Method for determining current-dependent inductances of a multi-phase electrical machine and frequency converter

Abstract

A method ascertains current-dependent inductances of a polyphase electrical machine. The method generates phase voltages for the polyphase electrical machine by means of a pulse width modulation such that currents of predefined current level flow through stator windings of the electrical machine. During a number of cycles of the pulse width modulation, the method generates a voltage pulse such that a change of current is brought about in a torque-forming axis of the polyphase electrical machine and/or in a field-forming axis of the polyphase electrical machine. The method measures the change of current, and ascertains the current-dependent inductances on the basis of the change of current.

Claims

1. A method for ascertaining current-dependent inductances of a polyphase electrical machine, the method comprising the steps of: a) generating phase voltages for the polyphase electrical machine by way of a pulse width modulation such that currents of predefined current level flow through stator windings of the electrical machine; b) during a number of cycles of the pulse width modulation, generating a voltage pulse such that a change of current is brought about in a torque-forming axis of the polyphase electrical machine and/or in a field-forming axis of the polyphase electrical machine; c) measuring the change of current; and d) ascertaining the current-dependent inductances based on the change of current.

2. The method according to claim 1, wherein steps a) to d) are repeated a number of times, the predefined current level being altered before step a) is repeated.

3. The method according to claim 1, wherein the number of cycles of the pulse width modulation is one.

4. The method according to claim 1, wherein a single voltage pulse is generated during a single cycle of the pulse width modulation.

5. The method according to claim 1, wherein the machine is a three-phase electrical machine.

6. The method according to claim 1, wherein the machine is a three-phase synchronous machine.

7. The method according to claim 1, wherein a rotor of the electrical machine is blocked before step a).

8. The method according to claim 1, wherein the current-dependent inductances are selected from: differential series inductances, differential parallel inductances, or cross-saturation.

9. A frequency converter designed for controlling a polyphase electrical machine, wherein the frequency converter comprises: three half-bridges, each having at least two switches, wherein the frequency converter is designed to carry out the acts of: a) generating phase voltages for the polyphase electrical machine by way of a pulse width modulation such that currents of predefined current level flow through stator windings of the electrical machine; b) during a number of cycles of the pulse width modulation, generating a voltage pulse such that a change of current is brought about in a torque-forming axis of the polyphase electrical machine and/or in a field-forming axis of the polyphase electrical machine; c) measuring the change of current; and d) ascertaining the current-dependent inductances based on the change of current.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a frequency converter designed for performing the method according to the invention,

(2) FIG. 2 shows a time characteristic for signals within one cycle of a pulse width modulation performed by means of the frequency converter from FIG. 1, and

(3) FIG. 3a/b show a time characteristic for different operating points and inductances ascertained at the operating points; and

(4) FIG. 4 shows a flowchart of an example method.

DETAILED DESCRIPTION OF THE DRAWINGS

(5) FIG. 1 shows a frequency converter 1 designed for controlling a three-phase electrical machine 2 in the form of a synchronous motor, wherein the frequency converter 1 conventionally has three half-bridges B1, B2, B3, each having two semiconductor switching means S1, S2; S3, S4; S5, S6. The switching means S1, S2; S3, S4; S5, S6 are controlled by a control unit 3 of the frequency converter 1.

(6) According to the invention, phase voltages U, V, W for the three-phase electrical machine 2 are generated by means of a conventional pulse width modulation such that currents of predefined current level flow through stator windings 2a, 2b, 2c of the three-phase electrical machine 2, or that a predefined or desired current is produced in the d-direction or q-direction in an iq coordinate system.

(7) Referring to FIG. 2, a single voltage pulse P is generated by virtue of a change of switching state for the phase V, while the switching states of the phases U and W are constant, during or within a single cycle or a single period TKT of the pulse width modulation such that a change of current Δid is brought about in a torque-forming axis (q-axis) of the polyphase electrical machine 2 and/or in a field-forming axis (d-axis) of the polyphase electrical machine 2.

(8) The changes of current Δid and Δidq brought about by the voltage pulse P are subsequently measured and this is taken as a basis for calculating current-dependent differential inductance(s), such as for example the series inductances Ldd and/or the parallel inductances Lqq. For the fundamental calculation specifications required for this purpose, reference should also be made to the relevant specialist literature.

(9) A rotor 5 of the electrical machine 2 can be blocked for performing the method.

(10) Referring to FIG. 3a/b, there follows a description of how the method steps described above are repeated for a plurality of different current operating points 1 to 29.

(11) FIG. 3a depicts a time characteristic for a current setpoint value S_SW and an arising current actual value S_IW in the d-direction or q-direction of a dq coordinate system. The current setpoint value S_SW exhibits a staircase-shaped characteristic, one of the operating points 1 to 29 having precisely one associated current setpoint value S_SW.

(12) A voltage pulse P is output for each of the operating points 1 to 29 in order to determine the differential series inductance Ldd and/or the differential parallel inductance Lqq at this operating point.

(13) The voltage pulse brings about an operating-point-dependent change of current Δid that is used for calculating the differential series inductance Ldd and/or the differential parallel inductance Lqq at this operating point.

(14) FIG. 3b shows the differential series inductances Ldd and differential parallel inductances Lqq calculated at the current-dependent operating points 1 to 29.

(15) The invention allows simple and reliable identification of the differential inductances, in particular the differential series inductances Ldd and/or the differential parallel inductances Lqq, by means of a single voltage pulse for one cycle of the PWM at different current operating points in a short time.

(16) The method 400 according to the invention has the following steps.

(17) In step S405: generating phase voltages for the polyphase electrical machine by means of a pulse width modulation such that currents of predefined current level flow through stator windings of the electrical machine.

(18) In step S410: during or within a number of cycles of the pulse width modulation, generating a voltage pulse such that a change of current is brought about in a torque-forming axis of the polyphase electrical machine and/or in a field-forming axis of the polyphase electrical machine.

(19) In step S415: measuring the change of current produced.

(20) And finally in Step S420: ascertaining the current-dependent inductances based on the change of current.