Aircon blower fan motor's speed ( voltage ) reduction via resistors, wastes electricity & produces dreaded heat.

My honda SM4's blower fan motor has a resistance of 0.8 ?, & mostly us
asted as heat by these resistors which presumably have a total resistance o
In Dec'12 I noticed a Mercedes 350 has the same problem ( like cheaper cars
have ) of heat inside users' space when aircon is off, i.e. no buck conver
ter is used to reduce blower fan motor's voltage, so these resistors heat u
p this space even after aircon is switched off.
Does any car maker use a buck converter to reduce this voltage ? Does any1
know where I can buy a buck converter with easily switchable output voltag
these wasteful resistors ?
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Not to my knowledge. Sometimes fan controls (thought perhaps not the heater fan - too large) are PWM but I've never seen a buck used. There really isn't a reason to spend the money.
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In the grand scheme of cooling a car a couple hundred extra BTU of sensible heat is not going to make any difference. The A/C unit itself is 24 KBTU or more. (Enough for a small house)
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You are doing calculations on the basis of V=(Re+Ri)I where Ri is 0.8 ohms and Re is 0.64 ohms- your calculations are on the basis of a stalled motor-not one that is turning Now consider that in fact you have V=(Re +0.8)I +E where E is the back emf produced when the motor is turning. (and much of the 7.5V measured is attributed to E), you will find that the resistance added will be quite a bit larger than 0.64 ohms (say 6 ohms)and the loss will be quite a bit lower-say in the 10 watt range(without knowing the current, this is ball-parking estimates at best). Yes, speed control with a resistor isn't the most efficient way to do it but for motors of that size- but it is cheap-it isn't worth spending more- the cost/benefit ratio for both the car maker and you isn't worth it. I suggest that, in addition to the voltage measurements-measure the current. Then you can get a handle on the losses and the total input power. While you are at it-check the panel lights and/or radio as heat sources.
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Don Kelly

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