Answer MET Question 34

Question: What is a soft starting of an Induction motor? Describe with a circuit using a thyristors used for soft starting. Discuss its advantages and dis-advantages.

Answer: "Soft" Starting: A motor soft starter is a device used with AC electrical motors to temporarily reduce the load, torque and electric current surge of the motor during start-up. It applies a gradually increasing voltage to the motors resulting in smooth acceleration of the motor and coupled load. It provides a linear power slope for smooth acceleration, which reduces in rush currents and excessive wear on mechanical driven components. The starter has a provision to adjust the starting voltage and acceleration time.

This method of supplying a gradually increasing a.c. voltale during start-up generally refers to an efficienl electronic switching technique. A basic method is to use back-to-back connected thyristors or triacs in the supply lines whiih are "gated" to delay "turn-on, within each a.c. half-cycle. This delayed switching applies a reduced average a.c. voltage to the motor.
The applied motor voltage is gradually ramped up by the starter software program until the full voltage level is reached. To achieve maximum efficiency, the electronic switching circuit can now be bypassed for normal running.
A "soft" starter may be further adapted to become a voltage controller over the motor operating load range. In this type of efficient "energy manager" application, the controller monitors the motor power factor which is a measure of the motor loading. On light load and full voltage, the power factor is low so the controllerreduces the motor voltage which reduces current while improving power factor and efficiency.

Advantages of soft starters over conventional starters
Energy Cost Reduction (Peak Demand Charges)
Voltage or current is applied gradually, without the voltage and current transients. Precise control over the current limit.
Improved power factor at all load cycles and there by reduction in kVA
Reduced power surge
Reduced heating of the motors and improves operating efficiency
Possibility of increased number of starts per hour
Increased contact life of starter contactors
Adjustable acceleration time
Elimination of starting mechanical shock to couplings and driven equipment
Smooth acceleration without the torque transients
Easy adjustment of start performance to suit the specific motor and load.
Reliable performance even if load characteristics vary between starts (eg. loaded or unloaded starts).

Disadvantages
High initial cost.
Difficulty in maintenance, require to sent ashore or require replacement when fault takes place.