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Home Made 3-ph Ac Motor Speed Control


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Hi there,

I have task to control 3-phase 400V AC squirel-cage motor with home made application. The objective is to achive just 2 steps of speed ( reduced speed operation- to about 50%, and nominal speed operation ).

Does anyone have some expirience with domestic made controller for 3-phase?

Can some variation of light-dimmers be used?


Thanks :huh:

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Hi darkos,

No, a dimmer cannot work because it only changes voltage (besides, 3 phase "dimmers" are hard to find or make anyway).

There really is no way to do this with "home made" components. Changing the voltage only will NOT change the speed directly, it only changes the torque output and if the load torque requirements are higher than the new motor output torque at a reduced voltage, then yes the motor will slow down, but the motor will do so at the cost of increased slip, which will severely overheat the motor and kill it in short order.


The only way to electrically change the speed is to change the frequency as you change the voltage. There is no "home made" device that is capable of doing that.


Consider changing the belt or gear drive system, or go to using one if you don't already have one.


Or, better yet, just buy a Variable Frequency Drive.

"He's not dead, he's just pinin' for the fjords!"
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Hello darkos


IF the motor is an integral part of a fan assembly, it is possible that the motor may be able to be controlled by reducing the voltage. Standard induction motors can not be speed controlled with voltage, but high slip motors can.


If it is a high slip motor, you can use and auto transformer to reduce the voltage and speed. A three phase solid state controller could also be used if you can find one.


The cheapest solution would be to use a small variable speed controller as Jraef recommended.


Best regards,

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  • 1 month later...

Depending on pitch, you can bring winding leeds out to contacter and turn say a 2 pole into a 4 pole by changing group cross-overs from say series to series consequent to change amount of poles. ie ..putting two norths together will create one north...This has been done many times in the past before silicon came about. Also other ways to do this exist depending on how motor is wound...you can supply three leeds and leave the other three open or star the prior supply and supply the other side. wow, that sounds a bit vague, your basically making internal connections externally. You would have to give details such as pitch (distance that coils within group span from slot to slot)...how many slots?. But it IS POSSIBLE. Just require details of motor!.



Hmm, thanks...

I was afraid of that.


Well, it seem's that I'll have to buy VFD...


Thanks a lot guy's...

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