Need of Starters
When induction motors are directly switched on to supply, it takes about five to six times of full load current. This initial excessive current causes damages to the motor and supply wires. Starters are used to limit the inrush of starting line current and full current is supplied when the motor picks up speed. It consists of protective elements like no volt coil and over load relays.
Different types of starters for Induction Motors
There are different types of starters used for induction motor.
- Direct – on – line starter (D.O.L Starter)
- Star – delta starter
- Auto transformer starter
- Rotor resistance starter (for slip ring motor)
Direct – on – line (D-O-L) starter
It is simple in construction when compared with other starters. It permits the motor to startup with full voltage on. It has protective elements to safeguard the motors against over loading and single phasing. Single-phasing means operation of motor with one line cut of accidentally. When the start button is pressed, the no volt coil circuit energises. The contacts are pressed against spring tension to connect the motor terminals to three-phase supply. The motor starts running.
Even if the pressing of ‘start’ button is stopped, the circuit is closed through the fourth conductor and the motor continues to run. When the motor is overloaded, the temperature of the heating elements becomes high to heat the bimetallic strip. This makes over load relay (OLR) to press a lever to open the no volt coil for a moment to switch off the supply to stop the motor.
If the OFF button is pressed, the supply to the No volt coil is disconnected and the motor is stopped. Direct-on-line starters are used for motors with capacity upto 5 hp.
Star – delta starter
This starter has two positions – ‘start’ and ‘run’. When the handle is pressed to ‘start’, the three motor windings are connected in star formation. Each phase winding gets only 58% of the line voltage. So the starting line current to the motor is reduced. As the motor picks up speed and attains nearly 80% of normal r.p.m., the starter handle is pushed to ‘run’ position. In this run position, the three winding get connected in delta formation so that each winding gets full supply voltage and full line current.
This starter is provided with usual protective devices such as overload relay, no volt release and single phase preventer. Star-delta starter is used for motors of capacity 5 hp to 15 hp.
Auto Transformer Starter
An auto transformer is fitted in this starter and so it is costlier. Star connected transformers with 40, 60 and 80% tapings, allows specific voltage to be pressed against the motor windings.
When the starter handle is thrown to start position-reduced voltage is supplied to the motor windings and the motor starts with reduced line current and reduced torque. When the starter handle is pushed to run position, full line voltage is applied to motor windings and the auto transformer is isolated from the circuit. This starter is provided with usual protec- tive devices such as overload relay and no volt coil. This type of starter is intended for starting three phase induction motors of capacity more than 15 hp.
Rotor Resistance Starter
Slip ring motors are always started with a resistance in series with each rotor phase. In this starter, full line voltage is applied across starter windings. Full value additional resistance is added in series with each phase of the rotor windings. These resistances are immersed in oil. As the rotor starts up, the handle is rotated slowly until the entire resistance is out. The motor then runs normally. At this stage, rotor windings are short-circuited eliminating the resistance. Now the motor runs like a squirrel cage motor.