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Motor Protection - Hot and Cold withstand curves

Motor Protection - Hot and Cold withstand curves

Motor Protection - Hot and Cold withstand curves

(OP)
I have recently been looking at some motor protection relay curves and noticed some curves that were set below both the the Hot and Cold locked rotor safe stall times, and other curves that were set in-between the two points.

I have heard that setting the overload curve between the two curves sometimes helped with achieving more starts, which in most cases is 2Cold, 1Hot start.

I was wondering typically where the relay curve should be set in relation to the two curves and if setting it between the two curves was risking damage to the motor?

RE: Motor Protection - Hot and Cold withstand curves

It depends.
The size of the motor,
The process,
Why are multiple starts needed?
How reliable are the operators?
What are the consequences of a motor failure?
Is it a condition of operation that the motor must make multiple starts?
This may happen if the motor is stopped to make a cut-and-try adjustment, that may have to be tweaked with multiple starts.
Unfortunately, the cases where the motor must be started multiple times in a short time, or where there is a strong possibility of multiple starts due to some process upset such as clearing a jam-up, are the cases where the motor is most likely to be damaged by an extra hot start.
Often, the more reason there is to go for a setting that allows one extra start, the same reasons are often a good indication that the extra start will eventually be made on a hot motor.
NEVER TRUST THE CUSTOMER'S VERBAL ASSURANCE THAT SOMETHING THAT CAN BE DONE, WILL NOT BE DONE.

By the way:

Quote (Nick)

I have heard that setting the overload curve between the two curves sometimes helped with achieving more starts, which in most cases is 2Cold, 1Hot start.
I have always considered that as one cold start followed by and two hot starts within an hour.
I have also seen motor rated for the number of starting seconds allowed per hour.
A high inertia load gets takes longer for each start and so gets less starts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Motor Protection - Hot and Cold withstand curves

Well, any decent motor protection relay should be able to model the motor heating which means the curve is set to be under the cold curve and the correct motor thermal heating and cooling time data is also entered. Then, the curve will shift to dynamically represent the overload capability remaining as the motor is operated.

RE: Motor Protection - Hot and Cold withstand curves

As LionelHutz said, most modern motor relays have stator and rotor thermal models that will adjust the safe stall time automatically. Best advice is to follow the recommendations in the instruction manual for the relay in question. The traditional tine-current curve approach does not tell the entire story of what is being done in the relay firmware. And the relays are all different.

Cheers,

Dave

RE: Motor Protection - Hot and Cold withstand curves

The above is good advice. If the motor protection relay you have does not adjust for the actual motor conditions (some function of (i2/t)), and setting below the more conservative curve does not provide a needed ability to restart, you could consider getting a modern motor protection relay (if the impact of a failed motor is costly enough to warrant it).

In the few similar cases I was involved in where 500HP motors gave trouble after operators repeatedly ignored the restart time limitation, the damage showed up as broken rotor bars. One or a few broken rotor bars did not prevent motor operation, the symptom was a difficult to diagnose vibration.

Fred

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