NEMA motor characteristics
Motors are designed with certain speed-torque characteristics to match speed-torque requirements of various loads. A motor must be able to develop enough torque to start, accelerate, and operate a load at rated speed.
Horsepower, torque, and motor speed
The relationship between horsepower (H), torque (T, lbf-ft), and motor speed (Nm , rev/min) is given by:
The National Electrical Manufacturers Association (NEMA) has established class designations for motors on the basis of motors’ starting-torque and accelerating loads. The four standard NEMA designs are NEMA A, NEMA B, NEMA C, and NEMA D. NEMA A motors usually are used for applications that require extremely high efficiency and extremely high full-load speed. NEMA A-design motors are special and are not used very often. NEMA B-design motors are considered to be normal-torque motors. They are used for low-starting-torque loads, such as with centrifugal pumps and fans. NEMA C and NEMA D motors are used for applications that require high starting torque (e.g., positive-displacement pumps and compressors).
Speed torque curve
The graph in Fig. 1 shows a typical speed/torque curve for a NEMA-B motor. Such curves show the relationship between motor speed and torque produced by a motor from the moment it is started until the time it reaches full-load torque at the rated speed.
Starting torque (Fig. 1, A) is also known as locked-rotor torque. It is developed when the rotor is held at rest with the rated voltage and frequency applied, a condition that occurs whenever a motor is started. When the rated voltage and frequency are applied to the stator, there is a brief time before the rotor turns. During this time, a NEMA B motor develops approximately 150% of its full-load torque.
Accelerating torque and breakdown torque
As a motor accelerates, torque decreases slightly (Fig. 1, A to B) before beginning to increase. As speed continues to increase, torque increases until it reaches a maximum at approximately 200% (Fig. 1, B to C). This torque is referred to as accelerating (or pull-up) torque. If this maximum is beyond the motor’s torque capability, the motor will then stall or abruptly slow down. Point C on the graph in Fig. 1 is referred to as the breakdown (or pull-out) torque.
Full load torque
Full-load torque is the torque that develops when the motor is operating with the rated voltage, frequency, and load. The speed at which full-load torque is produced is the slip speed or the rated speed of the motor (Fig. 1, D)
Starting current and full load current
Starting current also is referred to as locked-rotor current and is measured from the supply line at the rated voltage and frequency with the rotor at rest. Full-load current is the current measured from the supply line at the rated voltage, frequency, and load, with the rotor up to speed. Starting current typically is 600 to 650% of full-load current on a NEMA B motor. As the rotor comes up to speed, the starting current decreases to the rated full-load current (Fig. 2).
Special design motors
Multispeed motors and motors used in variable-speed applications are special motors that are uniquely designed or selected to fulfill specific load requirements. NEMA design classifications are not applicable to these specialized motors.
NEMA has standardized frame-size motor dimensions, including bolt-hole sizes, mounting-base dimensions, shaft height, shaft diameter, and shaft length. Existing motors can be replaced without reworking the mounting arrangement. New installations are easier to design because the dimensions are known. Letters are used to indicate where a dimension is taken. For example, the letter "C" indicates the overall length of the motor, and "E" represents the distance from the center of the shaft to the center of the mounting holes in the feet. Motor manufacturers provide tables in the motor-data sheet that reference the letter to find the desired dimension.
NEMA categorizes standard frame sizes as either fractional or integral. Fractional frame sizes are designated as 45 and 56, and mainly include horsepower ratings of < 1.0. Integral (or medium) horsepower motors are designated by frame sizes that range from 143T to 445T. A "T" in the motor frame size designation of integral horsepower motors indicates that the motor is built to current NEMA frame standards. Motors built before 1966 have a "U" in the motor frame size designation, indicating that they were built to previous NEMA standards.
The frame-size designation is a code to help identify key dimensions. For example, the first two digits are used to determine the shaft height. The shaft height is the distance from the center of the shaft to the mounting surface, given in inches. To calculate the shaft height, divide the first two digits of the frame size by four. For example, a 143T frame size motor has a shaft height of 3.5 in. (14 ÷ 4).
