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AC Motor speed control

  1. Jul 11, 2006 #1
    We have one of these Dayton single phase AC motors:

    And I am wanting to control the speed of the motor. I didn't take very many power classes but from what I remember the motor speed is dependent on the input frequency of the AC power. I am interested in slowing down the motor, not speed it up.

    Is there any reason why lowering the input frequency will not work with this motor? Can I damage it by messing with the input frequency?

    Also: I want to generate the frequency with a computer controlled analog output. Where can I find a good component amplifier? Any reason why a stereo amplifier won't work if I make sure I don't go over 115 Volts for my AC power signal (that is, if I wanted to be cheap and just use what I have around?). That would be 115 Volts, RMS right??

    Thanks for your input!
  2. jcsd
  3. Jul 11, 2006 #2


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    Staff: Mentor

    I don't think you can control the speed of a synchronous AC motor. Certainly not by varying the frequency (at least not much). If you cut the frequency in half, you will double the winding current, which will overheat the motor.

    For speed control on motors, I've seen PWM on the input power. I've seen it for both AC and DC motors, but I don't know enough about motor variations to be of help past that. I only wanted to post quickly to keep you from setting your motor on fire.

    Anybody else able to help Shadrack?
  4. Jul 11, 2006 #3
    I can do PWM pretty easily as the input power is tied to mechanical relays that I control with 24V DC signals. However, PWM'ing the motor will likely create more heat and power usage.

    What kind of motor would I use to accuratly control the speed? I have a stepper motor available, but I'm not sure if it will meet the torque requirements. My understanding of stepper is you can precisly move to each step (the one I have is a 1.84 degrees/step), but you have not control at the speed between steps.
  5. Jul 11, 2006 #4
    Don't synchronous AC motors rotate synchronously with the input AC frequency? Isn't that what makes them synchronous?
  6. Jul 11, 2006 #5


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  7. Jul 11, 2006 #6
    It is very common to control the speed of a sychronous AC motor by varying the frequency. It is done all the time. I think the voltage is reduced to prevent over current at lower frequencies. I suspect they use class D PWM to create a sine wave. As far as I know, this is NOT done with single phase motors. Only 3 phase. Single phase motors have centriful switches for starting and use a capacitor and starting winding, or in smaller HP motors like less than a half horse they will just use a high resistance winding for the starting winding. This is enough to get a phase shift difference to make the rotor turn. I'm not saying you can't vary the speed of a single phase motor at all, but I would only attempt to vary it by 10% or less. Also, most of the time when a motor with a speed controller is used the motor is soft started. I don't think that would work with a single phase capacitor start motor.
  8. Dec 11, 2009 #7
    Although once it's wired up with a capacitor it connects to 1-phase power, this is really a 2-phase motor. The capacitor in series with one winding creates the 2nd phase. The description refers to it as a "run" capacitor, not a "start" capacitor, and the motor type as "permanent split capacitor", implying that the capacitor is always in the circuit (no centrifugal switch).

    You could dispense with the capacitor and power the two windings from a stereo amplifier. You'd need to generate two phase-quadrature sinewaves, and the voltage should be proportional to the frequency because it has to track the back-EMF of the motor vs speed. For a starting point, I'd get the capacitor along with the motor and measure the actual voltage across the winding with the capacitor in series. Unless the instructions say you can put the cap in series with either winding, I would expect different numbers of turns on the two windings (you could also compare DC resistance to confirm this). This would mean they don't operate at the same voltage, but they will always operate at the same voltage ratio.

    You might be able to use a pair of 70v line PA amplifiers. They put out 70v RMS into 70 ohms at 70w. To get 115v, you'd need PA amplifiers that can put out
    70w * (115v/70v)^2, or 189 watts. Since 1/20 HP = 37 W, even allowing for inefficiency the amplifiers should run pretty cool. You could also use smaller amplifiers with step-up transformers.

    Why go to all this trouble and expense using an AC motor? A DC motor would be a much simpler and cheaper alternative. Take a look at: <http://www.sciplus.com/category.cfm/subsection/18/start/34/maxrows/11/srch.fp/1>. This is a small one and I don't know your torque requirements, but the speed range looks right. Ad says it stalls at 900 mA @ 24v, and unloaded speed @ 24v is 48 RPM.
  9. Dec 11, 2009 #8
    you would be better off using a dc motor for a quick variable speed setup, like others have said, reducing the input frequency will increase the current and overheat the motor. Since that motor is only 1/20 HP or 187watts, you should have an easy time finding a dc motor to replace it.
  10. Dec 12, 2009 #9
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