Is DOL starter a reduced voltage starter?

[saidaibala]

Dear all,

 

In star-delta starter, in star mode, voltage is reduced by 1/root3 times,due to its Phase & line voltage relation ship.Hence current is reduced proportionately and so its torque.

 

I'm now comparing DOL starter and Star mode in Star-Delta Starting. The only difference between these two is, in DOL starter the starpoint shorting is done in the motor,where as in Star mode in star-delta starter the star-point shorting is done externally in the control panel.

 

If this is the case, in DOL starting, voltage applied should be reduced by 1/root3 times and current torque reduced accordingly.

 

But practically this is not the case.When selecting overload relay for star-Delta starter for a motor of, say 5.5KW, i had to go for the relay which is rated 1/root3 times less than the overload relay for DOL application.

 

Can anyone explain this pls? It's very confusing to me being a beginner.

 

Thanks and Regards

saidaibala

 

 

 

 

[marke]

Hello saidaibala

 

If you take a motor designed to operate off your nominal supply voltage connected in delta, and connect it in star, you are effectively reducing the voltage across each winding by root 3 i.e. from 400V to 230V. The motor will draw 1/3 current and produce 1/3 torque that it would if connected in delta. When you now reconnect it in delta, the voltage returns to normal.

 

If you take a motor that is designed to operated in star at you nominal voltage, it will produce full torque and draw full current, it is not a reduced voltage starter.

 

The star/delta starter operates as a reduced voltage starter when the motor is temporarily connected in star due to the reduced voltage across the windings.

Direct On Line switching a motor in it's nominated configuration (star or delta) is not a reduced voltage start.

 

Best regards,

[saidaibala]

Hello Marke

Thanks for your reply and it answered my doubt fully. But i understand from your reply that a motor of same KW rating ,say 10KW,400V,50HZ,could come either in star or delta configuration.

 

Now my doubt is:

KW and voltage applied being the same,i think that there is to be some difference in the construction of the motor(size or weight or in the number of turns of winding). If yes ,Can you tell me how it differs?

 

Also i would appreciate if you could tell me technically, how that difference in construction and anyother difference between the motors of either configuraion,helps in achieving the required output (with the same voltage being applied).

 

Hope my question is not confusing?

 

Thanks and Regards

saidaibala

[marke]

Hello Saidaibala

 

Motors can be wound for either star connection or delta connection and there is no performance variation with either connection provided that it is operated at the design frequency and voltage.

 

If you make a three wire connection to the motor, you will not be able to tell the difference between a star designed motor connected in star and a delta designed motor connected in delta.

 

Delta designed motors are designed to provide the rated torque at rated voltage when delta connected. This requires that the iron is fully fluxed. If this motor is connected in star, the voltage across the winding is reduced, reducing the flux in the iron. The reduced flux will reduce the torque output from the motor.

 

Star designed motors are designed to provide the rated torque at rated voltage when star connected. This requires that the iron is fully fluxed. The flux is the same as in a delta designed motor.

 

The difference between the start winding and the delta winding, is the number of turns and the diameter of the wire used. For a constant flux, there is a fixed number of turns per volt. If you increase the voltage, you must increase the turns proportionally to keep the turns per volt constant. With more turns, the diameter of the wire must be reduced to enable the turns to fit into the same winding slot.

 

A motor can be wound as a star motor or a delta motor in exactly the same frame size.

In some countries, star connected motors are more common than delta. In the 50Hz world where star delta starters are common, it is normal to use delta connections due to the star/delta requirements. In other countries, star designed motors are more common.

 

Best regards,

[saidaibala]

Dear Marke,

 

Thank you so much for your time and reply.

 

 

Regards,

saidaibala