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Showing posts with the label Three Phase induction type squirrel cage motors

How are transformers sized to operate Three Phase induction type squirrel cage motors?

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