Starting motor thermal trip

A

Thread Starter

Amosiss

Hi

we have two GE frame 7 gas turbine with MARK VI.
The problem is that on cranking mode the starting motor trip after about 10 min by thermal protection.
The name plate mention nominal current of 102A but the motor is consuming 140A during cranking.
Is this normal?

Thanks for your help
 
Amosiss,

Presuming the Torque Adjustor limit switches are adjusted and working correctly during a normal START, purging draws (or should draw) approx. 150-160% of motor nameplate during purging, and when the purge time is complete the unit should coast down to firing speed (about 10-12%) and the current draw for that speed is about 60% of motor nameplate. When flame is established the torque adjustor pulses the torque output to maximum (again, typically about 160-170% of rated current) during acceleration up to about 60% of speed when the cranking motor is dropped out.

When CRANKing, the unit should still go through the purge at the higher speed and coast down to firing speed and hold that speed at the lower current draw indefinitely without tripping the cranking motor.

If, during cranking, someone has manually raised the torque adjustor to a higher position for a faster "cranking" speed that would increase the current drawn by the cranking motor. Another thing that slightly increased current draw and somewhat decreases speed while cranking is if the IGVs are forced to maximum operating position, but the current should only increase by about 10% to about 70%, still well below the thermal trip setpoint.

I have described the typical starting and cranking sequence for most GE-design heavy duty Frame 7EA gas turbines with Voith adjustable torque converters. There is always the odd unit here and there, but in general this is true of most Frame 7EA units in use around the world today. These large AC cranking motors should only be started two or three times per hour, maximum.

So, unless someone has been forcing logic or manually operating the Torque Adjustor Drive Motor's starter to raise torque/speed while CRANKing, the cranking motor should typically be drawing only about 60% of rated current--presuming the Torque Adjustor limit switches have been set correctly and are working correctly.

Hope this helps! Please write back to let us know the circumstances and how you resolve the problem.
 
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