PASCO SE-8658A PERMANENT MAGNET MOTOR - Manual (Page 26)

22
Permanent Magnet Motor 012-07210A
3. Now set the power source to furnish to the brushes on alternating current with a sinusoidal
waveform at a frequency of about 30 Hz and voltage of about 6 volts. Slowly rotate the armature
by hand through one complete revolution.
a) Describe the sensation you feel.
b) Explain why this happens.
4. Repeat this with 30 Hz and 4 volts.
a) Does this feel different? How?
b) Explain why.
5. Repeat with 15 Hz and 6 volts.
a) Does this feel different? How?
b) Explain why.
6. Turn the power off.
7. Set up the photogate or stroboscope to measure the speed of rotation of the motor, following your
teacher’s instructions.
If you are using a photogate, construct a “chopper” of a 7.5 cm piece of card stock to interrupt
the beam of light from the photogate as follows:
Cut a 7.5 cm square from corrugated cardboard and punch a hole that is 1 cm in diameter in the
center (a #4 cork boring tool works well). If the square slips, you may need to secure it with
➤ Troubleshooting: If there was no vibrating sensation in the previous step, either the brushes
were not contacting the slip rings, there was no AC voltage present at the brushes, or some
other defect existed. Recheck the connections and gently bend the brushes inward to make
better contact. If neither of these actions corrects the problem, get assistance. If there was a
vibrating sensation during only part of the rotation, turn off the power, remove the armature,
and examine both the split rings and brushes for corrosion and pitting. Cleaning these with
very fine (600 grit) emery paper will usually correct the problem and result in better