PASCO SE-8658A PERMANENT MAGNET MOTOR - Manual (Page 24)

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Permanent Magnet Motor 012-07210A
allow the commutator to slip down between them. If necessary, insert a pencil or similar
object down between the brushes. Use only the most delicate force to avoid bending the
brushes and necessitating adjustments or repairs.
Procedure—Part A
1. Remove the armature from the shaft by grasping it between your thumb and forefinger and
rotating it back and forth while lifting gently. If necessary, insert a pencil between the
brushes to gently separate them so they don’t prevent removal of the armature.
2. Examine the armature closely and imagine current entering
one of the two slip rings from a brush. Trace the path of the
current through the wire to the coil, through the coil,
through the wire to the coil on the opposite side of the
armature, through that coil, and through the wire to the
other slip ring and into the second brush. By carefully
examining the part of the coils where the leads emerge from
the coil, you should be able to determine the direction in
which the wire is wound on the coil (Figure 3.2).
3. Holding the armature in one hand, follow the wire from the
slip ring to the left coil of the armature and note the
direction the wire is wound in the coil. Note where the wire
enters and exits the coil.
4. Imagine that the AC current is in the positive half of the
waveform. This means that conventional current comes out
of the terminal marked positive of the power supply and
enters the terminal marked negative. Use the right-hand
rule to determine the direction the magnetic field will flow
at that instant: Grasp the coil with your fingers wrapped around the coil in the direction of the
current. (Current direction is described by convention as pointing from positive to negative.
Note that this is opposite to the direction of electron movement.) Your thumb will point in
the direction of the field (that is, toward the north pole of the coil). Put a small piece of tape
on the end of the coil that would be its north pole at that instant.
5. Follow the wire over to the right coil. Use the right-hand rule to find the direction of the
north pole.
a) In this situation, is the direction of the north pole the same for the right and left coils?
b) In that case, can you say that the north pole of either of the coils is in the same direction as
the north pole of the armature? Explain why.
c) True or False: In this AC motor, we can determine the direction of the magnetic field of
the armature at any instant by determining the direction of the north pole of either of the
two coils.
6. Imagine that the AC current is in the negative half of the waveform. This means that
conventional current comes out of the negative (black) terminal of the power supply and
enters the positive (red) terminal. Repeat step 5 to determine which end of the armature would
be its north pole at that instant.
In this case, is magnetic north on the same or opposite end of the armature?
Figure 3.2
Direction of the Wire Winding
on the Coil