PASCO SE-8658A PERMANENT MAGNET MOTOR - Manual (Page 19)

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012-07210A Permanent Magnet Motor
4. Label the end of the armature that connects to the upper slip ring with a small piece of tape.
5. Gently replace the armature onto the shaft. The dual slip-ring commutator should be down.
Carefully rotating the armature back and forth will often separate the brushes and allow the
commutator to slip down between them; otherwise, insert a pencil or similar object between the
brushes to separate them. Only the most delicate force should be used to avoid bending the
brushes and necessitating adjustments or repairs.
6. Position the armature so that it is at right angles to the N-S orientation of the field magnets. Then
rotate it 90 degrees so that the end of the armature marked with tape is near the north pole of the
magnet. The magnetic field of the magnet may be represented by arrows passing out of the north
pole and into the south pole.
a) What happens to the amount of this magnetic field that passes through the loops of the coils
during your 90-degree rotation above? If the amount changed, did it increase or decrease?
b) What does Faraday’s induction law say about this situation?
7. Continue rotating the armature another 90 degrees.
a) What happens to the amount of this magnetic field that passes through the loops of the coils
during your 90-degree rotation? If the amount changed, did it increase or decrease?
b) What does Faraday’s induction law say about this situation?
c) How would the induced emf during the rotation of step 6, be different from that of step 7?
8. The forces due to Lenz’s law in this equipment are much less than other effects and are not
readily noticeable. Nonetheless, the reasoning that led to Lenz’s law allows you to predict the
direction of current. Consider the 180 degree rotation you performed above:
a) To oppose the motion during the first 90 degrees of rotation, what pole (N or S) would the
taped end of the armature need to be?
➤ To
answer this question, you will need the “right-hand rule”, which can be used to
predict the direction of the magnetic field of a coil. Grasp the coil with the fingers
wrapped around the coil in the direction of the current. The thumb will point in the
direction of the field (i.e., toward the north pole of the coil). Current direction here is
described as being from the positive to the negative (conventional current). Note that
this is opposite to the direction of electron movement.