Introduction
Nameplate The nameplate of a motor provides important information necessary for proper application or we call Motor Specifications . For example,The following illustration shows the nameplate of a 30 horsepower (H.P.) three-phase (3 PH) AC motor.
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 amps.
Base Speed (R.P.M.) 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
The service factor is a number that is multiplied by the rated power of the motor to determine the power in
that the engine can be operated. Therefore, an engine designed to operate at or below the rating rating of the nameplate has a service factor of 1.0.
Some motors are designed for a service factor greater than 1.0, so sometimes they may exceed their rated power. For example, this engine has a service factor of 1.15. A 1.15 service factor motor can operate 15% higher than the horsepowerof the nameplate. Therefore, this 30 HP motor can be operated at 34.5 HP. Keep in mind that any motor running continuously above its rated power will have a reduced service life
Insulation Class
NEMA defines the insulation classes of the motor to describe the insulation capacity of the motor to handle the heat. The four insulators
the classes are A, B, F and H. The four classes identify the permissible temperature increase from an ambient temperature of 40 ° C (104 ° F). Classes B and F are the most used.
The ambient temperature is the temperature of the surrounding air. This is also the temperature of the motor windings before starting the motor, assuming that the motor has stopped long enough. The temperature increases in the motor windings as soon as the motor is started. The combination of the ambient temperature and the permitted increase in temperature is equivalent to the maximum nominal winding temperature. If the motor runs at a higher winding temperature, the service life will be reduced. An increase of 10 ° C in the operating temperature above the maximum allowed can reduce the life expectancy of the motor insulation by half
The following illustration shows the permissible temperature increase for motors operated with a 1.0 service factor at altitudes not exceeding 3300 feet. Each kind of insulation has a margin allowed to compensate for the hot spot of the motor, a point in the center of the motor windings where the temperature is higher For motors with a service factor of 1.15, add 10 ° C to the increase in temperature allowed for each class of motor insulation.
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
The efficiency of the engine is a topic of increasing importance, especially for AC motors. The efficiency of the AC motor is important because AC motors are widely used and represent a significant percentage of the energy used in industrial installations. The efficiency of the motor is the percentage of the energy supplied to the motor that is converted into mechanical energy in the motor shaft when the motor is running continuously at full load with the rated voltage applied. Because engine efficiencies can vary between engines of the same design, the NEMA rated efficiency percentage on the nameplate is representative of the average efficiency for a large number of engines of the same type. The motor in this example has a nominal NEMA efficiency of 93.6%. Both NEMA and the Energy Policy Act of 1992 (EPAct) specify the same process to test the efficiency of the engine. In 2001, NEMA established the designation of NEMA Premium for three-phase
AC motors that meet even higher efficiency standards than those required by EPAct. More recently, the Energy Security and Independence Act of 2007 (EISA) was passed. EISA requires that most engines manufactured after December 19, 2010 comply with NEMA’s premium efficiency levels. This includes engines previously covered by EPAct and some additional categories of engines.
Also read
Introduction to AC Motor Construction
Preventive maintenance – basics
