Experienced motor guys. Possible odd failure of 7.5HP 1P motor
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    Default Experienced motor guys. Possible odd failure of 7.5HP 1P motor

    Motor runs an air compressor. On several occasions over the past couple years, the compressor would randomly trip the breaker. Obviously something is wrong anytime that happens. We investigate for possible issues and flip the breaker and observe. Sometimes it would turn a little but have very high amps and trip very quick, other times it would start and run just like it should. It sort of "just started working right" and was put back in service without answers for many months.

    fast forward to today, I got involved because this time it seems to want to keep tripping. I currently have the motor completely uncoupled from the compressor. I have torn the motor down to the start switches and can confirm they work perfect. This is a cap start/cap run motor. All caps are good.

    I checked resistance on the aux winding at 1.0ohms. The main winding is showing 0.0 ohms. There is no crazy color to the main winding, no burnt smell, everything looks nice. Megger testing reports no leaks to ground. load testing shows no current on the ground leg either.

    In every way, it looks like there is an internal short in the main winding but I have never seen a motor do this where it was not obvious it was toast. It pulls 250A for about 1 sec before tripping. I know not to continue that abuse. The motor does not accelerate much at all and does not sound like a typical 1P motor where the caps are not kicking in. I am used to a hum. This just has a different sound.

    Any other tests I can perform? I am prepared to tear it down tomorrow just to look for the failure. I have NEVER seen a motor blow the breaker, then just start working again for MANY MONTHS! Not to mention, a 7.5HP 1P motor is NOT cheap!!!! We will likely convert it to 3P but I don't want to write off the motor if we can make it work.

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    Time I money and the time you waste being a detective may be cheaper to replace the motor or put in a 3 phase motor using a phase converter. YouTube or YouTube

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    The MOTOR may be OK, but you may have a bad connection somewhere. Maybe inside, maybe outside the motor. Hard to tell from here. crummy connections can arc and weld themselves back together for "some" time, then vibrate loose, and maybe ork again several times before they open and stay open. At higher currents that tends to be a bit more dramatic and noticeable, but....

    Are you certain you are getting full voltage at the actual motor during the start?

    Anyone looked at the contactor? The breaker?

    Larger motors often will read as zero ohms, you usually need a specialized meter or bridge to check them because the actual resistance is low. And ohms checking often will not find a shorted turn, there have to be many turns shorted to affect the resistance much.

    At some point, you will probably just decide to replace the motor, and you may then find the problem. Taking the motor to a motor shop may also help, if it is a good one and you tell them the issue in detail.

    Your motor would pull about 40A at full load, so 250A is not unusual at the start.

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    Quote Originally Posted by JST View Post
    The MOTOR may be OK, but you may have a bad connection somewhere. Maybe inside, maybe outside the motor. Hard to tell from here. crummy connections can arc and weld themselves back together for "some" time, then vibrate loose, and maybe ork again several times before they open and stay open. At higher currents that tends to be a bit more dramatic and noticeable, but....

    Are you certain you are getting full voltage at the actual motor during the start?

    Anyone looked at the contactor? The breaker?

    Larger motors often will read as zero ohms, you usually need a specialized meter or bridge to check them because the actual resistance is low. And ohms checking often will not find a shorted turn, there have to be many turns shorted to affect the resistance much.

    At some point, you will probably just decide to replace the motor, and you may then find the problem. Taking the motor to a motor shop may also help, if it is a good one and you tell them the issue in detail.

    Your motor would pull about 40A at full load, so 250A is not unusual at the start.
    Is it tripping just on start? If yes then my bet is the start switch. I would take a second look at the switch.

    If it has an internal short it is highly unlikely that it would run sometimes. It would burn up pretty quick.

    The other would losing a cap. Make sure all connections are good in the start and cap circuits.

    Standard tests would be a 250/500 volt DC ground insulation resistance test (sometimes called a meggar)to see if the ground wall insulation is good. A surge tester would be used to measure the insulation integrity between phase to phase and turn to turn. The surge test is not a perfect test but is best you got.

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    He does say he gets continuity in the start winding, which normally includes the switch, unless it is a timed start, or maybe has an external start contactor..

    For a timed start, then yes the external switch would also be a "possible".

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    Is the unloader valve working properly? (Bleeding off the main pressure line before it starts)
    I had the same motor on a shop air compressor blow breakers from a bad check valve. It kept the unloader line pressurized.

