Loop in magnetic field.
g. In figure 2 we have added commutator bars and a larger field. In view A
the left half of the loop is shown in black with an arrow to show the current is
flowing from its commutator bar to the loop. The right side loop is pictured in
white with an arrow showing the current flowing from it to its commutator bar.
With battery current supplied through the brushes to the commutator bars, the loop
would be forced to rotate clockwise. When the loop reaches and just passes the
position shown in view B, look what happens. The commutator bar for the black half
of the loop is now in contact with the positive (+) brush. The commutator bar
connected to the white half of the loop is now in contact with the negative (-)
brush. We have reversed polarity in the loop and it will continue to rotate
clockwise. When it rotates another 1/2 turn (180), it will again reverse polarity
and continue to rotate clockwise.
h. Of course a single loop would not produce enough torque to crank the
engine. But, by using many loops, each with its own commutator bars, we can have a
cranking motor that will produce all of the torque needed.
i. Most starter motors are series motors (fig 3). They are called series
motors because the rotating loop and the windings around the magnetic poles are
connected in one (series) path. The current flowing through the loop also flows
through the windings. In an actual motor, the windings around the pole shoes are
called field windings because they help produce the magnetic field. The purpose of
the field winding is to produce a strong magnetic field so that the loop will
receive a more powerful push. Note that the poles are curved. They are curved so
the conductors of the loop can pass as close as possible to the poles as they move
past. Since the magnetic field is strongest near the poles, the conductors in the
loops are given a stronger push.
j. In the actual starter motor there are many rotating loops assembled into
an armature. The armature consists of a shaft on which are mounted a laminated
iron core and commutator. The loops, or windings, of the armature are mounted in
the core and are insulated from one another and from the core. The commutator
segments have riser bars, like the generator, to which the ends of the armature
OS 010, 6-P3