Difference Between DOL, Auto-Transformer, Star-Delta, And Soft Starters

May 7, 2021July 3, 2022 CS Electrical And Electronics

Hello guys, welcome back to our blog. In this article, we will discuss the difference between DOL, auto-transformer, star-delta, and soft starters, why DOL (direct-online) starter is used, why the auto-transformer starter is used, why a star-delta starter is used, and why soft starter is used, etc.

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Difference Between DOL, Auto-Transformer, Star-Delta, And Soft Starters

These all starters are used for AC motors only. Based on HP of the motor theses types of starters are used. Now, let’s discuss the difference between direct-online starters, auto-transformer starters, star-delta starters, and soft starters.

Auto-Transformer Starter

A three-phase star-connected auto-transformer is used to reduce the voltage applied to the stator.
It consists of a charge-over switch.
The taps on the Auto-transformer limit the voltage( at starting).
When the switch is in the start position, the stator winding is supplied by the reduced voltage which is controlled by tapping on Auto-transformer.
The change-over switch is thrown into the run position by which rated voltage gets applied.
When motor gathers 80% of the normal speed.
The changes in making of the switch from start to run may be air break or maybe oil-immersed to reduce sparking.
The motor is disconnected from the supply when all the switches are opened.
This starter is disconnected at some point.
It is more expensive. Changing the switch is done automatically.
Power loss is less.
This method can be used both for star and delta-connected motors.
As compared to the direct online starters, for autotransformers the power factor will be less due to the inductive property.

Star-Delta Starter

This starter is used in the case of motors which are delta-connected stator winding.
By using the TPST switch, the stator is connected to the star at the start.
Therefore, the voltage per phase applied to the stator gets reduced by the factor of 0.577.
When the switch is thrown on the other side, the winding gets connected in the delta and rated voltage applied to the stator.
During starting period when the motor is star-connected, it takes 1/3rd as much starting current and develops 1/3rd as much torque as would have been developed were it directly connected in the delta.
The power factor will be reduced, reactive power will be taken in supply, and power tariff will be increased.
As the voltage is reduced the starting current is also limited.
Cheapest and maintenance-free operation.
The switch is operated automatically by using relays.
This method is suitable for delta-connected motors.
This method is used where high starting torque is not required and for machine tools, pumps, etc.

DOL (Direct-Online) Starter

For the motor whose capacity is less than 5 H.P the starting current is not very high and such motors can withstand high starting currents.
At the start, the NO (normally open) button is pushed for a fraction of a second due to which the coil gets energized and attracts the contactor and the stator directly gets supply.
When NC (normally closed) button is pushed the circuit gets opened and coil gets re-energized and the motor gets switch off from the supply.
Under overload, current drawn by motor increases, due to which excess amount of heat gets produced, then thermal relays get opened due to high temperature and protects the motor.
Here, there is no need of applying reduced voltages.
This starter is used to protect the motor against abnormal conditions like overloading, low voltage, etc.
Such a starter is called a Direct online starter.

The starting period, the temperature depends on,

i) Excessive load.
ii) Excessive voltage drop in a line.
iii) Rotor resistance.

Soft Starter

This starter tries continuously to control the voltage of the motor.
For controlling the motor’s phase separate equipment is used i.e., mechanical operating equipment.
Due to this equipment, the life of the motor increases and the workflows smoothly.
Using this starter we can adjust the load behavior of the motor.
For the line voltage applied to the motor, these starters must be connected in series.
Solid-state soft starters can control one or more than one phase of the voltage applied to the motor.

I hope this article may help you all a lot. Thank you for reading.

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