What Is The Difference Between DOL and Star-Delta Starter?

Why Use Direct On Line (DOL) and Star-Delta Starter?

DOL means the motor is connected Directly ON Line using one contactor with no starting circuit to lower the high starting current. Typically the Delta part of Star-Delta.

Star-Delta uses two contactors, one to start at a lower voltage in Star and one to run at a higher voltage in Delta configuration.

For example a 400V, 3-phase power supply Star applies phase to neutral or 230V. Delta applies the full phase to phase 400V.

Since power is proportional to voltage squared the starting current is three times less with Star-Delta.

Salient difference between DOL and Star-Delta starter

D.O.L( Direct On Line )Starter
1.    It does not decrease the starting current.
2.    It is cheap.
3.    It connects directly the motor with supply for starting as well as for running.

Star-Delta Starter
1.    It decrease the starting current to 1/3 times.
2.    It is costly.
3.    It connects motor first in star at the time of starting then in delta for running.


DOL Starter Motor Wiring Diagram

DOL-Direct ON Line Starter wiring Diagram
DOL Starter Wiring Schematic Diagram
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Significant Design Features of Star Delta Starter:

Sponsored:

1.    Contactor usually wide voltage band which is designed for longer endurance;
2.    Relay used for reliable overload protection;
3.    Easy to install facility;
4.    All edges should smooth and safe to use;
5.    Overall quality and performance should followed local standard.

Significant Design Features of DOL starter:

1.    DOL Starter Contactor should be wide voltage band;
2.    Relay for reliable overload protection;
3.    A latch on the push button should keep the stop button locked, that prevent accidental starting;
4.    Should be any hum Free;
5.    Easy & quick mounting facility;
6.    Coil should withstand wide voltage fluctuations;
7.    Earthing wire should screw up for a safety measure;
8.    Manually reset facility after each tripping;
9.    ON/OFF indications should be visible.

Some Demerits of DOL starter Compare to Star-Delta Starter

Sponsored:

1.    DOL starter draws very high inrush current at starting i.e. 6-8 times rated current of motor which restricts its use to motors upto 7.5 KW only;
2.    DOL starter will cause high Thermal Stress on motors that will reduce motor life;
3.    High Starting current will leads to dips in voltage;
4.    The unnecessary high starting torque required by the load that may cause increasing mechanical stresses on motor;
5.    DOL starter is not feasible for 10KW or above rated motors.



DOL starter is suitable and cheapest method to start up motor only for normaler sizes motor, for larger motor must use the star-delta starter.


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