After reviewing your discussion I would like to make the following observations:
This is the third motor installed in this application in 3 years.
That fact alone should focus your attention in four areas:
1. The circuit powering the motor.
2. The load driven by the motor.
3. The location where the motor is installed
4. Quality of work you are getting from the motor repair shop
Motors should last substantially longer than 1 year, 5-7 years is an industrial norm and that number should be much higher.
The first and easiest thing to check is the circuit powering the motor.
Use of Motor Circuit Analysis (MCA), Electrical Signal Analysis (ESA), and Motor Current Signature Analysis (MCSA) would be best utilized for this testing, however if you do not have this equipment or access to a contractor to provide the service, you can perform the following:
Check current in all three phases, simultaneously, if possible. If you cannot check all three phases at the same time, try to ensure hat you have a steady load and that the load is greater than 70% of full load rating for the motor. The balance of the phase currents should be less than 5%. Using the formula 1.2 x HP yields a full load of around 360 amps. With the unit drawing 320 amps you are within the proper range.
Next check voltage phase to phase and to ground. Your phase to phase voltage should be <3% unbalance and <1% to neutral.
Any unbalance in voltage and or current is indicative of problems in the power circuit. Use IR on all accessible portions of the power circuit, looking for connection problems. If none are found, de-energize the motor and check all connections for proper torque, crimp connections, look for signs of corrosion, burnt or embrittled insulation, broken or frayed conductor strands, discolored terminations. Also check local disconnects, if utilized. They are notorious for developing high resistance connections.
With the circuit de-energized, check the circuit protection devices, breaker trip points, fusing and overload sizing. It is possible that binding in the load may be occurring with possible overloading of the circuit causing thermal damage to the motor windings. Improper circuit protection will allow this to occur without tripping and lead to subsequent failure.
The next thing to check would be the gearbox:
If a new or refurbished gearbox is available, change it out and monitor temperature and current for a change in magnitude.
It may have worn or defective components causing excessive loading or intermittent binding which will cause the motor to run hotter and lead to subsequent failure.
The installation location should be rechecked for Softfoot or any anomalies that would prevent proper alignment. Since you have already checked this it is doubtful that this is the problem, however, it doesn’t hurt to be sure. After checking the above, perform vibration analysis on the complete drive train, at load, compare all acquired data to comparable equipment at comparable load.
Lastly, check the motor history:
Is it a used motor?
If rewound, how many times? Multiple rewinds may have caused serious damage to the core, resulting in excessive heating and premature failure.
Motor acceptance testing should be done when the motor is returned to you after rewind, this data will give you some idea of the quality of the rewind.
Don Donofrio
Technology Leader
MCA and Power Quality
Charlie Hudson
MCA and IR
The Snell Group
P.O. Box 6
Montpelier, Vermont 05601
United States
Tel: +1.802.229.9820
Fax: +1.802.223.0460
1.800.636.9820 (toll free US/Canada)