Soft start variable AC voltage starter

A soft starter is a solid-state AC voltage controller placed between the supply and an induction motor. It starts the motor at a reduced voltage and raises the voltage smoothly to full value, so the starting current and torque rise gradually instead of in one jump.

Principle of operation

• Each phase has a pair of anti-parallel thyristors (or a triac) in series with the motor winding.
• By changing the firing angle $\alpha$ of the thyristors, only part of each half cycle of the supply is applied to the motor. A large $\alpha$ gives a low rms voltage; $\alpha$ near zero (or the conduction angle near 180°) gives full voltage.
• The frequency stays at 50 Hz; only the rms voltage changes.

 R ---[SCR pair]----+
 Y ---[SCR pair]----+---  3-phase
 B ---[SCR pair]----+     induction
        |                 motor
   firing control  <--- ramp / current
   (alpha: 150->0 deg)    feedback
        |
   bypass contactor closes at full V

How the ramp-up is produced

1. At start the controller fires at a large angle (say 120–150°), giving about 20–40 % of rated voltage.
2. A ramp generator reduces $\alpha$ step by step over a set time (typically 2–30 s), so the rms voltage rises linearly from the initial value to full voltage.
3. Many starters use current-limit control: the current is measured and $\alpha$ is adjusted so that it never exceeds a set value (e.g. 2–4 times full load).
4. When full voltage is reached, a bypass contactor shorts the thyristors to remove their losses.
5. A soft stop works the other way: voltage is ramped down so that pumps and conveyors stop gently.

Impact on motor performance

Motor torque is proportional to the square of voltage, $T \propto V^2$, and starting current is roughly proportional to $V$.

Limitations: since torque falls with $V^2$, a soft starter is not suited to loads needing high starting torque (e.g. loaded crushers). It does not control speed in steady state; it only controls starting and stopping. It is ideal for fans, pumps, compressors and conveyors.