Posted by David Sherman on December 27, 1998 at 09:09:06:
In Reply to: Resistor to make 6V heater work in 12V system posted by Charlie on December 26, 1998 at 22:38:23:
If the motor has a wound field and a wound rotor
(no permanent magnets), it will run the "right" way
no matter which polarity of electricity you give it.
If it has permanent magnets for the field (attached to
the case) it will run "backwards" on opposite polarity
until you interchange the wires that go to the brushes.
To figure out the dropping resistor, hook the blower up to a
6V battery and measure the AMPS that it draws. 6 volts
divided by that many amps is how many ohms you need for
running if off of 12V. For example if it draws 10 amps,
you would need 6/10 = 0.6 ohms. You don't need to
be very exact on the resistance. Get within 20% and
it will be okay. Then you need to figure
out the wattage rating. This is the amps times the volts,
or 60 watts in this example. This is quite a lot. If you
get a little carbon-comp resistor from radio shack, the
kind with the stripes painted around it, it will
go up in smoke instantly. A 60 watt resistor will
probably consist of wire wrapped around a ceramic
tube about 4" long. The resistor will dissipate as
much energy as the motor is using since there's 6 V
across each. Multi-speed heaters use several resistors
to change the motor voltage. To reduce the size of
the resistor, they use an open wire coil mounted in
the blower's airstream. This also "recycles" the
wasted energy. However if the blower gets stuck, the
resistor burns up.