Title of Invention

A DEVICE FOR CONVERTING SINGLE PHASE AC TO THREE PHASE BALANCED AC

Abstract

A device for converting single phase AC to three phase balanced AC, according to this invention, comprises an induction motor receiving a single phase AC supply; a space vector controller and inverter connected to the output of the induction motor for sensing the unbalanced voltages and extracting the positive and negative sequence components and feeding the negative sequence components back in phase opposition to cancel out from the unbalanced voltages, such that the supply to the load is a balanced supply.

Full Text

This invention relates to a device for converting single phase AC to three phase balanced AC. Amo converter is a rotating machine that is used to convert single-phase AC to three-phase AC. It is extensively used in AC electric locomotives to supply auxiliary induction motors. It is essentially a three-phase squirrel cage induction machine that transfers power both through electric conduction and electro-magnetic induction. As only single-phase supply is available through the catenary in electric locomotives, the machine is started using a starting resistor. The three-phase voltage available at the Amo terminals is unbalanced. This unbalance increases monotonically as the Amo load increases. Analysis of unbalanced voltages or currents is made easy by breaking the unbalanced three- phase system into three sets of balanced systems (Fortescue-1918). So, the unbalanced three-phase voltages or currents can be decoupled into positive, negative and zero sequence components. The zero-sequence components are absent in a three - wire system with ungrounded neutral. The unbalance in the three-phase currents in a system not only depends on the voltage unbalance, but also on the type of load connected to the supply. For a passive load the current unbalance is the same as voltage unbalance. But an active load like induction motor draws currents that are more unbalanced than the voltages applied. This is because a balanced three-phase passive load offers same impedance to positive and negative sequence voltages; whereas, an active load offers much smaller negative sequence impedance as compared to the positive sequence impedance. This leads to a large unbalance in the three-phase currents for a small unbalance in the three-phase voltages. So the relation between positive sequence ande negative sequence components is non-linear for active loads like induction motors. To reduce the voltage unbalance at the output, several modifications have been suggested to the original squirrel cage induction motor working as Amo converter. One modified Amo converter proposed has more number of tums in one of the phases (12.5% more than the number of tums in the other two windings). This leads to much lower unbalance at light loads and monotonically increasing unbalance with increasing load. A later design proposed changes in the phase angle of the third winding along with number of tums. This led to higher voltage unbalance ratio at light loads, but the unbalance is found to reduce monotonically with load, with an unbalance ratio of less than 5% at loads higher than 40%. So the presently available designs of Amo converters give less than 5% voltage unbalance output either at light loads or at near full loads. The presence of large negative sequence currents lead to increased ohmic losses and non-uniform heating in the three phases, in addition to production of negative torque. This leads to poor performance and faster ageing of the machine. There is also a possibility of currents greater than the designed upper limit of the motor current flowing in one or two phases of the machine. It is known that presence of continuous voltage unbalance of as low as 5% leads to a derating of 70%. So, apart from supplying unbalanced voltages, the Amo converter is also highly de-rated. An Amo supplying 150 kVA has to be built with specifications of a 200 kVA induction motor which calls for the use of a machine of larger size, cost and weight The cost further goes up because the machine is custom-made with special winding specifications. This invention therefore proposes a device for converting single phase AC to three phase baianced AC in AC locomotives to replace the conventional Arno. It consists of an induction motor so modified as to give a baianced three phase output on being supplied a single phase AC input. Voltage unbalance means presence of negative and zero sequence components. These components are removed using an inverter of much smaller capacity. Single phase to three phase conversion is done either using a converter or an induction motor. The proposed scheme uses a Standard off the shelf three-phase motor and a converter. The negative sequence components aie extracted from the unbalanced output voltages and fed through an inverter back to the output terminals. If the system is a four wire system, the unbalanced voltage would also have zero sequence components. Space vector theory is used for the separation of sequence components and pulse generation for the inverter. The three-phase space vector voltage can be represented as v= 2/3(a + b ej2π/3 + c ej4π/3) where a, b and c are instantaneous values of the three - phase voltages. Considering a system with ungrounded neutral that does not contain zero sequence components, the three-phase voltage can be split into positive and negative sequence voltages. The voltage space vector is split into positive and negative sequence components The three-phase voltages can also be split into positive and negative sequence voltages. Where Va+, Vb+ and Vc+ are the positive sequence components and Va., Vb. and Vc. are the negative sequence components of Va ,Vb and Vc respectively. Three-phase voltages can be transfonned ixito orthogonal α-β components. So, the positive and negative sequence components can also be transformed into a-p components. The vectors α+, β+, α. and β. are represented as sinusoidal functions. They are then rotated by 90 degrees. By using algebraic operations the positive and negative sequence components can be calculated from the original tt+, P+, a. and p. and the rotated vectors. Once the positive and negative sequence components are separated, the negative sequence is fed back with opposite polarity so that it cancels out from the unbalanced voltages. The resultant output voltage has therefore reduced voltage unbalance. The device for converting single phase AC to three phase balanced AC, according to this invention, comprises an induction motor receiving a single phase AC supply; a space vector controller and inverter comiected to the output of the induction motor for sensing the unbalanced voltages and extracting the positive and negative sequence components and feeding the negative sequence components back in phase opposition to cancel out from the unbalanced voltages, such that the supply to the load is a balanced supply. This invention will now be described with reference to the accopmpanying drawings wherein Fig. 1 illustrates the basis schematic diagram of an amo converter Fig. 2 illustrates, by way of example, and not by way of limitation, an embodiment of this invention. The 3-phase induction motor M receives a single phase AC supply S. The motor has a startiiig resistor R. C indicates a space vector controller and inverter connected to the output of the induction motor M for sensing the unbalanced voltages and extracting the positive and negative sequence components. öf the separated positive and negative sequence components the negative sequence components are fed back to the output of the motor in phase opposition to cancel out from the unbalanced voltages, such that the supply to the load is a balanced supply. The main advantages of the improved Arno converter are:  No special design for the machine - a Standard 3-Phase induction motor off the shelf can be used  Rating of motor is equal to its output power (no de-rating)  Less voltage and current unbalances, leading to increased efficiency  The inverter used in the above embodiment has a capacity of only about 2% of the Amo converter as it is used to supply only the negative  sequence components. So, for a 150 kVA Arno, the inverter rating would be around 3 kVA.  Less heating.  All the above advantages lead to longer life when compared to the conventional amo. In addition, the auxiliaiy motors (the load) are supplied with balanced voltages and currents. There are no negative sequence currents and torque leading to reduced losses and increased efficiency. All these benefits lead to prolonged life of the load, more so, in the case of active loads like motors. We Claim: 1. A device for converting single phase AC to three phase balanced AC, according to this invention, comprises an induction motor receiving a single phase AC supply; a space vector controller and inverter connected to the output of the induction motor for sensing the unbalanced voltages and extracting the positive and negative sequence components and feeding the negative sequence components back in phase opposition to cancel out from the unbalanced voltages, such that the supply to the load is a balanced supply. 2.A device for converting single phase AC to three phase balanced AC, substantially as herein described and illustrated with reference to Fig.2 of the accompanying drawings

Documents:

1368-CHE-2005 EXAMINATION REPORT REPLY RECIEVED 07-10-2009.pdf

1368-che-2005-claims.pdf

1368-che-2005-correspondnece-others.pdf

1368-che-2005-description(complete).pdf

1368-che-2005-drawings.pdf

1368-che-2005-form 1.pdf

1368-che-2005-form 18.pdf

1368-che-2005-form 26.pdf