Colour Liquid Crystal Display. Screen size more than 10 inches. All the 8 captured waveform is displayed on the screen. Memory : 4 Mega word per channel. Trigger : Auto, Normal, Internal on Channel 1 to 8 with a settable trigger value, External edge trigger. External I/O : GPIB interface, RS-232 serial port, Printing of captured waveform on screen through Centronic Port. Capability to transfer and store the captured data on computer (OS Windows-98 or higher) with a software supplied to support the transfer and display. Accessories : Three current probes of clamp-on type with matching bandwidth and accuracy of the recorder and capacity 0 to 400 A. Four high voltage probes of matching bandwidth and accuracy of the recorder and capacity up to 600 V rms. 5. Contactor for application of power to motor winding: The contactor is a switch which performs both functions of application of power to the motor windings and also isolating it from the power source. Since the contactor is also used in conjunction with a protection device it should be capable if disconnecting the motor even while it is carrying large amount of fault currents. The contactor is a heavy duty and high voltage vacuum-switch of adequate fault handling capability. SPECIFICATIONS OF CIRCUIT BREAKER IS AS FOLLOWS: Voltage: 11 kV: Freq.: 50 Hz: Normal Current: 1250 A; System Breaking Capacity: 25 kA Short Time current: 25KA Duration 1.0 sec. Making Capacity 62.5 kAp P.F. Withstand 28 kV Impulse withstand 75kVp Shunt trip coil 110V Spring Release Coil 110V Interrupter VS 12014 6. Protection device: The protection device is a relay which can protect the motor in the event of any fault. The analogue relays for earth fault and over-current protection are commonly used in industry, but these days the analogue relays are replaced by microprocessor based multi-function numeric relay. In the present test setup a numeric relay was adopted with the following specifications. The relay provides a signal to the contactor to open. The switching ON of the motor is done. FEATURES OF NUMERICAL RELAY • Locked Rotor Protection based on impedance measurement. • Three phase o/c relay with selectable IDMT / definite time characteristics. • Earth fault relay with selectable IDMT / definite time characteristic. • Negative sequence relay • Thermal Overload protection • Overvoltage / Undervoltage alarm. • Event / Fault records • Wide setting range. • Suitable for medium and large motors. 7. Current and Voltage transformers: Since the currents and voltages which are to be monitored are large the current and voltage transformers are used. These are commonly known as CTs and PTs and are used in all motor testbed setups. The purpose of using these devices is not only to reduce the measured quantities to safe limits but also to isolate the high voltage signals for safety purpose. Care should be taken to use calibrated and accurate CTs of 0.5 class. 8. Control desk for switching ON and monitoring functions: The control cubicle is located at a distance from the motor and the place from where the complete testing is controlled and monitored. The signals from CTs and PTs are brought to the cubicle and is monitored. Since the CTs are sensitive and accurate enough to measure the transient current, the current transducer can also be connected to the CT secondaries. The control desk is also equipped with measuring meters. The voltage and frequency meters are also present on the control desk and the voltage and frequency prior to the testing is noted. The ON button to signal the contactor to close is located on the control desk. The transient recorder is also placed near to the control desk. On pushing the ON button the contactor closes and the motor receives the power, and the test begins. After the motor attains the full speed the OFF button is pushed and the motor is isolated and the test culminates. 9. High voltage cables inter-connecting the motor to the power source: The high voltage cables interconnect the motor to the power source. These are copper cables with insulation covering suitable for 11 kV and an armour for protection. It is however not advisable to go near the cables or the motor while it is being tested. The complete operation is conducted remotely. 10. Shielded cable connecting the transducer to the transient recorder: The cables connecting the transducer to the recorder are shielded so that they do not introduce noise into the measurement. 11. Shielded cable connecting the transient recorder to micro computer (14): The transient recorder is connected to the micro-computer (14) through high speed USB port and the cable to connect the two devices is also special nature supplied by the manufacturer of transient recorder. 12. Printer: A printer is connected to the port of the micro-computer and it is meant for taking hard copies of the torque speed characteristics. 13. Tachogenerator coupling: The tachogenerator coupling is a flexible coupling to allow about 10 mm of axial movement of the rotor of main machine. 14. Computer DETAILED DESCRIPTION OF METHODOLOGY The objective of the invention is to measure the electromagnetic torque from the measured currents and for this purpose a simulation model of the induction motor is adopted. The simulation model is the commonly used dq-model of induction motor and it can be found in the reference book, P.C. Krause, 0. Wasynczuk, S.D. Sudhoff, "Analysis of Electrical Machinery and Drive Systems" publisher IEEE press, Wiley Interscience, year 2004. For the simulation purpose the currents are transformed using the transformation method detailed in the said reference guidance. The transformed currents are given as follows: Where and are the transformed currents and are the three phase currents. The angle θ is the angular displacement of the reference frame with respect to the three phase current variables. Here it should be noted that the variables and and θ are all functions of time, in other words they change from instant to instant. The angle θ is given by the following expression: Where ω(ξ) is the angular speed of the rotor which is measured using the speed sensor and ξ is the dummy variable of integration. The angle θ(0) is the value of angle between the two frames at the instant zero, which can be fixed arbitrarily. The induction machine model, based on dq-transformation, given by the said reference book is given as follows in equation (2). This expression is a set of six differential equations which relates the transformed voltages with the transformed flux-linkages. Further, the flux-linkages are related to the transformed currents. Since the three stator currents are being measured, and considering the fact that it is a balanced three-phase winding with balanced currents, four of the equations above equations are eliminated by manipulations and simplifications. Finally we arrive at the following two set of differential equations which represent the induction motor case in question. Knowing the rotor flux linkages the stator flux linkages can be calculated from the stator currents and the rotor flux linkages. But these equations are of algebraic nature which is given below: In the same way the rotor currents can be calculated from the stator currents and the rotor flux linkages. And these equations are also of algebraic nature which is given below: With the flux linkages for stator and rotor circuits calculated the electromagnetic torque can be calculated from the expression given below: The electromagnetic torque and the speed are related by: The electromagnetic torque Te which is computed in the above expression is the output of all these computations. The above expressions are implemented in the software and with that the torque versus speed characteristics is computed. Embodiments of the proposed invention will be best understood from the following illustrative examples. The parameters pertaining to the induction motor equivalent circuit are generally found from some tests which are mentioned below: 1. Winding resistance: The resistances of the three phases windings are measured using a resistance bridge and the ambient temperature is also noted. 2. No load test: In this test the motor is supplied the nominal voltage while the motor is in free running condition. The voltages, currents, frequency, speed and the power are measured using standard meters. This test reveals the magnetizing reactance of the motor and also the no load losses. 3. Blocked rotor test: This test is conducted with the rotor in blocked condition and a reduced voltage applied to the stator. The voltages, currents, frequency and the power are measured using standard meters. This test reveals the stator and rotor resistances and the leakage reactances of the windings. 