Understanding low-voltage motor protection
After a mid-morning, building-wide blackout signaled a flood of frantic calls to the maintenance office in a manufacturing facility, the maintenance supervisor quickly learns that the cause of the outage is not due to the electrical utility. He quickly dispatches electricians to re-establish power and investigate the problem. Minutes turn into hours before power is restored, effectively causing a loss of business for the day. Upon learning that the outage was initiated by a ground fault in one of the HVAC motors on the roof, the maintenance supervisor rightly questions why the fault was not interrupted by downstream protective devices closer to the fault. All agree that an engineering study is needed, rather than waiting for another electrical equipment failure to take out the entire facility.
The first in a series that describe the nuts and bolts procedure to conduct a protective device coordination study, this article (based on the 2008 NEC) covers guidelines in the selection and coordination of branch circuit overload and short circuit protection of low-voltage motors. We begin with this type of electrical load, because it’s prevalent in most facilities and located farthest downstream in the electrical system.
An example of the protective device coordination procedure is provided for a 3-phase, 50-hp, 460V, 1.15 service factor, standard NEMA Design B squirrel-cage induction motor that is controlled by a full-voltage, non-reversing (FVNR) NEMA size 3 combination motor starter. Motor branch circuit overload protection is provided by a thermal overload relay with melting alloy (eutectic) heaters, while branch circuit short circuit protection is afforded with a motor circuit protector or dual-element (time delay) fuses. However, keep in mind that this presentation is brief and incomplete. Other crucial application-specific considerations are not addressed here due to space constraints.
In the absence of motor nameplate data, the full-load amperes (FLA) of the motor is 65A from NEC Table 430.250. This assumes that motor operation and torque-speed characteristic comply with the constraints of the table; and the motor locked-rotor amperes (LRA) is 363A from NEC Table 430.251(B), as opposed to the alternate method of finding the locked-rotor current from NEC Table 430.7(B) and knowledge of the motor locked-rotor indicating code letter.
Knowledge of the maximum starting amperes of the motor is necessary to properly select motor branch circuit short circuit protection. The maximum starting current not only accounts for the locked rotor current, but also considers the possible asymmetrical offset of the starting current waveform. In this example, the maximum starting amperes = locked-rotor current × offset factor = 363A × 1.5 = 545A.
Finally, for a standard NEMA Design B motor with class B or F temperature rise, the safe stall time should not exceed 20 sec, as noted in “AC Motor Selection and Application Guide,” GE Industrial Systems, Fort Wayne, Ind., Oct. 1999.
The one-line diagram of this case is shown in Fig. 1 (click here to see Fig. 1). The motor branch circuit overload protection is provided by a thermal overload relay with three (one per phase) melting alloy heaters, and the motor branch circuit short circuit protection is afforded by a motor circuit protector. The time-current plot of this case is shown in Fig. 2 (click here to see Fig. 2).
For this application, the manufacturer recommends a motor circuit protector with 150A frame, adjustable trip range from 58A to 130A, and adjustable instantaneous pick-up from eight to 13 times trip setting. (Note that this recommendation is based on outside ambient temperature not more than 40°C; infrequent motor starting, stopping or reversing; 10 sec or less motor accelerating time; and locked rotor current less than 6 × motor full-load current.) The trip is set at 58A (lowest dial setting), and the instantaneous pickup is set above the maximum starting current of 545A at 11 × trip setting or 638A, in compliance with NEC 430.52(C)(7). The Class 10 overload relay characteristic was chosen so as to lie below the safe stall point, thereby protecting the motor from damage due to a sustained locked-rotor condition.
The allowable ampacity of the 3 AWG copper conductors with 75°C XHHW insulation is 100A from NEC Table 310.16 (assuming an ambient temperature of 30°C or less), satisfying NEC 430.22. It should be noted that the actual thermal overload relay characteristic may not be completely to the left of the conductor ampacity due to its tolerance band. The motor branch circuit conductors are protected from short circuit damage by the motor circuit protector, because the conductors can withstand the maximum through-fault current of 15,000A for the time it takes the protector to trip (approximately one cycle = 0.017 sec). In other words, the point (15,000A, 0.017 sec) lies below the conductor short circuit heating limit curve in Fig. 2.
The one-line diagram of this case is shown in Fig. 3 (click here to see Fig. 3), and the corresponding time-current plot is given in Fig. 4 (click here to see Fig. 4). The 90A rating of the fuse complies with NEC 430.52(C)(1). Although a rating of 125A is acceptable under NEC 430.52(C)(1) Exception No. 1, the 90 A rating affords better protection from ground faults on the load side of the fuses.
This overview of the protective device coordination procedure for a low-voltage motor has shown how the ratings and settings of branch circuit and short circuit protective devices are chosen to provide adequate protection under overload and short circuit conditions. Next time, we’ll continue with the procedure at upstream levels progressively closer to the service entrance of the facility.
Frank Mercede, P.E., is vice-president and Joseph Mercede is president of Mercedes Electric Co., Inc., based in Lester, Pa. They can be reached at fmercede@mercedeelectric.com and jmercede@mercedeelectric.com.