GB2163264A - Measurement of multi-phase electrical machine torque - Google Patents

Abstract

The electromagnetic torque t0 of an a.c. machine is monitored continuously from measurements only of supply current and voltage, digital representations of the current and voltage being used for calculation of machine supply-side flux psi from which and the current measurements machine torque is computed 43 without the use of analogue techniques so that continuous drift-free operation is achieved. <IMAGE>

Description

SPECIFICATION Measurement of multi-phase electrical machine torque This invention relates to the monitoring of the electromagnetic torque of a multi-phase alternating current electrical machine in the steady state and transient modes of operation.

Torque monitoring based on the introduction of search coils and/or flux probes into the air-gap of an electrical machine is not very practical. A technique is known for monitoring the electromagnetic torque by utilising the supply currents and voltages of a two-or-threephase a.c. machine (Hungarian Patent Specification 167905). This technique is also of limited application as the measuring instrument depends on analogue circuits which are inherently liable to introduce errors and are not sufficiently stable.

It is an object of the invention to provide a torque monitoring technique and an instrument using such a technique which is suitable for general use on a wide range of multiphase a.c. electrical machines.

According to the invention there is provided a method of monitoring the electromagnetic torque of a multi-phase a.c. electrical machine including monitoring of the voltages applied and the currents flowing and computing the machine torque, characterised by monitoring the instantaneous voltages and currents as sets of measurements at intervals in time, forming digital representations of the measurements, processing said representations for different set of measurements to produce time-incremental values as an indication of machine supply-side fluxes, and using said flux indications and current measurements to compute machine torque on a continuous basis.

Preferably the voltage measurements are corrected for appropriate resistive stator voltage drops on a continuous, current related basis. The correction may be based on a measurement, monitoring, a computation, or a record of stator voltage drop variation with current.

The computation of machine torque can be continuous without drift-correction as analogue computation is not present.

In the special cases where the machine is operated with certain conditions constant the computation of machine torque can be made on a continuous basis without requiring an integration step.

According to the invention there is provided a method of monitoring the electromagnetic torque of a multi-phase a.c. electrical machine from electrical supply conditions including measuring the voltages applied and currents flowing, forming digital representations of the measurements, calculating machine supplyside flux from said measurements and then calculating machine torque from the current measurement and flux calculations, and performing the measurements and calculations intermittently or continuously without the use of analog computation so that continuous drift-free computation of torque is achieved.

According to the invention there is provided an apparatus to monitor the electromagnetic torque of a multi-phase a.c. electrical machine including means to measure the voltages applied and currents flowing in the electrical connections to the machine, means to form digital representations of the measurements, means to calculate from said digital representations the machine supply-side flux and to then calculate electromagnetic machine torque from said current measurements and flux calculations, the apparatus being arranged to perform the measurements and calculations intermittently or continuously without the use of analog computation to achieve continuous drift-free computation of torque.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a block diagram of a prior art method of torque measurement, Figure 2 is a block diagram of a circuit for a method of torque measurement according to the present invention, Figures 3a, 3b, 3c show circuit elements of a polyphase embodiment of the invention, Figure 4 shows an outline of an arrangement to measure torque in a polyphase machine from supply-side values, and Figure 5 shows equations relating to the invention.

Figure 1 is derived from Pigure 1 of Hungarian Patent Specification 1 67905 and shows an analogue method of calculating machine torque from supply side measurements of machine currents and voltages by unit 1.

Lines 21 and 23 carry analogue voltages e.g.

UR, UT, and lines 22 and 24 carry analogue currents e.g. In, UT. Corrections for stator voltage drops are made by voltage dividers 6 and 7 which produce voltages for subtraction from the voltage analogues in units K1, K2.

The corrected voltage analogues are integrated in anallogue integrators 8 and 9 to produce analogue values of machine fluxes.

These analogue values are supplied to respective inputs of analogue multipliers 10 and 11 to other outputs of which the analogue values of current are supplied. The flux-current product from each multiplier is summed in summing amplifier 1 2 to produce an analogue signal proportional to torque.

Such an arrangement, while effective as a laboratory research instrument, suffers from the inherent drift of analogue instrumentation.

Figure 2 shows a block diagram of an instrument for industrial test use which avoids the drift problems. Analogue values of supply side voltage and current are monitored in unit 21. However samples of these values are then taken by individual sample-and-hold circuits 22, 23, 24, 25. A multiplexer 30 selects sampled values and supplies these serially at its output. So far, although the signals are analogue, no processing able to cause drift has occurred. The output of the multiplexer 30 is connected to an analogue-to-digital converter 31. The output of converter 31 is thus a series of digital representations of supplyside conditions. These can then be processed in a drift-free manner to compute machine torque on a continuous basis.

