Motors are a source of short-circuit current. They will act like generators when a short-circuit occurs. At a minimum, motors must be specified by horsepower which will be listed on the motor nameplate. Motor specifications are as follows:
kV: Motor rated kV.
HP: Motor horsepower. Motors may be represented individually or as a lumped group.
FLA: The Full Load Amps.
PF: Motor operating power factor. This is used with the Efficiency to determine kVA. Motor groups should use an average value of power factor.
EFF: Motor operating efficiency. This is used with the Power Factor field to determine kVA. Motor groups should use an average value of efficiency.
kVA/HP: The motor or motor group kVA rating per horsepower. This is a simple way of defining the kVA when full load amperes, or efficiency and power factor are not known. Typical values for standard efficiency motors are listed below:
1.0 kVA/HP - Induction < 100 HP and 0.8 PF Syn Motors
0.95 kVA/HP - Induction 100 < 999 HP
0.90 kVA/HP - Induction > 1000 HP
0.8 kVA/HP - Synchronous 1.0 PF
Motor FLA Calculations
There are three ways to calculate motor Full Load Ampere (FLA) rating. The FLA calculation methods are shown below in priority order:
If the kVA is known, the motor FLA should be determined by the following equation:
If the motor KVA is not available but motor HP and (kVA/HP) values are known, the motor FLA should be determined from the following equation:
FLA = HP * (kVA/HP) / 1.73 / kV
If both the KVA and the (kVA/HP) are not available, the motor FLA should be determined from the following equation:
FLA = HP x 0.746 / (EFF x PF) / 1.73 / kV
Motor Impedance and X/R Ratio for Short Circuit Analysis
Reactance to resistance ratio X/R and impedance for short circuit analysis %Z can be read off from the motor nameplate. If motor X/R and/or %Z is not specified on the nameplate, the motor manufacturer may be able to supply this. When contacting the motor manufacturer, it may be helpful to have the motor serial number. If X/R or %Z cannot be obtained, check the SCA V1.0 program Help menu for reference values that can be used in the calculations.