From: dhky@shaw.ca   
      
   On 04/06/2014 1:16 PM, pbx5619@gmail.com wrote:   
   > My honda SM4's blower fan motor has a resistance of 0.8 Ω, & mostly used @   
   7½ v when alternator @ idle rpm produces 13½ v, i.e. 6v is wasted as heat by   
   these resistors which presumably have a total resistance of 6 ÷ 7½ x 0.8 Ω   
   = 0.64 Ω, i.e. #    
   of watts wasted must be 6v x 6v ÷ 0.64 Ω = 56¼ watt.   
   > 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 converter   
   is used to reduce blower fan motor's voltage, so these resistors heat up 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 voltages   
   of 4, 6, 9 & 12 v, of ( 12 ÷ 0.8 Ω = ) 15 amp, to replace these wasteful   
   resistors ?   
   >   
   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.   
      
   --   
   Don Kelly   
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