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    Just because the start switches appear to work with an ohm meter doesn't mean there isn't a problem with them.


    Verify you have current flowing through the start and run capacitors at startup, it should be a substantial amount of current. how many uF is the start capacitor(s)?

    edit: if you checked the start capacitor with a digital meter that has a capacitance check, that is often not good enough. instead just plug the capacitor into a 120v outlet and verify that the amps flowing is consistent with the uF.


    1uF is 45mA at 120vac or 2654 ohms. scale accordingly: 100uf is 4.5 amps. 1000uf 45 amps.

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    This is an internal start switch. I can guarantee mechanically, it works fine. I realize I am unable to properly test the contacts without load, but the DMM did not show a dribble of resistance. I cleaned the contacts just to be certain. IMO, if the contacts were at issue, just cleaning them and removing high spots should have yielded a difference.

    I do agree, the amps line up with what the motor should pull at start up. It does not instantly trip, but I would estimate slightly less than 1sec. Some of this is because the breaker is only a 30A and was sized up to help protect the motor. It has worked for years.


    Guys, the motor acted up like this before but as of right now, I cannot get it to start correctly at all. So this is no longer an intermittent issue.

    Also, the motor is uncoupled from the compressor. This is 100% a motor issue.

    Everything in me says the start circuit is at issue here. Maybe we need to swap caps just to be sure or do the actual electric current testing.

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    30 amps seems too small.

    What does the data plate on t hr e motor state?

    Sent from my SAMSUNG-SM-G930A using Tapatalk

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    Breakers are not immortal.....

    especially ones that have had a good deal of stress over time. Combine that with other issues, and you can have troubles.

    Yes, a motor by itself should start easily.

    I have been known to put a clamp-on meter on the start circuit to verify it is pulling current. And a voltmeter on the motor input would verify that the motor is not just having a voltage problem.

    If the motor pulls current on main and start windings, as it should, and the voltage to it holds up, which takes only a few minutes to check, then so long as it spins free, you have to assume an internal motor problem. A visit to a motor shop may ID the problem, if any, and give you facts to act on.

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    Quote Originally Posted by huleo View Post
    Motor runs an air compressor. On several occasions over the past couple years, the compressor would randomly trip the breaker. Obviously something is wrong anytime that happens. We investigate for possible issues and flip the breaker and observe. Sometimes it would turn a little but have very high amps and trip very quick, other times it would start and run just like it should. It sort of "just started working right" and was put back in service without answers for many months.

    fast forward to today, I got involved because this time it seems to want to keep tripping. I currently have the motor completely uncoupled from the compressor. I have torn the motor down to the start switches and can confirm they work perfect. This is a cap start/cap run motor. All caps are good.

    I checked resistance on the aux winding at 1.0ohms. The main winding is showing 0.0 ohms. There is no crazy color to the main winding, no burnt smell, everything looks nice. Megger testing reports no leaks to ground. load testing shows no current on the ground leg either.

    In every way, it looks like there is an internal short in the main winding but I have never seen a motor do this where it was not obvious it was toast. It pulls 250A for about 1 sec before tripping. I know not to continue that abuse. The motor does not accelerate much at all and does not sound like a typical 1P motor where the caps are not kicking in. I am used to a hum. This just has a different sound.

    Any other tests I can perform? I am prepared to tear it down tomorrow just to look for the failure. I have NEVER seen a motor blow the breaker, then just start working again for MANY MONTHS! Not to mention, a 7.5HP 1P motor is NOT cheap!!!! We will likely convert it to 3P but I don't want to write off the motor if we can make it work.
    Does this happen to be a repulsion start motor? These have caps but also brushes and commutator for the start circuit and a centrifugal switch.

    If so there are some extra problem areas to investigate.

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    How many times has this breaker tripped off in the past?

    10, 100, 1000?

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    Before you scrap the motor, have a look at the check valve between the pump and the tank. If it is leaking pressure back to the pump, the motor is trying to start against tank pressure. If this is the case, the unloader valve will not depressurize the pump head. Regards, Clark

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    The motor has had 2 other fits than I can recall in which nothing could be found wrong and it would trip quickly, then just started working and we planned to keep an eye on it. That was probably a year ago. Kicks on several times/day.

    Now the motor seems to want to trip out constantly.

    And to reiterate for the 4th time, the motor is not coupled to a load right now so the compressor is NOT the issue.

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    Have you measured the current through the start and run capacitors yet.


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