An Unusual Problem

The machinemonitor® team were invited to investigate a 4000kW 11kV compressor drive motor that failed to start reliably every time. The behaviour was intermittent, the machine accelerated towards full speed through the initial start sequence but timed out before the final start stage closed.

A Temporary Fix

To address the problem the machine owner resorted to boosting the voltage before attempting a start. Boosting the voltage was a temporary fix that stressed the other equipment on the busbar and although it recovered some capability for the motor to start, did not address the root cause of the problem.

Testing and Investigations

The investigation involved assessment of the main components, auto transformer, switchgear, excitation, motor and the compressor load and taking measurements for both a failed start attempt and a successful start. A reverse engineered machine model was created and the starting performance of the motor was calculated using the known power system fault level and capacity as a baseline, quantifying, starting currents, starting torque and acceleration time to full speed. The model was adjusted to match the measured parameters for the successful start to understand the voltage and flux levels at the measured current and starting time.  The model identified that when the measured start was simulated the applied voltage to the motor was low related to a connection anomaly  on the autotransformer.

A Solution In Sight

Our recommendation was to qualify this finding by fitting a voltage divider at the motor terminals, to measure the voltage directly during the start sequence transitions, to qualify the finding of the model before making the change. The investigation will be continued in July.

A Failed Start. Hover for a successful Start

Failed Start

Successful Start