Lesson 2
PURPOSE OF CRANKING MOTORS
Motors, like generators, are simply a means of changing energy from one form to another. In a
generator, we take the mechanical energy of the turning pulley and change it to electrical energy. A
cranking motor does just the opposite of the generator. Electrical energy sent to the motor is
changed to mechanical energy to crank the engine. A practical motor must produce continuous
rotary motion. In addition, it must develop a twisting or turning force called torque. In this lesson,
we will see how the starter motor develops torque and how it is used to crank the engine.
PRINCIPLES OF MOTORS
The magnetic principle of attraction and repulsion, or unlike poles attract and like poles repel, is the
principle applied in the development of the electric motor. Remember that a wire carrying an
electric current produces a magnetic field. When this wire is placed in the magnetic field of
another magnet, mechanical motion is produced because the magnetic field around the wire is
repulsed (pushed away) by the field around the other magnet.
Mechanical Motion Produced by Magnetic Repulsion
Lines of force move from the north pole to the south pole and travel in almost straight lines. In
fact, the lines would be straight if the ends of the magnets were flat instead of curved. The
magnetic lines of force moving between the north pole and the south pole of any magnet always
take the easiest path or route. The easiest path between the two poles is usually a straight line,
because a straight line is also the shortest path.
Remember, each of the magnetic lines of force moves parallel (side by side) to the other lines of
force. They will not cross each other.
The lines of force act a lot like rubberbands. If you stretch the bands between two pegs, they tend
to straighten out. Push down on the stretched rubberbands with your finger. If the bands are
stretched tight, you can feel them pushing back against your finger. Now move your finger away
quickly. The rubberbands will snap back to form straight lines again. Think of the magnetic lines
of force between the two poles of a magnet acting the same way as do the stretched bands.
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