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Old 07-11-2011, 02:18 PM   #29
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Bumping this only to close it out. I never reported back on the controller that worked after ordering 3 different ones. FYI.

Haven't messed with the sail switch yet.

Critical Velocity - 15 Amp High Frequency PWM Motor Speed Controller P/N: SPD-315
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Old 01-11-2012, 04:31 PM   #30
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Just realized after discussing this in another thread that I never posted the revised wiring mods. Here they are:

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Old 01-11-2012, 09:20 PM   #31
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I guess I'm lucky. I have a furnace without any electronic control. I just completed install a couple of small fans to circulate warm air into the dead spaces below and behind the cabinets. That's where the water lines run.
The fans made too much noise and ran too fast for my needs. The answer was to install a resistor in series with the motors to drop the voltage. Resulting in a lower motor speed and a lot less noise.
I wired the resistor between the negative lead of the motor and chassis ground. This results in the negative side of the motor standing above ground (an electrical term) by the amount of voltage dropped across the resistor.
In the case of the furnace blower. If I had a PWM controller which will make motors noisier than running on pure DC because of the notching characteristic. Especially at the lower speeds. Because the notching is at a lower rate and therefore more audible to the ear. I think I would install a relay to switch out the PWM and switch in a resistor to drop the fan speed to the desirable level. The motor would run quieter on the pure DC from the battery/converter. The circuit would be a simple one.
Just my 2 cents worth.
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Old 01-11-2012, 10:00 PM   #32
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Originally Posted by TG Twinkie View Post
I guess I'm lucky. I have a furnace without any electronic control. I just completed install a couple of small fans to circulate warm air into the dead spaces below and behind the cabinets. That's where the water lines run.
The fans made too much noise and ran too fast for my needs. The answer was to install a resistor in series with the motors to drop the voltage. Resulting in a lower motor speed and a lot less noise.
I wired the resistor between the negative lead of the motor and chassis ground. This results in the negative side of the motor standing above ground (an electrical term) by the amount of voltage dropped across the resistor.
In the case of the furnace blower. If I had a PWM controller which will make motors noisier than running on pure DC because of the notching characteristic. Especially at the lower speeds. Because the notching is at a lower rate and therefore more audible to the ear. I think I would install a relay to switch out the PWM and switch in a resistor to drop the fan speed to the desirable level. The motor would run quieter on the pure DC from the battery/converter. The circuit would be a simple one.
Just my 2 cents worth.
It would take some doing to figure out what resistor to use to get the speed just below the sail switch "on" speed. The last, high frequency, PWM controller makes no noise. It is above the frequency audible to humans. Doesn't seem to bother the dog either.
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Old 01-11-2012, 10:28 PM   #33
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Market Research

I'm impressed. Never thought I would see this kind of site on an Airstream site. I congratulate you guys for your initiative, as there probably aren't many out there that would do what you did.
Since you seem happy with the result and probably buying the PWM controller I won't get into the "ugly stuff" but make some observations of using Hight Side (P Channel FETS) and Low Side (N Channel FETS) like were used in your circuit diagram. The good news about pulling low it is a bit cheaper for same power rated FET and easier to slam on and turn off, which is why people do it. If your motor isn't pulling a lot of current a circuit similar to what you show is fine. Since I don't have a scope on the circuit with your application I can't "see the noise transients" when the motor turns off and when the pwm turns off. As you probably know a DC motor turns into a "generator" when spinning and you turn it off. The faster it spins, and the more current it is pulling, the bigger the voltages it generates. That said, if you look at your schematic, what happens when the two lower FETs turn off? Hmmm, the generator has no way to discharge to ground as it looses it's ground reference. Now in all fairness, the push pull transistors driving the FETs may not be a perfect square wave but may slowly ramp on and off, and thus kind of doing a slow turn off, in effect, giving a discharge path for the motor. That said, using a high side FET circuit keeps one leg of the motor to ground, the reverse diode provide discharge path when motor goes reverse of normal on current flow and bottom line is, less generate electrical transients.

All that said, I have done circuits both ways, have done some "active transient supressions" circuits for motors that have inrushes of like 60-90 amps, which a far more current than your little fan motor. So, in my opnion without spending time analyzing your circuits further, if you are pulling only a few amps you are probably OK, but you are probably also, depending upon PWM frequency, generating some transients on your power lines, which effectively are discharged by your battery and other stuff that is turned on. If it was a "big motor" it could get ugly, but with a small one you are probably OK.

