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.
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.