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Motor specifications

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Nameplate

A motor’s nameplate provides important information relevant to its selection and application. Fig. 1 is the nameplate from a 30-hp AC motor.

Voltage and amps

Alternating current motors are designed to operate at standard voltages and frequencies. This sample motor is designed for continuous-duty operation in a 460-VAC, three-phase system. At full-load, this motor would draw a 34.9-A current.

Horsepower and kilowatts

U.S.-manufactured AC motors generally are hp-rated, whereas European-manufactured equipment generally is kW-rated.

In kW, the power formula for a single-phase motor is:

 (Eq. 1)

The power formula for three-phase motor is:

 (Eq. 2)

The motor manufacturer provides the voltage, current, and power factor of the motors.

Base speed

Base speed is the nameplate speed—given in rev/min—at which the motor develops the rated horsepower at the rated voltage and frequency. It indicates how fast the output shaft will turn the connected equipment when fully loaded and supplied with the proper voltage and frequency. The base speed of the motor in Fig. 1 is 1,765 rev/min at 60 Hz.

Service factor

The service factor is a multiplier that may be applied to the rated power to allow a motor to be operated at higher than its rated hp. A motor designed to operate at its nameplate hp rating with a service factor of 1.0 would operate continuously at 100% of its rated hp without exceeding its operating temperature. Some applications might require a motor to exceed its rated hp. In such cases, a motor with a service factor of 1.15 can be specified, allowing the motor be operated 15% higher than its nameplate hp. For example, with a 1.15 service factor, the 30-hp motor in Fig. 1 can be operated at 34.5 hp. Note, however, that any motor operating continuously at a service factor > 1.0 will have a reduced life expectancy compared to one operating it at its rated hp. This is because of high winding temperature, which causes motor-winding insulation to age thermally at approximately twice the rate that occurs for a motor with a 1.0 service factor.

Insulation classes

The National Electrical Manufacturers Association (NEMA) has established motor-winding-insulation classes to meet motor-temperature requirements found in different operating environments. The four insulation classes are A, B, F, and H, as illustrated in Fig. 2. Class-F insulation is most commonly used. Class-A insulation seldom is used. Before a motor is started, its windings are at ambient temperature (the temperature of the surrounding air). NEMA has standardized on an ambient temperature of 40°C within a defined altitude range for all motor classes.

Temperature will rise in the motor as soon as the motor is started. Each insulation class has a specific allowable temperature increase. The combination of ambient temperature and allowed temperature increase equals the maximum winding temperature in the motor. For example, a motor with Class-F insulation has a maximum temperature increase of 105°C when operated at a 1.0 service factor. The maximum winding temperature is 145°C (40°C ambient plus 105°C rise). A margin is allowed to provide for the motor’s "hot spot," a point at the center of the motor’s windings where the temperature is higher.

The operating temperature of a motor is important to efficient operation and long life. Operating a motor above the limits of the insulation class reduces its life expectancy. For example, a 10°C increase in the operating temperature can decrease the motor’s insulation life expectancy as much as 50%.

The motor in Fig. 1 has Class-F insulation and is rated for continuous duty at 40°C ambient.

Motor design

NEMA has established standards for motor construction and performance. Standard NEMA designs are NEMA A, NEMA B, NEMA C, and NEMA D. NEMA B motors are the most commonly used. (See the NEMA Motor Design section below for more details of the NEMA designs.) Additionally, NEMA has assigned frame sizes for all three-phase induction motors built to NEMA standards. This includes motors from 0.5 to 250 hp. Each frame size has a specific frame design, set of dimensions, full-load amperage, efficiency, and power factor. See NEMA MG 1[1] for the details of frame sizes.

The motor in Fig. 1 is a NEMA B design and has a NEMA 286T frame designation.

Locked rotor code letters

NEMA has assigned code letters A through V to designate the locked-rotor kVA per horsepower. This is an amount of power drawn by the motor when it is started. Table 1 gives the designations of each code letter.

The motor in Fig. 1 is code letter G and has 5.6 to 6.29 locked-rotor kVA/hp.

Efficiency

AC motor efficiency is expressed as a percentage. It is an indication of how much of the input electrical energy is converted to output mechanical energy. The nominal efficiency of the motor in Fig. 1 is 93.6%. The higher the percentage, the more efficiently the motor converts the incoming electrical power to mechanical horsepower. A 30-hp motor with a 93.6% efficiency would consume less energy than a 30-hp motor with an efficiency rating of 83.0%. This can mean a significant saving in energy cost. In addition to lower energy costs, lower operating temperature, longer life, and lower noise levels are typical benefits of high-efficiency motors.

Nomenclature

Pa = active power, kW
E = voltage, V
I = current, A
Fp = power factor, cos θ

References

  1. MG 1 Motors and Generators, revised 2004. 2003. Rosslyn, Virginia: NEMA.

Noteworthy papers in OnePetro

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