The Process of Torque Reduction with Soft Starters For Motors

“Power is nothing without control”. You have probably read this quote in some motivational poster online and although it sounds cheesy, in my opinion it is actually quite true, especially when it comes to induction motors. This type of motors have the ability to self start due to the interaction between the rotating magnetic flux and and the rotor winding flux. This causes a high rotor current with the increase of torque. The result of this is the stator drawing high current by the time it reaches full speed, which may heat up the motor and eventually damage it. This is why motor starters are essential.

Soft starters for motors are any devices that control the acceleration of an electric motor by controlling the voltage applied. Basically, motor starting can be done in three ways: by applying full voltage at different intervals; by applying reduced voltage gradually (soft starters); and by applying part winding starting.

Soft Starters For Motors

So, to put it simply, soft starters for motors are devices that reduce the torque applied to an electric motor. It usually consists of devices such as thyristor that regulate the application of supply voltage. Starters work on the principle that the torque is proportional to the square of the starting current which is proportional to the voltage applied. By reducing the voltage at when the motor is starting the torque and the current can be adjusted.

Soft starters offer two types of control. With open control, a start voltage is applied regardless of the speed of the motor or the current drawn. It creates a delay which is gradually reduced over time until the applied voltage is increased up to the full supply voltage. This method known as Time Voltage Ramp System is not relevant since it doesn’t actually control the acceleration of the motor. Closed loop control on the other hand monitors motor output characteristic such as the speed or current drawn and it modifies the starting voltage accordingly in order to get the response required.

There are many advantages that come with a soft start which is why it is preferred over other methods. Fist of all, a soft start improves efficiency. The efficiency of a soft starter system that uses solid state switches is thanks to the low on state voltage. The controlled startup is another benefit. The starting current can be controlled by altering the starting voltage which ensures a smooth start of the motor without any jerking and avoiding damage. Smooth control of acceleration is another thing and of course, the low cost and size thanks to the use of solid state switches.