[EE] AC blower motor running hot

[Jason White]

Hello PICList,

I have jigged up a small (200W, 3600RPM, 10" blade) AC induction
blower unit salvaged from a high efficiency furnace in my basement in
order to "pump out" radon gas. I intend to run the system continuously
for one week while monitoring the levels of radon with a meter.

I have found that the blower motor is not rated for continuous
operation (the internal fan blades for cooling the motor are too
small). While running the motor gets so hot that I cannot touch it
without burning myself regardless of how the fan is loaded.

I would like to know if reducing the RMS voltage to the motor would be
an effective way to reduce the heat generated?

As it stands currently the blower moves more than enough air. If I
understand correctly reducing the applied voltage via triac basised
light dimmer (since I don't own a variac) will cause the motors slip
to increase and the overall efficency to decrease. But since the
applied power is lower and the frequency is the same the overall heat
generated (and work done) should decrease.

I have entertained the notion of adding a second cooling fan to the
motor, or redirecting some of the airflow over the motor, or reducing
the fan blade size, or using a VFD to reduce the speed, or even just
replacing the whole blower unit/motor. I would like to avoid those
particular solutions for the moment unless the current method turns
out to be impractical.

Comments and suggestions are welcome.

[RussellMc]

Can you pass the whole of the extracted air over the motor to cool it?
Why should this cause a problem?

A series resistance (such as a high wattage filament light bulb or a heater
element) MAY produce the desired results.

One method that does wok is to use a vacuum cleaner with a series wound AC
motor (ie none of these new fangled Dyson type thing-gummies) with a series
resistor,

- Or a vacuum cleaner on lower voltage - plus a series resistor

as current will tend to increase with lowered voltage and lower speed. )

Series wound motors work on AC or DC.
I have seen a long ago application where a 230 VAC 50 Hz vacuum cleaner WAS
operated instead on 50VDC with a series resistor to act as a ventillator to
remove Hydrogen Sulphide gas from small telephone exchanges.
An H2S filter was added in the VC dust bag space as a bonus.
H2S kills in a different manner to Radon but much more quickly.
Above a certain concentration the sense of smell is disabled by the gas,
thus hiding the usual characteristic "rotten eggs" smell, so impending
disaster is not noticed.
In our Rotorua thermal region people very occasionally die from this cause.

Russell

[Spehro Pefhany]

Since you salvaged the unit, check the capacitance of the run cap, a failure there will make the motor run really hot.

Best regards, SP

Post by RussellMc
Can you pass the whole of the extracted air over the motor to cool it?
Why should this cause a problem?
A series resistance (such as a high wattage filament light bulb or a heater
element) MAY produce the desired results.
One method that does wok is to use a vacuum cleaner with a series wound AC
motor (ie none of these new fangled Dyson type thing-gummies) with a series
resistor,
- Or a vacuum cleaner on lower voltage - plus a series resistor
as current will tend to increase with lowered voltage and lower speed. )
Series wound motors work on AC or DC.
I have seen a long ago application where a 230 VAC 50 Hz vacuum cleaner WAS
operated instead on 50VDC with a series resistor to act as a ventillator to
remove Hydrogen Sulphide gas from small telephone exchanges.
An H2S filter was added in the VC dust bag space as a bonus.
H2S kills in a different manner to Radon but much more quickly.
Above a certain concentration the sense of smell is disabled by the gas,
thus hiding the usual characteristic "rotten eggs" smell, so impending
disaster is not noticed.
In our Rotorua thermal region people very occasionally die from this cause.
Russell
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[Adam Field]

On Wed, Oct 17, 2018 at 9:24 PM, Jason White <

Post by Jason White
Hello PICList,
I have jigged up a small (200W, 3600RPM, 10" blade) AC induction
blower unit salvaged from a high efficiency furnace in my basement in
order to "pump out" radon gas. I intend to run the system continuously
for one week while monitoring the levels of radon with a meter.

You mention it's moving more than enough air. Would running it at 50% or x%
duty cycle be effective enough? One minute on, one minute off. A simple
circuit with a relay could do that, there are even some ready to go timer
relays. Also you could hook up a thermostat with some hysteresis to have it
turn off and cool when it hits a temperature set point.