What are DOL starters and their working principle?

What is DOL Starter?

A DOL starter also known as a Direct On-Line starter, is a method of starting a 3-phase induction motor. In a DOL Starter, a three-phase Induction motor is connected directly across its 3-phase supply, and the DOL starter applies the full line voltage to the motor terminals.

Connection of DOL Starter

DOL starter

1. Main Contact

  A Contactor is used to connect Supply Voltage, Relay Coil, and Thermal Overload Relay.

R Phase through MCCB1 of Contactor (NO)

Y Phase through MCCB3 of Contactor (NO)

 B Phase through MCCB5 of Contactor (NO)

NO Contact (-||-):

53-54 is a Normally Open NO contact in the contactor (closes when the relay energizes)

Main contactor terminal 53 is connecting to the Start Button terminal (94) and the 54 terminal of the main contactor is connected to a common wire between Start and Stop Button.

NC Contact (-|/|-):

Stop button – (95-96) is a normally closed NC contact (opens when the thermal overload relay trips)

DOL part – Thermal Overload Relay

Normally closed NC contact terminal (opens when the thermal overload relay trips)

2. Relay Coil Connection

A1 of Relay Coil is connected to anyone Supply Phase and A2 is connected to Thermal

overload Relay’s NC Connection (95).

3. Thermal Overload Relay Connection:

Motor terminals (T1, T2, T3) are connected to Thermal Overload Relay

Main Contactor and Motor is connected through Overload Relay

NC Connection of Thermal Overload Relay is connected to Stop Button and Common Connection of Start/Stop Button.

Working principle of DOL Starter

In the DOL starter, main terminals are connected with three terminals of the three-phase power supply, the control circuit is energized.

Current will flow through one phase to the control circuit and the contactor coil to the other phase when the start button (NO) is pressed. Contactor coil (A1-A2) gets energized due to this current that makes to close the contacts of the contactor and as a result of this three-phase supply is connected to the motor. Even though the start pushbutton (start) is released the control circuit of the DOL starter still provides the power supply via Hold-On-Contact. The current path via contactor coil (A1-A2) will break when the stop button is pressed and due to this, the contactor contacts drop out, thus breaking the power supply to the motor with the help of the overload relay coil.

Depending on the heating of the motor due to the load current thermal overload protection relay operates because the heating effect is directly proportional to load current. When the load current is high the heating effect is also more. Spring-loaded contact in the control circuit trips out only when the bimetallic strip used inside it expands due to heat. This all happens when the amount of load current is too much that it can heat the thermal coil. Relay operating speed depends on current adjustment. The current rating must be 3-5 times the rated current of the motor. Remotely operated DOL starters are also used to control the switching of the motor from any desired place.

Condition for remote control-based switching is, that all the “OFF” push buttons in the remote area always be connected in series with “OFF” pushbuttons of the starter and vice versa (all the remote “ON” push buttons always be connected in parallel to the “ON” pushbutton of the starter).

Advantages of DOL Starter

  1. Very cheap and economical Starter.
  2. More comfortable, simple to operate, and control.
  3. The control circuit is much simpler.
  4. Very Easy to troubleshoot and easily understandable.
  5. Direct on-Line starter provides 100% torque at starting.
  6. From starter to motor only one set of cables is required.
  7. It connects the supply to the delta winding of the motor.
  8. The size of DOL is so compact that it occupies very little space

Disadvantages of DOL Starter

  1. The starting current of the motor doesn’t reduce.
  2. Direct On-Line starter has very high starting current or too much current 6 to 8 times of full load current.
  3. Due to the huge thermal stress on the motor, the lifespan of the machine reduces.
  4. There is a significant dip in voltage in the electrical installation because of too much in-rush current and due to this other customers connected to the same lines affected, it suitable only for small motors.
  5. The mechanical stress on the mechanical systems is increased because of unnecessary high starting torque, even when not required by the load and this is very dangerous for machine life.
  6. DOL starter is only suitable for motors of rating less than 10kw.
  7. DOL starter disadvantage is that it gives the highest possible starting current

Applications of DOL Starter

DOL starter is used to start small water pumps, compressors, fans, and conveyor belts where high inrush currents do not cause any harm.

Where DOL Starter can be used?

If the high inrush current of the motor does not cause an excessive voltage drop in the supply circuit a direct Online starter can be used.

Comparison of D.O.L Starter and Star-Delta starter?

S noD.O.L StarterStar-Delta Starter
1It is used up to 5HP motor onlyIt is used from 5HP to 20HP motors
2It doesn’t decrease the starting currentIt decreases the starting current to 1/3 times.
3It is cheapIt is costly
4The motor is directly connected with DOL Starter with supply for starting as well as for runningIt connects the motor first in star connection at the time of starting then it changes to delta connection for running
Comparison between the DOL starter and star-delta starter

Some useful questions and answers on DOL Starter

1. What is the benefit of a DOL starter?

The benefit of a DOL starter is to avoid phase failure with the help of an overload relay, and it is also used to remote control the motor automatically; the circuit breaker must be connected before – the DOL Starter is connected to the power supply to avoid phase failure.

2. What are the data required to select contractor and overload relay?

A motor having a nameplate that includes

KW rating of the motor

Operating voltage

Application, KVA code (Available in NEMA code)

Motor nameplate

Parameters to be considered during the selection

  • The full load current of the motor P =√3 V.I. Cosφ kW
  • Operational voltage and coil supply voltage
  • Type of load and duty cycle
  • Breaking capacity of contactor required.
  • Overload Relay setting = 100% x FLC (Full load current)

3. Which type of motor doesn’t need Starters?

Motors below 0.746kw are directly connected without a starter because their armature resistance is very high. They can afford the high current, due to high resistance.  So, the Armature winding is safe from the high starting current.

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