# Auto Transformer Star Point

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I am in the Electrical training field and motors are run at no load. I have noticed that whether the three phase auto transformer star is open or closed makes no difference. Can somebody please give me the reason why the star point must be closed during start up?
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Hello Jakes

Welcome to the forum

When the star point is closed, the transformer supplies a reduced voltage to the motor where the voltage is determined by the transformer ratio.

When the star point is open, the transformer behaves as a primary reactor and the voltage applied to the motor is determined by the ratio of the motor impedance to the total impedance of the motor plus the reactance of the transformer.

There will be a difference in start current between the star point closed and the star point open with lower start current in the line for the star point closed.

Best regards,

Mark.

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• 4 months later...

As a follow-up to this question, I assume that for a standard 50/65/80% autotransformer, only 64% of the motor full load current rating will suffice for the current rating of the transformer contactor, as this covers the "worst-case scenario" of the 80% tap being selected for use (please correct me if I am wrong there).

What I would like to know though is what rating is recommended for the star contactor?

The current through it will be negligible, right?

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Hello Marco

The current flowing to the motor is directly proportional to the voltage reduction.

The current flowing in the supply is proportional to the square of the voltage reduction. Therefore the curent from the 80% tap is 80%, not 64%.

Best regards,

Mark.

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The current flowing to the motor is directly proportional to the voltage reduction.

The current flowing in the supply is proportional to the square of the voltage reduction. Therefore the curent from the 80% tap is 80%, not 64%.

Mark,

Yes, but the transformer contactor is installed on the primary side of the transformer, so it would see only 64% current flowing through it, right?

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Hello Marco

Yes, that is correct provided that you are using a single stage Korndorffer auto transformer starter and that the contactor is bypassed when full voltage is applied.

In some installations, that contactor is used as a main contactor and remains in circuit while the motor is running. This makes it easy to fit the overloads etc, otherwise you do not have a common path for the overloads to be connected and so they are out of circuit during start.

Best regards,

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Yes, fair enough.

Back to my original query then ... is there a standard "rule of thumb" used for sizing the star contactor?

For commonality of equipment I would probably just use the same rating as the transformer contactor, but I am also thinking that you could potentially make it a lot smaller to save on space & cost.

It is really just a voltage reference point, with negligible current flowing through it - is that correct?

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Hello Marco

The star contactor carries the motor current plus the transformer current so would need to be at least the size of the motor current.

Best regards,

Mark.

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