4. Retardation test: This test is conducted with the motor running at full speed and subsequently the supply being disconnected. The speed of the rotation is recorded with respect to time. This test reveals the moment of inertia of the rotating mass. Typical values of parameters of induction motors of different sizes are given below as an example in Table-1. From the parameters described in the table the torque speed characteristics of an induction motor can be estimated and a sample characteristics is given in Figure 3. The graph shown in Figure 3 is given for steady state condition. A steady state condition signifies a state when all quantities are steady, such as the voltage, current and speed. However, when an induction motor is started with direct-online application of voltage to the motor at its standstill condition the characteristics is different, as this is a transient condition. In a transient condition the current and speed are allowed to vary while the motor accelerates from its standstill condition and attains its full speed. The torque speed characteristics under the transient condition may be computed using the dq-model of the induction motor. A sample of the torque speed characteristics is given in Figure 4 . The torque produced by the induction motor is oscillatory in nature during the initial period and as the motor accelerates the torque attains a uni-directional value. If the torque produced by the motor is greater than the load torque the motor accelerates steadily and attains its rated condition. The measured values are fed to the computer program and the program estimates the torque speed characteristics from the measured values of the three-phase currents as estimated according to equation (6) and (7). A test carried out on an 1100 kW, 2 Pole, induction motor during the direct online starting transient is given in Figure 5. The invention as herein narrated with an embodiment through exemplary illustration should not be read and construed in a restrictive manner as many embodiments can be formed with various modifications of computer programme, alterations of the variables and parameters and adaptations for wide range of motor rating within the scope and ambit of the invention as defined in the appended claims. WE CLAIM 1. A method of estimation of torque from direct online starting at free running condition of an induction motor comprising: - disposing a rotor, a stator, at least one bearing supporting the rotor and connecting motor lead wire carrying electric current to motor (1); - connecting a measurement unit to electric motor comprising sensors (3) to record stator currents for the three phase R, Y, B windings and one sensor (2) to record the speed of the motor; - arranging a transient recorder (4) to record the sensor information at every time duration of 0.0002 seconds or less with an accuracy of 0.05% of the maximum value; - connecting a computer to the recording unit (4) to compute the torque speed characteristics of the induction motor; - disposing a contactor (5) as a circuit breaker and as a protection unit (to carry out the direct online starting of the induction motor in free running condition; - closing the contactor to start the motor with a control unit; - arranging the contactor (5) to act as a circuit breaker and also as a protection unit operating through a control desk; - making the circuit breaker on and causing a direct online voltage applied to motor terminals; - applying voltages to the motor terminals when the motor is in standstill and free to run condition; - allowing the motor to start and attain its rated speed, wherein the measurement unit comprising of sensors (2 and 3) record the signals during the transient process of the motor when the contactor is switched off from the controller after the motor achieves the steady free running full speed condition to complete the testing. - using four variables namely the three phase currents (ias, ibs and ics) flowing in the stator windings, the speed of the rotor (P) and parameters namely winding resistances (rs, rr), magnetizing reactance (Xm) and inertia of the induction motor; - obtaining change of variables of dq model induction machine by transforming them to dq - axis reference frame; - reducing the relationship to four equations based on voltages and currents flowing in the coil circuits; - further reducing the relationship to two sets of unknown rotor currents (iqr, idr) in dq reference frame; - further simplifying flux linkages (Ψqs, Ψds, Ψqr, Ψdr) in terms of currents (iqs, ids, iqr, idr); - finally representing the induction motor by two coupled differential equation; - solving the equations with measured currents and speed of the motor resulting flux linkages and - estimating torque of the motor on calculating product of these two flux linkages from the reduced simplified two coupled differential equations. 2. A method of estimation of torque of induction motor as claimed in claim 1, wherein the data is recorded in the memory of the high resolution transient recorder (4) on online manner till test is completed and the torque is estimated in the computer (14) in an off-line manner on transferring the recorded data containing three phase currents and speed of induction motor of the recorder (4) to the computer (14). 3. A method of estimation of torque of induction motor as claimed in claims 1 to 2, wherein voltages across the rotor winding is kept zero on shorting the squirrel-cage winding of the rotor. 4. A method of estimation of torque of an induction motor as claimed in claim 1, wherein balanced nature of voltages and currents flowing through the circuits is maintained by adopting the transformation to the dq - frame through a simulation model of the induction motor by transforming currents using mathematical relationship. Wherein ids, ids, and i0s are the transformed currents and ias, ids, ics are the three phase current, the angle θ being the angular displacement of the reference frame with respect to the three phase current variables where the variables ias, ibs, ics and iqs, ids, i0s and θ are all functions of time, changing from instance to instance, the angle θ being determined by the following expression: Where w(ξ) is the angular speed of the rotor which is measured using the speed sensor and ξ is the dummy variable of integration, wherein the angle θ(0) is the value of angle between the two frames at the instant zero, which can be fixed arbitrarily. 5. A method of estimation of torque of an induction motor as claimed in claim 1, wherein the induction machine model, based on dq - transformation is a set of six differential equations relating to transformed voltages {vqs, vds and vos, (stator voltage) and v'qr, v' or v'dr (rotor voltage)} with transformed flux (Ψqs, Ψds, Ψos, Ψqr, Ψdr, Ψor) and the flux linkages being related to transformed currents as expressed in mathematical relationship. estimating four of the said equation to arrive at the two sets of following differential equations: 6. A method of estimation of torque of an induction motor as claimed in claims 1 and 5, wherein knowing the rotor flux linkages Ψqr and Ψdr, the rotor flux linkages Ψqs and Ψds is calculated from the currents Iqs and Ids and the rotor flux linkages as expressed below: and in the some way the rotor currents are calculated from the stator currents and the rotor flux linkages as expressed from the following equation: in which 7. A method of estimation of torque of an induction motor as claimed in claims 1, 5 and 6, wherein the electromagnetic torque of the motor is calculated from flux linkages for stator and rotor circuits as expressed from the following mathematical relationship: and the electromagnetic torque and the speed of the motor is expressed by the following relationship: in which J is Polar moment of inertia of rotor in Kg.m2, TL is load torque in Nm and D = Xss X'rr - X2M ABSTRACT A METHOD OF ESTIMATION OF TORQUE FROM DIRECT ONLINE STARTING AT FREE RUNNING CONDITION OF AN INDUCTION MOTOR This invention related to a simplified measurement of the electromagnetic torque produced by an induction motor from measurements carried out while motor is started with the application of direct-online voltage to the motor terminals. The motor is supposed to be in standstill condition when the voltage is applied to the motor terminals and the three phase currents and speed are measured using a high speed recording equipment. The motor attains its rated speed in a few seconds while the recording is continuously made during the starting period. Later the data recorded on the transient recorder is transferred to a computer and the data is processed using a computer program offline to get an estimate of the electromagnetic torque produced by the motor during the no-load direct online starting. This computer program accepts the data recorded during the transient starting condition and the motor parameters measured from the no- load and block rotor test conducted on the motor as a routine test. The computation of torque is carried out from the measurement data based on the dq-model of induction motor. This invention proposes a simple method for measuring torque speed characteristics of large or medium size induction motors in a short time without incurring expensive equipment or consuming large amount of power. The test procedure is completed in a few minutes and the accuracy of measurement is within acceptable limits.

