PASCO SE-8657 MOTOR ACCESSORY - Manual (Page 11)

7
012-06247A Motor Accessory
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EQUIPMENT NEEDED:
•
Motor Accessory
•
multimeter
•
Variable Gap Magnet
•
patch cords
•
low voltage DC power supply, limited to 1 A
•
small piece of masking tape
Purpose
The purpose of this experiment is to demonstrate the operation of the DC motor in terms of
basic concepts of electromagnetism.
Theory
The Variable Gap Magnet is a permanent magnet possessing a north pole and a south pole
that interact with the north and south poles of the armature (an electromagnet when con-
nected to an electric current). Like poles repel, while unlike poles attract. The armature
rotates until its north pole is as close as possible to the south pole of the permanent magnet
(and also as far as possible from the north pole). Instead, if the rotational speed of the
armature matches the frequency of the alternating current, the direction of current in the
armature will reverse at that instant, so that the torque continues to act in the original direc-
tion.
A better explanation involves an understanding of fields. The variable gap magnet pro-
duces a magnetic field that passes through the gap between the pole pieces. When current
passes through the turns of the armature in the presence of the field, forces act to cause a
torque that rotates the armature. Inertia carries the armature past the position of no torque
to the point where the torque would force the armature back in the other direction. How-
ever, at that point the commutator reverses the direction of current in the armature so the
torque continues to act in the original direction.
Setup
➀ Be sure you have the flat iron pole pieces placed on the two neodymium magnets of the
Variable Gap Magnet. (The iron pole pieces spread the magnetic field over a wider
area.) Screw the larger threaded portion of the shaft into the threaded hole in the magnet
base. Insert the threaded end of the shaft from above, screwing it in until 1 mm, or
slightly less, of the threaded portion remains above the upper surface of the base.
➁
Turn the magnet over and screw the retaining nut onto the smaller diameter threaded
portion of the shaft that protrudes through the bottom of the magnet base. Use firm
finger pressure. Do not over tighten.
Experiment 1: Operation of the DC Motor