PM Motor theory - formulae etc.

Electric Motors and Controllers

Re: PM Motor theory - formulae etc.

Postby jumpjack » Fri Dec 02, 2016 6:35 am

ecotech wrote:Why post fourier transformation of trap or trig waves? I think you miss the point here.

To show how armonics increase when "going far" from sinusoidal wave.
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Re: PM Motor theory - formulae etc.

Postby ecotech » Sat Dec 03, 2016 5:26 am

Yes harmonics are not good, what's your view on torque ripple I see so many people trying to minimize it but you decrease motor performance by doing that.
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Re: PM Motor theory - formulae etc.

Postby jumpjack » Sat Dec 03, 2016 10:46 am

ecotech wrote:Yes harmonics are not good, what's your view on torque ripple I see so many people trying to minimize it but you decrease motor performance by doing that.


I don't know what torque ripple is.
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Re: PM Motor theory - formulae etc.

Postby ecotech » Sat Dec 03, 2016 2:53 pm

It's when a motor doesn't have a linear torque curve, it goes up and down in motors and if a small motor has a lot torque ripple you will feel vibation from the motor.
Last edited by ecotech on Sat Dec 03, 2016 2:53 pm, edited 1 time in total. View post history.
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Re: PM Motor theory - formulae etc.

Postby liveforphysics » Sat Dec 03, 2016 4:14 pm

Torque ripple approaches 0% in a well designed efficient motor/controller setup.

However, it's not something designed to be low torque ripple for the sake of being low torque ripple. It's simply because as you approach the highest efficiency system you create, you're using all the iron and all the magnet evenly and balanced throughout all rotation positions so torque ripple approaches zero.
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Re: PM Motor theory - formulae etc.

Postby ecotech » Sun Dec 04, 2016 5:43 am

Not all motors use iron and magnets. :mrgreen:
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Re: PM Motor theory - formulae etc.

Postby rogervize » Sun Dec 04, 2016 5:59 am

jumpjack wrote:One pitucre explains more than 1000 words :wink:
http://www.lorenzoroi.net/prelievi/sviluppiFourier.pdf

Are you relying on the winding inductance (including reflected mutual inductance) to resolve into a low harmonic current waveform?
At low RPMs that would not be the case and you must surely have high harmonics.. but maybe you are chopping the drive at a fixed high frequency?
Not sure what drive model you are using.
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Re: PM Motor theory - formulae etc.

Postby learningrc » Mon Dec 05, 2016 11:24 am

If you pair an ideal motor with trapezoidal back-emf with an ideal trap drive and do the math, you can see that the harmonics in the back-emf and the harmonics in the current actually work together to produce torque, not losses. In fact, if you compare an ideal trap motor+drive to an ideal sin motor+drive, you will get about an 87% decrease in K_m for a sin motor+drive.

The real issue is that it is really hard to make a motor with an ideal trap back-emf and it is really hard to make currents go from on to off instantaneously. This is where the losses and the torque ripple come in. It is much easier to make motors with a back-emf that is sinusoidal. It is NOT the harmonic content at every square wave edge that causes issues, it is our inability to make perfect square waves.
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Re: PM Motor theory - formulae etc.

Postby liveforphysics » Tue Dec 06, 2016 3:32 am

Making square voltage waves is all our controllers generate.

Getting motors to have sine BEMF is not easy and the dominant portion of motor stator design time. Trap-like BEMF is just a resultant component waveform from a handful of constructive-deconstructive wave forms overlapping.

You're right and I'm not disagreeing trap drive has excellent torque if you're willing to compromise efficiency and listen to a loud vibrating motor.
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Re: PM Motor theory - formulae etc.

Postby rogervize » Sat Dec 10, 2016 4:41 am

Best to drive the winding with a sine rather than rectangular...
Harmonics are bad.. more losses and stress.
Rectangular drive will cause essentially rectangular current especially at low revs
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