Full Text

FIELD OF THE INVENTION
The present invention relates to a method of estimation of torque from direct
online starting at free running condition of an induction motor. More particularly
the subject invention is related to measurement of electro magnetic torque
produced by an induction motor at any speed in the range from standstill to its
full speed.
BACKGROUND OF THE INVENTION
A motor which is an electromagnetic device that converts electrical energy to
mechanical energy. The electrical energy conversion device related to an
induction motor works on the principle of electromagnetic energy conversion and
while accepting electrical energy it converts it into a mechanical rotational energy
and involves the principle of electromagnetic induction. The equipment in
question is a large size induction motor which has a three phase winding on the
stationary part and a short circuited cage winding in the rotating part. The
stationary part is known commonly as stator and the rotating part as rotor. Since
there are various sizes of induction motors used for industrial and domestic
purposes it is helpful to
categories it into range of capacity as small, medium and large. While a small
capacity motor may only deliver a few watts to a few kilowatt of power, a
medium capacity motor delivers a few hundred kilowatts of power to about
thousand kilowatts and the large capacity motors would deliver the range from a
thousand kilowatt and above. This invention is concerned with medium and large
size induction motors.

The stator comprises of an iron part which consists of magnetic steel stamping sheets and
windings. The stampings are punched to form a toothed surface on the inner side for the
purpose of locating the windings. The function of the magnetic sheets is to carry the
magnetic flux while the windings carry the electrical currents. When the windings are
connected to an electrical power source the currents flow through these and a magnetic
flux is created which permeates everywhere. The shape of the stator is cylindrical with a
hollow bore. The rotor is also solid cylindrical in construction and resides within the stator
and thus the magnetic flux created by the stator also engulfs the rotor. Similar to the
stator the rotor is also made of magnetic steel stampings and windings. A small gap
separates the stator from rotor which is commonly known as air-gap, and it facilitates the
smooth rotation of the machine.
The configuration of the structure of motor described in this invention is a commonly
adopted one for large induction motors in industry. There are other configurations also
such as axial motor or spherical motors which are mostly for special purpose and are of
smaller capacity.
When the stator windings are connected to an electrical power source, while the motor is
in a stationary condition, heavy currents flow through the winding which brings the rotor
into a rotational state. Within a few seconds the motor attains a high speed, which is very
close to its synchronous speed, and is a function of the winding design. The time taken to
start a motor is short and during this short interval the motor accelerates to attain a
higher speed. The electromagnetic torque produced in the motor helps the motor to
accelerate and the amount of torque produced is a function of the speed. The torque
produced at any speed is an important feature of the machine. It is the endeavour of the
machine manufacturers and the user to measure the torque characteristics.

The torque versus speed characteristics of an induction motor is also estimated from the
measured rotor speed, knowing the inertia of the rotating mass. Since this method is not
so accurate the torque speed characteristics is directly measured using a dynamometer,
which is a loading device, and amounts to loading of the motor to its rated operating
power or more. Since the later method of loading requires expensive tests facility and
consumes large power, it is limited to only small capacity motors.
The proposed invention addressed to this difficulty of measurement of torque-speed
characteristics of an induction motor with the direct-online starting of a motor in free
running condition in order to control generation of torque to secure accurate torque speed
characteristics according to specification. The direct-online starting method of any motor
is a severe condition as heavy currents are drawn from the power source. However,
during this starting duration the motor traces the complete characteristic as it starts from
standstill condition and attains the full speed. In the free-running condition the full speed
is very close to its synchronous speed.
DESCRIPTION OF THE PRIOR ART
In a number of patents methods have been proposed which are basically for obtaining the
diagnostics of an induction motor while the motor is under operation. The U.S. Patent No.
6,262,550 by Kliman, et.al. relates to electrical motor monitoring system and method. In
this patent a sensorless torque monitoring system is described. In this method of torque
calculation phasors representing the current and voltage of the motor output are utilized.
The voltages and currents are acquired from the sampled data stored in the database.
The best approximation to a fundamental phasor is extracted by discrete Fourier transform
(DFT) or similar means very quickly for each voltage and current. Using the current and

