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  1. #1
    Join Date
    Oct 2012
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    Colorado Springs, CO
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    motor undervoltage and effect on current and torque

    I'm having trouble reconciling a couple different situations:

    1) Huge induction motors use reduced voltage autotransformer starting, resulting in lower current and lower torque. Does this essentially result in a new motor speed/torque curve (new LRA torque, new pull up torque, new breakdown torque)?

    2) Are multi-horsepower (multi-tap) single phase motors - common for small evaporators - doing the same thing with their speed taps as in #1? The current and torque both decrease.

    3) We know that when induction motors receive undervoltage conditions, the current increases. What's the difference between this and #1 and #2?

    Thanks,

    IJR

  2. #2
    Join Date
    Jul 2007
    Location
    Delta BC Canada
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    98
    Usually small evaporators use fractional horsepower psc motors. When you reduce the HP by changing the taps you are not reducing the input voltage.
    Get out your ohm meter and ohm out a PSC motor between common and the various incoming line(speeds/Horsepower). You will notice that common to high will give you the lowest resistance and common to low will give you the highest resistance.
    All you are doing is increasing resistance by increasing the amount of windings that the current must flow through. When that happens less current flow ,weaker magnetic field and less torque less HP produced by the motor. Exact opposite for highest HP settings same voltage, lower resistance,more current flow, stronger magnetic field more torque ,higher HP .
    Hope that makes sense .

  3. #3
    Join Date
    Aug 2006
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    Mixing oil and fire with a big spoon.
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    #1: yes. you get new LRA values and torque values

    #2: no. multi-hp (NOT multi-speed) motors do not do the same thing as autotransformers. motors that use autotransformers, change the resistance (sorta) but then after it is up to speed, it will bypass the autotransformer and the motor is connected across-the-line. multi-hp motors do not start in 1 configuration and then change to a different configuration later.

    #3: no difference. both motor types act the same during a low voltage condition.
    "If you pull one more stunt like you just pulled with Tommy, you won't have to get on a plane because I will personally kick your ass from here to Korea!" - Best of the Best

  4. #4
    Join Date
    Oct 2012
    Location
    Colorado Springs, CO
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    Right, eventually the autotransformer on #1 is taken out of the circuit, but I should have specified that I'm just talking about before that happens (in order to make the comparison), are #1 and #2 the same effect?

    And more importantly, why are they different than #3? That is, why does a reduced voltage in #1 and #2 result in less current and torque, and a reduced voltage in #3 result in more current and torque?

    IJR

    just for clarification to DPinst, for #2 to run in low speed I believe that you are reducing the input voltage to the main winding in fractional multi-hp motors, see AOSmiths "basics of multispeed motors":

    http://www.centuryelectricmotor.com/...pe=1033&id=766

  5. #5
    Join Date
    Aug 2006
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    Quote Originally Posted by IgnatiusJReilly View Post
    Right, eventually the autotransformer on #1 is taken out of the circuit, but I should have specified that I'm just talking about before that happens (in order to make the comparison), are #1 and #2 the same effect?...
    they are not the same thing (if i understand you correctly). a multi-tap motor and an autotransformer (which also has taps) are very different things.

    i think that you and i (probably mostly me) are talking about 2 different things under 2 different circumstances...but i think that i see your question in there.

    a multi-hp motor essentially has several windings within it. you pick the hp that you want for your application. with a lower voltage you would get a higher amperage WHILE RUNNING.

    a motor STARTING UP using an autotransformer uses the autotransformers windings for starting. this inserts extra inductance (AC resistance) into the circuit making the motor see lower voltage which causes it to use less amperage at START UP (this is the key) than it would if it were STARTING ACROSS THE LINE. both starters have lower voltage at start up and this causes the amperage to be higher...the difference is how is how much higher. the motor using the autotransformer DOES have a higher current at start up than running because it is starting...that part is the same as a multi-hp motor.

    multi-hp motors are not using different taps like autotransformers do...although both use the same terminology.
    "If you pull one more stunt like you just pulled with Tommy, you won't have to get on a plane because I will personally kick your ass from here to Korea!" - Best of the Best

  6. #6
    Join Date
    Jun 2010
    Location
    Miami
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    258
    Also.....lower voltage cause higher amperage "if" torque is required. In other words torque is proportional to square of voltage. Motors that are being started at reduced voltage are usually seeing little to no torque requirments....compared to what the see when fully loaded at line voltage.

    So conditons for them are very different than what you are suggesting in #3.

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