Off Topic / Motor Troubleshooting Help

I am trying to troubleshoot a motor that keeps blowing fuses on startup but does not trip the electronic overload on the contactor. The Overload has been swapped to make sure it is good so why does it blow the fuses and not trip the overload?

This is a 10 hp motor and is 1785 rpm 14 amps on the name plate.

When I check the motor t1-t3 with a megger I get 3.0 giga ohms so that is good but when I check phase to phase witha meter I get no resistance or infinity and on another motor of 15 hp I get about 3.5 ohms when I do t1 to t2 and t2 to t3 and t1 to t3.

Seems like the motor is bad or going bad? What would this kind of fault be called as it is not really a ground fault I assume?

open windings causing single phasing?

Tim Ganz said:

I am trying to troubleshoot a motor that keeps blowing fuses on startup but does not trip the electronic overload on the contactor. The Overload has been swapped to make sure it is good so why does it blow the fuses and not trip the overload?

This is a 10 hp motor and is 1785 rpm 14 amps on the name plate.

When I check the motor t1-t3 with a megger I get 3.0 giga ohms so that is good but when I check phase to phase witha meter I get no resistance or infinity and on another motor of 15 hp I get about 3.5 ohms when I do t1 to t2 and t2 to t3 and t1 to t3.

Seems like the motor is bad or going bad? What would this kind of fault be called as it is not really a ground fault I assume?

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The windings of a motor are just a coil of wire that produces an electromagnet.

One would expect such a winding to have a resistance equal to the length of wire used to produce the winding. I would expect the resistance of each winding to be at or below about 5 ohms.

In addition I would expect the resistance from each winding to the case of the motor to be very high, over 1 gigaohm.

Next check the field windings. Same idea applies, different readings are possible. Field windings could be 100 ohms, but all the windings should be very close each other. All the field windings should be similar or within 10% and then the motor windings should be similar to each other and are generally very low.

Some field windings use smaller wire and a lot more wire than the other windings, which accounts for the field having a higher resistance.

Open = Infinity or very high, over 1 gigaohm
Short = less than 2 ohms
Partial short could also cause problems or high resistance.


Open winding is bad.
Open field is bad.
Short to ground from any winding is bad.

A regular multimeter is not recommended for checking the ground readings, the megaohm meter applies a much higher voltage than a normal meter to test insulation breakdown.

If a normal meter says the motor is bad, the motor is most likely bad, if the normal meter indicates a motor is good, it means the motor might be good.

It sounds like you have a bad motor.

Remember to check the motor while completely isolated from all external wiring.

A quick check of the whole system would be to go to the MCC bucket that energizes the motor and test the motor from there.

This will not tell you if the motor is bad , but if the tests pass from the bucket, you have eliminated the motor and the wiring and you are right there at the contactor so you can then test the contacts in the motor starter and auxiliaries.

If you get a bad reading in the MCC, then go to the motor and do the test there with the wires disconnected.

Testing from the MCC must be done with the power to the bucket in question OFF.

If the motor is running, but trips from time to time , read the voltage drop across the contacts in the MCC feeding the motor. Overloads can fail as they age or get more sensitive and contacts can become resistive and drop voltage meant for the motor.

And remember the fuse is usually faster than the overloads.

The overloads actually heat up and then open a contact with a bi-metalic element that works like a thermostat in HVAC.

For this reason, the motor must pull too much current for a period of time to produce this heat and the overloads can not be reset until they have cooled off.

Hope some of that helps.

Tim you said
When I check the motor t1-t3 with a megger I get 3.0 giga ohms so that is good but when I check phase to phase witha meter I get no resistance or infinity and on another motor of 15 hp I get about 3.5 ohms when I do t1 to t2 and t2 to t3 and t1 to t3.

Now if t1 t3 are teh motor leads and you get giga ohms between them and you then us a VOM and get infinity then you have an open winding or such high resistance you may as well call it open. As you confirmed it should be about 5 or so ohms.

Why the fuses - they have a faster response time than the overloads.
Send out for rewind or replace. You may want to open motor up and see if you can get lucky and find a nearly broken motor lead or bad braze - dont hold your breathe though.

You can use a megger to check motor windings but not really the best choice - generally used for checking insulation resistance. VOM is better choice. Best is 4 wire put low voltage ie 6VDC across winding read current and voltage and crank into Ohms Law R = E/I
Dan Bentler

It is clear from the data in the OP that this is a 460V AC induction motor with extremely high efficiency (14amp instead of 17amp FLA).

Since this is an AC motor, there is no field or field leads.

The fuses are likely releasing due to exceptionally high inrush current. Maybe the OP could check the motor nameplate and give us the CODE LETTER on the nameplate. High inrush current is becoming very common due to the increasing use of premium efficiency motors. This is likely one of them.

Since this is likely a wye connected dual voltage nine lead motor, you would not expect any continuity between T1 and T3 or any other lead except T4. The motor is probably not bad.

My guess is that the premium efficiency motor is drawing unexpectedly high starting inrush amps and the time-delay fuse time curve is not long enough. Let's see what the motor nameplate code letter comes back as.

What's the type and rating of the fuses ie are they motor rated ? You only mention 1 contactor, is it a DOL starter - The inrush current can be 7 times the nameplate amps. This won't trip the electronic protection but will blow non motor rated fuses.
Paul

Lets keep it simple with how to check a motor circuit.

Assuming the motor leads are wired according to the nameplate illustration ( for a 9 lead motor high voltage it would be L1-T1, L2-T2, L3-T3, T4-T7, T5-T8, and T6-T9), then check the following from the bottom of the starter/overload.

First check T1-T2, T2-T3, T1-T3 with a VOM, all three reading should be low (<5ohms) and be equal to each other.

If that is ok, then check T1, T2, or T3 to ground with the VOM ( to find the in your face ground short)

If that is questionable then check above with the megger. The reading needs to be greater that 1Mohm (For across the line starters, VFDs generally like to see above 10Mohm)

If any of the above fails then the motor or wiring to it is bad. Remove the motor junction box cover and disconnect the line leads and check again to determine if motor or wires (sometimes you will get lucky and find a loose connection or a bare spot on the wires when you do this).

Sometimes the motor can still be bad and only show up when it rotates at speed due to faulty bearings causing the rotor to rub the stator windings. Spin the motor by hand if practicable to see if it spins free and smooth.

If the motor has been running in this application fine, and this just started happening then the fuses, overload, etc.. are probably not the issue.

If this is a new installation then the design may be an issue.

Be safe and make sure you have the power locked out whenever you are checking the motor circuit, including the power to the starter coil.

I hope this will help you or anybody else troubleshoot motors simply and quickly.

The starting inrush current USED to be 6-8 times FLA. With today's premium efficiency motors, its not uncommon to have inrush currents in the 13-16 times FLA range. This has been a matter of curious silence as the benefits of premium efficiency motors has been splashed across the headlines. There will be a lot more trouble with inrush currents in the next few years. It will be a great time for softstarter manufacturers.

DickDV said:

The starting inrush current USED to be 6-8 times FLA. With today's premium efficiency motors, its not uncommon to have inrush currents in the 13-16 times FLA range. This has been a matter of curious silence as the benefits of premium efficiency motors has been splashed across the headlines. There will be a lot more trouble with inrush currents in the next few years. It will be a great time for softstarter manufacturers.

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Dick good info will have to watch out for this.

In my response I assumed he had not opened up the motor leads ie just went across the line with all connected to get his readings.

Thanks for clarification(s).

Dan Bentler

As others have said, the simplest answer is that your discrimination is probably not set up correctly for the motor characteristics.