90. Motors and Generators

A large DC motor, with easily visible coils and split-ring commutator, operates from a 6 V battery. The motor can be rewired so that the field coils are energized. If the armature is now rotated by hand, a DC current is generated.

An AC-DC motor-generator has a single turn armature and both split and slip rings to act as a DC motor or an AC or DC motor or generator.

A hand-crank generator illuminates a neon bulb when cranked vigorously.

A small coil, mounted near the edge of a plexiglass disc, is connected to a peanut bulb. When the disc is spun between the poles of a powerful permanent magnet, the bulb is seen to be lit for that part of the circumference for which the coil passes through the poles of the electromagnet.

A tin can induction motor is demonstrated by holding a tin can on a special handle near two large coils at right angles. The coils are connected to the AC line, but one has a series capacitor to shift its phase 90° with respect to the other. This illustrates the starting circuit of the most common type of electric motor, the induction motor.

The hand-cranked generator is wired through a knife switch to an incandescent bulb. You can use a student volunteer to verify that it is much harder to crank the generator when the bulb is in the circuit, than when no current is being drawn. Ask your class why.

The Magnetic Levitator has two coils at right angles connected to 120 VAC so the coils are 90° out of phase. These will rotate the suspended ball as in the tin can motor.