In this case study, we look at an industrial customer with two 1250 hp motors on a 4kV circuit, whose motors would not start. To determine the problem, we installed a Dranetz PowerGuide 4400 at the motor input. The power monitoring instrument quickly identified the motor itself as the source of the problem. In fact, the motor start caused a deep sag to occur, which impacted the motor control circuitry and stopped the motor from starting. In effect, the motor was "shooting itself in the foot," creating a cycle of non-performance.
The customer was presented with several mitigation options, including adding more capacity to the circuit, installing a constant voltage transformer, or installing an uninterruptible power supply (UPS) system. The customer selected the UPS option, which was installed to protect the motor control circuitry during motor start-up and verified using the PowerGuide 4400.
Inrush currents, such as those associated with motor starting can cause interaction problems with other loads. When motors are started they typically draw 6-10 times their full load, which can cause voltage sags. These events can dim lights, cause contactors to drop out, disrupt sensitive equipment, and as in our case study, affect the successful start of a motor. The use of a power quality monitor that can capture waveforms during long duration start-ups will be quite effective in characterizing and optimizing motor starts.