Motor Specifications

Nameplate

The nameplate of a motor provides important information necessary for proper application. For example, the following illustration on Picture 1 shows the nameplate of a 30 horsepower (HP) three-phase (3 PH) AC motor. The following paragraphs explain some of the other nameplate information for this motor.

Picture 1: Nameplate of motor (example)

Voltage Source (VOLTS) and Full-load Current (AMPS)

AC motors are designed to operate at standard voltages. This motor is designed to be powered by a three-phase 460 V supply. Its rated full-load current is 35.0 A.

Base Speed (RPM) and Frequency (HERTZ)

Base speed is the speed, given in RPM, at which the motor develops rated horsepower at rated voltage and frequency. Base speed is an indication of how fast the output shaft will turn the connected equipment when fully loaded. This motor has a base speed of 1775 RPM at a rated frequency of 60 Hz. Because the synchronous speed of a 4-pole motor operated at 60 Hz is 1800 RPM, the full-load slip in this case is 1.4%. If the motor is operated at less than full load, the output speed will be slightly greater than the base speed.

Service Factor

Service factor is a number that is multiplied by the rated horsepower of the motor to determine the horsepower at which the motor can be operated. Therefore, a motor designed to operate at or below its nameplate horsepower rating has a service factor of 1.0.
Some motors are designed for a service factor higher than 1.0, so that they can, at times, exceed their rated horsepower For example, this motor has a service factor of 1.15. A 1.15 service factor motor can be operated 15% higher than its nameplate horsepower. Therefore this 30 HP motor can be operated at 34.5 HP. Keep in mind that any motor operating continuously above its rated horsepower will have a reduced service life.

Insulation Class

NEMA defines motor insulation classes to describe the ability of motor insulation to handle heat. The four insulation classes are A, B, F, and H. All four classes identify the allowable temperature rise from an ambient temperature of 40° C (104° F). Classes B and F are the most commonly used. Ambient temperature is the temperature of the surrounding air. This is also the temperature of the motor windings before starting the motor, assuming the motor has been stopped long enough. Temperature rises in the motor windings as soon as the motor is started. The combination of ambient temperature and allowed temperature rise equals the maximum rated winding temperature. If the motor is operated at a higher winding temperature, service life will be reduced. A 10° C increase in the operating temperature above the allowed maximum can cut the motor’s insulation life expectancy in half. The following illustration on Picture 2 shows the allowable temperature rise for motors operated at a 1.0 service factor at altitudes no higher than 3300 ft. Each insulation class has a margin allowed to compensate for the motor’s hot spot, a point at the center of the motor’s windings where the temperature is higher. For motors with a service factor of 1.15, add 10° C to the allowed temperature rise for each motor insulation class.

Picture 2: The allowable temperature rise for motors per class, operated at a 1.0 service factor

The motor in this example has insulation class F and a service factor of 1.15. This means that its winding temperature is allowed to rise to 155° C with an additional 10° C hot spot allowance.

NEMA Motor Design

NEMA also uses letters (A, B, C, and D) to identify motor designs based on torque characteristics. The motor in this example is a design B motor, the most common type. Motor design A is the least common type.

Motor Efficiency

Motor efficiency is a subject of increasing importance, especially for AC motors. AC motor efficiency is important because AC motors are widely used and account for a significant percentage of the energy used in industrial facilities. Motor efficiency is the percentage of the energy supplied to the motor that is converted into mechanical energy at the motor’s shaft when the motor is continuously operating at full load with the rated voltage applied. Because motor efficiencies can vary among motors of the same design, the NEMA nominal efficiency percentage on the nameplate is representative of the average efficiency for a large number of motors of the same type. The motor in this example has a NEMA nominal efficiency of 93.6%.
Both NEMA and the Energy Policy Act of 1992 (EPAct) specify the same process for testing motor efficiency. In 2001, NEMA established the NEMA Premium designation for three-phase AC motors that meet even higher efficiency standards than required by EPAct. More recently, the Energy Independence and Security Act of 2007 (EISA) was passed. EISA requires most motors manufactured after December 19, 2010 to meet NEMA Premium efficiency levels. This includes motors previously covered by EPAct and some additional categories of motors.
Siemens NEMA Premium Efficient motors meet NEMA Premium efficiency standards and Siemens Ultra Efficient motors with exclusive die cast copper rotor technology exceed NEMA Premium efficiency standards.