Title of Invention

A SENSORLESS VARIABLE SPEED DRIVE FOR A SLIP RING INDUCTION MOTOR AND A SLIP RING INDUCTION MOTOR INCORPORATING THE SAME

Abstract

A novel technique for sensorless, direct torque and frequency of doubly fed slip-ring induction motor is claimed. Novel frequency control are given to make the sensorless drive operation reliable and machine parameter independent at any rotor speed. Using this proposed controller, the drive full torque from standstill to twice the rated speed of the machine for either direction of rotation without any rotor transducer. Thus double the rated power can be extracted from the induction machine without overloading it. This claim has established the potential of double inverter fed slip-ring induction motor drive in the filed of variable speed drive application. In high power and high performance application areas like rolling mill, cement mill, earthmovers, cranes, traction, drives etc., this drive offers very attractive alternative to conventional drives.

Full Text

This invention relates to a sensorless variable speed drive for a slip ring induction motor and a slip ring induction motor incorporating the same. FIELD OF INVENTION This invention in general relates to Slip Ring Induction Motors. Further this invention is related to the control of speed sensorless doubly fed slip ring induction machines. More particularly this invention pertains to the control of Doubly Fed Speed Sensorless Slip Ring Induction Machine drives based on Direct Torque and Frequency Control for Double Power Output and Stable Zero Speed Operation. PRESENT STATE OF THE ART For high power drives whose ratings are less than several thousand of kilowatts, single multilevel converters are available. However, for more than several thousand kilowatts, two inverters are combined with interphase reactors. These interphase reactors have the problem of high acoustic noise and high losses. They are generally bulky and hence, have both economic and space disadvantages. Recently, it has been shown that grid-connected slip ring induction motors with current injection on the rotor side can be operated in super synchronous mode to produce up to two times the nominal power rating. More generally, both the stator and the rotor can be fed from variable frequency mverters. The controller, required for those two inverters should be reliable and should have very good dynamic performances. A new control algoritihrm as been developed for this. It controls the torque and the frequency of the motor directly. To increase the reliability of the drive, we have introduced a novel rotor speed sensorless operation. Generally, sensorless operation is unstable near zero rotor speed. However, we could achieve very good stable operation near zero rotor speed including stall operation. LIMITATIONS OF THE EXISTING SYSTEM 1. Slip ring induction motor drive, with stator connected to the grid directly and rotor fed with inverter does not have capability of reversal of speed. 2. The system with two separate torque current controllers for both stator and rotor side inverters can lead some instability problems, smce the torque components of the stator and rotator current are proportional. 3. The reported algorithm in the existing system is sensitive to machine parameters. 4. In the existing system there is no claim about the double rated speed operation without field weakening. 5. The existing system uses costly rotor speed transducer and associated circuitry. 6. The existing system has zero frequency operation on both the sides (stator and rotor) of the machine at some speed of operations. Hence, at these speeds, there will be thermal stress on the switching devices of a specific arm at full load condition. DESCRIPTION OF PRIOR ART The previous work on slip ring induction motor control using variable frequency sources on both the rotor and stator side are represented by the references [1], [2], [3] given at the end of the description. The present proposal is different from the work described in the references in the following way: A. All the references assume the availability of a rotor position signal from a rotor position sensor; the present work is the first to propose a sensorless control method; B. The previous work does not ensure that the fundamental frequency on the two sides of the motor is always more than a minimum value; in the present proposal the rotor frequency profiles of 4a, 5a and 6a and the corresponding stator frequency profiles of 4b, 5b, and 6b ensure that the frequencies on the two sides are always above a (selectable) minimum value. C. The previous literature uses the method of vector control with sensors, which are dependent to some extent on the knowledge of the parameters of the machine; in the present proposal, a direct method for the control of torque and flux is proposed which is very little dependent on the machine parameters. OBJECTS OF THE INVENTION It is a primary object of the invention to design a sensorless variable speed drive for a slip ring induction motor with Direct Torque Control on the