DOL Starter (Direct On Line Starter) Diagram And Working Principle

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Key learnings:
  • DOL Starter Definition: A DOL starter (Direct On Line Starter) is a simple electrical device that starts a motor by applying full line voltage directly to its terminals.
  • Starting Mechanism: By pressing the green start button, the motor is connected to the power supply, drawing a high inrush current to initiate motion.
  • Diagram Components: The DOL starter wiring diagram includes a circuit breaker, contactor, and overload relay, essential for motor protection and operation.
  • Working Principle: The starter directly connects the motor to a three-phase supply, utilizing a control circuit energized from two phases to manage start and stop functions.
  • Protection Strategy: Overload relays in DOL starters safeguard the motor by interrupting the power supply if the current exceeds safe operational levels.

What is DOL Starter?

A DOL starter (also known as a direct on line starter or across the line starter) is a method of starting a 3 phase induction motor. In a DOL Starter, an induction motor is connected directly across its 3-phase supply, and the DOL starter applies the full line voltage to the motor terminals.

Even with direct connection to the power supply, the motor remains protected. A DOL motor starter includes protective devices and, in some models, condition monitoring features. Below is a wiring diagram of a DOL starter:

DOL Starter Diagram

Since the DOL starter connects the motor directly to the main supply line, the motor draws a very high inrush current compared to the full load current of the motor (up to 5-8 times higher). The value of this large current decreases as the motor reaches its rated speed.

A direct on line starter can only be used in circumstances when the high inrush current of the motor does not cause an excessive voltage drop in the supply circuit. If a high voltage drop needs to be avoided, a star delta starter should be used instead. Direct on line starters are commonly used to start small motors, especially 3 phase squirrel cage induction motors.

DOL Starter Formula

As we know, the equation for armature current in the motor.The value of back emf (E) depends upon speed (N), i.e. E is directly proportional to N.

At start-up, the back electromotive force (E) is zero, resulting in a very high initial current. Small motors with longer axial lengths and smaller diameters accelerate quickly due to this effect.

Hence, speed increases and thus the value of armature current decreases rapidly. Therefore, small rating motors smoothly run when it is connected directly to a 3-phase supply.

If we connect a large motor directly across 3-phase line, it would not run smoothly and will be damaged, because it does not get accelerated as fast as a smaller motor since it has short axial length and larger diameter more massive rotor. However, for large-rated motors, we can use an oil-immersed DOL starter.

DOL Starter Wiring Diagram

The wiring diagram for a DOL stater is shown below. A direct online starter consists of two buttons, a GREEN button for starting and a RED for stopping purpose of the motor. The DOL starter comprises an MCCB or circuit breaker, contactor and an overload relay for protection. These two buttons, i.e. Green and Red or start and stop buttons control the contacts.

DOL Starter Wiring Diagram

To start the motor, pressing the green button closes the contact, applying full line voltage to the motor. Contactors may have 3 or 4 poles; the diagram below shows a 4-pole contactor.

It contains three NO (normally open) contacts that connect the motor to supply lines, and the fourth contact is “hold on contact” (auxiliary contact) which energizes the contactor coil after the start button is released.

If any fault occurs, the auxiliary coil gets de-energized, and hence the starter disconnects the motor from supply mains.

3 Phase Motor Starter with Overload Protection

When a motor draws excessive current to meet the load requirements such that this load requirement goes beyond the rated limit, this is known as overload.

Thermal overload protection safeguards the motor from damage by interrupting the power when excessive current leads to overheating. Overload relays help regulate the current to prevent this issue.

But that does not mean that protects the short circuit. Fuse or MCB used in the system protects the over current. Overload protection opens a circuit at relatively low currents that are a little higher than the rating of the motor.

Overload currents are likely to damage if they persist for a long time, i.e. it will not trip if a high value of current flows for a short period such as starting of the motor.

We often provide overload protection via an overload relay. Overload relays may be solid-state devices with adjustable trip setting also called as the electronic relay or by interacting with related temperature sensors called as a thermal relay or if only operates for excess current flow then called as a magnetic relay.

For most motors, the maximum rating of the overload protection device is 125% of the full load ampere rating.

DOL Starter Working Principle

The working principle of a DOL starter begins with the connection to the 3-phase main with the motor. The control circuit is connected to any two phases and energized from them only.

When we press the start button, the current flows through the contactor coil (magnetizing coil) and control circuit also.

The current energises the contactor coil and leads to close the contacts, and hence 3-phase supply becomes available to the motor. The control circuit for a DOL Starter is shown below.

DOL Starter Control Diagram

If we press the stop button, the current through the contact becomes discontinued, hence supply to the motor will not be available, and the similar thing will happen when the overload relay operates. Since the supply of motor breaks, the machine will come to rest.

Even after releasing the start button, the contactor coil (Magnetizing Coil) continues to receive power from the primary contacts, as shown in the DOL Starter diagram.

Advantages of DOL Starter

The advantages of a DOL starter include:

  1. Simple and most economical starter.
  2. More comfortable to design, operate and control.
  3. Provides nearly full starting torque at starting.
  4. Easy to understand and troubleshoot.
  5. DOL starter connects the supply to the delta winding of the motor.

Disadvantages of DOL Starter

The disadvantages of a DOL starter include:

  1. High starting current (5-8 times of full load current).
  2. DOL Starter causes a significant dip in voltage, hence suitable only for small motors.
  3. DOL Starter reduces the lifespan of the machine.
  4. Mechanically tough.
  5. Unnecessary high starting torque

DOL Starter Applications

The applications of DOL starters are primarily motors where a high inrush current does not cause excessive voltage drop in the supply circuit (or where this high voltage drop is acceptable).

Direct on line starters are commonly used to start small water pumps, conveyor belts, fans, and compressors. In the case of an asynchronous motor (such as the 3-phase squirrel-cage motor) the motor will draw a high starting current until it has run up to full speed.

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