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 Running AC Motor < Operating Voltage

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gvel

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Location : Howell, MI

PostSubject: Running AC Motor < Operating Voltage   Thu Oct 14, 2010 4:55 am

I need to slow down an AC motor, and it's been suggested that I connect it to a variac. It's a 120V motor and gets very little use. Loads are very small too. From what I understand, the motor has to be operated at at 11VAC, to "slow it down". Rolling Eyes

Anyway, what consequences would there be, if any, to run it under these conditions?
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pixellany



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PostSubject: Re: Running AC Motor < Operating Voltage   Thu Oct 14, 2010 5:36 am

It depends on the type of motor. For example, and inductive motor is designed to run at a speed just shy of the "synchronous speed"---eg something like 1780 RPM**. If you reduce the voltage, I think it will just stop running.

To control the motor speed with a variac, I think you need something called a "universal" motor. This will have brushes.

Theory aside---try it. As long as it doesn't overheat, it should be OK


**or some multiple thereof. (the synchronous speed is typically something like 1800, 3600, etc.)
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richfair

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PostSubject: Re: Running AC Motor < Operating Voltage   Thu Oct 14, 2010 5:49 am

If there is a decent load on an induction motor, such as a fan, you can easily control the speed with a variac or a solid state controller (like a light dimmer, but one rated for motor use). Your problem is that the load is tiny. An induction motor with no load will tend to run near synchronous speed all the time until the voltage is lowered so much that bearing resistance and electrical losses overcome the weakened torque, which slows it down, but since the load is so low, the motor is already close to stalling and makes it hard to control.

As said, just try it. With a small load, I highly doubt there will be any heating problem at all.
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radiotechnician



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PostSubject: Re: Running AC Motor < Operating Voltage   Thu Oct 14, 2010 9:05 am

From electricians point of view,

There are many types of AC motors. Some things in general. Overheating motors cause fires-everywhere. Loaded induction motors will draw a higher current, (run hotter) if the voltage is reduced. At breakdown torque they may stall and the current will rise greatly
which could damage a Variac.


de
VE7ASO
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Alan Douglas



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PostSubject: Re: Running AC Motor < Operating Voltage   Thu Oct 14, 2010 10:20 am

Depends entirely on the type of motor and load.
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pred



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PostSubject: Re: Running AC Motor < Operating Voltage   Sat Oct 16, 2010 6:03 pm

What's the application?
Why do you need to slow it down?
This is important since there are other ways some times.
For our big motors we use a VFD and reduce the HZ.
Peter
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gvel

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PostSubject: Re: Running AC Motor < Operating Voltage   Sun Oct 17, 2010 7:12 am

Sorry about the delayed response and thanks for everyone's input.

I'm done with the testing for now, and I used the variac as suggested by coworkers.

I can't tell you what kind of motor it is, since I never investigated! Rolling Eyes

Anyway, what I'm doing is referred to as a "bead pull". In short, it measures the electric field within an RF cavity. The motor is connected to fish line, and on the end there is a small weight, 2 or 3 ounces, just to keep the line straight. The motor needed to be slowed down to correspond to settings on the network analyzer. It only runs for about 20 seconds with reduced voltage, and it's fused.

Tomorrow, I'll see if there is any info on this motor. The set-up now, even though it works, is rather crude! There is another system at hand, using a stepper motor and Labview, but, according to some, it is not accurate? I'll look into that too.
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Alan Douglas



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PostSubject: Re: Running AC Motor < Operating Voltage   Sun Oct 17, 2010 10:15 am

Probably a small shaded-pole motor with a gear reduction? If so, you can go down to 70 volts and about half speed before the motor stops, and it won't hurt anything.
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radiotechnician



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PostSubject: Re: Running AC Motor < Operating Voltage   Sun Oct 17, 2010 4:59 pm

I would have salvaged a paper chart drive motor for this. If your network analyser
was like the Bruel and Kjaer stuff, you could of positioned the bead with a flexible shaft
stuck in to it.

de

VE7ASO
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