On Mon, 26 Jun 2006 04:49:31 GMT, John Savage
wrote:

My handheld hairdrier suddenly dropped to half-speed while in use. Does
the DC motor have two sets of parallel windings such that one can burn
out yet leave the other to continue operation but at half speed?

There was a slight "overheated paint/plastic" odour, but that could be
accounted for simply by the heater power not having similarly halved
thus causing the heater elements to glow red hot until I backed off the
heat setting. It is a good quality appliance, so I disassembled it to
investigate. All appears in order, visually, nothing blackened.

I traced the circuit. The heater has 3 elements, one is permanently in
series with the small DC motor, the other 2 elements are switched in/
out by a 3 position heater switch (0-1-2) and there's a series diode
associated with one of this pair of elements.

A separate 3 position switch selects the speed of the motor: OFF-MED-HI
by adding a diode in series with the motor (and, incidently, all elements
of the heater circuit so this means that in normal operation heater power
drops commensurately with the lowered fan speed).

Ah ha! Obviously it's part of the bridge-rectifier that has gone faulty,
I mused. But I connected a 30 volt DC wall wart to the 240 VAC pins of the
hairdrier and with the speed switch set to "HI" the motor runs smoothly and
with no discernible speed difference FOR EITHER POLARITY (though obviously
a lot slower than with 240 volts!). The bridge rectifier is a 4 pin epoxy
device mounted on the motor. With speed setting on "MED" a reversal of the
DC supply polarity causes the motor to not run because the series diode
blocks the current.

So, should I focus on the motor? Could there be part of a small DC motor
that burns out and causes its speed to halve? Otherwise, what else is
there to account for the fault? With the heating switched to the "0"
position there exists just a series string comprising the 3-position
speed switch, its diode, the motor, one heating element, and the overheat
bi-metal cutout switch.

Ideas are eagerly sought!!

I would stick with the full wave bridge, even though it works, it
sounds like just one diode has failed, and will still rectify ok, but
it will be half wave and not full wave rectification. The motor will
certianly run slower with one diode out of action.

Peter Dettmann