Nuisance Tripping with Premium Efficiency Motors

The National Electrical Manufacturers Association (NEMA) recognizes and describes two components of starting current: instantaneous peak inrush and locked rotor current(LRC).

Nuisance tripping primarily has been associated with the instantaneous peak inrush, which is a momentary current transient that occurs immediately (within half an AC cycle) after contact closure. LRC is the root-mean-square (RMS) current that is established following the peak inrush; the current remains near the locked rotor value during acceleration until the motor approaches its operating speed. (Note: The terms inrush or starting current are often used to mean locked rotor current). Premium efficiency motors have slightly higher LRCs and locked rotor code letters than lower efficiency motors of the same rating.
MotorMaster+ 4.0 software tool or deciphered from the locked-rotor code letter on the motor nameplate.

For example, the maximum LRC for a Code C, 460-volt, 100-hp motor is determined as follows:
LR Current = Motor Horsepower x (Maximum kVA/hp/Supply Voltage in kV) / √3
= 100 hp x {(4.0 kVA/hp / 0.46 kV) / √3}
= 502 Amps

Nuisance trips should not be an issue with motors that are controlled with soft starters or adjustable speed drives.

The ratio of peak inrush to LRC tends to increase with higher efficiency #motors due to their lower power factor under locked rotor conditions.Fortunately, peak-inrush current usually is not a problem because it lasts only a few milliseconds. However, it can be an issue when the #motor #controller uses instantaneous magnetic-only circuit protectors that react in less than a single AC cycle. This is because peak inrush can be as high as 2.8 times the RMS locked rotor current and may exceed the #circuit protector current setting.

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