(snip)


Further proof that little knowledge is worse than no
knowledge at all. He was taking about SYNCHRONOUS MOTORS.
Putting resistors in series with a synchronous motor is
folly.

**You would, of course, be wrong. It is a widely used practice, for slowing
FANS using shaded pole motors down to lower speeds. You seem to be
forgetting that although fans use sychronous motors,

No so, see below.

they are under
constant, heavy load. As such, they can never reach the theoretical
synchronous speed,

Which is why they are not synchronous motors.

which is determined by the number of poles and the
frequency of operation.

Correct as a definition of synchronous speed, viz.:
At 50 Hz:
2 pole machine, synchr. speed - 3,000 rpm.
4 pole machine, synchr. speed - 1,500 rpm.

At 60 Hz:
2 pole machine, synchr. speed - 3,600 rpm.
4 pole machine, synchr. speed - 1,800 rpm.

As such, use of a resistor (or even a cap) is a most
effective form of speed control.

Two points of information:
1. Shaded pole motors are not synchronous motors, they are
induction machines that run below synchronous speed by an
amount related to the torque load. This difference is
called "slip" and it's the means by which current is induced
into the rotor so that it can be pulled round by rotating
the stator magnetic field. No slip = no torque, so it can't
happen!

A synchronous machine must run at synchronous speed. If it
"pulls out" due to too much torque load, it stalls. In this
case the rotor field is provided by a DC current source.
Some synchronous machines are "induction start" - they
accelerate as induction motors under no load, then "pull
in".

2. A resistance in series with the motor stator field, in
this case the shaded pole field coil, will slow it down but
will reduce motor efficiency. A capacitance is more
efficient (no I2R loss) but the motor loss due to high slip
remains. This is never done in an industrial drive motor but
will work on a small fan where efficiency is not the prime
concern. I'll be the first to admit it's not elegant, but
you can do it.

(snip)

Trevor Wilson
www.rageaudio.com.au

Cheers,

Roger
--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada.

"Friends don't let friends vote Liberal"