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Want some advices about PWM for re-using small 3 phase motor

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Grossel

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Walking around on the floor these days pondering about if I should built myself a PWM controller for use on a cheap 3-phase motor from a washing machine (typically not more rated than 2 hp). I don't have one yet in my hands, so any question will be general for most induction motors avaiable.

I won't discuss any features or design of pwm controller (that will - if I decide to build one - be another issue) but focus on the motor itself.

The goal is nothing but to make this for demo only. That is - when the engine runs, for the only purpose od display it to others - I playing with the taught of using low frequens PWM within auditible range.

For the case of discussion - say I go find a used inductor motor rated 700W, is 15 years old but in working condition. I may put some load on it to make the engine output some physically wattage close to its rating.

The question I want to discuss on : Is it likely that this low frequenz PWM will cause damage to the motor over time. Have it ever being performed experiments that can give some insight in what I can do and can't do in order to make the engine last.
 

dknguyen

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Not wanting to talk about the features of the drive makes your question seem moot to me since plain PWM controllers don't work with 3-phase AC induction motors and they're not called PWM controllers anymore at that point. You need something that outputs a 3-phase sinusoid (like a VFD).

So your question about whether the low frequency PWM will damage the drive is a bit weird unless you actually mean a VFD, and what you're asking about is is the the frequency of the drive's sinusoidal output that is driving the motor (or perhaps the PWM'd higher frequency carrier wave that is being PWM'd to produce the sinusoid).

Please clarify what it is you are thinking of.
 
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unclejed613

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for a 3-phase motor you will want a 3-phase variable frequency driver of some sort, or, for a more archaic term that might also get good results in a search a "synchro drive" or "selsyn" technically synchros and selsyn drives are single frequency devices, relying on shifts in phase to operate the motors, but the drive and amplifiers used are very similar in principle.
 

Grossel

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Thank you. Yes, I meant a VFD (was not aware there was a separate term for that in english).

So I think I have found the readings I originally was looking for (now I know why wiki page for pmw don't mention that kind of problems), actually on wikipedia:
Variable-frequency drive
That links to two very informative articles in reference #75 anf #76.
So that boiled down to being a language problem (sorry for bothering you with silly mistakes).

Ok, I have rad some (but not all of the lectures) and I now know that long cables between VFD and motor may get problematic. And also for carrier frequency above 5kHZ is stated to be bad for bearings - thanks again.
 

dknguyen

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Well, my understanding is that VFDs can run induction motors down to zero speed so that would imply no damage would occur with very low frequency sinusoids (as far as the sinusoid is concerned, not the modulating frequency used to create it).
 

MaxHeadRoom78

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The typical VFD for AC motor takes the form of a sine wave at the frequency you intend the motor to run, with both the sine wave amplitude controlled and also each half cycle is PWM modulated.
Do they fit 3ph motors to W.M. in Norway?
The ones here in current use are based on the Fisher-Paykey outrunner 3ph motor, but they are not cheap, they are not induction motors, but P.M. poles..
Traditionally a 1ph AC induction motor does not control well with variable frequency as they tend to drop out of run at low speeds and/or when highly loaded.
Hence the popularity of VFD's running 3ph motors with a single phase supply.
3ph motors can be operated down to 0 rpm for applications such as elevator or crane lift operations.
A encoder typically is fitted for this.
Max.
 
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