electric motor problem

Discussion in 'General Electronics Chat' started by jammex, Jan 5, 2007.

  1. jammex

    Thread Starter New Member

    Jan 5, 2007
    9
    0
    Buenas Diaz from south of the border:

    Purchased a 4hp air compressor for use in Mexico. Problem is that it blows the motors breaker, exchanged the compressor for the exact same item with the same problem.

    The motor is rated at 220 volts and 12.5 amps. it has a 20 amp resettable breaker on the motor. Also, it is on a dedicated circuit of 50 amps and does not blow the main breaker. We have and over voltage situation here in Mexico which does not seem to be a problem with any other appliance or circuit, at least not on a daily basis.

    On checking the draw, with the limited resources available this is what we find:

    motor receives 252 volts and draws 25.6 amps before the breaker blows. It also seems to vary higher voltagewise from time to time.

    It is a dual capacitor motor. There is no schematic or wiring hookup chart.

    Question: could this motor be wired for 110v and hooked up to a 220v circuit? There is no indication that this is meant to work on 110v. Appreciate your insight.
     
  2. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    Welcome aboard. Look on the motor, where the feeder wires are attached, there should be an access cover. Look on the other side; most motor manufacturers put 120 and 240V connection info here. If its not there, let us know; I have a pocket reference booklet that has some motor connection diagrams. It sounds like it's trying to run on twice the operating voltage but I'm surprised there's no info for 120V on the name plate.

    As for 252V, that'll make the motor run warmer but it won't be a problem if it's otherwise hooked up right, particularly since it runs intermittantly. The 120/240VAC rating can safely vary by about 5%. Hope this helps.
     
  3. jammex

    Thread Starter New Member

    Jan 5, 2007
    9
    0
    No name or access plate visible. The motor case is a big heat sink. No doubt it is made in China. Has a plastic box mounted on top with the starting and running capacitor. Inside is a terminal block with the hook ups. If that is any help I'll be surprised. I would return the compressor except it is a ten hour drive each way to Phoenix and prefer to fix if possible. They are supposedly sending me new capacitors but I don't think that is the problem.

    I can send a digital photo of the wiring if that will help. Thanks.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    There should be a centrifugal switch in the motor that disconnects the start capacitor just after the motor starts to run. If it is not working, that might be the cause of your high current. If you take the compressor belt off, you might be able to spin the motor by hand fast enough to hear the switch click. Don't know if a meter on the input leads would tell you much from that, though.
     
  5. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    From what you're telling us it sounds like the motor's defective or it's made for 120V only, in spite of the 240V specs. Do you have a 120V, 20A circuit available to test this possibility?
     
  6. jammex

    Thread Starter New Member

    Jan 5, 2007
    9
    0
    Yes, have a 20 amp 120 curcuit available. That is the way they do them down here. Would that do any permanent damage to the motor if I run it on 120 for awhile? thanks,
     
  7. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    You might fry the windings within a minute running the motor under-voltage with a load if a breaker didn't trip first but it won't hurt to run the motor like that for a few seconds at a time. Before you try it, get an amprobe [or whatever you measured current with before] & run it long enough for the centrifugal switch to open, if it will, & watch the ammeter. If you don't hear the starting switch click [you might not be able to hear it above the compressor noise] or see the current drop to 12.5 or less within 3 seconds then the motor probably is wound for 240V or there's another problem. Disconnect it immediately.

    I can only think of one more test if possible: can you get at the motor/compressor shaft or isolate the two as Beenthere suggested? See if the compressor turns hard or if the motor runs OK with the compressor unhitched [at either voltage]; hopefully that'll tell you where the problem lies. Let us know what you find, I'm curious.
     
  8. jammex

    Thread Starter New Member

    Jan 5, 2007
    9
    0
    Borrowed the amp meter previously. May be a few days before I get the chance again but will let you know what happens. thanks for the replies and interest.
     
  9. rootboy

    Member

    Jan 4, 2007
    13
    0
    It sounds like it is either defective, or wired for the wrong voltage. As for burning the winding up, it may or it may not.

    We had a 480V motor wired to 240 volts for ages before we discovered the error. It never burned up.

    But I wouldn't press my luck.


    Whereabouts in Mexico are you?
     
  10. jammex

    Thread Starter New Member

    Jan 5, 2007
    9
    0
    We are in San Carlos, Sonora. About 265 south of the U.S. border. nine hours or more, depending on the border crossing which is very long now, to Phoenix. Been there?
     
  11. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    The reason a 480 V motor ran OK on 240 V is because it's probably 3 phase so, at undervoltage, it's drawing less current, even with a load. A single-phase motor, however, running at insufficient voltage with a load may not have enough power to gain sufficient speed to open the centrifugal switch. As such, the starting circuit & probably part of the run winding in series with it [when wired for 240 V] will stay energized longer than they're designed to & will likely overheat in short order.
     
  12. jammex

    Thread Starter New Member

    Jan 5, 2007
    9
    0
    Well, removed the belt from the compressor and tied a line around the pulley. Yanked several times like starting an old lawn mower. Never heard anything that sounded like a swithch engaging. Under power without the load of the compressor, there was nothing audible until it was turned off. As it slowed you could hear something that sounded like a clutch engaging or disengaging. Couldn't tell which. I assume that it was the centrifugal switch and it was probably operating as it should.

    Have not found an ammeter to borrow but will pick one up in the States toward the end of the month.

    Would it do any good for testing purposes to remove one of the capacitors?

    Thanks for your advice.
     
  13. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    It probably won't run without the run cap, but, with it unloaded, you might try spining the shafy by hand and see if you can get it to go without the start cap. That way you can see if the start cap is bad (or that the centrifugal switch is working).
     
  14. jammex

    Thread Starter New Member

    Jan 5, 2007
    9
    0
    Roger. will give it a try next time. thanks,
     
  15. jammex

    Thread Starter New Member

    Jan 5, 2007
    9
    0
    Well here is the latest. After getting nowhere with Harbor Freight I decided to yank the motor and take it to A1 Elec. Motors in Phoenix. They have rebuilt a half a dozen motors for me over the last 25 yrs. They found that with no load it was drawing over 25 amps. It looked good, according to them, when apart and past the first four tests. On the fifth test they found a short in the continuous windings. I'm sure that I have not expressed this correctly but any way. It is their contention that there must be a wire with cracked insulation rubbing against a similar wire when operating which causes the current to travel in a circle and therefore cause the excess load.

    At any rate, it was going to be another $125 to rewind the stator (I believe) and some other winding. I gave Harbor Freight another chance and they took a motor off a new unit as an exchange. Maybe the third time is a charm.

    Thanks for your interest and I will post the results after the motor is reinstalled. Could be a few weeks.

    Thanks,
     
  16. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    You've sure had your share of motor problems. It sounds like the motor shop finally found your problem; one or more shorted turns in the run winding will have current induced in it that will produce a magnetic field opposite to that of the rest of the winding. This condition increases both current demand & heat. Connecting it to 240 V would've magnified the problem in a hurry [& likely with a lot ot smoke] & would've voided your warranty. Plese do keep us posted.
     
  17. jammex

    Thread Starter New Member

    Jan 5, 2007
    9
    0
    Last update. The third motor was the charm. Harbor Freight took a motor off a new, in the crate, unit and this one worked perfectly. Draws 11.7 amps instead of the 25 the previous two drew.

    Thanks to all who showed an interest in this problem.
     
  18. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    Glad you got it fixed. It's rare that you get a replacement part with the same defect as the original; when that happens it throws you for a loop. Thanks for letting us know.
     
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