Torque Pulsations
Torque Pulsations
(OP)
I have a 24VDC BLDC servo motor that we are using for a force feedback application. One of the critical requirements is that there be as little cogging/torque ripple as possible. This is a 3 phase sinusoidal controlled motor.
We are testing a 12 slot/4 pole design now but would like to know:
1. What's better at reducing cogging - skewed laminations or skewed magnets?
2. Is it possible to wind a straight stack and then skew it? What particular design features should be evaluated - slot design, wire tension, etc.?
3. Would a 15 slot/4 pole motor design be better without the need to skew either the magnets or the stack? What degree of improvement could be expected between the two designs? Would a 18/4 be better yet provided that the slots could accommodate the wire (AWG #24 or #26)and the turns?
4. How accurate are 2D and 3D FEAs in predicting the possible cogging/ripple reduction?
5. Any good articles on reducing torque pulsations?
We are testing a 12 slot/4 pole design now but would like to know:
1. What's better at reducing cogging - skewed laminations or skewed magnets?
2. Is it possible to wind a straight stack and then skew it? What particular design features should be evaluated - slot design, wire tension, etc.?
3. Would a 15 slot/4 pole motor design be better without the need to skew either the magnets or the stack? What degree of improvement could be expected between the two designs? Would a 18/4 be better yet provided that the slots could accommodate the wire (AWG #24 or #26)and the turns?
4. How accurate are 2D and 3D FEAs in predicting the possible cogging/ripple reduction?
5. Any good articles on reducing torque pulsations?





RE: Torque Pulsations
I don't see what benefit at all comes from skewing the laminations.
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RE: Torque Pulsations
It is possible to wind straight and skew later, but you probably don't want to do it. You might want to consider the length of the stack to the diameter and number of slots (is it safe to assume that your interest is in an outside stator?). Slot pole configuration will also reduce reluctance torque. There is however more to it than just the number of slots and the number of poles. You must also consider the pole arc length with respect to the number of slots. The idea is to make the change in airgap reluctance as smooth a transition as possible. FEA (in the right hands) can be quite accurate in predicting the airgap reluctance. Have you considered a coreless motor?
If I can recall any liturature about the subject before you have solved your problem, I will post.
RE: Torque Pulsations
If the slots are skewed, I could see that could minimize torque pulsations. Is that what you're referring to?
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RE: Torque Pulsations
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RE: Torque Pulsations
To answer another question - this motor is a NEMA 23 frame size and has a peak torque of 2.3Nm (through a gearbox) with an averge torque of 0.23Nm.
RE: Torque Pulsations
The size and torques required are well within the capabilities of a coreless (slotless) motor.
RE: Torque Pulsations
RE: Torque Pulsations
I know multiple applications where a simple table-based correction (as a function of motor angle) reduced the velocity ripple resulting from cogging torque by about two-thirds. To find the table values, they just noted how much torque was needed to hold position of the (unloaded) motor at each point in the table. In operation, this torque amount was simply added to the computed servo command.
Curt Wilson
Delta Tau Data Systems
RE: Torque Pulsations
To cswilson:
Would you mind to share how to measure "cogging torque"?
Thanks.
RE: Torque Pulsations
Measuring cogging torque turns out to be very simple if you've got a digital positioning servo controller. I'll expand on my brief explanation above.
To hold an unloaded motor at a certain position with no error requires that the controller apply a torque that is equal and opposite to the cogging torque at that position. So you have the controller command that position and wait for it to settle at that position with zero error, then read the controller's servo-loop output value (which is the torque command if you are not using a velocity-mode amplifier).
You've got the value in the internal units of the controller now. If you want to convert that to physical units (e.g. N-m), you have about 3 system constants you need to multiply this value by. But typically you want the value in the controller's internal units, because now you can automatically add this value in to the active servo-loop command.
You will repeat this process -- of move, settle, read the torque command number, and put this number in a correction table -- for every few degrees of a motor commutation cycle or revolution. I have automated the process to be able to do it in less than a minute.
Curt Wilson
Delta Tau Data Systems
RE: Torque Pulsations
Thanks.
RE: Torque Pulsations
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RE: Torque Pulsations
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RE: Torque Pulsations
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Here on page 13/15 talks about cogging torque reduction for axial flux bldc machines:
http://journal.esrgroups.org/jes/papers/5_1_2.pdf
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RE: Torque Pulsations
These requirements are more easilly approached in slottless motor designs.