VSD (Variable Speed Drive)

A VSD (also called a VFD or inverter) sits between the supply and an AC motor and controls the motor's speed by changing the frequency it's fed.

An induction motor's speed follows the frequency of its supply, so varying the frequency varies the speed — smoothly, without the energy waste of throttling.

Three stages: a rectifier turns the incoming AC into DC; a DC bus (with capacitors) smooths and stores it; an inverter switches that DC rapidly to synthesise an AC output of whatever frequency and voltage is needed.

The drive keeps the voltage-to-frequency relationship sensible so the motor produces proper torque across the speed range, and it ramps speed up and down to limit current and mechanical stress.

Pumps, fans, conveyors, and any motor application where variable speed saves energy or improves process control. Drives also provide soft starting, protection, and monitoring.

The DC bus holds a dangerous charge after power-off — always wait the documented discharge time. Common faults are coded: overcurrent, overvoltage (on stop), undervoltage, earth fault, and over-temperature.

Drives mostly tell you what hurt them — read the fault code and the fault log before touching anything. Overcurrent points at too-fast acceleration, an output short, or a seized load. Overvoltage usually means deceleration set too fast (the motor regenerating into the DC bus). Undervoltage is a supply dip or tired bus capacitors. Over-temperature is a blocked heatsink or a dead cooling fan.

Hardware-wise: output transistors (IGBTs) fail short — typically an instant overcurrent trip or a bang at power-up — and DC bus capacitors age into undervoltage trips. Repeatedly resetting an overcurrent trip without finding the cause is how one dead IGBT becomes six.

Check supply at the input terminals, then trust the drive's own display over your meter: DC bus volts should sit around 1.4 × supply (roughly 560 V DC on a 400 V supply), and the drive's output-current readout is more honest than most meters — the output is chopped PWM, not a sine wave, and even a true-RMS meter reads it strangely.

Dead testing: isolate, wait the documented DC bus discharge time (check the drive's label — minutes, not seconds), and prove dead across the bus terminals before touching anything inside. Suspect the motor or its cable? Disconnect them from the drive first, then test them on their own.

Never megger a drive, or through a drive — 500 V DC into the terminals kills the electronics. Disconnect the motor cable at the drive before any insulation test, and test cable and motor only.

Drives leak to earth by design (EMC filters), so they nuisance-trip Type AC RCDs — that's a selection issue, not a fault. And the fault log is gold: codes, values and run data captured at the moment of trip beat any amount of guessing.