# PM sychronous motor parameter measurement

1. Feb 16, 2015

### awaiting

Hi everyone!

I am trying to find the parameters of a surface mounted PM synchronous motor using the application note AN4680 (PMSM Electrical Parameters Measurement) and had some queries regarding it.
The Ld process is mentioned as follows

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. See Figure 11.
5. Calculate inductance Ld.

Lq inductance measurement
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.

I wanted to know how does step 1 in both the cases ensure the alignment of the rotor with d and q axis respectively. I tried to work it out mathematically, for d axis inductance measurement, if we connect phase A to +5V and ground phase B and C,

Vd=(2/3)*Va*cos(theta),
(from Park's transformation)
now if we assume theta to be zero, only then will Vq will be zero.

Similarly, if we connect phase B to +5V, ground C and open phase A, how is it concluded that we have aligned the rotor in the q -axis.

Secondly,during Lq (q axis inductance measurement) measurement it is mentioned that the current response creates a torque in the q-axis and the motor may rotate but I did not find any rotation in the rotor during both Ld and Lq measurement.

Could you please point out if I missing any point in the document.

Thanks,

#### Attached Files:

• ###### Freescale AN4680.pdf
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2. Feb 21, 2015

### Greg Bernhardt

Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?

3. Feb 22, 2015

### milesyoung

The stator current vector is given by:
$$i_s = i_a + i_b a + i_c a^2, a = e^{j2\pi/3}$$
For the d-axis alignment step:
$$i_s = i_a - \frac{1}{2} i_a a - \frac{1}{2} i_a a^2 = \frac{3}{2} i_a$$
The stator current vector will be aligned with phase A, which means the magnetic axis of the stator field will be aligned with phase A, so the rotor will turn to align its d-axis with phase A.

For the q-axis alignment step:
$$i_s = i_b a - i_b a^2 = j\sqrt{3} i_b$$
The stator current vector will be at a right angle to phase A, which means the magnetic axis of the stator field will be at a right angle to phase A, so the rotor will turn to align its d-axis perpendicular with phase A, i.e. its q-axis is aligned with phase A.

4. Mar 18, 2015

### awaiting

I did not get an email notification of your post, hence missed it.
but I figured out the solution later on..