Reduced starting currents don't justify derated controllers, but different motor time ratings may affect required horsepower ratings.
An elevator service firm reported that it was seeing motor starters for some of the elevators it was working on being sized smaller, in horsepower rating, than the motor it was controlling. Specifically, the elevators in question were driven by 25-hp DC motors. The hoist motor was powered by a 15kW generator which in turn (the common "Ward-Leonard" configuration) was run by a 25-hp AC 3-phase motor. The AC motor was controlled by a magnetic starter with a top rating of 15 hp.
The service firm asked if this arrangement ever met NEC requirements, and more importantly, whether it would today. Its customers made light of the concerns, saying that since the motors started with little load in a reduced voltage (wye-connected) mode, the smaller controller size was perfectly adequate.
The firm wondered about the ability of the reduced size controllers to interrupt the locked-rotor current of the motors. There weren't any markings that would indicate some special evaluation had been made for the reduced-size controllers.
The EC&M panel's response
We agree with the service firm, and the fact that the AC motor may have a wye-delta controller is irrelevant. The relevant Code rule is Sec. 620-15:
The motor controller rating shall comply with Section 430-83. The rating shall be permitted to be less than the nominal rating of the elevator motor, when the controller inherently limits the available power to the motor and is marked as power limited.
This is a new rule in the 1996 NEC and modifies the general rule in Sec. 430-83 that motor controllers "shall have a horsepower rating at the application voltage not lower than the horsepower rating of the motor." The reduced rating is now available because some modern adjustable-speed drive systems are capable of controlling the elevator drive without demanding full-horsepower controller performance.
Prior to the 1996 NEC, however, Art. 620 didn't take any exception to the general rules in Art. 430 on sizing motor controllers. Sec. 430-83 has been in the Code, essentially unchanged, for more than 60 years. Therefore, even when the elevators were installed, the installation was questionable.
Could the rating be lower?
At this point, many readers may be wondering what the big deal is - a 25-hp motor needs a 25 -hp controller; this is an open and shut Code violation. In this case that may be true, but we think the AC drive motor might also be larger than necessary. The KW rating of the DC adjustable-voltage generator and the hp rating of the AC motor powering the drive don't bear any direct relationship to the rating of the actual DC hoist motor because of the differing time ratings of the motors involved.
The hoist motor is usually a 30-min or 60-min rated motor, based on the full-load-up running condition, the worst case. This type of motor rating is quite useful for the intermittent duty [per Sec. 620-61(b)(1)] required of elevators, where a normal duty cycle might involve a 40% run time with 120-80 starts per hour under various conditions of loading. The duty cycle limits the amount of heating, which removes the necessity for running overload protection (Sec. 430-33) and cuts the required conductor ampacity to the actual hoist motor (not the AC drive motor) to 90% of nameplate [Table 430-22(a), Exception].
The resulting hp rating of the drive motor, which is partly a consequence of its time rating, may therefore vary significantly from the AC drive motor. This is because, in contrast, the AC drive motor on Ward Leonard elevator drive systems must be continuously rated in accordance with the applicable NEMA standard. In addition, the DC generator may be nominally overloaded as well. Remember, these generators are usually designed to commutate up to three times their rated current without excessive arcing at their commutators.
Therefore, you might see 15-hp continuous-rated AC drive motors (with 15-hp controllers) for these elevators with larger hp time-rated hoist motors. However, if as in this question, the AC drive carries the same rating as the actual hoist motor, then that rating must be respected in sizing its controller.