How Soft starters Regulate Current in Induction Motors

When an induction motor is connected to a power supply, it draws a high speed magnetizing current irrespective of the load level.  The induction motor has less power factor compared to its rated power factor. This is because, as the load declines, the torque producing current decreases. As the changes take place, the magnetizing current remains the same. The power factor is lower in cases where the load is lighter. Most motors control panels have a higher load demand which goes beyond the maximum load demand. Unfortunately, the motors do not reach their rated power factor even when they have maximum load demand.

The value of soft starters

When an induction motor operates without soft starter panels, it leads to high starting current (as high as 7 times the expected current). When the motor begins at high current, it leads to mechanical shock which is felt by the immediate gearing. When the induction motor is operated without the soft starter over and over again, it leads to mechanical shock. This leads to high cost of maintenance, power consumption and the life of the motor is shortened.

Applying Starters

The heat produced after heating the motor up can damage the induction motor. Soft starters are used to prevent the damage. Motor can be power up in three ways. First, they are connected direct on line to start. This involves applying full voltage immediately to the motor. Secondly, it can be through autotransformer starter where part winding strategy is used. The last is applying reduced voltage slowly with a soft starter.

Also Read: The Appropriate Use of Motor Soft Starter

Construction of the Starter

Most soft starters consist of some type of solid state devices like thyristors to reduce the voltage which in turn reduces the current & torque while starting initially. This way motor starts slowly and the inrush current is controlled. When the motor terminal voltage reaches 90 – 96% and its full speed, the bypass magnetic contactor in the soft starter bypass the thyristors and connect the motor across line voltage for normal operation. After the disconnection, the motor operates at normal operating conditions until stopping and starting again. Something the soft stop feature is also used to ramp it down slowly.

Achieving Control

A soft starter helps to reduce the force applied on an electric motor. If the panel is in a hazardous place, then explosion proof or the purged panels should be used. For it to control the torque from the supply to the motor, it relies on a thyristors. The starter relies on the fact that the force is equal to the starting current’s square which is also equal to the applied voltage. To control the current or torque, the voltage is reduced when starting the motor. A soft starter can control the torque through open control or closed loop control.

Closed loop control is where the speed of the motor and the current drawn are monitored from the beginning and so is the starting voltage. The current achieved at each stage is observed and if it goes beyond the expected level, the time voltage ramp is stopped. With open control, there is a start voltage which is then reduced with time. The start current does not rely on the speed of the motor or the current drawn. The SCRs are connected one after the other. During the first half wave, they are connected at a 180 degrees delay. The delay is reduced until it reaches the applied voltage ramp. The method is not as good since it does not control motor acceleration.

Also Read: Reasons Why VFDs Are Preferable Over Soft Starters


When using electrical motors on various machines or applications, performance and cost saving are key factors. The cost should be as low as possible but the machine should have high performance with long life. Controlling the current in motors helps to achieve a high performance and to lower maintenance costs.