VSD (Variable Speed Drive)
Controls the speed of an AC motor by converting the supply to a variable frequency and voltage.
What it is
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.
How it works
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.
Where it's used
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.
Safety first
The DC bus stays charged after power-off — wait the documented discharge time and prove dead before touching internal terminals.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
VSD powered up but won't start the motor
The drive is energised and the display is alive, but it won't run the motor. No fault may be shown — it just sits in 'ready' or 'stopped' and ignores the start command.
VSD trips on overcurrent / overload
The drive trips with an overcurrent or overload code — on start, on acceleration, or under running load. It may restart and trip again on the same point in the cycle.
VSD trips on DC bus overvoltage
The drive trips on overvoltage — usually during deceleration or stopping, when a spinning load pushes energy back into the DC bus faster than it can be absorbed.
VSD trips on earth/ground fault
The drive trips with an earth/ground-fault code — typically the instant it outputs — indicating leakage to earth on the motor or output cabling.
Related definitions
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
Soft starter
Reduces motor starting current by ramping the voltage up, then often hands over to a bypass contactor.
Star-delta starting
Starts a motor in star (lower current) then switches to delta (full power) once it's up to speed.
AC vs DC
Alternating current reverses direction many times a second; direct current flows one way. Why it matters on site.