Can we pulse an AC motor?

Discussion in 'General Electronics Chat' started by vijaybala85, Jun 23, 2011.

  1. vijaybala85

    Thread Starter Member

    Jan 7, 2010
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    I have a 1/3 HP 120VAC motor with fixed RPM. It is controlled from a digital out channel of a DAQ card and a SSR. Basically, It is either ON or OFF. I want to know if it is OK to use pulse width modulation (PWM) on an AC motor. I want to avoid overshooting from target level so I am implementing a PID controller by varying duty cycle of pulse. I have heard that motors especially dont do well with sudden ON/OFF cycles. What are your thoughts?

    V
     
  2. PackratKing

    Well-Known Member

    Jul 13, 2008
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    What genre of motor.........you certainly cannot do this with a capacitor-start motor with a centripetal switch, unless you have it up near operating speed where the switch cuts the start windings out.

    Your capacitor would likely fail quickly on a PSC motor.

    You may be able to get away with it on a shaded-pole.

    Then again you would need a circuit with some real beef to pulse 120VAC, or DC for that matter. :D

    For the most part, AC motors are designed to run at their rated speed.......anything lower where the motor is dogging will very quickly release the magic smoke.:rolleyes:
     
  3. vijaybala85

    Thread Starter Member

    Jan 7, 2010
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    I am using a DC to AC SSR. The control signal is 5VDC and output is 120VAC. When ON (5V) Motor turns ON. and vice versa. I am facing overshoot problems. How to determine the ideal Pulse width that can be applied to the motor to avoid it failing? What do you mean by PSC motor or what is it that you are looking for when you ask what genre it is? Maybe some information on wiring or nameplate details? I am in software and using software to control the motor. I don't want to pulse it as fast as the software can. Thanks!

    V:)
     
  4. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Another thing to watch for, is on motors with centrifugal switches.
    A centrifugal swith is what turns off the start windings, if you run a motor that has one of theese ( and I don't know if yours does/doesn't ) and you run it too slowly for the switch to disengage you will burn out the start windings.
     
  5. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    the ideal pulse width is either 100% or 0%, such as you currently have. I assume your attempting to maintain a speed less than rated. You'll find that your motor will overheat unless you can control the volts/freq and the freq.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    AC motors can be controlled with VFD (variable frequency drives) or via Triacs depending on the motor you have.

    You should probably switch to DC motors.
     
  7. #12

    Expert

    Nov 30, 2010
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    That VFD mcgyvr mentioned...very sophistocated compared to a PWM. Also, they work!

    The least you should start with is knowing what kind of motor you are trying to run. The reason you can't PWM certain motors is that they have too much inductance to get significant current flowing in a millisecond.
     
  8. PackratKing

    Well-Known Member

    Jul 13, 2008
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    PSC = Permanent Split [ phase ] Capacitor...Employs a capacitor in the windings to give some advantage to starting torque, and running amps without the start windings and switch of a cap-start motor
     
    Last edited: Jun 24, 2011
  9. Dyslexicbloke

    Active Member

    Sep 4, 2010
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    Shaded pole and universal, shunt or seriese wound, motors will be fine with a modulated waveform.
    However you wouldnt want to be using PWM in the way you would on a DC system.
    If you look at industrial fan speed controllers which work like big dimmer switches you will find them being refered to in some cases as phase cut modules.
    To see the diferent aproaches google 'leading edge dimmer' and 'trailing edge dimmer'

    What you should be asking is what speed control options are available for your spaciffic type of motor and working out from there.

    Al
     
  10. vijaybala85

    Thread Starter Member

    Jan 7, 2010
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    Thank you all for your replies. All I can tell about the motor is that it is 1/3HP 120VAC single phase motor that draws 5.7A. We use the motor basically for ON/OFF. There is no frequency control on it. My requirement is to lift a device hydraulically by switching the motor ON until it reaches the target. The motor pumps water and lifts a cylinder to lift my device. It would have been so much more easier if it were analog controlled but it is not. :(

    Sometimes, when I need very little lift, I can't keep it ON for long, else I overshoot. Currently, I have a PID algorithm for lower value to take care of the overshoot and bring the device down to target. But, when raising, I was wondering if I could use PWM to raise it in steps. Like in a software loop of 200ms, I keep the motor ON for 50% duty cycle.

    What kind of information about this motor I should know apart from rating plate details to determine if this sort of PWM control is safe for the motor?Thanks!

    V
     
  11. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    The usual thing with hydraulics is to use a valve to control movement. The pump would run continuous making pressure and the control valve is used to change direction, speed of movement and to stop movement at a certain spot.

    Using just the pump like your doing means that the motor is starting under load, drawing more than normal current.

    When you reach the height you want how do you keep the device from retracting when shutting the motor off?

    I hope this device is not used around people or it could cause injury. Here is a link to hydraulic basics; http://www.capetronics.com/basics_of_hydrailics.htm
     
  12. vijaybala85

    Thread Starter Member

    Jan 7, 2010
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    I need as much as 5Hz ON/OFF couple times in succession. Is that acceptable? I see that most AC devices can respond to range of KHz switching. And motor is turned in only ONE direction.
     
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