A Website on Maintenance of Electrical Systems

NEMA Code letter     kVA/HP with locked rotor A 0 - 3.14 B 3.15 - 3.54 C 3.55 - 3.99 D 4.00 - 4.49 E 4.50 - 4.99 F 5.0 - 5.59 G 5.6 - 6.29 H 7.1 - 7.99 J 7.1 - 7.99 K 8.0 - 8.99 L 9.00 - 9.99 M 10.0 - 11.19 N 11.20 -12.49 P 12.5 - 13.99 R 14.00 - 15.99 S 16.0 - 17.99 T 18.0 - 19.99 U 20.0 - 22.39 V 22.4 and up With the data above it is possible to measure the In-rush current or locked rotor amps from  the Locked rotor code and Rated HP of the motor. Practically it is possible to do it by mechanically strapping down the rotor, then you press the start button and measure the stator current with your clamp-on ammeter. For this example let us assume a 3-phase 75HP motor designed to operate at 4...

The background story goes like this. Our gas turbine has an output rating of 11,000 volts but we had a problem with the Automatic Voltage Regulator (AVR) and it had to be replaced. After replacement, the HMI indicated that the output voltage was 11,000V but the actual voltage on the bus was about 10,400V and this had a ripple effect on the whole system as the output of our 3.3kV transformer was 3.1kV and the output of the 440v transformer was 400v.   After a while, the thermal overload relays on the well-pump motors began to trip one after the other. This prompted us to troubleshoot, and we discovered that the supply voltage to the motor was below the nameplate rating.   Why would a low supply voltage trip the thermal-overload protection relay, normally a when a motor is overloaded it draws more current from the supply and when the motor tries to pull current higher than its full load amperage (FLA) rating the overload opens the circuit to...