Starting Methods of Three phase induction Motors

 Starting Methods of induction motors 

Induction motors need certain types of starting methods to start without causing trouble to other utility consumers. When an induction motor begin to work, huge amount of current (6-8 times full load current) draw from the supply. The reason is the high emf induced in rotor bars/wingdings due to high rate of change of magnetic flux. Stator winding can withstand to this large current, but due to this large current receiving end voltage may reduce and other consumers may be affected by this low voltage supply. The increased torque results in sudden mechanical stress on the machine which leads to a reduced service life. To reduce this huge amount of current the voltage should be reduced. In order to do that supply voltage should be reduced. Hence various types of starting methods are used according to the application. 

•  Starting Methods 

1. Direct-on-Line (DoL) starter   

2. Star-Delta Starter  

3. Auto-transformer starter  

4. Soft Starter 

5. AC variable speed drive  

1. DOL Starter

Principle of operation 

In this method, 3-phase power supply is directly connected to the induction motor. Mechanical switch is used to switch power supply. This is the simplest method of starting an induction motor and also the cheapest method. Starters mainly consists of MCB, contactor and overload relay. When the mechanical switch is closed full line voltage is applied to the stator terminals. The contactors are closed against spring action. Very high inrush current will draw from the supply and it will reduce when the speed is increasing. Starting current is depend on motor design and supply voltage and totally independent of the load. Acceleration torque will determine the starting time of the induction motor. An isolator is used to isolate the starter from the supply in maintenance. Safety protections (Over-current, under-voltage, short-circuit, etc.) are provided to the motor to safeguard the motor. 

Characteristic curves during starting

12 Amp Direct On Line Starter Including Overloads METAL  IP65 Enclosure IEC 60947 / EN 60947 / IS 13947 / VDE 0660 Overload Options 0.10 Amps to 13.0 Amps 

Applications 

DOL method is normally limited to smaller cage induction motors, because starting current can be as high as eight times the full load current of the motor. Use of a double –cage rotor requires lower staring current( approximately four times) and use of quick acting A.V.R enables motors of 75 Kw and above to be started direct on line. DOL starter is used to start small water pumps, compressors, fans and conveyor belts.