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 motor terminals. If the ampere rating of the motor and transformer over current device falls within the motor's 50% RPM draw requirements, a problem is likely to develop. The over current device may not open under intermediate motor ampere loading conditions. Overheating of the motor and/or transformer would occur, possibly causing failure of either component. This condition is more pronounced when one transformer is used to power one motor and the running amperes of the motor is in the vicinity of the full load ampere rating of the transformer.
The following precautions should be followed:
1.When one transformer is used to operate one motor, the running amperes of the
motor should not exceed 65% of the transformer's full load ampere rating.
2. If several motors are being operated from one transformer, avoid having all
motors start at the same time. If this is impractical, then size the transformer so
that the total running current does not exceed 65% of the transformer's full load
ampere rating.
Comments
Post a Comment