stator (or rotor) side of the Doubly Fed 3-phase asynchronous machine and Direct Frequency Control on the rotor (or stator) side of the same machine to increase the reliability of the drives. It is another important object of the invention to design a variable speed drive, which is very much less dependent on machine parameters, so that the reliability of the drive improves considerably. It is another object of the invention to maintain high switching frequency operation at all times, even with high torque, without any thermal stress on any of the IGBT (or any other switching devices) arms for any rotor speed. It is another object of the invention to make the controller rugged and reliable by which the Doubly Fed 3-phase machine can run up to double the rated speed on either direction, without field weakening operation. It is another object of the invention to ensure full torque operation for the machine up to double the rated speed and hence, double the rated power can be extracted from the machine. It is also another object of the invention to invent the frequency profiles for Direct Frequency Control to ensure that the frequency on either side of the Doubly Fed machine never goes below to 12Hz. Another important object of the invention is to eliminate the need of costly, unreliable rotor transducer and to make drive more rugged. This invention thus provides a sensorless variable speed drive for a slip ring induction motor comprising two inverters of which one is connected to the stator and the other is connected to the rotor of the induction motor, a controller each for the rotor side and stator side, a third controller for speed control, two input means to input the speed reference and estimated rotor speed respectively, output means to output thetgrgue reference, means to feed the torque reference to a torque comparator, means to compare the estimated machine torque calculated from the stator currents and stator flux, means to estimate the stator flux which is fed to a flux comparator along with stator flux reference, and means to determine the optimal switching pattern for the stator inverter using the output from the flux and torque comparator along with the stator flux angle. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION Now this invention will now be described in detail with reference to the accompanying drawings of the complete specification. The descriptions will extensively deal with the nature of the invention and the manner in which it is to be performed. In the drawings: Fig. 1 illustrates the basic configuration of Doubly fed induction Motor drive; Fig. 2 illustrates the overall block diagram of the sensorless variable speed drive for the slip-ring induction motor; Fig. 3 shows the angular position of machine flux to understand suggested control algorithm; and Figs. 4a, 4b, 5a, 5b, 6a, and 6b show the proposed unique frequency profiles, which make the sensorless controller reliable and machine parameter independent at any rotor speed. The basic configuration of the doubly fed slip-ring induction machine drive is shown in fig 1. Stator and rotor are fed separately through 3 phase IGBT inverters. Both the inverters are connected to a common dc bus as shown in fig 1. Note that the IGBT is taken as one possible device; these inverters can also be constructed using other gate turn-off power semiconductor devices such as power MOSFETs, GTOs, IGCTs etc. Each inverter has three phases; each of these phases is taken to be of the two-level type in the present embodiment, consisting of two switching devices, each with its anti-parallel diode. However, higher-level inverters such as three level inverters can equally well be employed with suitable changes in the optimal switching logic (fig.2). The two inverters are connected to the same DC voltage source. In the present embodiment, the DC vohage source is realized as the energy stored in a large electrolytic capacitor, as is the practice in AC motor drives. This energy is usually derived from the three phase mains using a three-phase diode rectifier in case regeneration of power is not required. Figure 2 shows the block diagram of the control system for the drive shown in fig. 1. In this embodiment of the control, the gating signals of the stator side inverter, two of the output currents of the stator side inverter and the DC bus vohage are measured and taken as feedback signals. These feedback signals are fed to the "Motor Model" block; this block outputs the magnitude of the stator flux, the rotor speed, the developed torque and the instantaneous position of the stator flux. The rotor speed signal is compared with the rotor speed reference signal; the error between the two is given to a proportional integral (PI) controller, which outputs the reference for the developed torque. The reference torque and the feedback torque are compared in a two level hysteresis comparator called the "torque comparator". Likewise, the reference for the flux magnitude and the feedback flux magnitude are compared in another two level hysteresis comparator called