Soft starter
Reduces motor starting current by ramping the voltage up, then often hands over to a bypass contactor.
What it is
A soft starter limits the inrush and torque when a motor starts by gradually raising the voltage applied to it, rather than slamming full voltage on at once.
Unlike a VSD it doesn't control running speed — it only manages the start (and sometimes a controlled stop).
How it works
Power semiconductors (thyristors/SCRs) phase-control the voltage, ramping it up over a set time so current rises gently. Once up to speed, a bypass contactor usually takes over to carry the run current efficiently and keep the semiconductors cool.
Where it's used
Pumps and conveyors where a hard direct-on-line start would cause water hammer, belt snatch, or trip protection. It's a simpler, cheaper alternative to a VSD when only the start needs taming.
Safety first
Power semiconductors switch live current — isolate and prove dead before working on terminals. The motor can start when a fault clears.
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
Soft starter faulting or not ramping the motor
A soft starter won't ramp the motor up smoothly — it faults out, the motor jerks or doesn't reach full speed, or the bypass doesn't engage.
Motor trips protection on start but runs fine if it gets going
Protection trips during the start/inrush, but on the rare occasion it gets running it's fine — pointing at starting current, settings, or load inertia rather than a running fault.
Star-delta starter not transitioning to delta
A star-delta (wye-delta) starter starts the motor in star but never switches to delta — the motor runs weak/slow, or trips, because it stays in the starting connection.
Related definitions
VSD (Variable Speed Drive)
Controls the speed of an AC motor by converting the supply to a variable frequency and voltage.
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
Star-delta starting
Starts a motor in star (lower current) then switches to delta (full power) once it's up to speed.
Contactor
An electrically-operated switch that uses a coil to make or break a load circuit, usually three-phase power.