AC induction motor no load current?

Discussion in 'General Electronics Chat' started by alancj, Aug 11, 2009.

  1. alancj

    alancj Thread Starter New Member

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    I have a surplus single phase induction motor that I pulled out of a treadmill the other day and am trying to figure out if it's good. The face plate says it's 3/4 hp, 1725 RPM, 115 Volts, 10.4 Amps, and 60 HZ. When I wire it to my 120V mains, it starts right up and seems to work great. When I measured the current draw with no load, it draws about 9.1 amps. I know these are not very efficient motors but I would have guessed half that just to keep it running. I don't see any start or run capacitors on this but it does have a centrifugal switch built in for starting. It clicks if you turn it by hand, is that normal for this type of motor?

    So what should the idle current be for a motor this size and type? 9 amps? or is this drawing excessively?

    Part of the reason I ask is because I obtained a much older (1978) 1/2 hp motor of the same type a while back and upon testing found that it's start winding was burnt out, requiring a hand spin to get it to run. It was rated for 8 amps and at Idle used 7.2 amps, and got pretty hot pretty quick I thought, so I deiced that it must be drawing excessively because of a shorted winding or something. So either they are both good or both bad... I don't know.

    Thanks for your help,
    Alan
  2. alancj

    alancj Thread Starter New Member

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    Doing a little more reading those two motors I have must of the Resistance Split phase design. I did find something that might answer my question on google.

    The only thing I wonder is what design he is referring to by single phase. I just measured the current of a 3/4 hp disc grinder I have at no load and it is only 4.5 amps, while the rated full load current is 13.4 amps. This has a 20 uf capacitor wired in parallel to the incoming power lines and no centrifugal switch so I'm guessing it's a permanent split capacitor run motor.

    -Alan
  3. thatoneguy

    thatoneguy AAC Fanatic!

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    The disc grinder may be a DC Motor for easier variable speed.

    Induction motors for single phase need to start on their own, and due to the design, they use a lot of current at any load level.

    The article you found with the 25-35% is for 3 phase motors, which are a bit simpler by design as the different phases are already "moving" in the required direction, if that makes sense.
  4. rjenkins

    rjenkins AAC Fanatic!

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    I would have guessed a treadmill would be variable speed?

    If so, I'd have expected a 'universal' motor with commutator and field, like a hand drill or grinder etc., or a shunt wound like in a washing machine.
    The switch could be zero speed sensor as part of a safety interlock.

    If it's variable speed and does not have a commutator and brushes, then it's likely intended to run on an invertor or some form of electronic speed controller.
  5. alancj

    alancj Thread Starter New Member

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    From what I quoted the first sentence was for three phase second was for single phase. I.E. "Single phase motors generally are close to full load current."

    The treadmill affected variable speed by a variable pulley. It's definitely not DC, though I've seen surplus 120V DC motors for sale that were supposedly for treadmills. This was a very simple and cheap treadmill.

    My bench disc grinder is not variable speed... defiantly AC (nameplate says single phase 60hz 120v ac), but it is funny that it only uses 4.5 amps if single phase motors are supposed to draw near full load. Would a run capacitor improve it that much? Or maybe 13.4 amps full load is a lie? The starting current?

    Thanks,
    Alan
  6. JDT

    JDT Well-Known Member

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    I know that 9.1A at 120V for a motor with no load does not seem very good. But what you are forgetting is the Power Factor. An induction motor with no load is a very inductive load - the current is out of phase with the voltage.

    As the motor is loaded, the power factor increases, drawing more real power from the supply. Although the current doesn't change much.

    So the motor is probably OK.

    If you where to connect a capacitor in parallel with your unloaded motor you will reduce the total current drawn from the supply. This is because the capacitive current cancels out the inductive current.

    Large users of induction motors, factories for example, often have big banks of capacitors automatically switched across the supply to reduce this "reactive" current to a minimum. This is called "Power Factor Correction". This reactive current (inductive in your case) has to be supplied by the Electrical Supply Company, and carried in their wires, where it dissipates heat and loses them money. Therefore they charge the user for it - a lot! Most industrial users have two electricity meters: one for kWh (true power) and one for kVAr (reactive power). The automatic PFC aims to reduce kVAr to a minimum.

    Having said all that, small single-phase induction motors are quite inefficient and normally get fairly hot! This is due to losses in the copper windings and magnetic losses in the iron core.
  7. alancj

    alancj Thread Starter New Member

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    Thanks for that explanation. I haven't really read much about power factor or reactive power, so it's not much of a matter of remembering :).

    So a single phase motor may draw near full load current at no-load, but the power it draws is not "near" full load. As in it is not actually consuming 1092 watts at no load, since the average voltage is not 120 V when most of the current is drawn, but something a bit, or quite a bit less?

    If I were to yank out the 20 uF cap from the disc grinder (for sake or argument) and put it in parallel with my other motor do you think it would cut the current in half?

    Looks like the efficiency of my 3/4hp motor is about 45%. Not too impressive when you consider some motors can be in the high 90's. (though they may be big industrial size ones wound with superconducting wire.)

    Thanks,
    -Alan
  8. JDT

    JDT Well-Known Member

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    That's about it.

    It would be an interesting experiment. Have you got a clamp meter to measure the current? It would be possible, If you knew the current, voltage and phase angle, to work out what the capacitor should be. 20uF will definitely make a difference.
  9. Yuri B.

    Yuri B. New Member

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    The no-load current (that you measure with the clamp meter) should be much less than full load one : 9.1 A against 10.4 A seem indeed no good. Something seems to be keeping it back. Maybe the centrifugal switch does not let out the starting winding (because the motor cannot reach the speed needed, or the switch itself is faulty)?
    Also, how the motor is being cooled - is it an open type or have a cooling fan on its shaft ? Motors temp is decisive for its "life-span" and will tell you after some 10 min of running (at sensible ambient temp) is the motor good or not.
  10. Yuri B.

    Yuri B. New Member

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    Shortly, if the motor when loaded does not heat up more than 20-25K over ambient, let it be.
    No capacitors in parallel to the supply would have any impact on the motor's shaft output or on your electricity bills.
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