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Electric motor & rheostat to vary blower speed?

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Electric motor & rheostat to vary blower speed? darro 09-09-2006
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Posted by on September 9, 2006, 12:48 pm
The owners of a condominium apartment block in a temperate west coast
climate are looking for ways to adjust the incoming air volume of a
rooftop blower continuously supplying outdoor-temperature
pressurization air to the hallways (ie. low volume in winter, medium
volume in spring and fall, high volume in summer).

I'd appreciate your feedback on whether it will be practical and safe
to use a rheostat to vary the speed of the blower powered by an
electric motor, and whether there might be a better way to adjust the
volume of unheated/uncooled air which must be delivered non-stop.

1/4hp, 1725rpm, 6-pole 120vAC motor
Belt-driven Delhi G9 standard squirrel cage air blower

Thanks in advance for your replies.
Darro


Posted by Jeff Wisnia on September 9, 2006, 3:06 pm
darro@no_email.com wrote:
> The owners of a condominium apartment block in a temperate west coast
> climate are looking for ways to adjust the incoming air volume of a
> rooftop blower continuously supplying outdoor-temperature
> pressurization air to the hallways (ie. low volume in winter, medium
> volume in spring and fall, high volume in summer).
>
> I'd appreciate your feedback on whether it will be practical and safe
> to use a rheostat to vary the speed of the blower powered by an
> electric motor, and whether there might be a better way to adjust the
> volume of unheated/uncooled air which must be delivered non-stop.
>
> 1/4hp, 1725rpm, 6-pole 120vAC motor
> Belt-driven Delhi G9 standard squirrel cage air blower
>
> Thanks in advance for your replies.
> Darro
>

No, you can't use a rheostat in that application. The motor you describe
is designed to run at its rated speed when operated on 60 Hz power.

Some of the things you could do would be:

1. Change the motor to a multispeed (most have just two speeds) type and
install appropriate switching to change speeds when desired.

2. Get another slower speed motor and couple the two motors together
mechanically, maybe through the belt drive, then install switches to run
the proper motor for the speed you need.

3. Change the motor to a three phase type and buy a variable frequency
drive controller to suit it, one which derives its operating power from
the single phase power line. (Expensive, but speed is continuously
adjustable.)

4. Change pully sizes seasonally (A big PIA.)

5. Have you considered just installing a damper system on the input side
of the blower?

HYH,

Jeff



--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
"What do you expect from a pig but a grunt?"

Posted by Jeff Wisnia on September 9, 2006, 10:24 pm
darro@no_email.com wrote:
> The owners of a condominium apartment block in a temperate west coast
> climate are looking for ways to adjust the incoming air volume of a
> rooftop blower continuously supplying outdoor-temperature
> pressurization air to the hallways (ie. low volume in winter, medium
> volume in spring and fall, high volume in summer).
>
> I'd appreciate your feedback on whether it will be practical and safe
> to use a rheostat to vary the speed of the blower powered by an
> electric motor, and whether there might be a better way to adjust the
> volume of unheated/uncooled air which must be delivered non-stop.
>
> 1/4hp, 1725rpm, 6-pole 120vAC motor
> Belt-driven Delhi G9 standard squirrel cage air blower
>
> Thanks in advance for your replies.
> Darro
>

One more idea occurred to me while driving us home tonight.

How about a simple timer to vary the on-off duty cycle of the present
blower, cycling it on maybe three times an hour for time periods which
get adjusted seasonally?

Jeff

--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
"What do you expect from a pig but a grunt?"

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