CASE STUDY 9611
"Don't Restart This Motor!"
Summary: A high resistive imbalance at the motor can be fatal
Motor Information:
Type: Induction
Volt: 2300
HP: 1000
FLA: 227
RPM: 3600
Problem
While operating under normal load, this 1000 hp coke heater feed
pump tripped and opened two fuses. The maintenance history showed
that this motor had been rewound eleven months earlier at a cost
of $24,000.
Looking for the Root Cause
The Reliability technician conducted a standard MCE test at the
motor contractor. The results uncovered a large resistive
imbalance. To isolate the problem, the motor was tested at the
T-leads which recorded these readings:
Resistive Imbalance 19.66%
Inductive Imbalance 2.17%
Capacitance to Ground 52,000 pF
Resistance to Ground 3.6 Megohms
Since the resistive imbalance remained high at the T-leads, it was determined that the fault was the stator, possibly a turn or phase to phase short. The recommendation was "do not start the motor" and perform a breakdown investigation. The warning was ignored. During restart a fireball erupted from the motor and three substations subsequently tripped off-line and the back up power systems were surged.
Results
The motor was sent for repair. Disassembly of the motor revealed
turn to turn shorting in the stator. A warranty claim was
submitted to the rewind shop. Since part of the job had been
subcontracted, a settlement of $17,000 was reached.
Industry: Refinery
Application: Diagnostic