PASCO SE-8658A PERMANENT MAGNET MOTOR - Manual (Page 31)

012-07210 A Permanent Magnet Motor
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Part B: DC Generator
1. a) The voltage and current produced is always in the same direction (direct current) but is pul-
sating, not steady. (The pulsating nature will be difficult to note with a digital meter.) As-
suming as before that the negative lead of the meter is connected to the upper brush and the
positive lead is connected to the lower brush, then if armature is rotated clockwise (as
viewed from above) the meter will read positive.
b) It responds similarly to a but in the opposite direction.
c) Just as the current is about to reverse direction (as, for example, between steps 10 a and b),
the commutator reverses the connections between the coil and the brushes in order to main-
tain the direction of the current.
d) pulsating DC
2. a) The effect would be greater voltage and current.
b) Answers will vary. Two possible explanations include: (1) The free electrons present in the
wire of the coil move through the wire as a current due to the force given by
F=qV x B, where the force is seen as being proportional to the velocity of the wire, and thus
the electrons contained in it. At greater rotational speeds, the velocity of the wire would be
greater, and thus the force causing electron movement would also be greater. (2) Faraday’s
law states that the emf induced is proportional to the rate at which the flux in the loops of
the coils is changing. At higher rotational speeds, the rate of change of the flux is greater,
and thus so is the emf.
3. a) The graph of voltage versus time would be a “sine wave” whose amplitude and “wave-
length”, or
period, is decreasing to zero.
b) It would be the same as a except all portions that would be below the horizontal (time) axis are
instead reflected above it.
c) It would be the same as b except everything is below the axis.
Experiment 3
Notes concerning the setup:
• If the power source does not limit the current to 1 A, use an appropriate resistor in series
to limit the current.
• If the AC power source does not quantify the current, use an appropriate ammeter, or
calculate the current by measuring the voltage drop across a resistor of 0.51 ohm and a
power rating of 1 watt wired in series.
• Remind students not to prolong situations when the armature is not spinning and the
power is connected—the coils will overheat.
Answer To Questions:
Part A
5. a) Yes.
b) Yes. Since the coils are wound such that the north poles of each are in the same direction,
they work together to produce a net magnetic flux for the armature that is the sum of the
magnetic flux of each of the coils.
c) True