the "flux comparator". The two level outputs of these two comparators are then given to the "optimal switching block", which is a look-up table; this table outputs the gating signals for the three phases of the stator side inverter. The rotor speed feedback signal produced by the "motor model" block is also given as input to a frequency profile generator, labeled "rotor frequency" in fig.2. This generator produces the fi-equency to be developed by the rotor side inverter. Fig.4a, 5a and 6a show three possible fi-equency profiles that can be used. Using these frequency commands and the constant volts/hertz principle, the gating pulses for the rotor side inverter are generated. Because of the torque control on the stator side, the stator frequency will automatically adjust itself. The stator frequency profiles of fig.4b, 5b and 6b correspond respectively to the rotor frequency profiles of fig.4a, 5a and 6a. The rotor speed is calculated in the "motor model" block as the difference between the stator frequency and the rotor frequency, without the use of any mechanical speed or position sensors. There is DSP (Digital Signal Processor) based digital controller board. This control board controls the two inverters (stator and rotor) by generating the switching pulses for the IGBTs of each inverter. The control algorithms are described below. Stator side is controlled by Direct Torque Control (DTC) method. Rotor side is controlled by simple constant Voltage/Frequency (V/f) method. The control block diagram of the stator side and the rotor side controller is shown in fig 2. The drive has a PI controller for speed control. This speed controller has one speed reference (ωr) and estimated rotor speed (ωr) as two inputs. The output of the speed controller is the torque reference (md). This torque reference is directly fed to the torque comparator (fig2.). It is compared with the estimated machine torque (md). The machine torque is estimated from the cross product of stator currents (is) and stator flux (ψs) as follows - The stator flux vector is estimated from the following equation- The estimated stator flux is fed to the flux comparator along with stator flux reference (ψs ). The flux comparator and the torque comparator output along with stator flux angle (Ps) determine the optimal switching pattern (SI, S2, S3) for the IGBTs of the stator inverter. The stator flux angle (Ps,) is obtained by integrating the stator flux speed (ωs). The stator flux speed is estimated from the following equation- Thus, the stator flux and the machine torque are made equal to their reference values with some ripple, which is limited within the bandwidth of the comparators. Hence, this method is termed as direct torque control. Now, the rotor flux frequency command (ωmr) is generated from a frequency profile, based on the estimated rotor speed (ωr). This frequency profile will be discussed later on. There is a PWM controller block to generate the switching pattern (S1, S2, S3) for the IGBTs of the rotor inverter. The input of this PWM block is the frequency command « p (ω. This PWM block keeps the rotor flux (ψ) approximately constant by keeping the ratio of rotor voltage and rotor frequency constant. The rotor flux speed (ωmr), stator flux speed (ωms), and the rotor speed (ωr) are related as foUows- At any particular rotor speed (ωr), the PWM block directly controls the rotor flux speed (ωmr). The stator flux speed (ωms) also has a unique value confirming the equation (4) so, this algorithm is also termed as direct frequency control. Next, the analysis of how the direct torque control on the stator side always ensures the equation (4), is discussed. The developed torque (md) of the motor is proportional to the p cross product of stator flux (ψs) and the rotor flux (ψr). In our algorithm, rotor flux runs freely with predetermined frequency. Since the developed torque of the motor is kept constant to its reference value, the stator flux is forced to follow the rotor flux such that the angle 6 is always constant (fig3.)- This means that the relative velocity i.e. (ωr + ωmr)-ωms) between the stator flux and the rotor flux is zero. Hence at steady rotor frequency, equation (4) is obeyed with the help of direct torque control on the stator side of the motor. Now, a situation is considered, when the rotor frequency is rapidly changed. Then, it will be shown how the direct torque control method restores the relation (4). It is assumed that the motor is running with constant positive speed and also with constant load torque. It is also assumed that the rotor frequency is positive. Then, the stator frequency must also be positive and higher than the rotor frequency from (4). Now, suddenly, if the rotor p p frequency is reversed,ψr starts moving away from ψs, as the rotor speed does not change instantly. So, 5 starts increasing. This is reflected in torque rise of the machine. Direct torque control type of control on the stator side selects the proper switching vector for the inverter at each instant. So, it arrests the rise of torque (i.e. 5) by