The squirrel-cage motor
Suppose that a wire not connected to a battery is in a region where the magnetic field is
changing. As a result of the change in magnetic field, a current is induced in the wire.
Because there is a current in the wire—that is, since there are moving charges in a region
containing a magnetic field—there is a magnetic force on the wire causing it to move. If the
wire is bent into a squared-off U shape and the ends of the U are held on supports, the
wire pivots around the supports.
Suppose another U is connected rigidly 180 degrees away from the first. If the magnetic
field is changing uniformly, there will be the same induced current and the same force on
each. They would then try to swivel in opposite directions, and no net motion would be
possible.
If the field is not uniform, the magnetic field is different at the position of different wire
rods (see Figure E04.4.1). The forces at the positions of the different wire rods are
therefore different. If the magnetic field is stronger near the bottom wire, for example, the
magnetic force will be stronger on the wire on the bottom, causing it to begin to rotate
(Figure 4.4.1 a). Now, if similar U-shaped pairs are placed around the periphery, as in
Figure 4.4.1 b, the wires will start to rotate and will continue to do so. This field
imbalance will be enough to start the cage rotating and allow it to maintain its rotational
motion. This is the principle of operation of the squirrel-cage motor shown in Figure
E04.4.1 c.
Energy, Ch. 4, extension 4 The squirrel-cage motor
Fig. E04.4.1 The principle of electric motors.
a. A wire loop in a nonuniform magnetic field.
b. Several wire loops in a nonuniform magnetic field.
c. Squirrel cage configuration, for a squirrel-cage motor.
The squirrel-cage motor is an example of a “shaded pole” induction motor. The shading of
the pole supplies the nonuniformity of the magnetic field.
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Energy, Ch. 4, extension 4 The squirrel-cage motor
This changing field causes the cylinder to rotate. A picture of such a motor is shown in
Figure E04.4.2. The squirrel cage is made of hollow copper rods soldered to two disks
making a cage, which is free to rotate. A solid plate of copper is being held just above a
coil surrounding an iron core. The coil is connected to an AC source, so the current is
changing in the coil and so the magnetic field in the iron core also changes. The solid plate
affects the magnetic field and makes it non-uniform, which leads to differing forces on the
rods of the cage (see also Figure E04.4.1 c). Even if the squirrel-cage were a solid cylinder,
it would rotate in the same way. The section of the cylinder inside the region of magnetic
field is often made of laminated segments to reduce Joule heating (I2 R) losses from the
induced currents.
Fig. E04.4.2 A squirrel-cage motor in use. A coil is in the cylinder immediately under the platform. A
piece of steel rod runs through the center of this coil, and is an electromagnet. The magnetic field is
changing in the piece of steel because alternating current is fed into the coil through the two wires shown.
The copper disk held in my left hand distorts the magnetic field so it is different at different locations
nearby. The squirrel cage is held in the holder by bearings (one is visible near the top of the aluminum
piece. The plastic handle of the holder is being held by my right hand.
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Energy, Ch. 4, extension 4 The squirrel-cage motor
The AC motor is an induction motor. The squirrel-cage motor is different from a DC
motor.
Fig. E04.4.3 A DC motor.
Consider the motor shown in Fig. E04.4.3. A current flows in the circuit due to the
battery. The small metal plates touch a split cylinder. The halves of the cylinder are
connected by separate wires running from each half to a piece of iron (this arrangement
supplies a continuous flow of current) and magnetizes the piece of iron. Because the piece
of iron is between the pole pieces of the magnet, it experiences a magnetic force that will
lead it to rotate. The shaft can be hooked to a machine and do work. It is necessary to
switch the direction of the current to maintain the rotational motion in the same direction.
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