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Old 10-15-2013, 03:03 PM   #1
sar_dog_1
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Default need help with variable auto-transformer

I have a 1/2hp ac motor that was used in a drill press that I want to use in my grain mill. I also have a variable auto transformer (variac) that I want to use to control the speed of my ac motor. Has anyone ever used one? All I have found online is that they are used to test equipment and for fan speeds. I am worried that it will heat the motor to much and cause a fire from either use or grain dust. Any help I could get would be great. Thanks!

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Old 10-15-2013, 09:58 PM   #2
BarleyRocket
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Don't undervoltage an induction motor, unless you intend to create a fireball. Induction motors (95% of AC motors) speed is controlled by frequency, not voltage. To control the speed of such a motor, you need a VFD - variable frequency drive. Kicker is, there's likely a centrifugal start switch in your motor that will kick on below a certain speed, which would burn it up even if you did run it slower with a VFD.
So - you can run an AC motor slower than rated, but not with an autotransformer.
DC motor speed control can be controlled via voltage, but you would need to rectify the output of the variac to get DC anyhow.
These reasons are why most folks use either a gear drive motor or pulleys to get proper output speed.

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Old 10-16-2013, 08:43 PM   #3
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how about using a couple pulleys with a belt not too expensive and the motor can run at the speed its supposed to

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Old 10-17-2013, 09:55 AM   #4
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Thanks barley. I think ima try something different. Mabey some chain sprockets or something.

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