This article, second in a series, addresses design letters and code letters and how they are used in motor-circuit design and installation. It highlights the different characteristics of letters and why it is sometimes desirable to choose one over the other.
The starting torque of a motor varies with its classification. The National Electrical Manufacturers Association (NEMA) classifies motors as Design B, C or D type. These standardized types are the most used motors in the electrical industry. Other types of motors classified by NEMA are Design F or G motors.
Each class of motor has a different rotor design, which provides a different value of starting torque. Different values of torque, speed, current, and slip for starting and driving the various types of loads are produced when using NEMA Design B, C or D motors.
The design type of the motor to be selected depends on the starting and running torque required to drive the load.
Class B motors
The most used motor in the electrical industry is Class B. The starting torque of an induction motor will increase by 150 percent of the full-load torque (FLT) when using Class B motors. Most designers, however, ensure a starting torque of less than 150 percent when using Class B induction motors to start and run loads. Calculate the FLT by using the formula below for a Class C motor.
Design C motors
The starting torque of a squirrel-cage induction motor will increase about 225 percent of the FLT, when using Class C design motors; however, to keep from overloading the starting torque, designers will often load these motors to less than 225 percent.
For example: What is the FLT and starting torque of a 40 horsepower (hp), Class C design induction motor operating at 1,725 rpm?
Step 1: Find FLT
Torque = hp 5,252 ÷ rpm
Torque = 40 5,252 ÷ 1,725
Torque = 210,080 ÷ 1,725
Torque = 121.8 ft. lbs.
Step 2: Find starting torque
FLT increases by 225 percent
Torque = 121.8 ft. lbs. 225%
Torque = 274.05 ft. lbs.
Solution: The FLT is 122 ft. lbs., and the starting torque is 274 ft. lbs.
Class D motors
The starting torque of a squirrel-cage induction motor is increased about 275 percent of the FLT when using Class D motors. However, to keep from overloading the starting torque of a motor, designers will often load these motors to less than 275 percent. To determine the FLT of a Class D motor, use the formula above for Class C motors.
Code letters are given to motors by manufacturers for calculating the locked-rotor current (LRC) in amps based on the kilovolt-amperes (kVA) per horsepower per the motor’s code letter. Overcurrent protection devices (OCPDs) shall be set above the LRC of the motor to prevent the OCPD from opening when the motor’s rotor starts and begins to accelerate the driven load.
One may use code letters or horsepower to determine the motor’s LRC.
How do you determine LRC based on code letters? Code letters must be marked on the motor nameplates; these letters are used for determining the LRC. LRCs for particular code letters are listed in Table 430.7(B) in kVA per horsepower. For example, The LRC for a three-phase, 208V, 20-hp motor with a code letter B is found as follows:
Find LRC amps
Amps = kVA per hp 1,000 hp ÷ (V 1.732)
Amps = 3.54 20 1,000 ÷ (208V 1.732)
Amps = 70,800 ÷ 360
Amps = 197
Solution: The motor’s LRC is 197 amps.
Note that Table 430.7(B) must be used to find the LRC of motors based on their nameplate code letters.
What about an LRC that uses horsepower? The LRC of a motor can be found in Table 430.251(A) or (B) of the National Electrical Code. The LRCs for single-phase and three-phase motors are selected from these tables, which are based on the number of phases, voltage and horsepower rating of the motor. For motors with code letters A through G, the LRC is usually six times the FLC of the motor. Note that the code letter concept is not used when Tables 430.251(A) and (B) are used because the LRC is determined by the motor’s design letters.
Motors will be marked either as Design B, C or D to indicate which LRCs are to be selected from Tables 430.251(A) and (B) based on the horsepower, phases and voltages. For example, a 50 hp, 460V, three-phase motor from Table 430.251(B) has a LRC of 363 amps.
Engineers, contractors and electricians must select the LRC rating to allow a motor to start and run from Table 430.7(B) for code letters and Table 430.251(A) or (B) for design letters.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.