45 KW DC Motor overheating

shaneprasad

Member
Join Date
Feb 2017
Location
lautoka
Posts
14
Hi guys we having a bit of trouble with a 45 kw DC motor . The motor is controlled by a DC converter. The motor starts to heat up after about 2 hours and trips . The alarm displayed is ... motor overtemperature. We have checked the carbon brushes which are all ok. Also the three phase to the converter is ok . We have measured the current in all three phases which is 45 amp while running on load. We have checked the carbon brushes and cleaned the commutator and checked the blower motor and filter which is ok but still the motor is overheating .Also all the connections are tight. What could be the cause for overheating and what further tests do i need to do?
 
As Maxkling said, the first step is to find out if the motor is really overheating or if the temperature sensor is faulty. It sounds like you've checked things out pretty thoroughly so my guess is the temp. sensor, or it's wiring is at fault.

If the motor is really overheating, double check that the air paths through the motor are clear. Sometimes, even with a filter on the blower, dirt can accumulate inside the motor and cause overheating.
 
Hi guys....we have checked the motor temperature and it ranged from 58 degrees to 70 degrees celcius while running. All connections are ok. Air path is clear . Is there a possibility that there is some problem with the converter?
 
Some DC motors have an external cooling fan as you indicated. If the DC motor is running at lower RPM and not turning near base RPM (speed), make certain the external fan is on all the time that the DC motor is running. The cooling fan may also require a delay off after motor shutdown.


You might want to post the ambient temperature near the motor and the ambient temperature rating of the motor. Some motors are rated at 40 degrees Celsius above ambient. If this is the case, then your post of 70 degrees Celsius might be near that value provided the ambient temperature is 30 degrees Celsius or greater.
 
Last edited:
Hi guys....we have checked the motor temperature and it ranged from 58 degrees to 70 degrees celcius while running. All connections are ok. Air path is clear . Is there a possibility that there is some problem with the converter?

70C isn't even an overtemperature for the bearings. They should run 80 - 90C no problem.

I had to look up the motor ratings for insulation. As you can see, even the lowest class allows for much more temperature than you are seeing. I have never seen a DC motor (over 1 HP) with less than class F insulation

Class: A=105°C, B=130°C, F=155°C, and H=180°C

Where is the overload coming from? Is it the 3 phase converter that has AC overload blocks? Some sort of DC overload block? Or temperature sensors in the stator?

Do you have ground fault current measurement on the motor, or on the power supply to the motor? I could see ground fault current heating up part of the motor.

It is possible that you have a local hotspots in the stator, due to motor insulation partial failure, allowing current flow into the stator or across part of a winding. If it were beside a temperature sensor .. maybe that could trigger. I've never seen one. But you're looking for possibilities.
 
Does your motor run continuously? If it is a process that has periods of time where the motor is stopped you may want to check if your field supply is maintaining full current all the time. This will cause localised heating in the field windings as the blower normally stops after a short delay after your armature contactor (If fitted) drops out.
Most modern drives drop the field current down to 30-50% of running current while the motor is stopped. This is enough current to keep the windings warm, but not enough to cause the temperature sensor embedded in the field windings to trip.
 
I had a similar problem a few years ago and the issue ended up being the settings on the DC board. At zero commanded speed the board was outputting enough voltage to heat the motor but not make it turn.




Bubba.
 

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