Soft starter

A soft starter starts a squirrel-cage induction motor gently. Unlike a variable speed drive, it does not change the frequency or the speed. It ramps the motor voltage up from an initial voltage to the full mains voltage.

How it works

A soft starter has two anti-parallel thyristors in each controlled phase. At the start they let only the last part of each half-cycle through to the motor, and then a larger and larger part, so the voltage rises smoothly.

Current and torque fall with the voltage. The current is roughly proportional to the voltage and the torque to the square of the voltage: at half voltage the current is about half and the torque about a quarter. In a direct-on-line start the current is typically 7–8 times the rated current, with a soft starter 3–5 times.

Two or three controlled phases

Many soft starters control only two phases, and the third passes straight through. The phase currents are then unequal during the start and the stop, but balanced while running. A model that controls all three phases can also be connected inside the motor delta. It then carries about 58% of the motor current, and six cables are needed to the motor.

Bypass

When the ramp is finished and the motor has full voltage, the thyristors are bypassed by contacts. The thyristors then produce no losses and no heat. Many models have built-in bypass contacts. A separate bypass contactor is controlled by a relay output that closes at the end of the ramp (top of ramp, TOR).

Soft stop

When stopping, the voltage is ramped down. In pumps this reduces water hammer and pressure surges, and on a conveyor fragile goods are not damaged. Water hammer is eliminated completely only with torque control, which some models have. A stop ramp is not used for a fan, because the heavy impeller slows down slowly anyway.

Settings

• Start ramp. The time from the initial voltage to full voltage. It is not the same as the time the motor takes to start.
• Initial voltage. The voltage the ramp starts from. If it is too low, the torque is not enough and the motor does not start to turn.
• Stop ramp. The time from full voltage to the end voltage. Set to zero, the motor stops as in a direct stop.
• Current limit (on some models). When the limit is reached, the voltage stops rising until the current has dropped.

For example, the ABB PSR range has three potentiometers: start time 1–20 s, stop time 0–20 s and initial voltage 40–70%.

Wiring

• Power circuit. Supply to 1L1, 3L2 and 5L3, motor to 2T1, 4T2 and 6T3.
• Control. A control supply, for example A1–A2, and a start input that is kept energised for as long as the motor should run. On the ABB PSR the input is ST, on the Siemens 3RW30 terminal 1 (IN).
• Relay outputs. For example, Run is closed while the motor is supplied, and it can control a line contactor. There are often also TOR and fault outputs.
• Short-circuit protection. A motor protection circuit breaker, gG fuses or a circuit breaker give type 1 coordination. Type 2 coordination needs semiconductor fuses.
• Overload protection. Some models have it built in. Otherwise use a motor protection circuit breaker, or a thermal overload relay with a contactor.

Not an isolator

A soft starter does not isolate the motor from the mains. When the supply is on, there can be a dangerous voltage at the output even without a start command. For maintenance work the feeder is isolated with a separate switch. Some manufacturers also require a device that cuts the motor current when a fault occurs, for example a line contactor, because the soft starter cannot itself break the current if a thyristor fails short-circuited.

Soft starter, star-delta or variable speed drive

• Star-delta. The current is about a third of the direct-on-line current. In theory the torque is also a third, but according to ABB it is closer to 25% in practice. It cannot be adjusted, and switching from star to delta causes a current peak.
• Soft starter. A smooth ramp without a changeover peak, and a soft stop. It does not control the speed, and the starting torque is lower than in a direct-on-line start.
• Variable speed drive. Controls the speed and gives rated torque from the start at about rated current. For soft starting alone it is often an unnecessarily advanced solution.

Soft starters are used for example with pumps, fans, compressors and conveyors.