AC Motor Diagrams - Basic Stator and Rotor Operation
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
AC Motor Stator
AC Motor Rotor
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.
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.
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.
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