How is a Transformer selected to feed a motor?

Issue:
Transformers for motor loads

Product Line:
LV Transformers

Environment:
Applies to Low Voltage Transformers by SquareD/Schneider Electric

Cause:
Transformers are often required to power motor loads

Resolution:

This is provided on page 39 of the Low Voltage Transformers Selection Guide, document # 7400CT9601.  Please see attachment below.  When selecting a Transformer to feed a motor, it is important to note that the starting current of a motor can 6 to 7 times the full-load running current, or even higher if it is a high efficiency motor.  This initial high current can cause excessive voltage drop because of regulation through the Transformer.  Reduced voltage could cause the motor to fail to start and remain in a stalled condition, or it could cause the starter coil to release or ``chatter``.  A typical desirable voltage drop is to allow 10-12% voltage drop at motor start.  The voltage decrease during motor starting can be estimated  as follows:

Voltage Drop (%) = (Motor Locked Rotor Current / Transformer Secondary Full Load Rating) * Transformer Impedance (%)

Released for:Schneider Electric USA