DOL STARTER (DIRECT ONLINE STARTER)
Different starting methods are employed for starting induction motors because induction motor draws more starting current during starting. To prevent damage to windings due to high starting currents flow, we employ different types of starters.
The simplest form of motor starter for induction motor is the Direct Online Starter.
 |
| A Direct Online Starter |
DOL STARTER
Direct online starter consists of MCCB or Circuit Breaker, Contactor and an overload relay for protection, electromagnetic contactor which can be opened by the thermal overload relay under fault conditions.
The contactor will be controlled by separate start and stop button and an auxiliary contact is used across the start button as a hold contactor is electrically latched closed while motor is operating.
Principle of DOL Starter
To start the contactor is closed, applying full line voltage to the motor winding. The motor will draw a very high inrush current for a very short time, the magnetic field in the iron and then the current will be limited to the locked rotor current of the motor. The motor will develop locked rotor torque and begin to accelerate towards full speed.
As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed typically about 85% of synchronous speed. The actual starting current curve is a function of the motor design and the terminal voltage and is totally independent of the motor load.
The motor load will effect the time taken for the motor to accelerate to full speed and therefore the duration of the high starting current but not the magnitude of the starting current.
The acceleration torque is the torque developed by the motor mines the load torque and will change as the motor accelerates due to the motor speed torque curve. The start time is dependent on the acceleration torque and the load inertia.
This method of starting is applied to the motors of the power up to 7.5 KW, the motor are started by star-delta starter.
Wiring diagram of DOL Starter
For better understanding watch our video by clicking on the below link
http://dai.ly/x20urow
http://dai.ly/x20urow
