# Star/Delta voltage question

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Hi,

In another topic marke wrote

"In this country, a motor of that size would be supplied for 230/400 volt operation 230V in delta and 400V in star.

Larger motors are supplied for star/delta starting and are in effect dual voltage 400/660V and so are run in delta."

Given standard UK mains (230/400v) when you connect a motor in star the voltage on each of the phases is 230. where does the 660v come into into it? (obviously by summing but how exactly)

When a motor is connected in star can you actually measure 660v anywhere on it?

If a motor is in effect 400/660v maybe it should be called Delta/Star rather than Star/delta, tho I can see this would be confusing.

Regards

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Hello waterman

The voltage rating of a motor is related to the rating of the insulation and the flux density in the iron.

If we take a motor designed to operate at 400 volt in delta connection, we achieve design flux density with 400 volts across each winding.

If we now connect that motor in star, and we wish to have the full design flux density, we still need 400 volt across each winding. To apply 400 volt across each winding in star connection, requires a three phase voltage of around 660 volts.

Hence, to achive rated torque output, the motor must be supplied from 400Volt in delta or 660 volt in star connection.

If we apply 400 volt in star connection, we are effectively reducing the voltage across each winding from 400 volt to 230 volt. This reduces the flux in the iron, reducing the maximum torque and the starting current. It is ineffect a reduced voltage starter.

Best regards,

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Hello waterman

The voltage rating of a motor is related to the rating of the insulation and the flux density in the iron.

If we take a motor designed to operate at 400 volt in delta connection, we achieve design flux density with 400 volts across each winding.

If we now connect that motor in star, and we wish to have the full design flux density, we still need 400 volt across each winding. To apply 400 volt across each winding in star connection, requires a three phase voltage of around 660 volts.

Hence, to achive rated torque output, the motor must be supplied from 400Volt in delta or 660 volt in star connection.

If we apply 400 volt in star connection, we are effectively reducing the voltage across each winding from 400 volt to 230 volt. This reduces the flux in the iron, reducing the maximum torque and the starting current. It is ineffect a reduced voltage starter.

Best regards,

Hi marke

Yes your reply makes sense to me but I still dont understand where the 660v comes from. When a UK stardelta motor is started in star then the voltage on each of its windings is 230. As the voltage is low,all things being equal, the amperage is high. I`m quite happy to believe that this is good because torque is greater or somesuch but I`ve always been told that connecting stardelta is to reduce current on starting. How can this work?

Regards

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Hello waterman

There is no 660 volts when the motor is used in a star / delta starter.

What the 660 volt refers to is the voltage that would have to be applied to the motor when connected in star in order to achieve rated power output.

When connected in star in a star/delta starter, we are affectively applying 400 volt to a motor that needs 660 volte to achieve full flux. Hence the voltage is reduced, which results in a reduction in current and torque.

Best regards,

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Hello waterman!

I think you should forget all voltage you have only one power supply voltage in my example - 400V

See following drawing.

How you can see I use only power supply voltage, but in first case(connected to delta) this voltage comes to all windings and:

power supply voltage = voltage on winding

in second case(connected to start):

power supply voltage / 1.73 = voltage on winding.

It means that we reduce current.

It is not good idea for little motors, but if you have big current such as 100A and more it is very good thing to reduce staring current!

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