What will happen to the KW rating of the Induction motor when run in
star connection and then when run in delta connection?

Will the KW rating increase of decrease or remain the same????
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Is the line voltage the same, or is the line votlage being increased by root
3 when running delta?

Generally, when reconfiguring like this, its because you have a different
line voltage.
The overall current rating of a machine like this is based on the allowable
current through the windings (heat dissipation of the I^2R losses).
So going from star to delta, if you keep the phase current in the windings
the same, the line current can go to sqrt(3) times larger. To keep the
phase voltage applied to the windings the same, you need to reduce the
line-line voltage by sqrt(3). So net effect is the same power rating.
daestrom

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You need to be more specific. Are you reconnecting the motor windings, in
which case the line voltage is the same, or are you reconnecting the supply
transformer, in which case the line voltage changes, or both?
Ben Miller

Uh... the magic smoke may be released.
Reconnecting from star to delta is putting sqrt(3) times higher voltage on
the windings in each phase. This will increase the magnitizing current
needed. If the iron saturates at this higher voltage/coil, the current will
be excessive and burn up the motor.
If the iron *doesn't* saturate, and the magnitizing current is not too
large, then the motor would be capable of sqrt(3) times the power (same
current per phase in the windings, sqrt(3) times larger line current and at
the same time same line voltage but sqrt(3) times higher phase voltage).
But chances are any motor of appreciable size (integral hp and up), if not
specifically designed for the higher phase voltage, will burn up in short
order.
So "the effect on the motor rating' is most likely, "1.73 times higher
power, for about 10 minutes, followed by 0 power"
daestrom

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would have thought that the shaft torque at the motor is approx proportional
to current
In which case the kW rating in delta is higher
The kVA rating stays the same though

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The difference is negligable if you stick to ratings. Given a set of
windings, there is a rated current per winding as well as a rated voltage
per winding. Based on these the KVA rating doesn't change and as the
apparent R/X ratio, power factor at any given slip don't change, the KW
wont change. What does happen is that the rated voltage/current in Y is not
the same as the rated voltage/current in delta even though the product VI is
the same.
If you take a given set of windings in Y and reconnect them in delta at the
same line-line voltage, the current will be higher by root(3) and the torque
will also be higher by a factor of 3 but this is temporary-lasting only a
short time after you smell the magic smoke leaking out.

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Don Kelly snipped-for-privacy@shawcross.ca
remove the X to answer

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