Direct Online Starter (DOL)

An induction motor is started using a direct online starter. In a DOL starter, the stator of the motor receives the complete supply voltage. For the purpose of starting three-phase induction motors with small ratings, we prefer a direct online starter.

Direct Online Starter (DOL)
Direct Online Starter (DOL)

Introduction to Direct Online Starter

At the starting instant, a 3-phase induction motor acts as a poly-phase Transformer with an air gap and a short-circuited secondary. If the motor of higher capacity (larger than 2 kW) is connected directly to the supply mains, it will draw heavy current (about 5-8 times the rated full-load current) at the starting instant, which is offensive because it will cause a large voltage drop in the distribution network and thus adversely affect the operation of other electrical equipment connected to the same distribution network. Hence,to control the starting current, the use of a starter is essential for starting 3-phase induction motors of larger size (rating exceeding 2 kW).

The starter used for starting a squirrel cage induction motor is:

  1. Direct-on-line starter
  2. Line resistance starter
  3. Star-delta starter and
  4. Auto-transformer starter

Direct Online Starter Diagram and working principle

Squirrel cage motors of capacity up to 1.5 kW, double cage motors, and squirrel cage motors of large capacity having a large rotor resistance are started by the DOL method. This is one of the most economical methods of starting induction motors.

This method involves the direct switching of a polyphase squirrel cage induction motor to the supply mains. In this method of starting cage motors, the rate of temperature rise is high, and the motor may get damaged if the starting period is long, which may be due to

  • excessive load
  • insufficient rotor resistance or
  • excessive voltage drop in the supply mains

The starting period usually lasts a few seconds because starting torque for an induction motor is about twice the full-load torque.

Direct Online Starter Wiring Diagram

Wiring Diagram of a Direct-On-Line starter with protective Devices

The above shown is the push button type direct-on-line starter which is very common in use. This type of starter is simple, inexpensive, and easy to install and maintain. It consists of a set of `start` and `stop` push buttons, a contractor(an electromagnet) with its associated contacts, and usually an overload and under voltage protection devices.

The start button is a momentary contact switch that is held normally open by a spring. The stop button is held normally closed by a spring. When a start button is pressed the operating coil(or main contractor) gets energized through the overload relay contacts OL(normally closed). This closes the three main contacts M that connects the motor to the supply.

At the same time, a set of auxiliary or maintaining contacts MC is closed. When the maintaining contacts MC are closed, a new circuit is established through the stop button, maintaining contacts MC and operating coil. since the operating coil circuit is now maintained by the auxiliary contacts MC, the start button may be released. When the stop button is pressed, the operating coil gets de-energized, thereby opening all main contacts and auxiliary contacts.

The main contacts and the maintaining contacts get opened when the supply fails or the line voltage drops a certain value. When the supply gets returned, the contractor cannot close until the start button is again closed. the contractor is controlled by a three-wire control circuit which maintains the interruption of the circuit even after the supply is restored, this is the under-voltage protection for the motor.

Overload protection is employed for protecting the motor and control apparatus from excessive heating due to overloads on the motor.

Thermal overload relays are commonly used for motor overload protection. Thermal overload relays are of two types. Both types are operated from the heat generated in a thermal element through which the motor current flows. In one type, the heat bends a bimetallic strip and in the other, the heat melts a film of solder. Both act to open the motor control circuit and disconnect the motor from the source of supply.

A bimetallic strip is made of two different metals whose surface has been welded together. One of the metals expands rapidly as compared to the other when heated. When heat is applied, the strip is caused to deflect or curl up due to unequal expansion of the two metals. When the motor current attains a predetermined value, the heat generated deflects the strip far enough to trip a latch that opens the motor control circuit. When the strips cool down sufficiently, the relay may be reset and the motor re-started.

In the case of the solder film type of overload relay the heat generated at a given overload current melts the film, which releases a latch arrangement and opens the motor control circuit. When the solder has cooled enough to hold the latch, the relay may be reset.

In the case of thermal relay, the time required to operate is determined by the magnitude of current flowing in the heater. Thus, the relay operates slowly for light overloads but disconnects the motor in a short time for dangerously heavy overloads. Relays in which the time of operation is inversely proportional to the amount of current flowing are known as inverse time relays.

Torque Developed on Starting of Induction Motor By Direct switching

where I2 is the rotor current per phase, R2 is rotor resistance per phase, T is the torque developed,s is the slip and w is the supply angular velocity.

Since rotor current is proportional to stator current I1, so

At starting slip,s=1

so starting torque,

where Ist is the starting current

Full-load torque,

where If is the full load motor current and Sf is the full-load slip 

when the motor is connected directly across the supply mains, the starting current is equal to the short-circuit current, Isc.

starting torque,

Advantages of Direct Online Starter

  • Simple, inexpensive, and easy to install and maintain.
  • Full starting torque at starting.
  • DOL starter connects the supply to the delta winding of the motor.

Disadvantages of Direct Online Starter

  • Starting current is about 5-8 times of full load current.
  • This method is suitable for only small machines as there is a significant voltage dip.
  • Unnecessary high starting torque.
  • Mechanically tough.

Direct Online Starter Applications

  1. Used in small water pumps.
  2. conveyors belts
  3. fans and compressor


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