AC motors magnetic saturation etc

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acmotor
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AC motors magnetic saturation etc

Post by acmotor »

thread jumped from here.. viewtopic.php?t=824&p=15591#p15591

Good,
This saturation area is one I have long wanted to have a handle on.

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acmotor
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AC motors magnetic saturation etc

Post by acmotor »

Johny wrote:
woody wrote:I'd say by programming in some danfoss torque boost of our own into your coffee cup motor by telling the VFD it's a 500V motor, not a 400V, you'll get a fair whack more peak torque (not as much as you'd expect < 44%) but a fair whack less efficiency.
I wonder whether you can treat this like a transformer. With a transformer under no load, wind up the voltage until there is a non-linear rise in current. That generally is the last value before the no-go zone, where is generates lots of heat - saturation.

With an ACIM and no load, set a middle range speed and increase the v/f ratio (holding speed/frequency constant) until the motor current exceeds what you would project for the applied voltage.

Cumbersome to try because you will have to stop the motor each time and set the motor voltage parameter.

When you hit the "kink" in the current curve does this then correspond to the maximum v/f that you can use under load?
Can we then calculate the resulting torque increase before saturation?


I like the proposal. I would need to test it on a known motor.

I have often wondered how to best establish the nominal operating voltage of a motor by electrical test. Sounds like this may be a way !
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Johny
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AC motors magnetic saturation etc

Post by Johny »

I also wonder whether this is one of the techniques used by VFDs when they "auto-tune" to a motor. When I observe the auto-tune in the Lenze, part of the process involves the current beginning at zero then increasing over about a 5 second interval to maximum motor current (as per user entered parameter) - all with no motor rotation. There are probably many measurements made during this current ramp-up.

Is auto-tune using this to determine the saturation point of the stator for vector control?

BTW Torque boost is not allowed in vector mode in the Lenze - kind of supports this line of thinking...

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AC motors magnetic saturation etc

Post by Martin »

Seeing how the topic was saturation, hopefully this is relevant...

Some say an AC motor won't react badly to using a higher voltage than it is rated at,

While others say the extra voltage will be wasted as the rotor can only handle so much induced magnetism.

At least with brushed DC it was only sparking at the brushes that was the problem!

Add to this the fact the higher voltage is to be delivered via a PWM inverter, so unless it's required to go flat out - does the overall maximum voltage affect the efficiency/performance/life expectancy?
Last edited by Martin on Tue, 22 Sep 2009, 08:12, edited 1 time in total.
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AC motors magnetic saturation etc

Post by woody »

I see this as two parts:
A) higher absolute voltage - IE running a 400v50Hz motor at 480v60Hz
This I think is more than fine, the torque/efficiency etc should be very similar as long as the winding insulation is Ok for the higher voltage, which it should be if you're not jumping too far.

B) higher voltage for frequency - IE running a 400v50Hz motor at 480v50Hz.
This I see as having diminishing returns. Industrial motors are conservatively set up - over bolting by 20% will probably* give 15% more torque but that missing 5% is all lost as heat. Double voltage probably* only 50% more torque but the missing 50% goes to heat!

Rewinding for lower voltage/higher current or connecting a star motor in delta lets you do A to get more rpm at peak torque with the same voltage controller..

Danfoss torque boost is B, at low rpm.

So A) I reckon will work up to a certain point (the voltage rating of the winding insulation)
B) will add less and less torque and more and more heat the further you push it.
This may be fine for drag racing, it is basically the same thing that DC motor + Zilla is doing with 1000 amps through a 200 amp motor. A lot more torque and a bloody lot more heat...
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AC motors magnetic saturation etc

Post by Sparky Brother »

Woody I just PMed you would you please check it out?

Thanks

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