Motor Instantly Tripping Breaker

[jerjwillelec]

This is a spinoff from "Megging Motors". I'm troubleshooting (existing and aged) grain elevator exhaust fans. Six silos, two exhaust fans per silo. The motors are 460 volt 2 HP. I've found the issues in ten of the twelve motors. The last two are tripping the MCC bucket breaker instantly on startup. 498 vac on all three phases...flip the switch and trip.

Bad motor? Or bad conductors that cause tripping only under load? I tried megging the motors with my Fluke 1507 but quite honestly haven't had much training on megging so I don't know exactly what I'm doing. I haven't megged the conductors (because I just thought of it) yet.

Thank you!

 

[ActionDave]

You are right, it is either the motor or the conductors feeding it.
Did you disconnect the motor and try the switch?

 

[kwired]

My bet is on bad motor windings.

When you use your megger test from motor lead(s) to motor frame(ground), make sure there are no disconnected leads that are touching the frame or you may get false reading through that lead.

Anything below the Megohm ranges - double check for disconnected lead touching frame or something similar, if that is not happening then you have a toasted motor. If you are in the megohm ranges you have to consider the readings a little more carefully, but generally you should still have over 5-10 meg or the motor is probably bad.

 

[GoldDigger]

My bet is on bad motor windings.

When you use your megger test from motor lead(s) to motor frame(ground), make sure there are no disconnected leads that are touching the frame or you may get false reading through that lead.

Anything below the Megohm ranges - double check for disconnected lead touching frame or something similar, if that is not happening then you have a toasted motor. If you are in the megohm ranges you have to consider the readings a little more carefully, but generally you should still have over 5-10 meg or the motor is probably bad.

Also, none of the tests suggested so far will detect a turn to turn short in one of the windings.
One way to detect that is to see a much higher peak starting current in just one winding.
Another is to measure the inductance of each coil using a high frequency signal and an impedance bridge.

 

[kwired]

Also, none of the tests suggested so far will detect a turn to turn short in one of the windings.
One way to detect that is to see a much higher peak starting current in just one winding.
Another is to measure the inductance of each coil using a high frequency signal and an impedance bridge.

Correct, turn to turn shorts are a little trickier to diagnose, but often will end up becoming either open circuit or a ground fault. Sometimes just taking off the cover off the terminal housing and putting your nose up close and smelling can give you an indication of motor insulation condition - at least a go/no go type of test result.

 

[Ohms law]

This is a spinoff from "Megging Motors". I'm troubleshooting (existing and aged) grain elevator exhaust fans. Six silos, two exhaust fans per silo. The motors are 460 volt 2 HP. I've found the issues in ten of the twelve motors. The last two are tripping the MCC bucket breaker instantly on startup. 498 vac on all three phases...flip the switch and trip.

Bad motor? Or bad conductors that cause tripping only under load? I tried megging the motors with my Fluke 1507 but quite honestly haven't had much training on megging so I don't know exactly what I'm doing. I haven't megged the conductors (because I just thought of it) yet.

Thank you!

Have you tried disconnecting the conductors at the motor pecker head, then energizing the motor starter. That would allow you to see if its the conductors feeding the motor or if its the motor.

 

[jerjwillelec]

You are right, it is either the motor or the conductors feeding it.
Did you disconnect the motor and try the switch?

Yeah...the "switch" is a brand new MMS that I wired into the circuit because there isn't a local disconnect.

 

[jerjwillelec]

Also, none of the tests suggested so far will detect a turn to turn short in one of the windings.
One way to detect that is to see a much higher peak starting current in just one winding.
Another is to measure the inductance of each coil using a high frequency signal and an impedance bridge.

How would I get a high peak starting current if the breaker won't allow the motor to start?

 

[jerjwillelec]

Have you tried disconnecting the conductors at the motor pecker head, then energizing the motor starter. That would allow you to see if its the conductors feeding the motor or if its the motor.

Yes...with my temp MMS at the motor...voltage is good but when I flip the switch, the breaker trips out. I'm just wondering if weak conductors (that might hold under no load) might still be a possibility.

 

[Little Bill]

Yes...with my temp MMS at the motor...voltage is good but when I flip the switch, the breaker trips out. I'm just wondering if weak conductors (that might hold under no load) might still be a possibility.

You said the voltage was good, but did you actually disconnect the feed to the motors and try the switch?

 

[jerjwillelec]

You said the voltage was good, but did you actually disconnect the feed to the motors and try the switch?

No...actually I haven't considering it is brand new. I will do that first thing tomorrow.

 

[Jraef]

Yes...with my temp MMS at the motor...voltage is good but when I flip the switch, the breaker trips out. I'm just wondering if weak conductors (that might hold under no load) might still be a possibility.

Conductors don't get "weak", but the insulation can leak. But it would leak whether or not it was under load. It's more likely that you have a problem in the motor. One thing that sometimes does not show up on a megger test is if the connections in the peckerhead are touching the frame, but only when the motor jerks at start-up. Older split-bolt and tape connections, or those not done right, can rub against the walls and wear through. Have you been able to open up and inspect each peckerhead?

 

[ptonsparky]

Disconnect the motor at the peckerhead. While there, inhale. You will recognize the smell forever after, if you don't already. Most bin top fans are the last thing on list of preventive maintenance. Look around while your up there, not just for the view but for broken couplings, dampers that don't work or missing, bad bearings, etc. The fans we work with are 3600 rpm and debris on the blade can cause bearing failure in one season or less.

 

[kwired]

Disconnect the motor at the peckerhead. While there, inhale. You will recognize the smell forever after, if you don't already. Most bin top fans are the last thing on list of preventive maintenance. Look around while your up there, not just for the view but for broken couplings, dampers that don't work or missing, bad bearings, etc. The fans we work with are 3600 rpm and debris on the blade can cause bearing failure in one season or less.

My experience outside of phase loss and improper overload protection, is that bearing failure is the number one cause of failure of such a fan. Imbalance from debris deposits on the fan blades doesn't help, but they are also in a pretty severe environment when it comes to daily temperature changes and the resulting condensation issues that go along with that. A fan that runs 24/7 will usually last longer than one that is only run for a few weeks every year because it doesn't see the daily temp fluctuations.

 

[ptonsparky]

Debris on a Grain Dryer fan happened to be about 4 mice, hard to tell, that happened to be inside the blade hub when it started. Birds, rats that decided it was time to leave or were sucked through when the fan started. Wear gloves it can be a bit messy.

Your 1507 has an ohms range that is as accurate as most of us in the field will ever need. Ohm, A-B, B-C, C-A to check basic resistance. Odd reading is either you or the motor/conductors to it.

 

[jerjwillelec]

My experience outside of phase loss and improper overload protection, is that bearing failure is the number one cause of failure of such a fan.

Would you please expand on what you mean by this? This is a shaft mounted fan. Guess I'm wondering where bearing failure comes into play. Thank you!

 

[ptonsparky]

Would you please expand on what you mean by this? This is a shaft mounted fan. Guess I'm wondering where bearing failure comes into play. Thank you!

Enough failure that the rotor will rub into the windings.

 

[jerjwillelec]

Enough failure that the rotor will rub into the windings.

Ahh ok thanks!

 

[ptonsparky]

Armature.

 

[jerjwillelec]

Armature.

Is that really necessary? Thanks for the help