voltage phasors, the torque calculation is done algebraically in terms of the phasors. Since
the voltage excitation is nearly a pure sine wave, nearly all of the information needed for
torque calculations is carried by the current data. Little information is lost (but much
efficiency is gained) by extracting the best fit of the voltage fundamental phasor
calculation, which may then be analytically integrated to produce the flux linkage phasor.
The flux linkage phasor is used to produce a time function that is combined with the
current waveform to yield the time varying torque. Moreover, this calculation is done
without numerical integration and, thus, is accomplished without the substantial
processing resources used with prior torque calculations. In particular, instantaneous
torque is computed by multiplying several factors times the "cross" product of flux and
current. The method relies on measurement of voltage and current phasors, whereas in
the proposed invention only currents are measured and the torque speed characteristics is
estimated from the parameters of the induction motor. The parameters of the motor are
in turn measured from standard routine test methods detailed in standards IS-325, IEC-
60034 Part 12, 2002 and IEEE Standard 112, 1996. The proposed invention relies on the
measurement of the currents in the three phase windings at a fast sampling rate and
accurate measurement system. The recording process is on-line, but the torque
calculation is an off-line process which is described in the body of the specification of this
invention.
DESCRIPTION OF THE INVENTION
One object of the invention is measurement of the electromagnetic torque produced by an
induction motor at any speed in the range from standstill to its full speed.
Another object of the invention is determination of torque of an induction motor from
measurement yielding the torque versus speed characteristics of the motor which is vital
for its operation in industry and field.

A yet another object of the invention is to make it possible to carry out the measurement
process in a fairly inexpensive manner by test setup which is proposed to utilize only the
normally used test facilitates, and the complete test procedure is conducted in a short
time, within a few minutes.
A further objective of the invention is to measure torque speed characteristics under the
transient condition by simple means, adopting known testing procedure.
A still another objective of the invention is to compute torque of an induction motor by
using four variables the three currents flowing in the stator windings, the speed of the
rotor and using measured parameters namely the winding resistance, winding leakage
resistance, magnetizing reactance and inertia of rotor, obtaining change of variables of dq
model machine by transforming them dq-axis reference frame, reducing the relationship to
four equations based on voltages and currents flowing in the coil circuits, further reducing
to two sets of unknown rotor currents in dqr-reference frame, further simplifying flux
linkages in terms of the currents, finally representing the induction motor by two coupled
differential equations, solving the equations with measured currents and speed of the
motor resulting flux linkages and estimating torque on calculating product of these two
flux linkages form the reduced simplified two differential equation.
The present invention relates to the estimation of torque speed characteristics of an
induction motor from the measured transient currents and speed of the motor during free
running and direct online starting of the motor. The following parameters are measured
on a transient recorder with a sampling time of 0.0002 second, an accuracy of +/-0.5% on
full scale, with 12 bit resolution and frequency response of DC to 3 MHz (-3 dB point):

1. R-phase current
2. Y-phase current
3. B-phase current
4. Speed of motor
The currents and speed are recorded for a duration which allows the induction motor to start
from a standstill condition and reach its final steady operating speed. Later the data recorded
on the transient recorder is transferred to a computer and the data is processed using a
computer program offline to get an estimate of the electromagnetic torque produced by the
motor during the no-load direct-on-line starting. This computer program accepts the data
recorded during the transient starting condition and the motor parameters measured from the
no-load and block rotor test conducted on the motor as a routine test which are given below:
1. rs - stator winding resistance
2. xs - stator winding leakage reactance
3. rr - rotor winding resistance referred to stator winding
4. xr - rotor winding leakage reactance referred to stator winding
5. xm - magnetizing reactance
6. J - polar moment of inertia of rotor.
The parameters K1 and J1 for the deep-bar effect are also required by the computer program
(refer to Alger's book for deep bar effect and parameters K1 and J1).
The computation of torque is carried out from the measurement data based on the dq-model
of induction motor. The voltage-current relationships of the windings in the motor are
functions of machine inductances which change with the rotor position, consequently the
coefficients of the differential equations describing the voltage-current relationships vary with
rotor position and time. In the proposed invention change of variables is used to reduce the

complexity in the relationship. The change in variable is obtained by transforming them to a
reference frame with two perpendicular axes which are called direct and quadrature axis and
are commonly known as dq-axis. The machine model based on dq-axis is called dq-model.
In this method all variables such as voltages, currents, flux etc. and all parameters such as
inductances are transformed to the dq-frame. The subsequent relationships become simpler
and are easier for implementation in computer.
The voltages applied to the stator winding and the resulting currents flowing in the winding of
the motor are transformed to the dq-reference frame. In the three-phase squirrel-cage
inducting motor there are three set of windings made form interconnection of several coils.
Because of the symmetry of these windings and the balanced nature of the voltages and
currents flowing in these circuits and by adopting the transformation to the dq-frame it is
possible to reduce the relationships to four numbers. Among these four relationships two are
for the stator windings and two for rotor windings. Further since the squirrel-cage winding of
the rotor is shorted, hence the voltage across these are zero. In addition it should be noted
that the stator winding voltages are known and the stator winding currents are measured
during the test. Consequently, there are only two set of unknowns which are the rotor
currents in the dq-reference frame. A further simplification is carried out by representing the
flux-linkage in terms of the currents. This helps in finally representing the induction motor by
two coupled differential equations. Solution of these equations with the measured currents
and speed of the motor will result in flux linkages and a product of these flux linkages results
in the torque.
In this procedure for estimation of the torque, four variables are used namely the three phase
currents flowing in the stator windings, the speed of the rotor. The parameters used are the
winding resistances, winding leakage reactances, the magnetizing reactance, and the inertia
of the rotor. These parameters are calculated from standard test procedures which are
covered in the standard IS-325, IEC-60034 Part 12, 2002 and IEEE Standard 112,1996.

