DOL Starter vs Star Delta Starter

DOL Starter vs Star Delta Starter

I work with many electrical motors. The most common starting method I have ever seen is DOL , direct online.

However, Star Delta and VSD starters, are also used for larger motors. In this article let’s discuss DOL starter vs Star Delta starter.

What is DOL starter?

The word DOL means Directly On Line. The starter in which the motor is connected directly in line with the switch is called DOL starter.

It has only one contactor. There is no starting circuit to reduce the amount of starting current.

The simple DOL starter example is your home water pump, it has a motor which you can run directly without any method of soft starting. All you need is a switch to connect the motor to the electricity directly.

What is Star-Delta Starter?

Star delta starter is a starting method of larger induction motors, in this method the motor starts as a star connected then switches to delta after it starts.

This method does not connect the motor directly. It consists of two contactors. One contactor is used to start the motor with reduced voltage, i.e Start connection,  and the other one, Delta contactor, is used to run the rated voltage of the motor.

At starting position, the motor is in the Star, its common known as Y connection, configuration.

While at higher speed the motor is at delta configuration to reduce the starting current of the motor.

For more details I recommend reading my article Motor starting methods, it has all wiring diagrams of power and control circuits.

The main differences between the DOL starter and star Delta starter are following.

Star Delta vs DOL table

DOL Starter Star Delta Starter
Motor starts on its full starting torque Starts motor on third of its starting torque
Motor starts at full starting current Reduces starting current
Very simple and needs just one contactor More complicated in design and needs 3 contactors
Suitable for high starting torque applications Not good for applications with high starting torque
Lower cost, and requires less maintenance Costs more, and requires more maintenance
Works for motors with Delta or Star connection Motor must be designed to work on  Delta connection
Full voltage is applied to the motor at the starting The voltage is reduced to a lower value

Now let’s dive into some details of DOL starter vs Star Delta starter

Starting Current

  • The DOL starter gives the highest possible current to start the motor. It doesn’t reduce the starting current, which is a big disadvantage of DOL starter.
  • While working with the DOL starter, values of 9 or 10 times of current the rated current exist, i.e motor starting current.
  • In case of star delta starter, the starting current is reduced. It draws the star connection starting current which is about 30% of the starting current provided by DOL. This is also a big advantage of star delta starter over the DOL.

Starting Torque

When we talk about DOL starter vs star delta starter in terms of relative starting torque, we came to know that DOL starter gives the starting torque which is higher than the rated motor torque.

On the other hand a star delta starter reduces the starting torque of the motor.

Star delta starter is used when the application is not highly loaded during the start because, Star Delta does not provide the motor its full starting torque value.

Operating Principle and wiring diagram

DOL starter circuit
DOL starter circuit

DOL principle and circuit

DOL starter circuit consists of, a contactor, overload relay, start button and a stop button.

These components are used for the control and protection purposes.

In case of a fault condition, the contactor opens with the help of thermal overloaded relay.

The contactor is also controlled by the start and stop buttons.

When the motor is working the contactors are electrically closed. To stop the motor, just press the stop button to de energize the contactor.

Star delta starter principle and wiring circuit

StarDelta circuit
Star Delta power and control circuit

In case of star delta starter, the power supply source to the motor is connected to the delta connection from star with the help of wiring.

When the current passes the motor started in star configuration and then converted to the delta configuration.

In this process full voltage is applied to the motor when it is running and in this way full torque is obtained as an output.

The star delta starter is used to start the motor and when it gets the speed after starting it is converted to the delta.

Star connection starts the motor with less voltage and motor takes time to get the speed in this mode.

As we know the torque is proportional to the square of voltage that’s why the starting torque also reduces in the star mod.

Wiring and Motor Terminals

In case of star delta starter the motor is connected in such a way that it can be switch-over from star to delta.

To complete this task the six ends of winding of motor are connected on the terminals. Then the starter switch of star delta starter performs its action of switching over the motor accordingly.

Delta connected motor
Delta connected motor
Star connected motor
Star connected motor

The working system of DOL starter is different. It requires only one set of cable from starter to the motor.

That wire and the single switch perform all the tasks. That is the reason the DOL starter is easier to troubleshoot and understand.

Cost

DOL starter is the cheapest motor starting method. It is most economical starter option. On the other hand star delta starter is more expensive when compared to the DOL starter. Its all about the components used for each circuit.

When we find the applications of both types of starter, the star delta starter does not look too much costly because, it has great functions as compared to the DOL starter. i.e when the application require the use of star delta starter, it should worth the cost.

Starting time

Both types of starters also have great differences in case of starting times. The starting time of star delta starter is larger than the DOL starter.

Star delta starter takes the run up time of approximately 2 to 15 seconds. On the other hand the DOL starter takes the starting time of approximately 2 to 5 seconds.

These starting times are also affected by the amount of the load. The startup time values are increased in case of heavy-duty.

circuit Component

  • The formation of DOL starter is simple. It consists of only thermal overloaded relay and a main contactor. There is no further component or any circuit in it and all the working system is carried out by only these components.
  • Star delta starter is not that simple in case of components. The components are consists of an overload relay, three contactors and a timer.

The timer is used to set the time in the start position. In order to use the star delta starter the motor should be delta connected during the normal run.

Significant Design Features

Star Delta starter

  • Contactors have longer endurance due to wide voltage band.
  • Reliable overload protection.
  • Easy to install.
  • Safe to use with smooth working

 DOL starter

  • Contactor should be wide.
  • Easy and Quick mounting facility.
  • Relay for reliable overload protection.
  • Withstand wide voltage fluctuations.
  • Screwed up earth wire for safety purposes.
  • Manually reset facility after trip

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