switching the inverter in proper sequence. Thus, the torque is maintained constant with small ripple, limited by the torque error bandwidth in the torque comparator. So, the stator frequency must automatically become smaller to maintain the torque (equivalently 6) constant to the reference value. Thus, the relation (4) holds good for all operation conditions and controls the stator frequency indirectly. p p Thus, the stator flux (ψs,), the rotor flux (ψr) and the developed torque of the motor (md) are kept under control directly under all the operating conditions. These, in turn, ensure that there are no transient in current on both the sides (stator and rotor) of the motor. The dynamic operation of the motor becomes very fast. Finally, the rotor frequency profile generation method is presented to ensure the reliable sensorless operation of the motor. The reliability of the sensorless operation is dependent p p on the estimation of the stator flux vector (ψs) and the rotor flux (ψs). At very low stator frequency (below say, 5Hz), the estimation of stator flux vector becomes very p much unreliable. Due to variations of stator resistance drop (isRs) and inaccurate p estimation of stator line voltage Vs , the output of the equation (2) drifts away from the actual machine flux. Similarly, at very low rotor frequencies also, controller does p not ensure constant rotor flux ((ψs). So, a new rotor flux frequency profile is developed to limit the lowest frequency on either side of the motor to 12Hz (for 50 Hz machine) at all operating conditions. This first scheme of rotor flux frequency (ωmr) is shown in fig 4a. The corresponding stator flux frequency (ωms) plot is shown in fig 4b. As mentioned earlier, the stator flux frequency is developed automatically, whereas the PWM controller develops the rotor flux frequency directly. Fig4a suggests that the rotor flux frequency is never below 15Hz. There are some frequency reference jumps. But, they do not lead to any current jump in the motor as long as the stator side current control (torque control) is very fast. In our system, direct torque control based torque controller on the stator side ensures that. So, the controller automatically generates the stator side frequency by confirming the equation (4) as shown in fig 4b. The stator side DTC based confroUer works fine when the stator flux frequency is high. In fact, the stator resistance drop error has negligible effect at high frequency. In this scheme, at low speed, both the stator and rotor side have minimum frequencies. So, the low speed performance, especially during sudden speed changes, is not very good. Hence, another scheme (scheme II) is suggested in fig 5a. Here, at zero rotor speed, rotor fiux frequency is 44Hz. From equation (4), the stator frequency is also 44Hz. Clearly, low speed operation is less dependent on machine parameters. So, the low speed operation is very smooth. In this scheme, beyond 35Hz rotor speed, the magnitude of the rotor frequency is equal to the half of the rotor speed like the previous scheme I. Below 30Hz, the rotor frequency is generated from the following equation- Now, from equation (4) and (5), it can be said that the stator frequency is automatically developed around 47Hz through out the low speed operation. This is an important advantage over the previous scheme. Now, the low speed operation is less dependent on machine parameter and it is smooth even during speed transients. The stator frequency, developed by the controller, is shown in fig 5b. At +/- 3Hz, there are two frequency jumps from 44Hz to -44Hz and vice versa. These frequency jumps are quite big and so, have small torque jumps. These are overcome in the next scheme III. Fig.6a gives the rotor frequency command profile of the scheme III. Here, rotor frequency command has hysteresis loop. There is no abrupt change over of frequency command near zero rotor speed. Hence, the low speed operation is very smooth and rugged. Fig6b. shows the corresponding frequency developed on the stator side. The frequency profiles shown for the all the schemes are interchangeable. That means for each scheme, the given stator frequency plot can be used as a rotor frequency command. Then, the generated stator freauencv plot will be the nresent rotor frequency command. References [IJY.Kawabata, E.Ejiogu, and T.Kawabata, "Vector-Controlled Double-Inverter-Fed Wound-Rotor Induction Motor Suitable for High-Power Drives," IEEE Trans, on Industry Applications, vol. IA-35, September-October 1999, pp. 1058-1066. [2]D.Lecocq, Ph.Lataire and W.Wymeersch, "Application of the double fed asynchronous motor (DFAM) in variable speed drives," European Power Electronics Conference, Brighton, 1993 [3]Gerald M.Brown, Barna Szabados, Gerard J. Holbloom, and Michel Poloujadoff, "High-Power Cycloconverter Drive for Double-fed Induction Motors," IEEE transactions on Industrial Electronics, vol.39, no.3, June 1992. WE CLAIM: 1) A sensorless variable speed drive for a slip ring induction motor comprising two inverters of which one is connected to the stator and the other is connected to the rotor of the induction motor, a controller each for the rotor side and stator side, a third controller for speed control, two input means to input the speed reference and estimated rotor speed respectively, output means to output the torque reference, means to feed the torque reference to a torque comparator, means to compare the estimated machine torque calculated from the stator currents and stator flux, means to estimate the stator flux which is fed to a flux comparator along with stator flux reference, and means to determine the optimal switching pattern for the stator inverter using the output from the flux and torque comparator along with the stator flux angle. 