Therefore, the value of torque from these measurements results in a close estimation to the
real value of torque. Further since the value of currents and speed change every instant the
estimated torque also changes from instant to instant. For the currents which are alternating
at a frequency of 50 cycles per second the time duration of one cycle is 0.02 second,
therefore, the current samples are collected after a time duration of 0.0002 seconds which
gives 100 samples during every cycle. In order to record the samples of the currents and
speed at a fast rate a high resolution transient recorder is used. The accuracy of transient
recorder used is 0.05% of the full scale, and the resolution is 12-bit. In other words while a
current of 100 amperes can be measured with an accuracy ranging from 99.95 to 100.05
amperes the resolution will be 1 in 4096 levels. Such transient recorders are available in the
market.
The Induction motor is started by direct application of the voltages to the motor windings
when it is in standstill condition. The motor starts rotating and eventually reaches its full
speed which is close to its full speed which is close to its synchronous speed. The time taken
for the motor to reach its full speed from standstill is the starting time of the motor. The
starting time of induction motors generally ranges from 1 to 20 seconds in case of motors in
the range of 100 to 10,000 kW, and 1000 to 3000 rpm motors. This is only to indicate the
order of starting time for a motor and the amount of data which is measured and has to be
stored on the recorder. The data is of the order of 5 mega bytes from a test record.
A computer is used for the estimation of torque from the measured data. Since the recording
is carried out online and has to be done at a fast speed the computer connected to the
recorder waits till the test is completed. Thereafter the data which is recorded in the memory
of the transient recorder is transferred to the computer and the torque is estimated by the
computer in an off-line manner. However, the complete procedure of data transfer and
computation of torque is finished within a minute.

DESCRIPTION OF THE INVENTION
WITH ACCOMPANYING DRAWINGS
Figure 1 represents existing torque speed characteristics of 520 KW
4 pole induction motor at different voltages.
Figure 2 represents schematically the test set up according to the invention.
Figure 3 represents measurement record of the speed versus
time characteristics according to the invention.
Figure 4 represents measurement of record of the three phase currents
versus time characteristics according to the invention.
Figure 5 represents estimated torque from the measured
currents according to the invention.
Figure 1 shows a typical torque-speed characteristics of an induction motor. In the
torque-speed characteristics there are two important values which are of more interest to
any user, namely, the starting torque and the maximum torque. The starting torque
occurs in standstill condition and the maximum torque at a speed close to the synchronous
speed.
The test setup shown in the Figure 2 is a schematic diagram of the motor under test and
the connection of the measurement equipment for recording the data during the starting
transient. The description of the items numbered from 1 to 13 in the Figure 2 are given
below:

Item No. Description
1. Induction motor
2. Speed sensor / tachogenerator and transducer
3. Current sensor and transducers
4. Transient recorder
5. Contactor for application of the power to motor winding
6. Protection device a numeric-relay
7. Current and Voltage transformers (CTS & PTS)
8. Control desk for switching ON and monitoring functions
9. High voltage cables inter-connecting the motor to the
power source
10. Shielded cables interconnecting the transducers with
transient recorder
11. Serial cable interconnecting transient recorder to micro-
computer
12. Printer
13. Tachogenerator coupling
14. Computer
DETAILED DESCRIPTION OF EACH ITEM IS GIVEN BELOW:
1. Induction motor: The induction motor is the test motor for which the measurement is
being carried out. In the figure it is shown in an uncoupled condition, which signifies its
free running state. It is a three phase induction motor and has a provision for connection
of the speed sensor, which is a tachogenerator, on the non-drive end of the shaft. The
induction motor on which the test was carried out was of capacity 1100 kW, 6.6 kV, 3000
rpm, flame proof squirrel cage induction motor with sleeve bearings.

2. Speed sensor / tachogenerator and transducer: In measurement of any quantity
two aspects are involved, one is the conversion of the physical quantity into a measurable
electrical signal and the second aspect is converting this electrical signal into a voltage /
current signal which may be transmitted over cables to a recorder which is located at a
distance from the test setup. The device which converts the signal from the sensor into a
signal for transmission is called a transducer. Usually the sensor and the transducer are
two separate devices. However, in this test setup a DC tachogenerator is used which is
commonly available in a testing laboratory. The DC tachogenerator performs both the
functions of sensor and transducer and produces a voltage signal which is powerful enough
to be transmitted over cables. The DC tachogenerator used for the testing described here
was of capacity 4000 rpm 200 V DC.
3. Current sensor and transducer: A hall current sensor is used for converting the current
signal into a measurable electrical voltage signal and a transducer is also built into the
same device which derives its power from the alternating magnetic flux emanating from the
current carrying high voltage cable. The device is a clamp type equipment and produces at
its terminals a milli-volts signal which can be communicated over shielded cable to the
transient recorder. The current sensor capacity is 400 A rms, frequency 20 Hz to 20 kHz
and the transducer output is 10 mili-volt per ampere. The accuracy of the transducer is +/-
0.5 %.
4. Transient recorder: The transient recorder is a recording instrument of the following
specification. The device performs the function of digitising the analogue signals, namely
the speed and currents, at a fast sampling rate of 0.0002 seconds and store it in its
memory. The recorder also displays the recorder speed and currents on its screen. Since
the recording time is short and the signals have to be recorded at a fast rate the recording
function is done online along with the test when the motor is started by application of the

direct-online voltages. Subsequently the recorded signals are transmitted to a micro-
computer over a serial communication link on an off-line condition. It is important to note
here that the transient recorder has an inbuilt timer with is accurate and provides the sample
time. This time-record is used in the computation of torque.
THE SPECIFICATIONS OF TRANSIENT RECORDER IS AS FOLLOWS:
DESCRIPTION: A transient recorder measures transient voltages and currents in transient
as well as in steady state conditions. The equipment is capable of recording quantities from
DC up to a frequency of 100 kHz storing and displaying the waveforms. It posses capability
to measure / compute functions such as phase angle, time period, frequency, FFT etc.
Inputs to be measured: Voltages and currents. Direct voltage input up to 250 Volts
rms, currents and higher voltage inputs through Clamps Type
probes / sensors.
Accuracy of measurement : +/- 0.5 % of full scale
No. of channels : 8 channels
Transient measurement : Sampling rate of 1 Mega Samples per second,
and storage of data for all the 8 channels.
Analog / Digital (A/D): 12 bit resolution
Converter
Display : Display screen type TFT Colour Liquid Crystal Display.
Screen size more than 10 inches. All the 8 captured
waveform is displayed on the screen.
Memory : 4 Mega word per channel.

