Motor Speed Controller

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Motor Speed Controller

Post by tarnall » Sat Dec 08, 2012 7:45 pm

I have been searching for a device that will control the speed of a standard 1/2 horse, 120 VAC, 8 Amp brush-less motor - induction type. I have several in my shop I would like to be able to slow it down, like my band-saw, drill press, bench grinder etc. I have heard a VFD , variable frequency drive, is needed. Any ideas? Something fairly inexpensive I can build? I have heard those fan speed controllers are for bush type motors. ??


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Bob Scott
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Re: Motor Speed Controller

Post by Bob Scott » Sun Dec 09, 2012 11:14 am

tarnall wrote:I have been searching for a device that will control the speed of a standard 1/2 horse, 120 VAC, 8 Amp brush-less motor - induction typeThanks
Terry Hayward
Hi Terry.

There are several types of induction motors. Some are synchronous and will seek to keep their speed phase-locked to the power line frequency. These are (were) used in tape recorders and speed sensitive electro-mechanical equipment. This is probably not the type you use in a shop.

Your induction motors in shop equipment usually have a high current starter winding. You can hear a loud "click" from a centrifugal switch when the starter winding kicks in or out when the motor is speeding up or slowing down. Turn on your motor for a few seconds, then turn off. Listen for that click as the motor slows down. You don't want to use a speed controller and slow down this type of motor because the starter winding may kick in at slow speed, and it is only rated for a few seconds of power to help start the motor. If this winding is kept on all the time by keeping the motor speed slow, the start windings burn.

Another type of induction motor has no starter winding. This type can be found in home furnace blowers. I know that they are available in several HP ratings (1/4, 1/3, 1/2). You can probably use a speed controller on these. I would use this device available from Fry's Electronics. Get to the Fry's website

Do a search for this Fry's product number: 5422799
It is a 1kW variable voltage autotransformer, a "Variac" type. The output of this autotransformer is a nice smooth 60 Hz sine wave.

I know that there is still another type of induction motor that uses a starter capacitor. This type is identifiable by the "bump" on the motor's housing where the capacitor is located. This type was a high starting torque option for operating the compressor in my old central air conditioner. I'm not familiar with this type and whether it also uses a centrifugal switch or not. Maybe someone else in here knows. Anybody?
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Re: Motor Speed Controller

Post by Bygar » Wed Nov 13, 2013 3:48 pm

You can NOT vary the speed many AC motors with voltage change!
One exception is universal wound motors.
Poly phase and most single phase Ac motors require the frequency to changed.
Bodine a company which also makes motors can supply 3phase controllers for various
sizes of motors. Variable speed drives are available for many sizes of motors. Both AC
and DC. There is other voltage variable AC motors usually in large HP ratings.

Hope this gives you some insight of what you seek.

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Re: Motor Speed Controller

Post by dacflyer » Thu Nov 14, 2013 4:41 am

as for the cheap fan speed controllers ( looks like a dimmer switch )
they are mainly designed for ceiling fans...and they are not brush type motors.
i have used them on motors with capacitors such as ceiling fans, stand fans, window fans, furnace blowers
and many small motors with out capacitors.
but they are not for use with motors that use start / run switches.

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Re: Motor Speed Controller

Post by haklesup » Thu Nov 14, 2013 11:26 am

You are correct that only varying the frequency can change the speed of the motor and only in a narrow range (consider that your 60 Hz is getting you what? 3600 RPM) The type of motor you explained is cheap and is so for a reason, it does one thing well and is not variable speed. A variable frequency power supply pumping out 12A at 120V would be expensive and maybe large. I think it would be cheaper to search the granger catalog for a different type of motor that is variable speed and is about the same size and mounting.

Reducing the voltage or PWM the signal (like a lamp dimmer) will only reduce torque, it will not reduce speed. Even in a DC motor where reducing voltage does reduce speed, you get a corresponding loss of torque. With tools, it is important to maintain torque.

The traditional way to adjust speed on these tools is to change the belt position on the multi size pulleys for a lower gear ratio. The drill press especially should already have this feature even my cheapest harbor freight press has that. Band saw might have adjustable pulleys, bench grinder probably does not but they do have variable ones you can get.

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