Thus a programmed microprocessor or specific dedicated digital circuit can be provided, at 32, to carry out the following procedure, using a series of digital representations of two phase voltages (UA, UB) which can be true phase voltages or their two-axis components and two supply currents (1A, 1 B) which can be true phase currents or their two-axis components: X = O; Y =0; FOR J = 2 TO N STEP 1 XA = H/2"(UA(J - 1) + UA(J) R*(lA(J - ) + lA(J))); XB= H/2*(UB(J1) + UB(J) - RlB(J - 1) + IB(J))); X = XA + X; Y = XB + Y; T = C*(X*IB(J)Y*IA(J)); NEXT J This algorithm does not provide for nonzero integration constants but these can easily be incoporated if required.

The digital representations of the ua, Ub, iaw 1b analogue values at (j - 1 )th and jth instants are UA(J - 1), UA(J). IA(J -- 1), IA(J). R is the primary resistance, the prinary winding being usually but not always on the stator. H is the time between two samplings, C is constant and N is the number of samples. T is the electromagnetic torque. The effects of m.m.f. space harmonics are neglected.

The digital representation of T is provided at the output of element 32. An analogue value of torque can be provided in any suitable manner from its digital representation.

In a practical device the unit 21 would include means to reduce the supply potential and current to a suitable level and means to isolate the supply, such as isolating amplifiers.

If such means introduce d-c offset errors, of drift errors, nulling techniques or reduction of the sampled value can be applied to correct the position. In the algorithm the digital representations are those after correction.

If required an algorithm giving greater accuracy can be used, as follows: X = O; V =0; FOR J = 3 TO N STEP 1 XA = H/3'(UA(J - 2) + 4UA(J - 1) + UA(J) - RlA(J - 2) + 441A(J - 1) + IA(J))); XB= H/3*(UA(J - 2) + 4*UB(J - 1) + UB(J) - R*(lB(J - 2) + 4*lB(J - 1) + IB(J))); X=XA+X; V=XB+V; T = C*(X4lB(J) - V*lA(J)); NEXT J The above comment on non-zero constants of integration applies here, as does the meaning of the symbols used.

Here three successive samples are used.

Clearly more processing or a more complex specific circuit is required than that for the previous algorithm.

Circuits for obtaining the L-phase voltages and currents and their two-axis components from phase quantities are shown in Figures 3a, 3b and 3c. Alternatively these components can be obtained from the line quantities.

Figure 3a shows a circuit portion to detect the instantaneous values of the phase voltages ua, ub,...u,. Figure 3b shows a circuit portion to condition these values to the two-axis components ux, uy (and ix, iy) of the voltage spacevector ú (and the current space-vector i). Figure 3c shows a circuit portion to detect the instantaneous values of the phase currents ia ib,. . . i L.

Figure 4 shows the schematic of the general form of a torque monitoring device for a polyphase machine, such as one energised by L phases, above. Reference 41 indicates the unit to produce analogue values and condition them as two-axis components as indicated for Figure 3. Unit 42 operates on the components to obtain the modulus and phase angle of the machine flux spacevector equations and the modulus and phase angle of the machine current space-vector using equations 1 to 3 of Figure 5. Unit 43 derives the machine torque from the modulus values and phase angles using equation 4 of Figure 5. Here c is a constant and p the number of pole-pairs.

Using the above techniques digital representations of machine supply-side values can be manipulated to produce consistent, driftfree, continuous torque monitoring for machines for any number of phases.

In certain circumstances one of the nachine operating conditions can be constant. Thus a nachine may be operated at constant magnetising flux or constant supply side total flux or in a sinusoidal stady state. In such circumstances the monitoring of torque using the invention can be simiplfied and the use of an integration process avoided. These must be considered to be special cases but they are non-the-less of practical importance as they can occur quite often in machine use.

Some of the various circumstances will now be considered.

1. Constant magnetising flux The total flux linkage space vector in equa tion 1 can be resolved into two components.

One is the leakage flux linkage space vector and is the product of the leakage inductance L, of a phase winding or the supply side of the machine and the current a of equation 3.

The other is the magnetising flux linkage space vector. Thus the e.m.f. due to the main flux can be determined by substracting the ohmic and leakage volt drops from the supply voltage.

The real and imaginary components of ú and X can be obtained from the phase values as described above. The instantaneous values of the direct (urn) and quadrature (urns) axis e.m.f. components of the voltage Urn induced by the respective magnetising flux linkage components can now be obtained as shown in equations 5 and 6. The last term of equation 6 is zero if saturation of the leakage flux paths can be neglected.

