Polyphase motor
design
Perhaps the most important benefit of
polyphase AC power over single-phase is the design and operation of AC
motors. As we studied in the first chapter of this book, some types of
AC motors are virtually identical in construction to their alternator
(generator) counterparts, consisting of stationary wire windings and a
rotating magnet assembly. (Other AC motor designs are not quite this
simple, but we will leave those details to another lesson).
If the rotating magnet is able to keep up
with the frequency of the alternating current energizing the
electromagnet windings (coils), it will continue to be pulled around
clockwise. However, clockwise is not the only valid direction for this
motor's shaft to spin. It could just as easily be powered in a
counter-clockwise direction by the same AC voltage waveform:
Notice that with the exact same sequence
of polarity cycles (voltage, current, and magnetic poles produced by the
coils), the magnetic rotor can spin in either direction. This is a
common trait of all single-phase AC "induction" and "synchronous"
motors: they have no normal or "correct" direction of rotation. The
natural question should arise at this point: how can the motor get
started in the intended direction if it can run either way just as well?
The answer is that these motors need a little help getting started. Once
helped to spin in a particular direction. they will continue to spin
that way as long as AC power is maintained to the windings.
Where that "help" comes from for a
single-phase AC motor to get going in one direction can vary. Usually,
it comes from an additional set of windings positioned differently from
the main set, and energized with an AC voltage that is out of phase with
the main power:
These supplementary coils are typically
connected in series with a capacitor to introduce a phase shift in
current between the two sets of windings:
That phase shift creates magnetic fields
from coils 2a and 2b that are equally out of step with the fields from
coils 1a and 1b. The result is a set of magnetic fields with a definite
phase rotation. It is this phase rotation that pulls the rotating magnet
around in a definite direction.
Polyphase AC motors require no such
trickery to spin in a definite direction. Because their supply voltage
waveforms already have a definite rotation sequence, so do the
respective magnetic fields generated by the motor's stationary windings.
In fact, the combination of all three phase winding sets working
together creates what is often called a rotating magnetic field.
It was this concept of a rotating magnetic field that inspired Nikola
Tesla to design the world's first polyphase electrical systems (simply
to make simpler, more efficient motors). The line current and safety
advantages of polyphase power over single phase power were discovered
later.
What can be a confusing concept is made
much clearer through analogy. Have you ever seen a row of blinking light
bulbs such as the kind used in Christmas decorations? Some strings
appear to "move" in a definite direction as the bulbs alternately glow
and darken in sequence. Other strings just blink on and off with no
apparent motion. What makes the difference between the two types of bulb
strings? Answer: phase shift!
Examine a string of lights where every
other bulb is lit at any given time:
When all of the "1" bulbs are lit, the
"2" bulbs are dark, and visa-versa. With this blinking sequence, there
is no definite "motion" to the bulbs' light. Your eyes could follow a
"motion" from left to right just as easily as from right to left.
Technically, the "1" and "2" bulb blinking sequences are 180o
out of phase (exactly opposite each other). This is analogous to the
single-phase AC motor, which can run just as easily in either direction,
but which cannot start on its own because its magnetic field alternation
lacks a definite "rotation."
Now let's examine a string of lights
where there are three sets of bulbs to be sequenced instead of just two,
and these three sets are equally out of phase with each other:
If the lighting sequence is 1-2-3 (the
sequence shown), the bulbs will appear to "move" from left to right. Now
imagine this blinking string of bulbs arranged into a circle:
Now the lights appear to be "moving" in a
clockwise direction because they are arranged around a circle instead of
a straight line. It should come as no surprise that the appearance of
motion will reverse if the phase sequence of the bulbs is reversed.
The blinking pattern will either appear
to move clockwise or counter-clockwise depending on the phase sequence.
This is analogous to a three-phase AC motor with three sets of windings
energized by voltage sources of three different phase shifts:
With phase shifts of less than 180o
we get true rotation of the magnetic field. With single-phase motors,
the rotating magnetic field necessary for self-starting must to be
created by way of capacitive phase shift. With polyphase motors, the
necessary phase shifts are there already. Plus, the direction of shaft
rotation for polyphase motors is very easily reversed: just swap any two
"hot" wires going to the motor, and it will run in the opposite
direction!
- REVIEW:
- AC "induction" and "synchronous"
motors work by having a rotating magnet follow the alternating
magnetic fields produced by stationary wire windings.
- Single-phase AC motors of this type
need help to get started spinning in a particular direction.
- By introducing a phase shift of less
than 180o to the magnetic fields in such a motor, a
definite direction of shaft rotation can be established.
- Single-phase induction motors often
use an auxiliary winding connected in series with a capacitor to
create the necessary phase shift.
- Polyphase motors don't need such
measures; their direction of rotation is fixed by the phase sequence
of the voltage they're powered by.
- Swapping any two "hot" wires on a
polyphase AC motor will reverse its phase sequence, thus reversing its
shaft rotation.
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