20th September 2020

Direct On-line(DOL) Motor Starter.

Induction motor draws more current while starting, So the starting of the induction motor is a critical process. Many types of starters are used for starting of the motor. Direct On-line(DOL) is the simplest and most economical starter. This article will cover selection, construction of DOL starter, advantages, and disadvantages.

Selection of Direct On-line(DOL) Motor Starter

Motor feeders can be classified into two types: Fuse and Fuseless based on the type of short circuit protection devices used.

Fuse system consists of fuses while Fuseless systems have either molded case circuit breakers (MCCBs) or Motor Protection circuit breakers (MPCBs).

While these devices are normally fast in clearing short circuit faults and they do take a finite time to operate. By the time device interrupts short circuit current, a certain amount of fault energy passes through the protected circuit. All the downstream devices and cables in the protected circuit are subjected to stresses corresponding to this energy.

H.R.C fuses, the most efficient and popular, like conventional fuse-switch units and are capable of switching and protecting electrical circuits.

MCCB was primarily used for protection of distribution circuits. However, with the development of current limiting MCCBs, it has become possible to employ MCCBs in motor feeders also. 

Motor protection circuit breakers (MPCBs) has both short circuit and overload protection in a single compact unit. More information about this type of combination of devices will be covered in the Construction section of Direct On-line(DOL) Motor Starter.

Mostly motor failures are due to following electrical faults :

  • Overcurrent
  • Single Phasing 
  • Short circuit

It is extremely important to select effective motor protection devices to safeguard motors against any of the above faults, that will make motor windings to exceed safe working temperature. More importantly, protection devices should be co-ordinated.

Construction Direct On-line(DOL) Motor Starter

The motor is controlled with the help of the control circuit, the control circuit consists of the following components:

  1. Indication lamps(ON, OFF and Fault indicator) – 3 Numbers
  2. Push Buttons(START and STOP) – 2 Numbers
  3. 3 Pole MCCB or circuit breaker or H.R.C fuses
  4. Contactor – acts as a switch
  5. Overload Relay

When an induction motor starts, it draws a very large current around 6 to 9 times the full load current of the motor. Energy efficient motor like IE3 will have higher inrush current.

Thermal overload relay should protect the motor against overloading or blocked-rotor condition. At the same time, it should withstand starting current for a duration equal to the starting time of the motor. 

Fused protection is done with H.R.C fuses or Fuseless protection is done with MCCB and MPCB because devices are generally fast in clearing Short circuit faults.

Typical diagram of DOL Motor starter

Control Scheme of DOL

DOL starter requires two supply 3phase supply for Power circuit and 110V AC supply for the control circuit.

Related Article : Control scheme of DOL Motor Starter

Advantages of On-line(DOL) Motor Starter

  • DOL starters are cheaper.
  • High starting torque is achieved at the beginning.
  • The operating and controlling of this starter are very easy.
  • Identifying and troubleshooting are very easy.

Disadvantages of On-line(DOL) Motor Starter

  • Starting current is very high
  • Dip in voltage while starting due high in-rush current at starting.
  • The lifespan of the machine can be reduced due to high starting torque.

Related Article: Type 2 Co-ordination and Type 1 Co-ordination

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