Trigger : Auto, Normal, Internal on Channel 1 to 8 with
a settable trigger value, External edge trigger.
External I/O : GPIB interface, RS-232 serial port, Printing of captured
waveform on screen through Centronic Port. Capability
to transfer and store the captured data on computer
(OS Windows-98 or higher) with a software supplied to
support the transfer and display.
Accessories : Three current probes of clamp-on type with
matching bandwidth and accuracy of the
recorder and capacity 0 to 400 A. Four high
voltage probes of matching bandwidth and
accuracy of the recorder and capacity up to 600
V rms.
5. Contactor for application of power to motor winding: The contactor is a
switch which performs both functions of application of power to the motor windings and
also isolating it from the power source. Since the contactor is also used in conjunction
with a protection device it should be capable if disconnecting the motor even while it is
carrying large amount of fault currents. The contactor is a heavy duty and high voltage
vacuum-switch of adequate fault handling capability.
SPECIFICATIONS OF CIRCUIT BREAKER IS AS FOLLOWS:
Voltage: 11 kV: Freq.: 50 Hz: Normal Current: 1250 A;
System Breaking Capacity: 25 kA
Short Time current: 25KA Duration 1.0 sec.

Making Capacity 62.5 kAp
P.F. Withstand 28 kV
Impulse withstand 75kVp
Shunt trip coil 110V
Spring Release Coil 110V
Interrupter VS 12014
6. Protection device: The protection device is a relay which can protect the motor in
the event of any fault. The analogue relays for earth fault and over-current protection are
commonly used in industry, but these days the analogue relays are replaced by
microprocessor based multi-function numeric relay. In the present test setup a numeric
relay was adopted with the following specifications. The relay provides a signal to the
contactor to open. The switching ON of the motor is done.
FEATURES OF NUMERICAL RELAY
• Locked Rotor Protection based on impedance measurement.
• Three phase o/c relay with selectable IDMT / definite time characteristics.
• Earth fault relay with selectable IDMT / definite time characteristic.
• Negative sequence relay
• Thermal Overload protection
• Overvoltage / Undervoltage alarm.
• Event / Fault records
• Wide setting range.
• Suitable for medium and large motors.

7. Current and Voltage transformers: Since the currents and voltages which are
to be monitored are large the current and voltage transformers are used. These are
commonly known as CTs and PTs and are used in all motor testbed setups. The purpose
of using these devices is not only to reduce the measured quantities to safe limits but also
to isolate the high voltage signals for safety purpose. Care should be taken to use
calibrated and accurate CTs of 0.5 class.
8. Control desk for switching ON and monitoring functions: The control cubicle is
located at a distance from the motor and the place from where the complete testing is
controlled and monitored. The signals from CTs and PTs are brought to the cubicle and is
monitored. Since the CTs are sensitive and accurate enough to measure the transient
current, the current transducer can also be connected to the CT secondaries. The control
desk is also equipped with measuring meters. The voltage and frequency meters are also
present on the control desk and the voltage and frequency prior to the testing is noted.
The ON button to signal the contactor to close is located on the control desk. The
transient recorder is also placed near to the control desk. On pushing the ON button the
contactor closes and the motor receives the power, and the test begins. After the motor
attains the full speed the OFF button is pushed and the motor is isolated and the test
culminates.
9. High voltage cables inter-connecting the motor to the power source: The
high voltage cables interconnect the motor to the power source. These are copper cables
with insulation covering suitable for 11 kV and an armour for protection. It is however not
advisable to go near the cables or the motor while it is being tested. The complete
operation is conducted remotely.
10. Shielded cable connecting the transducer to the transient recorder: The
cables connecting the transducer to the recorder are shielded so that they do not
introduce noise into the measurement.

11. Shielded cable connecting the transient recorder to micro computer (14):
The transient recorder is connected to the micro-computer (14) through high speed USB
port and the cable to connect the two devices is also special nature supplied by the
manufacturer of transient recorder.
12. Printer: A printer is connected to the port of the micro-computer and it is meant for
taking hard copies of the torque speed characteristics.
13. Tachogenerator coupling: The tachogenerator coupling is a flexible coupling to
allow about 10 mm of axial movement of the rotor of main machine.
14. Computer
DETAILED DESCRIPTION OF METHODOLOGY
The objective of the invention is to measure the electromagnetic torque from the
measured currents and for this purpose a simulation model of the induction motor is
adopted. The simulation model is the commonly used dq-model of induction motor and it
can be found in the reference book, P.C. Krause, 0. Wasynczuk, S.D. Sudhoff, "Analysis of
Electrical Machinery and Drive Systems" publisher IEEE press, Wiley Interscience, year
2004. For the simulation purpose the currents are transformed using the transformation
method detailed in the said reference guidance. The transformed currents are given as
follows:

Where and are the transformed currents and are the three phase
currents. The angle θ is the angular displacement of the reference frame with respect to
the three phase current variables. Here it should be noted that the variables and
and θ are all functions of time, in other words they change from instant to
instant. The angle θ is given by the following expression:

Where ω(ξ) is the angular speed of the rotor which is measured using the speed sensor
and ξ is the dummy variable of integration. The angle θ(0) is the value of angle between
the two frames at the instant zero, which can be fixed arbitrarily.
The induction machine model, based on dq-transformation, given by the said reference
book is given as follows in equation (2).

This expression is a set of six differential equations which relates the transformed voltages
with the transformed flux-linkages. Further, the flux-linkages are related to the
transformed currents. Since the three stator currents are being measured, and considering
the fact that it is a balanced three-phase winding with balanced currents, four of the
equations above equations are eliminated by manipulations and simplifications. Finally we
arrive at the following two set of differential equations which represent the induction
motor case in question.

