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Facts At Your Fingertips: Alternating-Current Induction Motors

| By Scott Jenkins

Alternating-current (a.c.) induction motors are widely used throughout the chemical process industries (CPI) and other manufacturing sectors to convert electrical energy into mechanical energy to power rotating equipment, such as centrifugal pumps, compressors and fans, as well as other industrial machinery. This one-page reference provides information on the construction and operation of three-phase induction motors (Figure 1).

FIGURE 1. In this cutaway image of the interior of an electrical induction motor, the rotor can be seen inside the stator, which surrounds it

Motor components

The basic construction of a typical industrial a.c. induction motor includes the following elements: rotor, stator, stator windings and enclosure.

Rotor. The rotor has an iron core made from a cylinder-shaped stack of laminated rings around a motor shaft to which it is attached. The rotor has conducting end caps on either end and conducting bars that run through slots in the laminated metal stack between the end caps. The assembly rotates inside the stator on bearings, which ensure that the rotor remains centrally positioned with the stator. The appearance of the rotor and conducting bars gives rise to the name “squirrel-cage motor.”

Stator. The stator is made from a series of stacked steel sheets that are ring-shaped and encircle the rotor, while allowing the rotor to move freely. The stacked metal slices are laminated with insulating material and have spaces cut out around their diameter to accommodate the copper windings.

Windings. Windings made from copper wire are distributed in the slots on the interior of the stator to carry the supply electrical current that will induce a magnetic field that will penetrate the rotor inside.

Enclosure. The enclosure, consisting of a frame and end bells, protects the motor. There are several types of enclosures for different applications, as designated by the National Electrical Manufacturers Association (NEMA; Rosslyn, Va.; www.nema.org), a trade association and standards-making body. The four main NEMA enclosures are the following: open, drip-proof (ODP) enclosures; totally enclosed, non-ventilated (TENV); totally enclosed, fan-cooled (TEFC); and totally enclosed, blower-cooled (TEBC) motors.

Induction motor operation

With correct design of the stator windings and stator slots, applying alternating current to the stator will generate a rotating magnetic field. When electrical current is applied, the stator produces a rotating magnetic field needed to rotate the motor shaft. Typically, three-phase a.c. electric power is supplied to the stator so that the three phases are electrically separated from each other by 120 deg.

Meanwhile, the rotor sits inside the stator core and its design gives it the ability to conduct electromagnetic current. As the rotating magnetic field moves about the rotor, it induces voltage in the conducting bars of the rotor. Because of the conductive end caps on each end of the rotor, current can flow though the rotor bars. The rotor then produces a magnetic field opposed to that of the stator. The opposing magnetic fields set up a situation where the opposite poles of the rotor and stator attract each other while the like poles repel. As the stator’s magnetic field rotates, the rotor chases it, driven by the attractive and repellent magnetic forces.

Motor terminology

The following terms are useful for understanding the real-world operation of an induction motor.

Magnetic poles. The stator can be designed to have a varying number of magnetic poles around the stator body. The number of poles has an impact on the speed and torque of the motor. For a smaller number of poles, the speed of the rotating magnetic field is faster, but the torque is lower. Adding magnetic poles lowers the speed of the magnetic field, but increases the level of torque that is possible for the motor to generate. The manufacturing cost for the motor is higher for larger numbers of poles, so most motors are two- or four-pole motors.

Synchronous speed. This is the speed of the rotating magnetic field in the stator. Synchronous speed is calculated by the equation Na = 120 f/ P, where f is the frequency of the a.c. supplied to the stator and P is the number of motor poles. For example, the synchronous speed of a four-pole motor powered by 60-Hz a.c. current would be 1,800 rpm.

Rated speed. The rated speed is the rotational velocity of the rotor inside the motor housing. The rated speed is always less than the synchronous speed because the rotor always rotates slower than magnetic field of the stator. In fact, the rotation speed of the rotor must be lower than the synchronous speed or else there would be no induction and the rotor would not be able to create a magnetic field.

Slip. The slip is the difference between the speed of the rotating magnetic field of the stator and the mechanical speed of the rotating rotor assembly. The size of the slip depends on the load that is on the motor. For a larger load on the motor, the slip will be greater than for smaller loads.

Torque. The term torque refers to the rotational force generated by the rotating motor. Induction motors draw more current as the load is increased, and consequently produce more torque.

Editor’s note: Some material for this column is from Yaskawa America Inc., Induction Motor Basics, video e-learning module, accessed at www.yaskawa.com/support-training/training/elearning-curriculum