I have very limited electrical knowledge.
Can I get some ideas/information on what might cause a 100hp DC motor's field windings to fail?

This is the second time in 14 months this has happened.

What tests if any can be done to catch this early?

Thanks

Yes.

Some drives keep the field windings energized when the armature is not turning. This is meant to keep the winding warm so that it pulls the correct current and provides the proper torque and good speed control. Modern drives that keep the fields energized usually time out after a certain period of time to avoid overheating the field coils.

Usually there is a fan that keeps airflow over the coils both when the motor is running and when the armature is stationary but the field is energized. If the filter is dirty or the fan is not operational, the fields will fail due to overheating from the inside out.

Because of the type of failure, insulation to ground testing will often not work very well. You can use motor circuit analysis (ALL-TEST Pro or PdMA) instruments to compare one set of coils to the next (simple resistance readings will not detect the problem soon enough).

Howard

Yes, there is a cooling fan on this motor.

Low air flow was noted after the motor was changed yesterday, filter was changed with no effect, then someone else decided to check the rotation of the cooling fan. The rotation of the cooling fan was backwards after replacing the motor.

Not sure if the filter was dirty enough to have caused the initial motor failure.
But, if heat is the main cause of field winding failures, that may very well have been the issue.

I think this motor may not last very long as it got pretty warm and did not smell good on startup with the cooling fan running backwards.

Thanks

REJ,
Unfortunately you were bitten by a very common problem. People assume that because the rag is 'sucked' up against the cooling fan inlet screen that rotation is correct. That is a false assumption. The squirrel cage blower wheels will still draw inlet air, but the flow could be as much as 40% less.

Good point, Ron.

REJ, what kind of testing technology do you have?

Motor diagnostics, multimeter, milli-ohm meter and/or meg-ohm meter?

Howard

We have/use multimeters and meg-ohm meter.

We also have a MTC Smart Advisor continouis vibration monitoring system, and an old Entek system.

REJ

Smells good or bad is very subjective. I bet it would smell good to me. But I am in the motor repair business.
Besides technologies mentioned elsewhere in this thread there is one that has been used with great success for past about 100 years.
In most cases the field winding has 4 leads. F1, F2, F3 and F4. If it is your case feed F1 and F4 from 120 V AC line. Connect together F2 and F3. Then measure from F1 to F2 and record the voltage and also from F3 to F4 and record the voltage. If the voltages differs more than 5%, there are likely few turns shorted. Further investigation would be necessary. The initial investment in this "technology" is fairly favorable and the results are just as accurate, if not better. Just remember that the safety is your responsibility.
jank

Some other things to consider:

1. How clean or dirty is the environment around the motor? We have found that, even though we keep filters in place on our blowers, crud still manages to find a way in, and it builds up on the squirrel cage fan blades, and reduces air flow over time. We currently disassemble our blowers quarterly for cleaning.

2. We have started taking frequent looks at our larger motors with the IR camera, and are trending the results. A motor that grows warmer over time may be experiencing reduced air flow from the blower.

3. We have also started trending the inductance of our field windings on those motors. A reduction in inductance will give a much earlier indication of developing turn to turn shorts than resistance checks.

Did you ever confirm whether the fields are energized during idle times?

Howard