Simultaneous Burnout of Motor and Control
Copyright © 2005 Francis J. Martino
Case Number 1: Control Fails, Motor Burns Out
On and off control of a submersible pump with an across-the-line full voltage starter was accomplished by remote signal transmitted by the telephone system. The telephone utility acknowledged that the utility system had malfunctioned and was the cause of the starter repeatedly starting and stopping the motor.
The typical motor can be expected to start and stop no more than six times per hour, or, once every ten minutes, without excessive overheating. A motor starter will have a similar rating. However, with a repeated cycling on and off so that the motor starter is chattering, as was the case, both the starter and the motor will have excessive heating to the point of contacts melting and motor insulation failure due to excessive heating.
The resulting effect was that the starter contacts burned as the starter continuously opened and closed the motor circuit under repeated motor inrush conditions.
Thus, in Case Number 1, control failure caused the motor to burn up.
The only protection against a motor starter chattering for any reason, including worn mechanical linkage, is a low overload class protection of less than class 10. A low overload class would have allowed both detection of sustained high currents and a quick shutdown that would have kept the motor from burning out.
Case Number 2: Motor Fails, Control Burns Out
A 75 HP submersible pump motor burned up due to either bearing failure or normal insulation degradation after years of service. The control was an autotransformer reduced voltage starter that was set for a three second start.
The resulting high currents during motor failure did not generate sufficient heat to cause the starter overload to trip quickly. In addition, the upstream branch circuit protection failed to remove current fllow. Thus, the motor burned before it was taken off line. The high current flow caused by the motor failure also caused the reduced voltage starter to burn up with the motor.
The only protection available for the controller to protect against bearing seizure and failure would have been an overload class lower than that of Class 10. If the motor winding were the cause of the failure then a lower rating on the upstream circuit breaker or fuses would have caused the motor to be taken off line in just a few seconds and the starter would not have been damaged.
For information on overload protection see:
Motor Protection: Preventing the Burn-out
Order a Solid State Overload Relay
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