Knowing the rotor flux linkages the stator flux linkages can be
calculated from the stator currents and the rotor flux linkages. But these
equations are of algebraic nature which is given below:

In the same way the rotor currents can be calculated from the stator currents and the
rotor flux linkages. And these equations are also of algebraic nature which is given below:

With the flux linkages for stator and rotor circuits calculated the electromagnetic torque
can be calculated from the expression given below:

The electromagnetic torque and the speed are related by:

The electromagnetic torque Te which is computed in the above expression is the output of
all these computations. The above expressions are implemented in the software and with
that the torque versus speed characteristics is computed.

Embodiments of the proposed invention will be best understood from the following
illustrative examples.
The parameters pertaining to the induction motor equivalent circuit are generally found
from some tests which are mentioned below:

1. Winding resistance: The resistances of the three phases windings are measured using a
resistance bridge and the ambient temperature is also noted.
2. No load test: In this test the motor is supplied the nominal voltage while the motor is in
free running condition. The voltages, currents, frequency, speed and the power are
measured using standard meters. This test reveals the magnetizing reactance of the motor
and also the no load losses.
3. Blocked rotor test: This test is conducted with the rotor in blocked condition and a
reduced voltage applied to the stator. The voltages, currents, frequency and the power
are measured using standard meters. This test reveals the stator and rotor resistances
and the leakage reactances of the windings.
4. Retardation test: This test is conducted with the motor running at full speed and
subsequently the supply being disconnected. The speed of the rotation is recorded with
respect to time. This test reveals the moment of inertia of the rotating mass.
Typical values of parameters of induction motors of different sizes are given below as an
example in Table-1.

From the parameters described in the table the torque speed characteristics of an
induction motor can be estimated and a sample characteristics is given in Figure 3.
The graph shown in Figure 3 is given for steady state condition. A steady state condition
signifies a state when all quantities are steady, such as the voltage, current and speed.
However, when an induction motor is started with direct-online application of voltage to
the motor at its standstill condition the characteristics is different, as this is a transient
condition. In a transient condition the current and speed are allowed to vary while the
motor accelerates from its standstill condition and attains its full speed. The torque speed
characteristics under the transient condition may be computed using the dq-model of the
induction motor. A sample of the torque speed characteristics is given in Figure 4 .
The torque produced by the induction motor is oscillatory in nature during the initial period
and as the motor accelerates the torque attains a uni-directional value. If the torque
produced by the motor is greater than the load torque the motor accelerates steadily and
attains its rated condition.
The measured values are fed to the computer program and the program estimates the
torque speed characteristics from the measured values of the three-phase currents as
estimated according to equation (6) and (7). A test carried out on an 1100 kW, 2 Pole,
induction motor during the direct online starting transient is given in Figure 5.
The invention as herein narrated with an embodiment through exemplary illustration
should not be read and construed in a restrictive manner as many embodiments can be
formed with various modifications of computer programme, alterations of the variables
and parameters and adaptations for wide range of motor rating within the scope and
ambit of the invention as defined in the appended claims.

WE CLAIM
1. A method of estimation of torque from direct online starting at free
running condition of an induction motor comprising:
- disposing a rotor, a stator, at least one bearing supporting the rotor and
connecting motor lead wire carrying electric current to motor (1);
- connecting a measurement unit to electric motor comprising sensors (3)
to record stator currents for the three phase R, Y, B windings and one
sensor (2) to record the speed of the motor;
- arranging a transient recorder (4) to record the sensor information at
every time duration of 0.0002 seconds or less with an accuracy of 0.05%
of the maximum value;
- connecting a computer to the recording unit (4) to compute the torque
speed characteristics of the induction motor;
- disposing a contactor (5) as a circuit breaker and as a protection unit (to
carry out the direct online starting of the induction motor in free running
condition;
- closing the contactor to start the motor with a control unit;
- arranging the contactor (5) to act as a circuit breaker and also as a
protection unit operating through a control desk;
- making the circuit breaker on and causing a direct online voltage applied
to motor terminals;

- applying voltages to the motor terminals when the motor is in standstill
and free to run condition;
- allowing the motor to start and attain its rated speed, wherein the
measurement unit comprising of sensors (2 and 3) record the signals
during the transient process of the motor when the contactor is switched
off from the controller after the motor achieves the steady free running
full speed condition to complete the testing.
- using four variables namely the three phase currents (ias, ibs and ics)
flowing in the stator windings, the speed of the rotor (P) and parameters
namely winding resistances (rs, rr), magnetizing reactance (Xm) and inertia
of the induction motor;
- obtaining change of variables of dq model induction machine by
transforming them to dq - axis reference frame;
- reducing the relationship to four equations based on voltages and currents
flowing in the coil circuits;
- further reducing the relationship to two sets of unknown rotor currents
(iqr, idr) in dq reference frame;
- further simplifying flux linkages (Ψqs, Ψds, Ψqr, Ψdr) in terms of currents
(iqs, ids, iqr, idr);
- finally representing the induction motor by two coupled differential
equation;

- solving the equations with measured currents and speed of the motor
resulting flux linkages and
- estimating torque of the motor on calculating product of these two flux
linkages from the reduced simplified two coupled differential equations.

2. A method of estimation of torque of induction motor as claimed in claim 1,
wherein the data is recorded in the memory of the high resolution
transient recorder (4) on online manner till test is completed and the
torque is estimated in the computer (14) in an off-line manner on
transferring the recorded data containing three phase currents and speed
of induction motor of the recorder (4) to the computer (14).
3. A method of estimation of torque of induction motor as claimed in claims
1 to 2, wherein voltages across the rotor winding is kept zero on shorting
the squirrel-cage winding of the rotor.
4. A method of estimation of torque of an induction motor as claimed in
claim 1, wherein balanced nature of voltages and currents flowing through
the circuits is maintained by adopting the transformation to the dq - frame
through a simulation model of the induction motor by transforming
currents using mathematical relationship.

