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!!
--
John Savage (my news address is not valid for email)

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

"John Savage" wrote in message
om...
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!!
--
John Savage (my news address is not valid for email)


I would say not the DC motor. Check switches and all diodes/rectifier (out
of circuit if possible).

In my initial post I wrote:
My handheld hairdrier suddenly dropped to half-speed while in use.

In the week since that happened, twice while it has been in use the
speed of the hairdrier has slowly risen to almost normal operating
speed with it running at that speed for about 20 to 30 seconds
before dropping back to half-speed operation. During this normal
speed operation there was a slight odour of an overheating electronic
component (quite different from a burnt rubber smell).

If it was an open circuit diode in the bridge rectifier then the motor
would not operate with reversed polarity DC input like I tested it
with. If it was a short-circuited diode, I'd expect the bridge package
to self-destruct with a bang.

The mystery continues.
--
John Savage (my news address is not valid for email)





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!!
--
John Savage (my news address is not valid for email)

John Savage wrote:
In my initial post I wrote:

My handheld hairdrier suddenly dropped to half-speed while in use.


In the week since that happened, twice while it has been in use the
speed of the hairdrier has slowly risen to almost normal operating
speed with it running at that speed for about 20 to 30 seconds
before dropping back to half-speed operation. During this normal
speed operation there was a slight odour of an overheating electronic
component (quite different from a burnt rubber smell).

If it was an open circuit diode in the bridge rectifier then the motor
would not operate with reversed polarity DC input like I tested it
with. If it was a short-circuited diode, I'd expect the bridge package
to self-destruct with a bang.

The mystery continues.

Hi John...

Just thinking out loud...

There should be some sort of safety thermostat in there, I think
usually buried inside heater coils. That doesn't describe well
enough - let me say in the center of the air flow, close to the
heater coil.

Possible that the contacts are sufficiently cooked that there's
a few ohms worth, perhaps a bit intermittent?

Take care.

Ken

Ken Weitzel wrote:


John Savage wrote:

In my initial post I wrote:

My handheld hairdrier suddenly dropped to half-speed while in use.



In the week since that happened, twice while it has been in use the
speed of the hairdrier has slowly risen to almost normal operating
speed with it running at that speed for about 20 to 30 seconds
before dropping back to half-speed operation. During this normal
speed operation there was a slight odour of an overheating electronic
component (quite different from a burnt rubber smell).

If it was an open circuit diode in the bridge rectifier then the motor
would not operate with reversed polarity DC input like I tested it
with. If it was a short-circuited diode, I'd expect the bridge package
to self-destruct with a bang.

The mystery continues.


Hi John...

Just thinking out loud...

There should be some sort of safety thermostat in there, I think
usually buried inside heater coils. That doesn't describe well
enough - let me say in the center of the air flow, close to the
heater coil.

Possible that the contacts are sufficiently cooked that there's
a few ohms worth, perhaps a bit intermittent?

Take care.

Ken

If the user of the dryer has long hair, and brushes her hair while
blow-drying it, it is not uncommon for deteached hairs to get sucked
into the intake end of the blower. When that happens, they suddenly
wind around the blower shaft, slowing the blower almost instantly. If
they are caught on something and stick in place, the blower shaft will
spin within a cocoon of wound-up hair, and the heat of the friction can
cause the cocoon to heat up and smell funny. As the hairs in the cocoon
burn up, they grip the shaft less tightly, allowing the blower to slowly
speed up.

I ran into this several times when my wife had long hair.

Bill