Answer / pillay

a)Loose terminations at the auto transformer itself .
b)intermittent failure of the change over timer or starter.
c)excessive load at time of starting .
d)insulation failure of the auto transformer.

Answer / k.prakashchandra

Autotransformer is for starting of large motor.If you use a
starter additionally ,it requires a certain minimum voltage
for the hold coil.As this voltage was high enough for heavy
starting current which burnt the autotransformer.

Answer / amit kumar singh

The one reason may be that at the time of starting,
induction motor sucts very high inrush current for the
establishment of magnetic field. Thus the total MVA
requirement at the time of motor starting is about 6 to 7
times the rated MVA of motor (if voltage is considered to
remain constant at the time of starting).

As, a transformer can only withstand 1.5 times its rated
MVA (i.e. also only for short duration), the heavy
requirement of MVA by motor could have damaged the
transformer (in case of failure of tap changer operation to
reduce voltage applied before starting of motor).

Answer / d.lakshmi narayana

To check the transformer tap voltage
persantage.(or)Unbalance the tap voltage (or)To take proper
tap voltage for motor starting load

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