In the special case of the magnetising flux being constant the modulus of the magnetising flux linkage space vector is constant. The electromagnetic torque is then expressed by equation 7 where c, is also a constant and the function f3m is expressed in equation 8. Circuit units of the general form mentioned above can be used to obtain the real and imagninary components of ú and t, evaluate therefrom the instantaneous values of umDand umQand then evaluate function zero A simple multiplication of this function by the constant c, and the constant modulus of the magnetising flux linkage space vector then gives the result in equation 7 of the electromagnetic torque te By extension of the analysis phases the electromagnetic torque for an L-phase machine operated at constant magnetising flux can be obtained.

In the above calculations no integration is required, simplifying the circuits required.

2. Constant total flux When the modulus of the total flux linkage space vector is constant on the supply-side of the machine an analysis analogous to the one above for constant magnetising flux yields a similar expression for electromagnetic torque.

The e.m.f. rate induced in a supply winding due to the rate of change of total flux can be obtained by subtracting the ohmic voltage drop from the terminal voltage. This e.m.f. is related to the electromagntic torque. A function 35 analogous to 3m above can be expressed in terms of the direct and quadrature components of this e.m.f. and the electromagnetic torque is the product of this function, a constant and the modulus of the total flux linkage space vector. Again no integration is required and straightforward circuit units can be used to evaluate the components and carry out the calculations. The details will be apparent to those skilled in the art from the earlier discussion, as will the extension to an L-phase machine.

3. Sinusoidal steady state When the machine is being operated in the sinusoidal steady state the air-gap power can be determined and used as the basis for the calculation of electromagnetic torque. As in the previous cases the real and imaginary components of the voltage and current space vectors are obtained from the supply values.

These direct and quadrature components are used to calculate the total input power and the ohmic losess. The air-gap power is the total input power, less the ohmic losses. The electromagnetic torque is the product of the air-gap power and a constant for the machine, divided by the angular frequency of the supply voltage. Again simple circuit units can perform the required calculations, without integration or differentiation in this case. Also the analysis can be extended to L-phases as before.

The above special cases are all examples of the appiication of the invention to provide drift-free monitoring of electomagnetic torque from measurements of voltages applied and current flowing to a machine.

Claims (11)

1. A method of monitoring the electromagnetic torque of a multiphase a.c. electrical machine including monitoring of the voltages applied and the currents flowing and computing the machine torque, characterised by monitoring the instantaneous voltages and currents as sets of measurements at intervals in time, forming digital representations of the measurements, processing said representations for different set of measurements to produce time-incremental values as an indication of nachine supply-side fluxes, and using said flux indications and current measurements to compute nachine torque on a continuous basis.

2. A method according to Claim 1 in which the voltage measurements are corrected for appropriate resistive stator voltage drops on a continuous, current related basis.

3. A method according to Claim 2 in which the correction is based on a measurement, monitoring, a computation, or a record of stator voltage drop variation with current.

4. A method according to Claim 1 using digital computation of machine torque to provide continuous computation without introducing drift-correction as analogue computation is not present.

5. A method according to Claim 1 for monitoring a machine operated with certain conditions constant including computation of machine torque be made on a continuous basis without requiring an integration step.

6. A method of monitoring the electromagnetic torque of a multiphase a.c. electrical machine from electrical supply conditions including measuring the voltages applied and currents flowing, forming digital representations of the measurements, calculating machine supply-side flux from said measurements and then calculating machine torque from the current measurement and flux calculations, and performing the measurements and calculations intermittently or continuously without the use of analog computation so that continuous drift-free computation of torque is achieved.

7. An apparatus to monitor the electromagnetic torque of a multi-phase a.c. electrical machine including means to measure the voltages applied and currents flowing in the electrical connections to the machine, means to form digital representations of the measurements, means to calculate from said digital representations the machine supply-side flux and to then calculate electromagnetic machine torque from said current measurements and flux calculations, the apparatus being arranged to perform the measurements and calculations intermittently or continuously without the use of analog computation to achieve continuous drift-free computation of torque.

8. A method of monitoring the electromagnetic torque of an a.c. electrical machine from supply-side conditions in a continuous manner substantially as hereinbefore described by a digital calculation providing continuous driftfree operation.

9. A method of monitoring torque substantially as herein described with reference to one or more of Figures 2 to 5 of the accompanying drawings.

10. Apparatus to monitor torque substantially as herein described with reference to one or more of Figures 2 to 5 of the accompanying drawings.

11. The features herein described or their equivalent, in any novel selection.