The interaction between a rotating magnetic field flux and a rotor winding flux allows an induction motor to self start. In response, torque increases and results in high rotor current. When there is high rotor current generated, the motor reaches full speed ending up heated to high degrees which eventually damages it. A soft starter motor exists to prevent such situations.

A motor can be started in one of the following three ways:

• with full load voltages in specific time periods, also known as direct on line starting;
• gradually applying reduced voltage or soft starting;
• with an autotransformer starter or winding starting.

So basically, any device that can reduce the torque applied to an electric motor is called a soft starter. This device mainly consists of solid state devices which are known as thyristors that control the application of the supply voltage to the motor. There are two types of control that can be applied using soft starters:

1. Open control – the speed of the motor and the start voltage are not relevant here. Two soft motors with SCRs are connected back to back and initially, they are conducted with a 180 degrees delay during the two half wave cycles. With time, this delay gets reduced until the applied voltage ramps up to full supply voltage. However, this method does not really control motor acceleration, so it is not that relevant.
2. Closed loop control – the response from the motor you need to get is designed as a result of monitoring the speed and the current. To get the required response, the current in all phases is monitored and the voltage ramp gets halted as needed.

How do these devices work?

Their main characteristic is that they’re semiconductor devices. They can reduce the motor terminal voltage for a short period of time, which results in one of the above-mentioned types of motor current control. As the voltage of the motor is controlled and reduced, the torque is also reduced, thus allowing the motor to start more gradually.

Applications

Soft starters can be used anywhere where a smooth and slower starting process is required:
starting and stopping processes where control of speed and torque is required;
where starting a large motor is associated with high inrush currents which need to be limited;
when torque spikes need to be avoided in order for a gradual controlled starting to be conducted;
in piping systems, for avoiding pressure surges and hammering.