Mrbikerman

Hey all,
I just swapped my blower motor resistor because the low speed wasn't working. I was driving for about an hour with the fan on low, and I accidentally touched the resistor itself when I was finishing up installing it (HOT!!). I kinda got freaked out by how hot it was, must've been at least 350F. It got me thinking, would it be possible to control the blower motor with a pulse width modulator instead of this archaic resistor? That way the circuit wouldn't draw max power at all speeds, like it does now. Also you could make tiny adjustments to the blower speed instead of only having 4 settings. Any ideas?

4Crawler

iTrader: (3)

I do think the resistor gets hot in use, that is why it is made of nichrome wire and has the ceramic insulation and why it is installed in the fan air stream:



On my VW, they use a big wire wound power resistor (about the size of a hot dog - maybe 50 watt rating) and that one stays cool.

Sure, if you can retrofit a PWM or similar solid state speed controller, go for it. The resistor circuit does not really draw max power at all settings, just that the resistor takes a greater percentage of the lowered power and the motor takes less. The current should drop lower with each lower speed setting.

Mrbikerman

At anything but max speed, wouldnt the whole the circuit (resistor + blower motor) be using way more power than it needs by converting the unused current from the blower to heat energy? Or is there something I'm missing here?

Also would it damage the motor if it was constantly turning on and off with a pwm? It seems like there's gotta be a reason they chose the resistor setup other than price.

You're an electrical engineer right? I'm just now taking basic electrical and programming classes for my mechanical engineering degree, so I'm really not very knowledgable on the finer details of circuitry. Thanks for your help!

4Crawler

iTrader: (3)

Yes, you are correct, but the key item is that the resistor+motor resistance, the overall resistance is greater, thus the current through both is less than at full speed with no resistor. So on a 12 volt system, less current = less power. So each slower speed results in that much less current and that much less power. Now granted, at the slower speeds, the resistor is taking more and more percentage of that less and less power and converting it to heat.

If you ignore the motor's back EMF and just model it as a resistor, you could plug in some numbers to see what is happening. Lets say the motor is 1 ohm, so with 12 volts across it, you get 12 amps through the motor and 12A x 12V = 144 watts. So now add a 1 ohm resistor in series with the motor, you now have 2 ohms with 12 volts across it and thus 6 amps of current and now only 6 volts across the resistor and 6 volts across the motor since they are both in series. So now the motor and resistor each have 6A x 6V = 36 watts of power, so 72 watts total power with half that in the motor and half that in the resistor. With half the current and voltage, the motor is running about 1/2 the speed. So you can see that the more resistance you add the slower the motor runs and the less current (and power) flow in the circuit.

Won't hurt the motor with PWM, but yes, cost is the reason most likely for the resistor and switch (and while Toyota charges a boat load for those resistors, they probably cost Toyota under $1). Maybe today, with microprocessor chips in everything and likely solid state speed controls in blower motors it is a no-brainer, but back in the '80s-'90s, that was an expensive option. Plus a switch and resistor setup is fairly reliable. Yes they do fail but do so gradually, you lose the lower speeds first then gradually are left with high speed only. I have a number of halogen lamps in my house with built in dimmers and those often go from infinite brightness control to nothing/dead when they fail.

But here is one you might try:
- http://store.qkits.com/moreinfo.cfm/MX033

Mrbikerman

That's great info, thank you.

One more thing, would I be able to put the pwm in the circuit before the speed controller switch so that I could retain the factory operation of the heater by setting the pwm to 100%? Then I could just put the heater control switch on maximum (bypassing the resistor) when I want to use the infinite adjustment, right?

4Crawler

iTrader: (3)

Suppose you could do that. But then with both devices in series, you do cut the reliability, since a failure in either part will cause the fan to stop. So yes, more flexible that way, but less reliable. It is not like it is a life-critical device, but have it go out on you in the middle of a raging blizzard and it could be inconvenient to say the least.