Relative to burning power with a series resistor (heater) per some other comments, just make sure your resistor can dissipate sufficient power and stay cool and you should be fine. A small muffin fan and the like doesn't pull a lot of power, so for that kind of stuff a resistor could be OK. If it was me, I would PWM that puppy, but then I do products that already do that stuff and would use some old protoype board to make my "cheap motor control". After all, some can run at 30 amps forever!

I will be a vendor to this market soon and would just like to know; "So what is the demand for a solid, high side, pwm controller and is it anything some one would be willing to buy?" Obviously it appears it was of some value for the creative tank warmer, but how many applications on an RV woudl someone need a gorilla PWM controller?

Relative to frequency I am always running with a micro, can change frequencies to what best works for the motor based upon number of poles, RPM, current draw and so on. I can't say as I ever had a hum problem, but then again, I never did a product where that might be an issue! So on that topic I really can't provide any "informed input".

Per early discussions sounds like the operation of the trailer furnances are pretty much the same as a house? Thermotat calls for heat, start exhaust fan, once air flow turn on gas, once plenum to temp turn on circulating fan, once thermostat temp reached turn off gas (an maybe exhaust fan after small delay), once plenum drops to low temperature turn off circulating fan and wait for thermostat to ask for heat again. Is this true or am I adding or missing something?

Regards,
Don
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Old 01-12-2012, 12:52 AM   #34
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As far as the furnace operation goes, you are close. The difference is the exhaust fan and combustion fan are driven by the same motor. In the case of the simple furnace with a standing pilot. The procedure is to lower the thermostat to the minimum setting, light the pilot, set the thermostat to the desired temp. Setting the thermostat starts the motor for the blowers . When the sail switch is satisfied, the main gas valve will open and the burner comes on. When the thermostat is satisfied, the contacts in the thermostat open which closes the main gas valve, however the blower motor continues to run until the plenum temperature switch is satisfied that the plenum is now cool enough to be safe.
This cycle is repeated, minus the requirement of lighting the pilot.


As far as the sizing of the resistor in motor speed control. It's fairly straight forward. Let's say the 12volt motor draws 4 amps. Volts times Amps = Watts. Therefore the motor consumes 48 watts of power at full speed. If you want the motor to run at half speed, size the resistor to drop 6 volts at 2 amps. The heat dissipation requirement of the resistor would be; 6x2 or 12 watts. Any resistor with a rating of 15 watts or higher would work to dissipate the heat. Using ohms law one can determine the resistance required since it is a DC circuit. Where resistance = Voltage divided by Current or R= E/I. This is just an example.
I had a 10 watt 5 ohm resistor on hand so that is what I used. It slowed the motor down to less than half speed, which was fine for my application and desire to reduce the noise. One should keep in mind that the voltage in the so called 12 volt system will vary from something under 12 volts if you are on battery only power and the battery is low on charge to something around 14 volts if you are on shore power and the converter is kicked up to the high output while charging the battery.
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Old 01-12-2012, 09:17 AM   #35
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As far as the furnace operation goes, you are close. The difference is the exhaust fan and combustion fan are driven by the same motor. In the case of the simple furnace with a standing pilot. The procedure is to lower the thermostat to the minimum setting, light the pilot, set the thermostat to the desired temp. Setting the thermostat starts the motor for the blowers . When the sail switch is satisfied, the main gas valve will open and the burner comes on. When the thermostat is satisfied, the contacts in the thermostat open which closes the main gas valve, however the blower motor continues to run until the plenum temperature switch is satisfied that the plenum is now cool enough to be safe.
This cycle is repeated, minus the requirement of lighting the pilot.


