As we have seen in the working of three-phase induction motor, when three-phase supply is given to the stator, a rotating magnetic field produced and the rotor starts rotating. At time of starting, the slip of motor is unity and starting current is very large. The induction motor is self-starting motor, then also it needs starters to perform following functions,
- To provide the heavy starting current
- To provide overload and underload protection
There are three starting methods or starts are used for the squirrel cage induction motor.
- Direct On-line starter
- Star-delta starter
- Autotransformer starter
In this article, we will learn about direct on-line starter.
Direct On-line Starter
In this method of starting, squirrel cage induction motor connected with starter across the full supply voltage. Connection diagram of Direct On-line starter is as shown in below figure.
The DOL starter consist of a coil operated contactor C. This contactor controlled by the start and stop push buttons. These push buttons are places at convenient place.
The start push button is denoted as switch S1 and it is normally held open by a spring. And stop push button is denoted as switch S2 and it is normally held close by spring.
Three-phase line conductors connect to the motor with the main contacts M and there is one auxiliary contact A.
When the start push button is pressed, the contactor coil C energized from two line conductors L1 and L2.
Therefore, three main contacts M and the auxiliary contact A close and terminals ‘a’ and ‘b’ are short circuited.
Now, the motor is connected with supply. When the pressure on S1 released, it moves back under spring action. Even in this condition, the coil C remains energized through ‘a-b’.
Therefore, main contact M remains close and motor continuous to get supply. Because of this reason, auxiliary contact known as “hold-on contact”.
When stop push button S2 pressed, the supply through the contactor coil C is disconnected. Since the coil C dis-energized, the main contact M and auxiliary contact A are opened. The supply through motor is disconnected and motor stops rotating.
The starter provides smooth start to motor and also provide undervoltage and overvoltage protection.
When the voltage falls below a certain value, or in event of failure of supply during motor operation, the coil C is de-energized. The motor is then disconnected from the supply.
In case of overload, one or all overload coils are energized. The normally closed contact D is opened and the contractor coil C is de-energized to disconnect the supply to the motor.
Fuses are provided in the circuit for short-circuit protection.
DOL starting is a simple and cheap method. The starting current may be 10 times the full load current and the starting torque is equal to full-load torque.
The large starting current produces excessive voltage drop in the line supplying the motor. Small motors up to 5 kW rating may be started by DOL starter to avoid supply voltage fluctuations.
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