Tracing a 440 V Charger Ripple Fault to Two Open Thyristors

This published field-service record starts with a low-voltage alarm on a 440 V DC power system. One of two parallel chargers had stopped delivering current, leaving the plant on the remaining unit while troubleshooting began.

From alarm to waveform

The equipment used six-pulse rectifier circuits. The team isolated the faulty charger and measured the output. The source reports that it had no output current and a high ripple voltage on the DC bus.

Tracing the ripple into the rectifier circuit led to two open thyristors. The published record says the parts were replaced and the charger was tested again. Ripple fell from 4 V to 0.5 V before the unit was returned to parallel operation.

What the record teaches

A high ripple reading is not enough to condemn a charger. It can point to the rectifier, the capacitor bank, the control gate or the measurement setup. In this record, the oscilloscope trace narrowed the fault to the semiconductor path.

The source also notes that a library of known-good waveforms would have shortened the work. That is a practical maintenance asset: capture normal waveforms at defined test points after commissioning, then compare them with the first fault record rather than starting from memory.