As far as the sizing of the resistor in motor speed control. It's fairly straight forward. Let's say the 12volt motor draws 4 amps. Volts times Amps = Watts. Therefore the motor consumes 48 watts of power at full speed. If you want the motor to run at half speed, size the resistor to drop 6 volts at 2 amps. The heat dissipation requirement of the resistor would be; 6x2 or 12 watts. Any resistor with a rating of 15 watts or higher would work to dissipate the heat. Using ohms law one can determine the resistance required since it is a DC circuit. Where resistance = Voltage divided by Current or R= E/I. This is just an example.
I had a 10 watt 5 ohm resistor on hand so that is what I used. It slowed the motor down to less than half speed, which was fine for my application and desire to reduce the noise. One should keep in mind that the voltage in the so called 12 volt system will vary from something under 12 volts if you are on battery only power and the battery is low on charge to something around 14 volts if you are on shore power and the converter is kicked up to the high output while charging the battery.

Does it not consume less power to use a PWM over a resistor....from a boondocking battery conservation standpoint?
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Old 01-12-2012, 09:24 AM   #36
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RTOS, I think there would be a market, if it wasn't too expensive. IMO, you would need to figure out a way to have the base system verify proper sail switch operation inputs, so the add on could be operated at any fan speed AND have the furnace not throw a code and not light when the sail switch is closed before the board anticipates it.
I'm not sure how you could do that in a way that doesn't defeat that particular safety feature.....and be within code.
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Old 01-12-2012, 10:24 AM   #37
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In my case the fan motor draws only 1/4 amp at full speed. By reducing the speed I have reduced the power consumed.
I would guess that the PWM would consume as much or more power under a half speed situation. Electronic devices are not 100% efficient, so the inefficiency factor of the PWM would be added to the power consumed by the motor.
When a resistor is involved it is a different scenario. If one looks at the circuit of the motor wired in series with a resistor. If the 12 volt motor draws 4 amps at full speed it will consume 48 watts of power. W=E x I. By installing a resistor in series with the motor of a size that would restrict the flow of current to 2 amps whereby reducing the speed of the motor to half. There is still a 12 volt drop across the circuit; 6 volts across the motor and 6 volts across the resistor. When you calculate the power usage in this circuit. Watts = Volts x Amps or 12 x 2 = 24 watts. Half the power half the speed. The motor consumes 12 watts and the resistor dissipates 12 watts of heat.
In either case whether a PWM or resistor is used to slow the speed of the motor we are not talking about a huge amount of power. Cutting the motor speed in half will save considerably more power than any inefficiencies mentioned above.
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Old 01-12-2012, 10:53 AM   #38
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I was told somewhere, sometime that a PWM is more efficient because it turns the power on and of and resistors get hot..... That PWM has some pretty large heat sinks!!!

Like you said I think on this small a motor, it's rather irrelevant anyway.
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Old 01-12-2012, 11:10 AM   #39
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Hi Don

There are a fair number of engineers, physicists, chemists, boatbuilders, and airframe mechanics, retired and otherwise, on this board, so you see people do stuff beyond what you might expect.

Market wise, Atwood does make an RV furnace with a two-speed fan. It's expensive as a retrofit, though, and there are reports that the PWM in it is poorly applied so that there is audible noise. One of the many areas where the RV market has not engineered their products to the extent we might like.
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Old 01-12-2012, 11:22 AM   #40
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One might ask. With the notching characteristic of a PWM. Is it possible or does anyone know if this would cause static on a radio or possibly interfere with television reception.
I was in error I believe in the previous post. If the power consumed by the motor is 12 watts it would run at 1/4 speed not 1/2 speed. Since at full speed it consumes 48 watts.
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Old 01-12-2012, 11:26 AM   #41
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One might ask. With the notching characteristic of a PWM. Is it possible or does anyone know if this would cause static on a radio or possibly interfere with television reception.
I was in error I believe in the previous post. If the power consumed by the motor is 12 watts it would run at 1/4 speed not 1/2 speed. Since at full speed it consumes 48 watts.
I get no interference with the PWM motor controller. I do, however get interference in the stereo only with a PWM LED controller. Gotta work on that sometime. It's minor, but bugs me.

Again, I believe that is a function of the frequency.
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Old 01-12-2012, 11:34 AM   #42
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One other note about motors running on variable speed electronic drives, ie PWM's for DC and variable frequency inverters for AC motors. These drives can be hard on motors that are not specifically designed to run on these devices.
The variable frequency inverter is a different animal from the inverters used in RV 's. This type converts AC to DC then back to a modified sign wave AC. Much more complexed device than what is used in an RV.
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