Inverted Brushless DC Motor Surging

[rakeshkdhawan]

We have a 3-Phase brushless inverted motor. The rotor is 10.5" in diameter and 2" in length.

The rotor is made of COLD ROLLED DEEP DRAWN LOW CARBON ALLOY 1008/1010 STEEL PER ASTM A-A1008A/A109, 16 GAUGE ( .059").

This has 20 poles - 10 pole pairs. The max spped of the motor is about 500 rpm.

We see surging/pulsing which occurs once per revolution. We do not know what the cause is?

I have the following questions:

1. What does the aspect ratio (D/L) do to a motor in terms of vibration?
2. Having a 10.5" diameter steel, we do see some run out. How can run out contribute to surging/pulsing/vibration?
3. The rotor consist of 2 housings. Left housing and right housing. The right housing is single straight sheet of steel. Can that contribute to surging.
4. How do I go about finding the cause of the surging. We have looked at electrical currents. They are well balanced. We do see 10 times the rotational rpm due to electrical excitation but do not think that contributes to the mechanical issue at hand.
5. What is the effect of uneven air gap on surging/pulsing?

Any help would be greatly appreciated.

Thanks
Rakesh

 

[itsmoked]

Total guess: Could you have a shorted/open set of poles? Is this in a bunch of motors or is this a one of a kind?

If you have several can you swap parts, say rotors, for comparision?

Sure be nice if you could scope an accelerometer, and with a strobe get the actual angular position of the anomaly.

 

[UKpete]

Are you talking about torque ripple? I haven't heard it referred to as surging, which makes me wonder if you have a fault condition rather than a design deficiency.

Are you also referring to a brushless dc motor i.e. it has a permanent-magnet rotor?

A brushless dc motor always has a certain level of torque ripple, i.e. the PM rotor is subject to torques at speeds harmonically related to the synchronous speed. It is normally evident as noise and vibration (a degree of "growling"), but it isn't normally so bad that it causes oscillations in the motor speed.

There are in fact two contributions to torque ripple:
a cogging or reluctance torque which is due to changes in the magnetic circuit reluctance around the rotor (this can be felt by turning the rotor by hand, if it has definite preferred positions like a stepper motor then it has cogging torque); and electromagnetic torque ripple, due to the stator producing rotating fields at higher harmonics of the synchronous speed.

In answer to your questions:
1. The small D/L aspect ratio would of course be more susceptible to misbalance, but this is easily corrected by a simple static balance procedure (as it is a low speed machine).
2. I don't think that the large diameter will contribute to any torque ripple (assuming the run-out is small-ish relative to the airgap). It will only effect the mechanical balance.
3. I can't really picture what you describe, but the key test is: what does the smoothness of rotation feel like when rotating by hand? This will indicate any reluctance problem due to assymetry.
4. and 5. - as covered above.

There are various means of reducing torque ripple at the design stage, these are described in books such as D C Hanselman "Brushless Permanent-Magnet Motor Design" and possibly J R Hendershot, T J E Miller "Design of Brushless Permanent Magnet Motors". They are mainly concerned with altering the pole shape, skewing (rotor or stator), increasing the number of slots per pole, and using a larger airgap.

 

[ScottI2R]

UKPete,
I dont think he is talking about torque ripple since this only occurs once per rev. as stated in the OP. Sounds like a mechanical failure to me. Possible bearing problems? Over what degree of rotation is this occuring? Is that equivalent to pole spacing? This could point to the scenario stated by itsmoked.

Out of swags.

Scott

In a hundred years, it isn't going to matter anyway.

 

[UKpete]

Scott, I guess you're right. I also notice that the poster hasn't logged in since he posted a week ago, could be a waste of time anyway.

 

[rakeshkdhawan]

Guys:

Thanks for your reply. We are investigating hall-sensor tirggering and magnet ring effects.

There seems to be cumulative error which may be the cause of surging. Do not know for sure.

I will get back with results soon.

Thanks
Rakesh