Starting of induction motor

The

induction motor at the time of starting, we can see the motor behaves like an electrical transformer with short circuited secondary winding, because at the time of starting, the rotor is stationary and the back emf due to the rotation is not developed yet hence the motor draws the high starting current. Almost the induction motors draw 6 to 7 times Full-Load current while starting direct on line. Therefore, we use starters in order to limit the high starting current. for small size induction motor, because the load torque during starting and motor load inertia are not large, the starting process is over in a few seconds and therefore, motor temperature does not exceed the permissible value therefore, the motor can always be started direct on line. However, for large size of the motor, when either the load torque during starting is high or load inertia is large, the starting process takes long time, if motor carries large current during starting it will get damaged due to overheating. Therefore, large induction motor cannot be started direct on line. We use different starters for both the type of induction motors. we have three main considerations and these are

a) A particular type of starter is selected on the basis of power capacity of the power lines

b) The type of starter selected on the basis of the size and the design parameters of the motor

(c) The third consideration is the type of load on the motor (i.e. the load may be heavy or light

We classify starting methods for induction motor into two types on the basis of voltage. The two types are

i) Full voltage starting method

v  Direct on line starting

ii) reduced voltage method

• Star-Delta Starter
• Stator Resistor Starting Method
• Auto Transformer Starting Method
• Soft Start starter
• Part winding starting
• Rotor resistance starter