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Strange effect with fan speed control

T

Tom Peel

Jan 1, 1970
0
Hi

We have a kitchen extractor fan which had the speed control burn out- it
was a pretty crude affair using a huge resistor around 200 Ohm with
sections shunted out with a switch, and this in series with the motor. I
threw it out and bought a solid state speed control unit, the kind with
an external potentiometer to control the motor speed. It works great at
low inputs, however if you try to wind the speed up, the motor starts
"chugging", that is, it speeds up for about 2 seconds, then the speed
dies and drops back, repeating over and over. I guess it must be back
emf from the motor that is causing the current to drop, but it's just a
guess. I have no information about the motor, I guess it's a brushless
induction motor. The speed control is some kind of sealed unit with a
thyristor circuit.

Any idea how I can stop the motor surge and get more output? It seems
odd to me, even if it is back emf, that the speed never stabilises.

T.
 
S

Steve Oakes

Jan 1, 1970
0
The damn thing is broke! Take it back and get a replacement fan from a
different manufacturer.

Steve
 
T

Tom Peel

Jan 1, 1970
0
Back EMF would happen much faster, like on the order of
once cycle of the power at 60 Hz.

Sounds more thermal based on the cycle time.
It would also make sense a thermal problem would
show up at high power. Does this controller need
a heat sink. Does it warm up a lot. Is it rated to
handle the power of your motor.
Sounds like it's overheating, and then backs
off the power for safety. Then it cools,
and goes back to normal operation.

If it's got a metal side, it probably needs a heat sink.
You may be able to use a metal case on the fan.
Or you may need a "real" heat sink. Mount this
in the fan's airflow, if possible.

The controller is rated it 2.2Kw max, 1.6 Kw continuous IIRC.
I have the controller mounted on a heat sink, but it does not even get warm.

T.
 
T

Tom Peel

Jan 1, 1970
0
Steve Oakes wrote:

It is not broken.
The damn thing is broke! Take it back and get a replacement fan from a
different manufacturer.

Steve
 
T

Tom Peel

Jan 1, 1970
0
Steve said:
If it wasn't broken, you wouldn't be asking about it making the
"chugging" sound when you wind up the speed. It isn't normal operation
for fans to make "chugging" sounds. Take it back or call customer
support so they can tell you it's broken.

Steve

Steve, looks like I'll have to explain it to you slowly.
With the old resistor speed control in series, the fan works fine. The
chugging effect happens with the solid state motor control.

I repeat: there is nothing wrong with the fan.

T.
 
S

Steve Oakes

Jan 1, 1970
0
See! I told you the thing is broken!!! Keep discussing it with people
until it is out of warranty and you are out of luck getting a free
replacement.

Steve
 
S

Steve Oakes

Jan 1, 1970
0
He also thinks it's broken! Do you get the hint yet?

Steve
 
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