Direct On Line (DOL) Motor Starter

The simplest form of motor starter for the induction motor is the Direct On Line starter. The Direct On Line Motor Starter (DOL) 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.

Advantages of DOL Starter

  • Most Economical and Cheapest Starter.
  • Simple to establish, operate and maintain.
  • Simple Control Circuitry.
  • Easy to understand and trouble‐shoot.
  • It provides 100% torque at the time of starting.
  • Only one set of cable is required from starter to motor.
  • Motor is connected in delta at motor terminals.

Features of DOL starting

  • For low- and medium-power three-phase motors.
  • Three connection lines (circuit layout: star or delta).
  • High starting torque.
  • Very high mechanical load.
  • High current peaks.
  • Voltage dips.
  • Simple switching devices.

Direct On Line Motor Starter (DOL) is suitable for:

  • A direct on line starter can be used if the high inrush current of the motor does not cause excessive voltage drop in the supply circuit. The maximum size of a motor allowed on a direct on line starter may be limited by the supply utility for this reason. For example, a utility may require rural customers to use reduced-voltage starters for motors larger than 10 kW.
  • DOL starting is sometimes used to start small water pumps, compressors, fans and conveyor belts.

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