2) A sensorless variable speed drive for a slip ring induction motor as claimed in claim 1, wherein the two inverters, which in turn is connected, to a common dc bus. 3) A sensorless variable speed drive for a slip ring induction motor as claimed in claim 1 or 2, wherein means are provided to make the stator flux and the machine torque equal to their reference values with some ripple, which is limited within the bandwidth of the two comparators. 4) A sensorless variable speed drive for a slip ring induction motor as claimed in any one of claims 1 to 3, wherein the stator flux angle is obtained by integrating the stator flux speed. 5) A sensorless variable speed drive for a slip ring induction motor as claimed in claim 4, wherein the stator flux speed is estimated from the following equation: p wherein ((ψs) is the stator flux, Vs is the stator line voltage and igRs is the stator resistance drop. 6) A sensorless variable speed drive for a slip ring induction motor as claimed in any one of claims 1 to 5, wherein the stator side controller uses direct torque control. 7) A sensorless variable speed drive for a slip ring induction motor as claimed in any one of claims 1 to 6, wherein the rotor side controller uses simple constant voltage/frequency. 8) A sensorless variable speed drive for a slip ring induction motor as claimed in any one of claims 1 to 7, wherein the stator flux angle is obtained by integrating the stator flux speed. 9) A sensorless variable speed drive for a slip ring induction motor as claimed in any one of claims 1 to 8, wherein the inverters are selected from Insulated Gate Bipolar Transistor inverter or inverters constructed using other gate turn-off power semiconductor devices such as power MOSFETs, GTOs, IGCTs. 10) A sensorless variable speed drive for a slip ring induction motor as claimed in claim 9, wherein each inverter has three phases, each of these phases being of the two level type consisting of two switching devices, each with its anti-parallel diode. 11) A sensorless variable speed drive for a slip ring induction motor as claimed in any one of claims 1 to 9, wherein the inverter phases being of three level type with suitable modification. 12) A slip ring induction motor including a sensorless variable speed drive as claimed in any one of claims 1 to 11. 13) A method of sensing and controlling the speed of a slip ring induction motor which is fed by variable frequency inverter on both the rotor and the stator side, comprising the steps of; a. measuring the gating signals as "feedback signals" of the stator side inverter, two of the output currents of the stator side inverter and the DC bus voltage; b. obtaining the magnitude of the stator flux, the rotor speed, the developed torque and the instantaneous position of the stator flux by passing the "feed back signals" to a "motor model" block; c. comparing the rotor speed signal with the rotor speed reference signal; d. calculating the reference for the developed torque from the error between the two speed signals in step (c) using an proportional integral controller; e. comparing the feedback torque and the reference torque in a two level hysteresis comparator called the "torque comparator"; f. comparing the flux magnitude and the feed back flux magnitude in a second two level hysteresis comparator called the "flux comparator"; g. passing the outputs of both the comparators to the optimal switching block to obtain the gating signals for the three phases of the stator side inverter; h. generating the frequency profiles by passing the rotor speed feedback signal to a frequency profile generator; i. generating the gating pulses for the rotor side using the frequency profiles and the constant volts/hertz principle; j. calculating the rotor speed as the difference between the stator frequency and the rotor frequency.

Documents:

0360-mas-2001 abstract duplicate.pdf

0360-mas-2001 abstract.pdf

0360-mas-2001 claims duplicate.pdf

0360-mas-2001 claims.pdf

0360-mas-2001 correspondence others.pdf

0360-mas-2001 correspondence po.pdf

0360-mas-2001 description (complete) duplicate.pdf

0360-mas-2001 description (complete).pdf

0360-mas-2001 drawings duplicate.pdf

0360-mas-2001 drawings.pdf

0360-mas-2001 form-1.pdf

0360-mas-2001 form-13.pdf

0360-mas-2001 form-19.pdf

0360-mas-2001 form-26.pdf

0360-mas-2001 form-4.pdf

0360-mas-2001 form-5.pdf