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Hoots Weird Science

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Hoot

Inactive Moderator
Joined
Feb 13, 2001
Location
Twin Cities
Several people have expressed an interest in how much air the Grainger 2C646 DC blower will deliver at voltages below its rated 170cfm @ 12V.

Not having an anemometer, I had the idea to see how fast it would blow up a "lawn & leaf" bag. I know some Euros did this to test no-name blowers on RF Power amplifiers. Out to the garage and back with an Ironman 39 gallon "clean-up" bag.

A gallon is 231 in3, so that is 9009 in3. A cubic foot is 1728 in3, so the bag is 5.2 ft3. Let round down to 5 to account for constricting it to the orifice. Now the fan motor is as close to instant-on as you can get, so it was a matter of squeezing out the air, plugging the motor in, releasing the wheel and run the stopwatch. My 7 year old was watching me, LHAO. Infidel!

Here's the results, averaged 5 tries each.

5V-----4.50 seconds = 66cfm = .202c/w @ 96W
6V-----3.60 seconds = 83cfm = .190c/w @ 96W
7V-----3.05 seconds = 98cfm = .181c/w @ 96W
8V-----2.80 seconds = 107cfm = .176c/w @ 96W
9V-----2.50 seconds = 120cfm = .170c/w @ 96W
10V---2.30 seconds = 130cfm = .165c/w @ 96W

Now, I'll be the first to admit that the cfm measurements are "loose science", but the c/w were very carefully measured with a thermocouple drilled into the heatsink and at the blower intake.

I used my Tbird @ 10x150 @ 1.95V running Prime95 Torture to generate the heat. Wattage was determined using Radiate2.

Here's a picture to chuckle at

Hoot
 

CalCoolage

Registered
Joined
Jun 30, 2001
Hoot (Jul 02, 2001 11:01 p.m.):
. I know some Euros did this to test no-name blowers on RF Power amplifiers. Hoot

Clever... I mean Cool.
Looks like the cfm is very roughly linear with voltage. I wonder if the RPM is too.

I wondered how much current the blower pulled at 5V. I put a .1 ohm 5 watt resistor in series with the 5V lead. It measured .16V unblocked and .09 with my hand completely blocking the outlet. That means it draws .16/.1 =1.6 amps wide open and .9 amps completely bogged down, which also means it uses 5 x 1.6 = 8 watts max and 5 x .9 = 4.5 watts min. If the motor is only 80% efficient, the motor would waste 1.6 watts to heat. No wonder it feels cool to the touch.

It is interesting that blocking the air flow puts LESS load on the motor, not more This is confirmed by the fact that the pitch of the motor sound goes UP when the air is blocked.
 
OP
H

Hoot

Inactive Moderator
Joined
Feb 13, 2001
Location
Twin Cities
It is interesting that blocking the air flow puts LESS load on the motor, not more This is confirmed by the fact that the pitch of the motor sound goes UP when the air is blocked.

It's like spinning the wheels of your car. Once they break traction with the surface, your motor has less load on it and they will spin faster.

I put an ammeter in series with the motor once. it was a while ago, but I believe it was something like 1.3A at 5V. Certainly not a large load on the +5V output of your PSU, compared to what the core voltage regulator draws.

FWIW: I don't ride horses. Damned things make me nervous. They are bigger and stronger than me. ;D

Hoot