Heater Resistor

1959 - 1978, M151, M151A1, M151A2, Technical questions and discussions, regarding anything related to the M151.
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Jeeps4Brains
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Heater Resistor

Post by Jeeps4Brains » Wed Jan 24, 2018 4:10 am

I have been toying with the idea of a heater. The 24 volt blower motor appears to have only one winding, so only one speed. I see folks asking about resistors for the low side.

I know a resister will lower the voltage and let the motor run slower. But has anyone tried using a modern Pulse Width Modulator?

From what I know the modulator will supply 24 volts but only in pulses. That way the winding will get the proper 24 volts and not generate heat inside the motor winding. Thoughts?
45' MB, ??' MBT, 47' CJ2A, 48' CJ2A,
51' M38 #1, 51' M38 #2, 51' M100,
52' M37, ??' M101A1 (1st Gulf War Vet),
53' M38A1, 53' M211, 65' M151A1, 67' M416,
MVPA #31724


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Jeeps4Brains
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Posts: 1082
Joined: Wed Jul 20, 2005 8:56 am
Location: Upstate SC

Re: Heater Resistor

Post by Jeeps4Brains » Wed Jan 24, 2018 9:29 am

The reason I am thinking about this is that I can by a 12v - 24v PWM fan control for very little. Much less than the official military resistor for the same purpose. Plus many of them allow you to tune the speed of the fan. So I would set up the PWM for low speed and hide it under the dash. Then run the wires to a military Hi / Low switch you could see to choose the heater fan speed.
So how do we control the flow of current through the motor. Well many people attempt to control the speed of a DC motor using a large variable resistor (Rheostat) or a fixed rated resistor in series with the motor. (As the military unit.)

While this may work it generates a lot of heat in the motor winding and wasted power in the resistance. One simple and easy way to control the speed of a motor is to regulate the amount of voltage across its terminals and this can be achieved using “Pulse Width Modulation” or PWM.

As its name suggests, pulse width modulation speed control works by driving the motor with a series of “ON-OFF” pulses and varying the duty cycle, the fraction of time that the output voltage is “ON” compared to when it is “OFF”, of the pulses while keeping the frequency constant. This allows the motor winding to stay at or below the normal operating temperature.
45' MB, ??' MBT, 47' CJ2A, 48' CJ2A,
51' M38 #1, 51' M38 #2, 51' M100,
52' M37, ??' M101A1 (1st Gulf War Vet),
53' M38A1, 53' M211, 65' M151A1, 67' M416,
MVPA #31724

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