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A short explanation of operation is that the stator creates a
rotating magnetic field which drags the rotor around.
The theory of operation of induction motors is based on a rotating
magnetic field. One means of creating a rotating magnetic field is to
rotate a permanent magnet as shown in Figure below. If the moving
magnetic lines of flux cut a conductive disk, it will follow the motion
of the magnet. The lines of flux cutting the conductor will induce a
voltage, and consequent current flow, in the conductive disk. This
current flow creates an electromagnet whose polarity opposes the motion
of the permanent magnet-- Lenz's Law. The polarity of the
electromagnet is such that it pulls against the permanent magnet. The
disk follows with a little less speed than the permanent magnet.
Rotating magnetic field produces torque in
conductive disk.
The torque developed by the disk is proportional to the number of
flux lines cutting the disk and the rate at which it cuts the disk. If
the disk were to spin at the same rate as the permanent magnet, there
would be no flux cutting the disk, no induced current flow, no
electromagnet field, no torque. Thus, the disk speed will always fall
behind that of the rotating permanent magnet, so that lines of flux cut
the disk induce a current, create an electromagnetic field in the disk,
which follows the permanent magnet. If a load is applied to the disk,
slowing it, more torque will be developed as more lines of flux cut the
disk. Torque is proportional to slip, the degree to which the
disk falls behind the rotating magnet. More slip corresponds to more
flux cutting the conductive disk, developing more torque.
An analog automotive eddy current speedometer is based on the
principle illustrated above. With the disk restrained by a spring, disk
and needle deflection is proportional to magnet rotation rate.
A rotating magnetic field is created by two coils placed at right
angles to each other, driven by currents which are 90o out of
phase. This should not be surprising if you are familiar with
oscilloscope Lissajous patterns.
Out of phase (90o) sine waves produce
circular Lissajous pattern.
In Figure above, a circular Lissajous is produced by driving the
horizontal and vertical oscilloscope inputs with 90o out of
phase sine waves. Starting at (a) with maximum “X” and minimum “Y”
deflection, the trace moves up and left toward (b). Between (a) and (b)
the two waveforms are equal to 0.707 Vpk at 45o.
This point (0.707, 0.707) falls on the radius of the circle between (a)
and (b) The trace moves to (b) with minimum “X” and maximum “Y”
deflection. With maximum negative “X” and minimum “Y” deflection, the
trace moves to (c). Then with minimum “X” and maximum negative “Y”, it
moves to (d), and on back to (a), completing one cycle.
X-axis sine and Y-axis cosine trace circle.
Figure above shows the two 90o phase shifted sine waves
applied to oscilloscope deflection plates which are at right angles in
space. If this were not the case, a one dimensional line would display.
The combination of 90o phased sine waves and right angle
deflection, results in a two dimensional pattern-- a circle. This circle
is traced out by a counterclockwise rotating electron beam.
For reference, Figure below shows why in-phase sine waves will not
produce a circular pattern. Equal “X” and “Y” deflection moves the
illuminated spot from the origin at (a) up to right (1,1) at (b), back
down left to origin at (c), down left to (-1.-1) at (d), and back up
right to origin. The line is produced by equal deflections along both
axes; y=x is a straight line.
No circular motion from in-phase waveforms.
If a pair of 90o out of phase sine waves produces a
circular Lissajous, a similar pair of currents should be able to produce
a circular rotating magnetic field. Such is the case for a 2-phase
motor. By analogy three windings placed 120o apart in space,
and fed with corresponding 120o phased currents will also
produce a rotating magnetic field.
Rotating magnetic field from 90o phased
sine waves.
As the 90o phased sine waves, Figure above, progress from
points (a) through (d), the magnetic field rotates counterclockwise
(figures a-d) as follows:
- (a) φ-1 maximum, φ-2 zero
- (a') φ-1 70%, φ-2 70%
- (b) φ-1 zero, φ-2 maximum
- (c) φ-1 maximum negative, φ-2 zero
- (d) φ-1 zero, φ-2 maximum negative
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