PWM Motor Speed Controller Regulator - Page 2 - Fiberglass RV


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Old 06-16-2014, 01:27 PM   #15
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To get half the voltage at the fan, it is necessary to double the resistance in the circuit.

At full power the fan consumes 36 W, if you draw 3 A at full power, (W = V x I). This also means your fan is 4 ohms, (I=V/R, R=V/I, V=I x R). If you add another 4 ohms in series, the current is cut in half, to 1.5A. The voltage at the fan is now split between the resistor, and the fan, so 6 V each. The fan is consuming 9W, (W = V x I), and so is the resistor. 18 W total, or half the power consumption, but 1/4 the fan power. 9W wasted as heat.

The PWM wastes heat when it switches between full current, and no current. The higher the current flow, the more the waste heat. It will not make nearly as much heat as a resistor though.

And as for suggesting I am wrong, don't worry about it. I frequently am.
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Old 06-16-2014, 01:40 PM   #16
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Quote:
Originally Posted by David Tilston View Post
To get half the voltage at the fan, it is necessary to double the resistance in the circuit.

At full power the fan consumes 36 W, if you draw 3 A at full power, (W = V x I). This also means your fan is 4 ohms, (I=V/R, R=V/I, V=I x R). If you add another 4 ohms in series, the current is cut in half, to 1.5A. The voltage at the fan is now split between the resistor, and the fan, so 6 V each. The fan is consuming 9W, (W = V x I), and so is the resistor. 18 W total, or half the power consumption, but 1/4 the fan power. 9W wasted as heat.

The PWM wastes heat when it switches between full current, and no current. The higher the current flow, the more the waste heat. It will not make nearly as much heat as a resistor though.

And as for suggesting I am wrong, don't worry about it. I frequently am.
Ok, perhaps I misunderstood. Just to confirm, both will reduce power consumption but one will lose more through heat and won't be as efficient?
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Old 06-16-2014, 03:50 PM   #17
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Correct.
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Old 06-16-2014, 08:25 PM   #18
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I watched my current draw and had a few other things on--an inverter and some possibly some other small draws I did not disconnect the DC power and there are a few circuits hooked directly to the battery which were not included in my measurements.

I did not see a huge power savings, maybe half an amp at most on high. The best part was being able to dial it down low and quiet.

Right now I need the fan running because my puppy has a very serious eye infection and had surgery to open up two drains in her face. The infectious pus stinks despite all my efforts to keep her clean. The fan helps. I want to keep it from spreading to me it the other dog too.

In any event. I'm pleased with the results and hope it holds under prolonged use.
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Old 06-17-2014, 10:43 AM   #19
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There should be no savings at full power. The PWM is basically off at that point. No pulses, just straight 12VDC. The power savings I am referring to only apply at lower speeds.
At 10% voltage to the fan, the resistor in the previous example would be ~36 ohms.

12V/40ohms = 0.3A
0.3A x 4 ohms (fan) = 1.2V
0.3A x 36 ohms (resistor) = 10.8V
0.3A x 1.2V (fan) = 0.36W
0.3A x 10.8V (resistor) = 3.24W

The worst case for power loss is actually my first example. 50/50 between the resistor, and fan. 9W would be about 0.75A at 12V.
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Old 06-18-2014, 01:21 AM   #20
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I just installed my pwm in my fan today. For some reason, when I turn it all the way down it still wants to turn the fan slightly, drawing current. So I wired it in after the factory switch so I could turn it off. Another thing I noticed is that the potentiometer does not seem to be linear. most of the adjustment happens in the first 45 deg of rotation. I may have a defective unit if it won't turn off.
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Old 06-18-2014, 01:27 AM   #21
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The specs from the ad state 5% to 100%. It never shuts off. I plan to put it after the power switch on the fan itself.
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Old 06-23-2014, 07:16 PM   #22
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I tried a PWM on my fantastic fan and where the three speed switch is I mounted a variable resistor that also had a off switch, like a radio volume switch that's also the on and off switch. Kind of like this one but a little stouter I think.
10K-Ohm Audio Control Potentiometer with SPST Switch : Potentiometers | RadioShack.com
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Old 06-24-2014, 10:41 AM   #23
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My only issue so far is I switched the fan direction from in to out when on full speed and popped a fuse. I think was a 7.5 amp fuse. All I had was a 15 amp glass fuse to replace it. I rarely run it on high. I should take notes in this stuff.
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Old 06-24-2014, 11:08 AM   #24
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Quote:
Originally Posted by Joe Romas View Post
I tried a PWM on my fantastic fan and where the three speed switch is I mounted a variable resistor that also had a off switch, like a radio volume switch that's also the on and off switch. Kind of like this one but a little stouter I think.
10K-Ohm Audio Control Potentiometer with SPST Switch : Potentiometers | RadioShack.com
Joe, that potentiometer is not rated to 10W. Odds are it is a 1/4W.

As discussed in the eariler part of this thread, the power on the resistor can reach 9W. This is enough to melt a 1/4W pot.
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Old 06-24-2014, 11:10 AM   #25
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My only issue so far is I switched the fan direction from in to out when on full speed and popped a fuse. I think was a 7.5 amp fuse. All I had was a 15 amp glass fuse to replace it. I rarely run it on high. I should take notes in this stuff.
I have done that with a Fantastic fan repeatedly, (by accident). No fuse blew. I wonder if the PWM caused the current spike?
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Old 06-24-2014, 11:32 AM   #26
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David,

I would like to reduce the motor speed on my Suburban furnace. Do you think a PWM would be worth experimenting with? I want to achieve about a 10% reduction, but still enough to close the sail switch? From previous discussions the furnace seems to run fine and much quieter at 12 or even 11 volts. Am I crazy?

Thanks, john
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Old 06-24-2014, 11:37 AM   #27
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Not crazy John. As long as the sail switch closes, I can't see a problem with what you are proposing. But I am no expert on your furnace. These PWM's will probably provide too much adjustment for your purposes.
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Old 06-24-2014, 01:17 PM   #28
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Quote:
Originally Posted by David Tilston View Post
I have done that with a Fantastic fan repeatedly, (by accident). No fuse blew. I wonder if the PWM caused the current spike?

It has to be a spike. I like to fuse things as low as possible. I would have thought that fuse would have held. I will probably swap it for a 10 Amp and see how that works.
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