real electric motors work
home should be without one – or maybe a dozen. You'll
find an induction motor in the fan, fridge,
washing machine, dishwasher, clothes drier, and the little
pump that circulates water in the fish tank to stop the
water turning green and the fish going belly-up. Chances
are there's also one in the air conditioner – unless
it's a particularly high-tech one.
• Long lasting
• Creates no interference
• Wants to turn at constant speed (50Hz divided by
half the number of poles)
• Cannot turn faster than 1500rpm (4-pole motor)
• Draws a massive starting current, or is inefficient,
• Kind of big and bulky for the power it develops
one came out of a fan.
the bearings and end-caps of the motor have already been
removed. (In retrospect, I should have used something more
delicate than an axe to disassemble the fan.) We can pull
the rotor out and this is what we're left with. There are
four windings, and they are all simply in series.
not quite simply – the current comes in the white
wire, then the first winding (top right) is clockwise, the
next one (bottom right) is anticlockwise, bottom left is
clockwise again, top left is anticlockwise, then out the
other white wire. So, imagine a positive half-cycle of the
mains, with the current actually coming in that first wire.
The first winding produces a north pole facing in; the second
a south pole facing in; etc, like this: N-S-N-S.
a mains cycle later (10 ms) the current has reversed and
so must the magnetic sense of the poles, which are now:
S-N-S-N. The rotor is an electrical conductor, and therefore
tries to follow this field. To do so it has to rotate through
90 degrees. The rotor thus takes two full cycles of the
mains (40 ms) to make a complete rotation, and so revolves
at 1500 rpm. At least, it would if it could keep up with
the rotating field. But it can't, quite, and in fact it's
only because it's slipping behind that any torque is developed
at all. So, it rotates a bit slower than 1500 rpm (typically
1440 rpm) depending on how much torque it is being called
upon to produce.
that the motor, as described so far, could rotate happily
clockwise or anticlockwise. This kind of motor therefore
needs some kind of internal cleverness to ensure it only
turns in the right direction. This is achieved, in this
motor, by the use of shaded poles.
the winding at the top of the picture. See how there is
a small additional pole (or set of iron laminations) off
to the left of the main pole. It's excited by the same winding
as the main pole, but is "shaded" from it by a
thick copper band that wraps around the laminations and
acts like a shorted electrical turn. The current induced
in this band by the magnetic field generates a phase shift
so that the shaded pole can generate a small component of
magnetic field at right angles to the main field, and with
the correct phase to ensure the fan turns the right way
(otherwise the fan would suck instead of blowing).
fact our induction motor is using induction already, and
we haven't even got to the rotor yet!
we look at the rotor. This is a real disappointment –
it looks nothing like the "squirrel cage" in the
text book! Where's the squirrel supposed to go, for starters?
happened here is that the rotor is actually made up of a
stack of disc-shaped laminations of soft iron. That's right
– it's solid. This concentrates the magnetic field
(generated by the windings) into the region where it will
do the most good (the conducting bars of the rotor).
can actually see the edges of the bars that run along the
axis of the rotor, but they're at an angle of maybe 30 degrees
to the shaft. What's going on here? Bad day at the factory?
Chances are it's been designed that way to reduce cogging
torque. If the bars ran parallel to the axis, the torque
would rise and fall as each bar passed under the windings.
By slanting the bars, the torque is kept more uniform as
the rotor turns.
let’s look at a different type of induction motor.
induction motor came out of an astronomical telescope. It
was part of the photographic film transport, and needed
to be able to turn both forwards and backwards. It therefore
has two separate windings, and four wires coming out. One
winding is fed directly from the mains (or "line"
as our US colleagues call it); the other is fed through
a capacitor that provides the necessary 90 degree phase
shift. Swap the windings over, or reverse the connections
to one of the windings, and the motor goes the other way.
when we take it apart, although note a very different winding
pattern to the previous motor. It has more poles, and therefore
again, the rotor is solid, and we can't see what's inside.
The aluminium plate at the end of the rotor has been stamped
and turned up into a series of small fins to make a crude
cooling fan. (This wasn’t necessary with our first
motor – it kept itself cool by the simple expedient
of placing itself in the middle of, well, a fan.)
astronomical telescopes no longer use film, we may as well
cut the rotor in half and see if there's a squirrel in there.
but a magnificent set of aluminium conducting bars, just
like in the text books. If you think of the rotor bars as
forming (via the end rings) a single-turn secondary winding
of a transformer, the primary of which (the windings on
each pole) has some 50 ~ 100 turns, it is clear that the
current through the rotor bars can be very high –
as much as 100 amps for a 240 watt motor. This explains
the need for really chunky bars!
disadvantage of the shaded pole motor is that the starting
torque is rather low. This doesn’t matter for
something like a fan, where the load when stationary is
almost zero. For other applications, like a washing machine,
it would be a disaster. Such motors therefore use a capacitor
to generate the required phase shift for the quadrature
windings, as in this example.
motors also come in other variations, but the two described
above are the most common in domestic use.
serious grunt, however, you need a three-phase
induction motor. This takes advantage of the fact that commercial
3-phase power is delivered by three conductors, each of
which carries a 50 Hz sine wave with 120 degrees of phase
shift relative to the other two [See 3
phase power]. A 3-phase motor simply places three windings
at 120 degree intervals around the casing, and a rotating
magnetic field is automatically produced. Three-phase induction
motors are the “workhorse” of industry, with
large units having ratings well in excess of a megawatt.
new Millenium trains use 3-phase induction motors, each
rated at 226 kW, breaking away from the traditional DC motors
used on Tangara trains and earlier models. However, since
the overhead power to the train is 1500 volts DC, each Millenium
train must use an inverter to create the three
AC phases to feed to its motors.