An AC Motor converts electric energy into mechanical energy. An AC Motor uses alternating current - in other words, the direction of current flow changes periodically. In the case of common AC that is used throughout most of the United States, the current flow changes direction 120 times every second. This current is referred to as "60 cycle AC" or "60 Hertz AC" in honor of Mr. Hertz who first conceived the AC current concept. 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 electro-magnetic coil, increases and decreases with the increase and decrease of this alternating current flow.
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. The term, "stator" is derived from the word stationary. The stator then is the stationary part of the AC 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 AC motor's shaft. The term "rotor" is derived from the word rotating. The rotor then is the rotating part of the AC motor. 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 A-1 and C-2 are north poles and the opposite poles, A-2 and C-1, are south poles. The 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 C-2 and B-1 and N-poles and C-1 and B-2 are S-poles. The rotor then is forced to rotate 60 degrees to line up with the stator poles as shown. At time 3, B-1 and A-2 are N. At time 4, A-2 and C-1 are N. 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.
AC Motor Stator
One way to produce a rotating magnetic field in the stator of an AC Motor is to use a three-phase power supply for the stator coils. To produce a rotating magnetic field in the stator of a three-phase AC motor all that needs to be done is wind the stator coils properly and connect the power supply leads correctly. The connection for a 6-pole stator 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 electro-magnet 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 A2 is S and when pole B1 is N, B2 is S and so forth.
Figure 12 shows how the rotating magnetic field is produced within an AC Motor. At time 1, the current flow in the phase "A" poles is positive and pole A-1 is N. The current flow in the phase "C" poles is negative, making C-2 a N-pole and C-1 is S. 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 C-2 and B-1 both N and C-1 and B-2 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.
AC Motor Rotor
Up to this point not much has been said about the rotor. In the previous examples, it was assumed the rotor poles were wound with coils, just as the stator poles, and supplied with DC to create fixed polarity poles. This, by the way, is exactly how a synchronous AC motor works. However, most AC motors used today are not synchronous motors. Instead, so-called "induction" motors 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. This is no external power supply. As you might imagine from the motor's name, an induction technique is used instead.
Induction is another characteristic of magnetism. It is a natural phenomenon, which 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. 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 other words, 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 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. This magnetic field around each rotor conductor causes each rotor conductor to act like the permanent magnet in the Figure 9 example. As the magnetic field of the stator rotates, due to the effect 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.