Wherein ids, ids, and i0s are the transformed currents and ias, ids, ics are the
three phase current, the angle θ being the angular displacement of the
reference frame with respect to the three phase current variables where
the variables ias, ibs, ics and iqs, ids, i0s and θ are all functions of time,
changing from instance to instance, the angle θ being determined by the
following expression:

Where w(ξ) is the angular speed of the rotor which is measured using the
speed sensor and ξ is the dummy variable of integration, wherein the
angle θ(0) is the value of angle between the two frames at the instant
zero, which can be fixed arbitrarily.
5. A method of estimation of torque of an induction motor as claimed in
claim 1, wherein the induction machine model, based on dq -
transformation is a set of six differential equations relating to transformed
voltages {vqs, vds and vos, (stator voltage) and v'qr, v' or v'dr (rotor voltage)}
with transformed flux (Ψqs, Ψds, Ψos, Ψqr, Ψdr, Ψor) and the flux linkages
being related to transformed currents as expressed in mathematical
relationship.

estimating four of the said equation to arrive at the two sets of following
differential equations:

6. A method of estimation of torque of an induction motor as claimed in
claims 1 and 5, wherein knowing the rotor flux linkages Ψqr and Ψdr, the
rotor flux linkages Ψqs and Ψds is calculated from the currents Iqs and Ids
and the rotor flux linkages as expressed below:

and in the some way the rotor currents are calculated from the stator
currents and the rotor flux linkages as expressed from the following
equation:

in which

7. A method of estimation of torque of an induction motor as claimed in
claims 1, 5 and 6, wherein the electromagnetic torque of the motor is
calculated from flux linkages for stator and rotor circuits as expressed
from the following mathematical relationship:

and the electromagnetic torque and the speed of the motor is expressed
by the following relationship:

in which J is Polar moment of inertia of rotor in Kg.m2, TL is load torque in
Nm and D = Xss X'rr - X2M

ABSTRACT

A METHOD OF ESTIMATION OF TORQUE FROM DIRECT ONLINE
STARTING AT FREE RUNNING CONDITION OF AN INDUCTION MOTOR
This invention related to a simplified measurement of the electromagnetic torque
produced by an induction motor from measurements carried out while motor is
started with the application of direct-online voltage to the motor terminals. The
motor is supposed to be in standstill condition when the voltage is applied to the
motor terminals and the three phase currents and speed are measured using a
high speed recording equipment. The motor attains its rated speed in a few
seconds while the recording is continuously made during the starting period.
Later the data recorded on the transient recorder is transferred to a computer
and the data is processed using a computer program offline to get an estimate of
the electromagnetic torque produced by the motor during the no-load direct online
starting. This computer program accepts the data recorded during the
transient starting condition and the motor parameters measured from the no-
load and block rotor test conducted on the motor as a routine test. The
computation of torque is carried out from the measurement data based on the
dq-model of induction motor.
This invention proposes a simple method for measuring torque speed
characteristics of large or medium size induction motors in a short time without
incurring expensive equipment or consuming large amount of power. The test
procedure is completed in a few minutes and the accuracy of measurement is
within acceptable limits.

Documents:

01086-kol-2007-abstract.pdf

01086-kol-2007-claims.pdf

01086-kol-2007-correspondence others 1.1.pdf

01086-kol-2007-correspondence others.pdf

01086-kol-2007-description complete.pdf

01086-kol-2007-drawings.pdf

01086-kol-2007-form 1.pdf

01086-kol-2007-form 18.pdf

01086-kol-2007-form 2.pdf

01086-kol-2007-form 3.pdf

01086-kol-2007-gpa.pdf

1086-KOL-2007-(26-12-2012)-ABSTRACT.pdf

1086-KOL-2007-(26-12-2012)-CLAIMS.pdf

1086-KOL-2007-(26-12-2012)-CORRESPONDENCE.pdf

1086-KOL-2007-(26-12-2012)-DRAWINGS.pdf

1086-KOL-2007-(26-12-2012)-FORM 2.pdf

1086-KOL-2007-(26-12-2012)-FORM-13.pdf

1086-KOL-2007-(26-12-2012)-OTHERS.pdf

1086-KOL-2007-CORRESPONDENCE.pdf

1086-KOL-2007-EXAMINATION REPORT.pdf

1086-KOL-2007-FORM 18.pdf

1086-KOL-2007-GPA.pdf

1086-KOL-2007-GRANTED-ABSTRACT.pdf

1086-KOL-2007-GRANTED-CLAIMS.pdf

1086-KOL-2007-GRANTED-DESCRIPTION (COMPLETE).pdf

1086-KOL-2007-GRANTED-DRAWINGS.pdf

1086-KOL-2007-GRANTED-FORM 1.pdf

1086-KOL-2007-GRANTED-FORM 2.pdf

1086-KOL-2007-GRANTED-FORM 3.pdf

1086-KOL-2007-GRANTED-SPECIFICATION-COMPLETE.pdf

1086-KOL-2007-REPLY TO EXAMINATION REPORT.pdf

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Patent Number

258054

Indian Patent Application Number

1086/KOL/2007

PG Journal Number

48/2013

Publication Date

29-Nov-2013

Grant Date

29-Nov-2013

Date of Filing

06-Aug-2007

Name of Patentee

BHARAT HEAVY ELECTRICALS LIMITED

Applicant Address

ITS REGIONAL OFFICES AT REGIONAL OPERATIONS DIVISION (ROD),PLOT NO: 9/1, DJBLOCK 3RD FLOOR, KARUNAMOYEE, SALT LAKE CITY, KOLKATA - 700091, HAVING ITS REGISTERED OFFICE AT BHEL HOUSE, SIRI FORT, NEW DELHI -110049,

Inventors:

#	Inventor's Name	Inventor's Address
1	BHATTACHARYA, JAYANT LAL	EMC, CORP, R&D, BHEL, HYDERABAD-500 593
2	RAMAKRISHNA, GUDURU	AME, HEP, BHEL, PIPLANI, BHOPAL-462 022

PCT International Classification Number

H02P 5/40

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA