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IndustryArena Forum > CNC Electronics > General CNC Machine Related Electronics > accurate single phase ac motor speed control
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  1. #1
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    accurate single phase ac motor speed control

    I would like to digitally set an RPM for a 2hp, 60 hz, single phase AC motor and have it hold that speed within something around 3%.

    Most of the controllers I've seen for simple single phase motors are more like a light dimmer or a fan control that is analog and doesn't adjust things based on load. Are there any digital speed controls that are affordable and that will let me set a RPM and then keep the motor at that RPM during a varying load on the motor?

    (I know there are vector type drives for three-phase motors but I don't know about single phase motors)

  2. #2
    Community Moderator Al_The_Man's Avatar
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    I assume this is a 1ph induction motor?
    If so, you may be out of luck for a VFD, as although there are VFD's out there for 1ph motors, they are in general not that successful, they have been known to drop out of run under load on lower rpm's.
    If it happens to be a Universal motor, you could look at the SuperPID that has links here also.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  3. #3
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    Hi, you have a very narrow speed range with single phase motors due to the centrifugal switch switching the start windings out as soon as the motor gets up to speed.

    If you slow the motor down the start windings cut back in again and burn out due to the fact that they carry high current for starting for a short time only.

    If it is possible you would be 100% better off getting a 3 phase motor, from a metal scrap yard (very cheap) or brand new off EBAY, 1 HP for about $100, and buying a small VFD that will slow the motor down to a crawl or rev it up to double the rated speed with dynamic braking and load sensing ....BTDT......who needs a gearbox?

    The VFD (converts single phase 240 volt in mains supply to 3 phase out and is frequency variable) for a 1 HP motor will cost about $300, but once you go down that path you'll never use single phase motors again.

    If you overload a single phase motor it slows down and then the start windings go west.....BTDT.
    Ian.

  4. #4
    Community Moderator Al_The_Man's Avatar
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    The only type of 1ph motor recommended for the 1ph VFD's is the single cap start/run style, where the start capacitor is in circuit at all times.
    But the performance does not equal the 3ph type.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  5. #5
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    Hi Al, I have a number of theses small motors that have a capacitor in the circuit....no centrifugal switch for starting etc, all fractional HP around 1/10 Hp etc, and have wondered about the possibility of using these types with a variable frequency adjustment for controlling the speed......some of them are 3,000 rpm so it might allow reasonable speed control without stalling as the synchro speed is still tied to the frequency which if you vary it will control the motor revs.....is this incorrect?

    If you vary the frequency, up or down from the 50 or 60HZ supply, will the motor attempt to match the new frequency for synchronisation and "adjust" it's speed when a load is increased by drawing more amps from the supply.....getting hot in the process but still trying to match the frequence applied?

    I know for certain that in a tape deck I have the speed of the capstan increased when it ran on a 240 volt 60Hz supply as opposed to the 240 volt 50 Hz supply........the motors were the shaded pole type, probably with a capacitor......I can't remember.
    Ian.

    I also have a number of the shaded pole motors in quite small frames..... the types that drive fans and small pumps etc.

    By experiment, you can hold the shaft of these motors and slow them down quite a bit without causing any drama.
    Ian.

  6. #6
    Community Moderator Al_The_Man's Avatar
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    The rpm of any induction motor is governed by the applied frequency, but induction motors can never be synchronous, they operate just below the applied freq from ~4-12 cycles, called the slip frequency.
    Many small critical rpm mains motors have a P.M. rotor in order to operate at synch.
    The small shaded pole motors use the shading ring to produce a phase shift the way a capacitor does, these small cap and shaded pole motors can often be stalled without too much happening.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  7. #7
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    Thanks Al, would I be right in saying that any of the single phase induction motors that did not have a centrifugal type start set-up and separate start windings can be frequency/speed adjusted.....right down to zero and above without drama?
    Ian.

  8. #8
    Community Moderator Al_The_Man's Avatar
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    I have never tried it, and frankly would only try it if there were no alternatives.
    The split phase phase-angle relies on the matching of the capacitor to the winding at 50/60hz, which in turn is a function of the winding inductive reactance & capacitive reactance,
    These two values are of course are dependent on frequency, so as the freq approaches a low value, conceivably you would lose any split phase effect?
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  9. #9
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    Hi Al, thanks for the info, I'm going mainly 3 phase now since I bought a small 3/4 HP single to 3 phase converter a couple of years ago and that does the speed control right down to a crawl and up to twice the rated rpm or 200% etc.

    I invested in a 2 HP rotary 3 phase converter to drive my Ajax Mill and that has complete adjustment for the phase balancing through the circuitry inside the box....costs about 2 grand, but the mill loves it.

    There are a lot of new motor bargains on EBAY in the under 1 HP range for as little as $100, so all those single phase 1/4 and 1/2 HP motors I bought at auctions for a few buck will probably go to EBAY now.
    Ian.

  10. #10
    Community Moderator Al_The_Man's Avatar
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    Also there is a glut of T.M. DC motors on ebay, and the likes of KB controllers handle these well, max torque down to zero rpm, and are just connected across the incoming AC, I know you are 240v there so I don't know if the T.M. used there use the same voltage motor or are the 190vdc versions?
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  11. #11
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    Thanks for all the replies. I'm kind of stuck with the single phase motor since it comes on the equipment and I don't want to have to swap (long story).

    What I was thinking is that a simple router speed control or a cheap fan speed control adjusts the RPM with my fingers turning the knob. Why isn't there something electric that has closed loop feedback to know the RPM and then just adjusts the speed control setting to keep the RPM constant?

    I'm not really an electrical guy but figure if I can adjust something by hand to a set speed then an electronic device should be able to do the same thing.

    Comments?

  12. #12
    Community Moderator Al_The_Man's Avatar
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    Is your motor a split phase motor or a Universal motor, (has brushes)?
    There is controllers for Universal 1ph AC motors from simple 'Dimmer Style' to the SuperPID.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

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