Thursday, 28 January 2016


  • Different starting methods are employed for starting induction motors because Induction Motor draws more starting current during starting. To prevent damage to the windings due to the high starting current flow, we employ different types of starters.
  • The simplest form of motor starter for the induction motor is the Direct OLine starter. The DOL starter consist a 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.
  • Typically, the contactor will be controlled by separate start and stop buttons, and an auxiliary contact on the contactor is used, across the start button, as a hold in contact. I.e. the contactor is electrically latched closed while the motor is operating.

Principle of DOL:

    •  To start, the contactor is closed, applying full line voltage to the motor windings. 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.
    •     Advantages of DOL Starter:
    1. Most Economical and Cheapest Starter
    2. Simple to establish, operate and maintain
    3. Simple Control Circuitry
    4. Easy to understand and trouble‐shoot.
    5. It provides 100% torque at the time of starting.
    6. Only one set of cable is required from starter to motor.
    7. Motor is connected in delta at motor terminals.
  •      Disadvantages of DOL Starter:

    1.  It does not reduce the starting current of the motor.
    2. High Starting Current: Very High Starting Current (Typically 6 to 8 times the FLC of the motor).
    3. Mechanically Harsh: Thermal Stress on the motor, thereby reducing its life.
    4. Voltage Dip: There is a big voltage dip in the electrical installation because of high in-rush current affecting other customers connected to the same lines and therefore not suitable for higher size squirrel cage motors
    5. High starting Torque: Unnecessary high starting torque, even when not required by the load, thereby increased mechanical stress on the mechanical systems such as rotor shaft, bearings, gearbox, coupling, chain drive, connected equipments, etc. leading to premature failure and plant downtimes.

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