Posted on May 17, 2011 at 10:15Hi everyone,
There's a 1HP permanent magnet motor. I've got reason to suspect that its magnets are surface-mounted. The manufacturer doesn't know or isn't willing to supply details about the motor. The ''official'' controller is a low cost 6-step drive, and there are at least three possible reasons for that:
-the manufacturer may want to reduce the cost of the controller as much as possible,
-the manufacturer may be technically unable to build a better controller,
-or the motor may really have trapezoidal back emf.
I viewed the back emf as measured from phase to phase, and it seemed to be sinusoidal (with some distortion, of course). I didn't regenerate the ground with 3 resistors because the sinusoidal wave looked convincing enough.
1) In view of what follows, do you think I should recheck this fact but regenerating the ground this time?
The motor has 120 degree halls. The halls are advanced by some amount, presumably to offset inductance at high speeds, presumably because the cheap controller can't do it. (The motor doesn't work well with 6-steps when going in reverse at high speeds.)
I assume the advance angle is to offset inductance. Since the advance is constant with speed, it follows that field weakening wasn't what they were after, as they'd be better off simply by using weaker magnets.
2) Do you concur?
I set off to measure the advance. I fed the motor with 3 phase AC at around 0.1Hz. I meant to observe the hall edges and measure their phase compared to the slow sinusoidal inputs.
I expected the motor to turn mostly at constant speed but it didn't: it turned in a 6-step patten, quickly advancing a step then staying almost still for the rest of the 60 electrical degree period.
I'm fairly new with these machines. I could think of two reasons for this:
-the motor is really a BLDC with trapezoidal back emf and not a PMSM,
-or the motor, used this way, is acting as a reluctance motor.
But aren't SM PMSM supposed to have constant flux linkage with respect to rotor angle? Shouldn't they have zero reluctance torque?
3) Why is this happening? Do you believe this is a BLDC motor or an intenral PMSM? Or maybe this behavior is normal for surface-mounted PMSM?
4) I need to know if I can use field oriented control on this motor. Any ideas?
Thank you very much for your help.
Regards,
Lanchon