The third digit in the integral "T" frame-size number is the NEMA code for the distance between the center lines of the mounting bolt-holes. The dimension is determined by matching the third digit in the frame number with a table in NEMA MG-1. 
Above NEMA motors
Motors that are larger than the NEMA frame sizes are referred to as above-NEMA motors. These motors typically range in size from 200 to 10,000 hp.
There are no standardized frame sizes or dimensions for above-NEMA motors because above-NEMA motors typically are constructed to meet the specific requirements of an application.
The customer typically supplies specifications for starting torque, breakdown torque, and full-load torque on the basis of speed-torque curves obtained from the driven-equipment manufacturer; however, there are some minimum torques that all large AC motors must be able to develop. These are specified by NEMA MG-1:
- Locked-rotor torque ≥ 60% of full-load torque.
- Pull-up torque ≥ 60% of full-load torque.
- Maximum torque ≥ 175% of full-load torque.
Altitude and ambient temperature
Above-NEMA motors require the same adjustment for altitude and ambient temperatures as do integral frame-size motors. When the motor is operated at an altitude of above 3,300 ft, a higher class of insulation should be used or the motor should be derated. Above-NEMA motors with Class-B insulation can be modified easily for operation in ambient temperatures between 40° and 50°C. Operation at ambient temperatures of > 50°C requires special modification at the factory.
Enclosures for above-NEMA motors
Environmental factors also affect large AC motors. Enclosures used on above-NEMA motors are different from those on integral frame-size motors.
Open drip proof (ODP)
The ODP enclosure for an above-NEMA motor provides the same amount of protection as the one for the integral frame-size open motor. As with the integral frame-size ODP, the above-NEMA ODP provides the least amount of protection for the motor’s electrical components and typically is used in contaminant-free environments.
Horizontal drip proof weather-protected type I
The horizontal drip-proof weather-protected type I enclosure is an open enclosure with ventilating passages that are designed to minimize the entrance of rain, snow, and airborne particles that could come into contact with the electrical and rotating parts of the motor. All air inlets and exhaust vents are covered with screens. It is used on indoor applications in low-humidity environments.
Horizontal drip proof weather-protected type II
Horizontal drip-proof weather-protected type II enclosures are open enclosures with vents that are constructed so that high-velocity air and airborne particles blown into the motor are discharged without entering the internal ventilating passages that lead to the electrical parts of the motor. The intake and discharge vents are designed to have at least three 90° turns and to maintain the air velocity at < 600 ft/min. It is used outdoors on motors that are not protected by other structures.
Totally enclosed, fan cooled (TEFC) for above NEMA motors
A TEFC enclosure for an above-NEMA motor functions the same way as the TEFC enclosure for integral frame-size motors. It is designed for indoor and outdoor applications in which internal parts must be protected from adverse ambient conditions. Above-NEMA TEFC enclosures are available for motors up to 900 hp on 580 frames and up to 2,250 hp on 708 to 880 frames.
Totally enclosed, air to air cooled
Motors using the totally enclosed, air-to-air cooled enclosure use the air-to-tube type of heat exchangers for cooling.
Totally enclosed, water to air cooled
In some situations, the motor frame cannot adequately dissipate heat, even with the help of a fan. The totally enclosed, water-to-air cooled enclosure cools the motor using a water-to-air heat exchanger and thus requires a steady supply of water.
Totally enclosed, fan cooled, explosion proof
Large AC motors also are used in hazardous environments. The totally enclosed, fan-cooled, explosion-proof enclosure meets or exceeds all applicable Underwriter’s Laboratories (UL) Standard 1203 requirements for hazardous (Division 1) environmental operation. 
|horsepower [Eq. 1]
|motor speed, rev/min
- NEMA MG 1 Motors and Generators, revised 2004. 2003. Rosslyn, Virginia: NEMA.
- UL Standard 1203, Explosion-Proof and Dust-Ignition-Proof Electrical Equipment for Use in Hazardous (Classified) Locations. 2000. Northbrook, Illinois: Underwriters Laboratories Inc.
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