Basic AC Generator

Basic AC Generator

The basic components and operation of an AC (Alternating Current) generator are shown here. Its operation applies the principle of electromagnetic induction as previously explained.

In this case the moving permanent magnet is the armature and the stationary non-permanent magnet is the stator.

In the graph the red curve indicates strength if the field induced by the stator. Note how the induced field strength changes in both magnitude and polarity as the armature magnet rotates. This is illustrated by the changing size of the N and S.

The blue curve indicates the output voltage which is proportional to the rate of change of the field strength.

Note how the output voltage is related to the rotation of the armature magnet. As either pole of the armature magnet swings nearest a pole of the stator (points A/E and C) the rate of change of the strength of the induced magnetic field in the stator is smallest and the resulting output voltage is passing through zero.   On the other hand, when the swinging armature magnet is at right angles to the poles of the stator (points C and D), the induced flux is changing most rapidly and the voltage across the coil is at its highest value (positive or negative).

As the armature completes one revolution after another, the two curves on the graph repeat themselves. The form of these curves in known as a sine curve (or sine wave). One complete cycle of the sine curve relates to one revolution of the armature or 360 degrees of rotation. We can see that the voltage curve is a quarter of a cycle behind the field strength curve. In other words, the two curves are out of phase by 90 degrees.

AC generators with permanent magnet armatures are generally small such as bicycle generators (in the pre-LED era). Large AC generators, such as those used for power generation, do not have permanent magnet armatures. They have an electromagnet powered by a small DC generator (called an exciter) usually located on the drive shaft.

Electromagnetic Induction
Moving Magnet
Field Strength Curve Detail
The animated illustration above demonstrates the principles of electromagnetic induction on which electric generators are based.
The moving magnet is a permanent magnet.
The stationary magnet is a non-permanent magnet made of soft iron. It becomes magnetized only when immersed in the magnetic field that surrounds the passing permanent magnet.  The strength of this induced magnetic field rises from a relatively low value to a maximum density and then falls back to the low value again. This is indicated by the N and S that temporarily appear while the moving permanent magnet is at its nearest.
In the graph the red curve shows the strength of the induced magnetic field. The blue curve shows the electric potential (voltage) that appears across the coil. This voltage is proportional to the rate of change of the induced field strength. The generation of an electric potential by an induced and changing magnetic field is known as electromagnetic induction.
Consider the strength of the induced magnetic field as the moving permanent magnet passes from point A to point E. The field strength first rises from nearly zero slowly at first. But, as point B is reached, the field strength rises at a faster and faster rate. This rate of increase is highest at point B as is shown by the slope of the tangent line b. This is where the output voltage is at its maximum positive value.
From point B to C the rate of increase if the induced magnetic field decreases until it reaches zero at point C. The slope of the tangent line, c, at point C is zero. The corresponding output voltage is zero at point C also.
From point C to point E the situation is reversed - the field strength falls back to nearly zero at point E. Its maximum rate of fall is at point D where the voltage is at its maximum negative value as indicated by the negative slope of the tangent line d.
Note that the magnetic field strength curve (red curve) as shown here is generalised - the actual shape will depend on: (1) the amount of separation between the moving permanent magnet and the stationary soft iron core magnet, (2) the shape and size of both magnets and (3) the rate of travel of the moving magnet.