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Thread: Buffing Motor on a Variac.?

  1. #1
    Join Date
    Jul 2019
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    Sacramento, CA
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    Buffing Motor on a Variac.?

    I have several buffer and grinding motors and all run in the 3450rpm range. Does anyone know if I run a 3450 motor through a variac at around 50% power will that work to reduce the rpms? Will it damage the motor?

    Thanks much,

    dj

  2. #2
    Join Date
    Apr 2017
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    Tucson, Arizona
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    Which Eastwood buffer do you have? Maybe post the part number and a photo.
    David

  3. #3
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    Dec 2006
    Location
    Toronto Ontario
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    If it's an induction motor, which it probably is, yes you'll damage the motor.

    If it's a 3 phase induction motor you can run it at reduced frequency and voltage with a VFD to reduce the speed.................Regards, Rod.

  4. #4
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    Jul 2019
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    Haven't even opened the box yet... But this is the link: https://www.eastwood.com/eastwood-1-2-hp-buffmotor.html

  5. #5
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    Yes David, it's an induction motor.......Rod

  6. #6
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    Jul 2019
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    Sacramento, CA
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    Thanks, I read up on capacitor motors. I guess 3450 it is until I find a slower motor.

  7. #7
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    A Variac does not change the frequency. It changes the voltage.

  8. #8
    You can use a VFD to control the speed of a single phase induction motor - if you can find a VFD that will output single phase 120 volts. There are a few other things you need to do. If you decide to go that way let me know and I'll discuss what you need to do.

    Mike

    [Here's a single phase output, 120 volt input, VFD. But it looks like it outputs 220 to 240 volts, not 120 volts.]

    [So let me discuss what you need to do to use this on a standard single phase induction motor. The problem with a VFD on a single phase induction motor is the starting circuit, normally a capacitor and coil. I'm going to specifically address a capacitor start induction motor.

    When a single phase, capacitor start, induction motor starts, the start capacitor is in series with the start coil. This generates a second phase which creates a torque on the rotor, causing the motor to begin rotating. When the motor reaches about 70% of full speed, the centrifugal switch opens and removes the starting circuit. The problem with using a VFD on a single phase motor has to do with this starting circuit and whether it's in the circuit or not.

    What you want is for the starting circuit to be in the system when you're first starting up the motor, but to then remove it in such a way that even if you reduce the speed below 70% of the full speed RPM it does not come back into the circuit.

    Here's what you can do.

    1. Get a momentary close single pole, single throw (SPST) push button switch.

    2. Get access to your starting capacitor. It's usually on the outside of the motor so it's easy to get to.

    3. Cut one wire going to the starting capacitor and, using some wire, hook in your SPST push button switch. Put the switch next to the VFD.

    4. Hook the power wires of your motor to the VFD.

    When you go to start your motor, set the VFD to close to the line hertz (usually 60 hertz). Hold your SPST switch down and turn the motor on through your VFD. This will start the motor normally, just as it it was connected to wall power. If you try to start at a low hertz, the starting capacitor may not conduct sufficient current to provide sufficient torque to start the motor. You can experiment to see how low (in hertz) you can go and still get the motor to start. Start with 50 hz, then go to 40 hz, etc. and see where the motor just won't start.

    Once the motor is up to speed, let go of the SPST switch. The starting circuit is now completely disconnected from the power. Go ahead and reduce the speed of your motor. When you reach 70% of the rated RPM the centrifugal switch will close but that will not affect anything because the starting circuit is disconnected by the SPST switch.

    Use the motor and VFD normally, just as you would with a 3 phase VFD.

    Induction motors on a VFD are generally constant torque systems when you're at an RPM less than the rated RPM. But since horsepower is a factor of torque and RPM, the motor will not be able to do as much work at lower RPMs because the HP will be lower.

    Also, most general purpose induction motors are cooled by an internal fan. At lower speeds the fan does not move as much air so you have to be concerned about heat (true for both single phase and 3 phase motors) so monitor the heat and put an external fan pointing at the motor if it starts getting hot.]
    Last edited by Mike Henderson; 08-12-2019 at 12:46 PM.
    Go into the world and do well. But more importantly, go into the world and do good.

  9. #9
    Here's a buffer motor that runs at 1725 RPM. About the same price.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

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