Powering industry since replacing the steam engine
From tiny hysteresis motors that drive electric clocks to huge synchronous motors that develop more than 50,000 hp, the alternating current (AC) motor has become an indispensable part of everyday life. These omnipresent devices come in many varieties, but generally fall into two categories: synchronous and induction machines. The term “machine” emphasizes the fact that these devices can operate in two different modes. Electrical power causes the device to produce torque, making it a motor. But when the same machine receives input rotational energy, electricity is produced and it becomes a generator.
All rotating machines have two main sets of windings. One set of windings is mounted on a freely rotating shaft, or rotor. The other set, called the stator, is attached to the motor frame and remains stationary. In the case of a motor, the stator windings are energized with single-phase or polyphase voltage. Single-phase voltage is adequate for small machines up to about 5 hp, but polyphase voltages provide superior torque characteristics and simplify the starting process. Larger machines commonly use 3-phase voltage.
The current flowing through the stator windings produces a magnetic field, the polarity of which rotates about the center axis of the stator. When the rotor windings are magnetized, pairs of magnetic poles form and the rotor spins to keep the rotor poles aligned with the stator field. The number of poles determines the speed of the rotor, according to the equation
n = (120 x f)/p
where (n) is the synchronous speed in revolutions per minute, (f) is the electrical frequency in hertz, and (p) is the number of poles. When the machine is used as a generator, the “motoring” process basically is reversed.
Many generators and very large motors are categorized as synchronous machines, in which the rotor turns at the same speed as the stator field. This is desirable when the rotational speed or generated frequency must be precisely controlled. The synchronous design also lends itself to large devices. The largest synchronous generators are rated in excess of 2 million hp. But a means of applying voltage to the rotor windings is required, usually using carbon brushes and slip rings.
The slip ring is a conductive surface on the rotor shaft that is electrically connected to the rotor windings. The brushes are stationary and held snugly against the slip rings by spring pressure. These components complicate the motor construction, add cost to the motor design, and require considerable maintenance. As a result, a majority of the AC machines in service, particularly small- to medium-sized motors, belong to the other general category — the induction machine.
The induction machine uses electromagnetic induction to energize the rotor circuit. To further economize the design, most induction motors use cast rotor bars instead of a wound rotor. Because of their unique appearance, motors that use these rotor bars are called “squirrel cages” (Photo above).
The induction machine is essentially a transformer with the secondary winding free to rotate. The speed of rotation, however, isn't the same as that of the rotating stator field. The difference is called the slip, and determines the torque produced. A speed-torque curve illustrates the relationship between rotor speed and torque, and is essential when specifying a motor to drive a particular load.
Countless AC motor designs have come and gone over the years, but the principles of operation are common to all.