Machine Parts

The minimum transformer KVA rating required to operate a motor is calculated as follows:  Minimum Transformer KVA  Running Load Amperes x 1.73 x Motor Operating Voltage _____________________________________________________                                          1000  NOTE: If motor is to be started more than once per hour add 20% additional KVA.  Care should be exercised in sizing a transformer for an induction type squirrel cage motor as when it is started, the lock rotor amperage is approximately 5 to 7 times the running load amperage. This severe starting overload will result in a drop of the transformer output voltage. When the voltage is low the torque and the horsepower of the motor will drop proportionately to the square of the voltage. For example: If the voltage were to drop to 70% of nominal, then motor horse power and torque would drop to 70% squared or 49% of the motor nameplate rating. The underlying problem is low voltage at the moto