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AC Motor - Basic Properties, Terminology and Theory

AC Motors

AC motors convert electric energy into mechanical energy. An AC motor uses alternating current (AC). For AC, the direction of the current changes periodically. In the case of common AC that is used throughout most of the United States, the current changes direction 120 times every second. This current is referred to as "60 cycle AC" or "60 Hertz AC.” Another characteristic of current flow is that it can vary in quantity. For example, the flow can occur in 5 amp, 10 amp, or 100 amp.

It would be rather difficult for the current to be flowing at say 100 amps in a positive direction one moment and then flow at an equal intensity in the negative direction. Instead, as the current is getting ready to change directions, it tapers off until it reaches zero flow and then gradually builds up in the other direction.

The maximum current flow (the peaks of the line) in each direction is more than the specified value (100 amps in this case). Therefore, the specified value is given as an average. What is important to remember is that the strength of the magnetic field, produced by an AC electromagnetic coil, increases and decreases with the increase and decrease of this alternating current flow.

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BASIC AC MOTOR OPERATION

An AC motor has two basic electrical parts: a "stator" and a "rotor" as shown in Figure 8. The stator is in the stationary electrical component. It consists of a group of individual electro-magnets arranged in such a way that they form a hollow cylinder, with one pole of each magnet facing toward the center of the group.

Example of Circuit Breaker Construction

The stator then is the stationary part of the motor. The rotor is the rotating electrical component. It also consists of a group of electro-magnets arranged around a cylinder, with the poles facing toward the stator poles. The rotor is located inside the stator and is mounted on the motor's shaft. The objective of these motor components is to make the rotor rotate which in turn will rotate the motor shaft. This rotation will occur because of the previously discussed magnetic phenomenon that unlike magnetic poles attract each other and like poles repel. If you progressively change the polarity of the stator poles in such a way that their combined magnetic field rotates, then the rotor will follow and rotate with the magnetic field of the stator.

As shown in Figure 9, the stator has six magnetic poles, and the rotor has two poles. At time 1, stator poles A1 and C2 are the north poles (N-pole), and the opposite poles, A2 and C1, are the south poles (S-pole).

AC Motor rotating magnetic field

S-pole of the rotor is attracted by the two N-poles of the stator, and the two south poles of the stator attract the N-pole of the rotor. At time 2, the polarity of the stator poles is changed so that now C2 and B1 are N-poles, and C1 and B2 are S-poles. The rotor then is forced to rotate 60 degrees to line up with the stator poles as shown. At time 3, B1 and A2 are N-poles. At time 4, A2 and C1 are N-poles. As each change is made, the opposite poles on the stator attract the poles of the rotor. Thus, as the magnetic field of the stator rotates, the rotor is forced to rotate with it.

SYNCHRONOUS MOTORS

Synchronous motors employ rotor poles wound with coils, just as the stator poles, and supplied with DC power to create fixed polarity poles. This is how a synchronous AC motor works. A rotating magnetic field in the stator of an AC synchronous motor uses a three-phase power supply for the stator coils.

AC Motor rotating magnetic field

To produce a rotating magnetic field in the stator of a three-phase AC synchronous motor, wind the stator coils properly and connect the power supply leads correctly. The connection for a 6-pole stator motor is shown in Figure 11.

Each phase of the three-phase power supply is connected to opposite poles and the associated coils are wound in the same direction. The polarity of the poles of an electromagnet is determined by the direction of the current flow through the coil. Therefore, if two opposite stator electro-magnets are wound in the same direction, the polarity of the facing poles must be opposite. When pole A1 is N-pole, pole A2 is S-pole, and when pole B1 is N-pole, pole B2 is S-pole, and so forth.

Figure 12 shows how the rotating magnetic field is produced. At time 1, the current flow in the phase "A" poles are positive, and pole A1 is N-pole. The current flow in the phase "C" poles is negative, making C2 an N-pole and C1 is S-pole.

Rotating magnetic field and 3-phase power produced

There is no current flow in phase "B", so these poles are not magnetized. At time 2, the phases have shifted 60 degrees, making poles C2 and B1 both N and C1 and B2 both S. Thus, as the phases shift their current flow, the resultant N and S poles move clockwise around the stator, producing a rotating magnetic field. The rotor acts like a bar magnet, being pulled along by the rotating magnetic field.

INDUCTION MOTORS

Most AC motors used today are not synchronous motors. They are "induction" motors, and they are the workhorses of industry. So how is an induction motor different? The big difference is the manner in which current is supplied to the rotor. There is no external power supply.

Induction is another characteristic of magnetism. It is a natural phenomenon that occurs when a conductor (aluminum bars in the case of a rotor, see Figure 13) is moved through an existing magnetic field or when a magnetic field is moved past a conductor.

AC induction motor rotor construction

In either case, the relative motion of the two causes an electric current to flow in the conductor. This is referred to as "induced" current flow. In an induction motor, the current flow in the rotor is not caused by any direct connection of the conductors to a voltage source, but rather by the influence of the rotor conductors cutting across the lines of flux produced by the stator magnetic fields.

The induced current that is produced in the rotor results in a magnetic field around the rotor conductors (as shown in Figure 14).

Current flow in rotor conductors

This magnetic field around each rotor conductor causes each rotor conductor to act like a permanent magnet (as in the Figure 9 example). As the magnetic field of the stator rotates, because of the three-phase AC power supply, the induced magnetic field of the rotor is attracted and will follow the rotation. The rotor is connected to the motor shaft, so the shaft rotates and drives the connection load.

AC MOTOR TYPES

AC motors are available in 3 types; 3-phase IEC, single-phase NEMA, and three-phase NEMA. These AC motors have horsepower available from one-eighth to 750. Voltages span from 115-575. Before making your final selection, consult a motor application specialist. Proper motor sizing can save energy and reduce costs over time for your system's operation.

AC Motor rotating magnetic field
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