In the following attached files, you can find the method for calculating the direct axis inductance (Ld) and quadrature axis inductance (Lq) with respect to a star wound rotor. To explain briefly, in order to measure Ld, I would block my rotor in a position where the direct axis of the permanent magnet is in line with the A-Phase of the rotor. At this point, I can conduct a DC Decay Test (pls refer attached document - ACTA-2013-1-07.pdf). In case of a star winding, I would multiply the obtained value by (2/3) and thus get my Ld value. Same goes for Lq, but I would block it in a position which is 90° electrically ahead. But in my case, I have a PMSM with a delta wound rotor. Can anyone suggest, how to calculate the Ld and Lq incase of a delta winding.
The way the motor is wound does not actually make much difference. In both cases you have only three wires coming out of the motor. The easiest way to find correct rotor position for measurements may be to connect the positive output of a DC current supply to phase a and the negative output to phases b and c (connected together). Increase the DC current until the rotor rotates itself to a position where it is aligned with the stator field. Mark the mechanical position. Repeat the procedure few times from different initial positions in order to minimize the error caused by friction. The average of the positions obtained is then the position you can use to find Ld. Rotating to rotor by 90 electrical degrees from that position you can measure Lq.
Dear Bhupendra Desai,
How can I conduct slip test on a PMSM motor, where there is a permanent magent in place of the field? As far as I know, slip test is generally done for alternators or induction motors. Please enlighten me with a little elaborate explanation.
Thank you very much
Dear Juoko Niiranen,
Thank you for that input, but I already know what you have explained. My intention is basically to plot the following graphs:
Ld vs Id,
Lq vs Iq,
Ld vs Iq,
Lq vs Id.
Incase of using an RLC meter to measue the inductances, I am not sure if I can use an RLC meter when I have high values of current flowing in the windings. Also even if I could probably use the RLC meter, the point of measurement is another question. If you can imagine the setup of the procedure you have advised, you would probably realize that with the motor connected to the DC Supply, I do not have a point to measure with the RLC meter.
I hope you can understand my problem. Please suggest further methods of achieving my graphs.
Thank you very much
Regards
Naresh Kakani
As you note the RLC meter cannot be used to measure the inductance when there are high DC voltages and currents present. The methods suggested in ACTA paper are OK. However, they can identify only Ld vs Id and Lq vs Iq.
Regarding cross coupling, that is Ld vs Iq and Lq vs Id, the only way may be to use a vector controlled frequency converter to measure currents and estimate flux linkages of the machine at different voltages and loads. As commercial frequency converters usually don't show Id, Iq and Psi_d and Psi_q in their user interface, you should get some research oriented drive for your study.
Follow the steps given below to measure the d-axis inductance Ld:
1. Align the rotor to phase A. Phase A is connected to the positive potential (+) and phase B and C are grounded (-).
2. Lock the rotor shaft.
3. Apply negative step voltage. Phase A is grounded (-) and phases B and C are connected to the positive potential (+). Usual level of the current is about 10% of the rated phase current.
4. Measure the step response of the current by a current probe.
5. Calculate inductance Ld.
Follow the steps given below to measure the q-axis inductance Lq:
1. Align the rotor to the q-axis. Connect the phase B terminal to the positive potential (+) of the voltage source and phase C is grounded (-). Phase A terminal is floating.
2. Lock the rotor shaft firmly because current step response in q-axis creates torque.
3. Generate a current step response in this configuration: phase A is connected to the positive potential (+) of the voltage source and phases B and C are grounded.
4. Calculate inductance Lq
in the same way as Ld.
Now shortest way to determine the Ld and Lq is to know the type of motor first.Either it is IPM or SPM.
SPM PMSMs are generally having same value of Ld and Lq, which is (2/3)*Ls.
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