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  1. #1
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    Switching motor speeds with relay

    I have a large 2-speed motor and I'm planning to switch speeds using a double throw relay in the control circuit, NO will be low and NC will be high. My question is, is there any issue with switching windings too quickly? Or is it desirable in any way to have a delay, even milliseconds, when going from the slow to the fast speed?

  2. #2
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    Mar 2018
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    If the motor is already spinning and it will continue to spin in the same direction when the speed is changed it should not be a problem at all. We are talking about alternating current motor.

    Since AC induction motor is behaving like a transformer with shorted secondary right before rotor moves, while it is already moving you should have much less inrush current at the moment of the switch to a different winding.


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  3. #3
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    Slowing the motor too quickly can do damage!

  4. Likes rjk_cmh liked this post
  5. #4
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    This is no different than an older residential gas furnace where NC would be wired to low speed and in front of the relay is the fan/limit switch which would bring on the fan after the heat exchanger heats up. NO would be wired to high speed and the relay coil is powered by the green wire off the stat.
    The 2nd most important amendment is to protect the 1st...

  6. #5
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    How large can this motor be if you are using a relay? Any large two speed motors (20HP 3phase and Up) I have worked on have a delay between high to low speed changes. They also use a mechanical interlock between the two contactors to make sure both speed are never energized at the same time.

  7. #6
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    Quote Originally Posted by Oodly View Post
    I have a large 2-speed motor and I'm planning to switch speeds using a double throw relay in the control circuit, NO will be low and NC will be high. My question is, is there any issue with switching windings too quickly? Or is it desirable in any way to have a delay, even milliseconds, when going from the slow to the fast speed?
    Are you an HVAC pro or an interested homeowner?
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    2 Tim 3:16-17

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  8. #7
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    Quote Originally Posted by timebuilder View Post
    Are you an HVAC pro or an interested homeowner?
    I'm both

  9. #8
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    Quote Originally Posted by VTP99 View Post
    I'm both

  10. #9
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    Quote Originally Posted by VTP99 View Post
    I'm both
    I'm thinking this is DIY...
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist
    Member, IAEI

    AOP Forum Rules:







  11. #10
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    Quote Originally Posted by timebuilder View Post
    I'm thinking this is DIY...
    Im thinking tripped breaker at best, blown motor worse case

  12. #11
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    Quote Originally Posted by timebuilder View Post
    I'm thinking this is DIY...
    Yep
    The 2nd most important amendment is to protect the 1st...

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