Electricity for the 4-H Scientist Part 6

Instead of using only one pole of a stationary magnet, we can use both.

This is done by shaping the stationary magnet around the path of the rotating electromagnet. This way we have the benefit of the attracting and repelling forces from both poles. The effect is doubled.

We can also wrap wires around this circular iron and make an electromagnet of it. But when we wire this magnet we use no brushes because we want the current to flow in one direction only.

The stationary electromagnet is called the _field_. The rotating electromagnet is the _armature_.

WHAT TO DO: Make A Motor

_Tools Needed:_

Pocket knife, hammer, vise (or 2 pairs of pliers).

_Materials Needed_:

1 roll of No. 24 enameled wire 1 roll of electrician's tape 3 - 4" (20-penny) nails 4 - 2-1/2" (8-penny) nails 4 - 3" brads (10 penny) Wood board for motor base 2 staples or 4 small brads 2 tacks 2 - 3 volt dry cell batteries (or a 6 volt transformer).

Step No. 1-Armature

Wrap about 1-1/2" of a 4" nail with two layers of tape. This will be the shaft.

The iron core will be made of two pairs of 2-1/2" nails. Wrap tape around each pair with heads and points alternated.

Center both pairs on each side of the shaft. Place them about 1" from the head of the shaft nail. Wrap them together with two layers of tape from tip to tip.

Start at the shaft and wind No. 24 enameled wire to one end and back.

Then do the same on the other end. Always wind in the same direction.

Leave 6" of spare wire at start and finish.

Step No. 2-Commutator

Sc.r.a.pe all insulation off the ends of the wire. Bend the bare ends back and forth as shown. Lay them flat over the taped shaft-one on each side of the shaft.

Hold the commutator down with narrow strips of tape. Wrap tightly near the core and at the opposite end.

Step No. 3-Field

Make the core by bending two 4" nails in the middle at right angles.

s.p.a.ce the heads about 3" apart to form a horseshoe. Wrap together with two layers of tape.

Wind about 400 turns of wire around the center. Leave 4" of spare wire at start and finish. Attach to wood base with staples at each end of the wire. Small brads, bent over, will do just as well.

Step No. 4--Armature Supports and Brushes

Sc.r.a.pe the insulation from the ends of two 6" pieces of wire. Tack them to the base and bend them as shown to make brushes.

Drive two pairs of 3" brads into the base about 3-1/4" apart and in a line midway between the field poles. Wrap wire around the supports to form armature bearings.

Sc.r.a.pe insulation off ends of wire from the field. Connect one end to a brush wire.

_a.s.semble As Shown_

Adjust the position of commutator and tension of brushes against it for best operation.

Take the armature off the motor and connect the commutator wires to a dry cell battery. Test the polarity of each end of the armature with a compa.s.s. Switch the connections on the commutator and test again. See how the compa.s.s needle changes direction?

With the armature still off, connect the field coil directly to the dry cell. Test the polarity of each end of the field with the compa.s.s. How can you reverse the polarity? Try it. It's easy.

Rea.s.semble the motor again and start it. Push the field poles slightly out of alignment with the turning armature. What happens to the motor's speed? Can you tell why?

This time, push the field poles completely out of the way. Test the polarity of the armature as you slowly turn it by hand. Do you see what happens and why it does?

Try to reverse the direction of rotation of your motor by reversing the connections at the battery. What happens? Can you explain why?

Demonstrations You Can Give

Make a display board showing the parts of the toy motor and explain how each part works compared with the parts of a commercial motor.

For Further Information

There are several other types of toy motors you can build. Your club leader or power supplier can help you find information about them.

1. Did your toy motor run?

2. Did your motor speed up or slow down when you pushed the field poles out of line? Why?

3. What happens to the magnetic polarity of the armature when you turn it slowly by hand and check it with a compa.s.s?

4. How can you reverse the direction of rotation of your toy motor?

Is there another way too?

What is it?

LESSON NO. B-6