motors


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Induction motors
No modern home should be without one – or maybe a dozen. You'll find an induction motor in the
fan, fridge, vacuum cleaner, 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.

Advantages:
• Cheap
• Quiet
• Long lasting
• Creates no interference

Disadvantages:
• 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, or both
• Kind of big and bulky for the power it develops


No squirrel, 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!
One 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.
Induction motors also come in other variations, but the two described above are the most common in
domestic use.
For 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 <a
href=”http://phys.unsw.edu.au/~jw/power.html”3 phase power</a>]. 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.
Sydney’s 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.


"Universal" motors
No home should be without a dozen or so of these as well. They'll be where you want something
compact that spins fast (food processor, coffee grinder, electric nose-hair trimmer) or needs variable
speed (sewing machine, hand-held electric drill).
Advantages:
• Will turn at any speed you want it to, including really fast
• A lot of power in a small package
Disadvantages
• Horrible
• Arcing brushes create radio interference, ozone, noise.
A universal motor has both a wound field (on the stator) and a wound armature (on the rotor). It
cannot use a permanent magnet to create the stator field because it needs to reverse its magnetic
polarity every half cycle of the mains. The voltage being fed to the commutator brushes is also
changing polarity every half cycle, and in fact is simply in parallel with the field winding.
This one ran for 30 years in a sewing machine.