Electric Motors

1.1 THREE-PHASE INDUCTION MOTORS In the integral horsepower sizes, i.e., above 1 hp, three-phase induction motors of various types drive more industrial equipment than any other means. The most common three-phase (polyphase) induction motors fall within the following major types: NEMA (National Electrical Manufacturers Association) design B: Normal torques, normal slip, normal locked amperes NEMA […]


1.2 There are many types of single-phase electric motors. In this section, the discussion will be limited to those types most common to integral-horsepower motor ratings of 1 hp and higher. In industrial applications, three-phase induction motors should be used wherever possible. In general, three-phase electric motors have higher efficiency and power factors and are […]

Energy-Efficient Motors (Electric Motor)

2.1 STANDARD MOTOR EFFICIENCY During the period from 1960 to 1975, electric motors, particularly those in the 1- to 250-hp range, were designed for minimum first cost. The amount of active material, i.e., lamination steel, copper or aluminum or magnet wire, and rotor aluminum, was selected as the minimum levels required to meet the performance […]


2.2 The escalation in the cost of electric power that began in 1972 made it increasingly expensive to use inefficient electric motors. From 1972 through 1979, electric power rates increased at an average annual rate of 11.5%/yr. From 1979 to the present, the electric power rates have continued to increase at an average annual rate […]

WHAT IS EFFICIENCY? (Electric Motor)

2.3 Electric motor efficiency is the measure of the ability of an electric motor to convert electrical energy to mechanical energy; i.e., kilowatts of electric power are supplied to the motor at its electrical terminals, and the horsepower of mechanical energy is taken out of the motor at the rotating shaft. Therefore, the only power […]


2.4 Until recently, there was no single definition of an energy-efficient motor. Similarly, there were no efficiency standards for standard NEMA design B polyphase induction motors. As discussed earlier, standard motors were designed with efficiencies high enough to achieve the allowable temperature rise for the rating. Therefore, for a given horsepower rating, there is a […]


2.5 Efficiency is defined as the ratio of the output power to the input power to the motor expressed in percent; thus, TABLE 2.4b Full-Load Nominal Efficiencies of Three-Phase Four-Pole Energy-Efficient TEFC Motors Nominal efficiency range Average nominal efficiency hp Min. Max. 1 77.0 85.5 S2.5 2 82.5 86.5 84.2 3 82.5 89.5 87.8 5 […]


2.6 Coincident with the NEMA test program, it was determined that a more consistent and meaningful method of expressing electric motor efficiency was necessary. The method should recognize that motors, like any other product, are subject to variations in material, manufacturing processes, and testing that cause variations in efficiency on a motor-to-motor basis for a […]


2.7 In 1990, based on the experience gained by the electric motor manufacturers in producing energy-efficient polyphase induction motors and interest by industry, NEMA adopted a suggested standard for future design defining energy-efficient motors and setting efficiency levels for energy-efficient motors. These standards are as follows:* MG1-2.43 Energy Efficient Polyphase Squirrel-cage Induction Motor. An energy […]

Fundamentals of Electric Motor Drives

Electric machines are an essential part of industry. They provide the necessary mechanical-to-electrical or electrical-to-mechanical conversion. In the United States, more than 50% of the electric power is consumed by electric motors. The motors perform many different functions, from small applications like cooling fans in your personal computer that consume only a few watts of […]