Electric motor Basics - Reversing polarity with alternating current

Reversing polarity with alternating current

Magnetic polarity is continuously reversed by means of alternating current, (AC). Later, we will see how the rotating magnet is replaced by the rotor by means of induction. Alternating current is important in this regard, so a brief presentation should be useful:

Alternating current

By alternating current, we mean an electrical cur-rent that reverses in intervals and has alternating positive and negative values.

A rotating magnetic field can be created by using three-phase power. This means that the stator is connected to an AC source which supplies three separate current flows (also known as phases), all of them applied to the same circuit. A complete cycle is defined as having 360 degrees, which means that each phase is different from the others by 120 electrical degrees. They are illustrated in the form of sinus curves such as those presented to the right.

The poles change

On the following pages we will explain how the rotor and the stator interact and thus make the motor turn. In order to illustrate this clearly, we have replaced the rotor by a magnet that turns and the stator by a stationary part with coils. The illustration on your right-hand side, should be considered as a two-pole three-phase motor. The phases are connected in pairs like in a real motor; phase 1 consisting of A1 and A2, phase 2 consisting of B1 and B2 and phase 3 consisting of C1 and C2. When current is applied to the stator coils, one coil becomes a north pole and the other becomes a south pole. So, if A1 is a north pole, A2 is a south pole. The principle we can derive from this is that when the current is reversed, the polarity of the poles is also reversed.

                       The stages of movement.

Three-phase AC

Three-phase power is a continuous series of over - lapping alternating current (AC) voltages.

The rotating magnetic field on slow-motion

Applied alternating current

The phase windings A, B and C are placed 120 degrees apart. The number of poles is determined by the number of times a phase winding appears. Here, each winding appears twice, which means that this is a two-pole stator. It follows, then, that if each phase winding appeared four times, it would be a four-pole stator and so on.

When power is applied to the phase windings, the motor starts running with different speeds depending on the number of poles.

The rotor rotates

The following pages deal with how the rotor rotates inside the stator. Again, we have replaced the rotor with a magnet. Of course, all of these changes in the magnetic field occur really fast, so we need a step-by-step breakdown of the course of events.

To the right, we see how the current in winding A1 creates a north pole at this particular point in time. The magnet moves to make its south pole line up with the stator's north pole.

Having begun its rotation the magnet will try to follow the rotating magnetic field of the stator. .

As the purpose of this process is to keep the mag-net moving, the stator field will now change so that the process is continued. This maintains rotation in the same direction.

We have now begun to touch upon the matter of induction. The next section provides much more detail about this concept.

The phase winding's and number of poles

The phase windings A,

B and C are placed 120

degrees apart.

3-phase, 2-pole motor      3-phase, 8-pole motor

When an AC

supply is applied

Current flow in the

positive direction

Current flow

at zero

Current flow in the

negative direction

    Time 1          Time 2          Time 3