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Slip Ring Motor


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I experienced a very strange but interesting phenomenon. We have two 600 k W slip ring motors driving a common load. The two motors are identical motors. The two motors have always been sharing the load equally around 300 k W on each one. Since a couple of days, however unnoticed by anyone, one of the motor had been running with one phase of the rotor almost opened due to a loose cable. This was shown on the trends with one of the motor lagging behind the other in power consumption. It is easily understood that under this condition, the motor with the loosed rotor will be slipping more than the other and therefore was not contributing fairly to the load. My worry is that this condition was not detected by the protection relay, a CET4 one. Another amazing feature was that the stator currents on the faulty rotor were fluctuating more but with no noticeable imbalance.

How could such a condition be avoided ?





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Hi Bob,


Motor protection relays (be they CET4 or alternative type) will only trip out if the abnormality exceeds a user defined set point. Trip delays may have also been programmed.


Take for example a situation where a protection relay has been programmed as follows;


Motor FLC - 100 amps

Undercurrent trip level - 75 amps

Undercurrent trip delay - 10 seconds


The run current would have to fall below 75 amps for a period of 10 seconds for the relay to trip. If the run current fell to 20 amps for a period 2 seconds, the protection relay would not trip. In other words both conditions need to be met.


Bob, based on the above I would have to ask; i it possible that the CET4 has not been programmed to trip under the conditions you are witnessing?




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Hi GGoss,


Thanks, I understand your points. However, what is of concern to me is that one of the motor was running with one leg of the rotor opened and without any appreciable change in stator current. The kW consumed on the motor was fluctuating and the other one was taking the major part of the laod.

I understand that if the motor was driving the load on its own, it would have tripped beacuse it would have been impossible for it to sustain the starting and running torques.

My question is could a slip ring motor run with one of the rotor leg opened on no- or small load ?





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