Any time we have an electrical conductor moving through a magnetic field, an electrical current will be induced in the wire. That's one of the science lessons from about middle school as I recall.
If we take our heater motor, hook the wires to a voltmeter and hook a variable speed drill to the shaft and turn the drill on, what happens? Yup, we get a voltage on the voltmeter. If you turn the drill faster, the voltage goes higher, if you stop the drill, the voltage disappears. If you reverse the drill, the voltage reverses. This voltage is called the 'speed voltage' and is caused by the movement of the conductors through the magnetic field left in the steel parts of the motor.
The magnitude of this speed voltage depends on two things, how fast the motor is turning and how strong the magnetic fields are. Faster motor -> more voltage, stronger magnetic field -> more voltage.
This should all seem somewhat intuitive as far as a generator goes. Increase the engine speed and the ammeter shows more charge or have your voltage regulator fail and supply to much field voltage to the generator and the generator will overcharge your battery.
In the case of a motor where we supply the voltage and current to move the rotor what happens to the speed voltage? It's still there!
Not only that, it still behaves just like it does in the generator with one new wrinkle. The 'speed voltage' OPPOSES the voltage supplied from the outside source.
If we apply full voltage to our heater motor, at the moment the switch is thrown, the current through the motor will be in accordance with Ohm's law and limited by the non-rotating resistance of the motor.
As the motor speeds up, the 'speed voltage' will increase and act to 'reduce' the voltage available to push current through the motor. As the motor continues to speed up, the current flow through it will drop off as the speed voltage increases. At some point, the motor will reach an equilibrium point where there is not enough current flowing to continue to accelerate the motor and it runs at a steady speed.
The motor is using just enough current to spin the unloaded rotor and is running at it's maximum speed. What's the resistance of the motor at this point?
The same as it was before, around .9 ohm. Why is there MUCH less current flowing through it than expected? Because the 'speed voltage' created by the motion of the motor armature is opposing the voltage supplied by the battery and effectively reducing the voltage available to push current through the motor.
What happens if we put a load on the motor? The shaft will slow down, the 'speed voltage' will drop and the battery will push more current through the motor. Since the resistance of the motor is quite small, it takes only a small drop in motor speed to create a substantial increase in current and mechanical power delivered.
What happens if we supply mechanical power to the shaft of our motor and increase the speed and the associated 'speed voltage' above the supply voltage? Our motor has now become a generator and will PUSH current in the opposite direction back into the battery and charge it.
My brain is strained, and if you got this far, I'll bet yours is to. I hope this long-winded blurb helps a few people get a better grasp on 'juice'.
The original question about why the resistors on the back of the switch get hot? They're supposed to, that's how the motor speed is lowered. Newer cars place the blower resistors inside the air duct to help keep them cool and I suspect to keep wires and such from touching them.
If they seem way to hot, then something is overloading the motor and causing it to draw to much current, the oilite bushings my be dried out and gummed up or there may be trash rubbing on the fan wheel.
John
