ECN Forum Forums General Topics of the Trade General Discussion Area Combination Resistive/inductive circuits question

If I have a DC motor with 4 speeds controlled by a multi tap resistor such as in an vehicle HVAC blower this makes the circuit resistive/inductive. My question is, when the motor is running on a lower speed, and say the motor is getting older with bearing wear, will the motor draw more current through the resistor? I know that voltage is required to push current thru a resistor and I know that a motor will pull more current when voltage drops to maintain power, but will a motor load pull additional current through a fixed resistor to maintain power as it ages?

I guess another way to put it is when the motor gets loaded heavier will it pull more current through the fixed resistor if voltage remains the same?

Here is the argument, a claim is being made that on the 4 speed system with a resistor pack, the lowest speed is more likely to experience overcurrent/overheating issues due to the fact that it has the most resistance which will force the motor to draw more current to maintain speed than it would on the higher speeds with less resistance.

My thought is that the resistor drops voltage which in turn limits current and when more heavily loaded, the motor will simply slow down (which will of course result in a reduction of counter EMF and increase current draw but to what extent can it increase current draw with the resistor in series?)

I argue that the highest speed will be more likely to experience overcurrent/overheating because it bypasses the resistor and thus isn't as limited in available current.

I suppose in the end, it would depend on the load on the motor. You certainly see more heat in the resistor box when the resistors are in the circuit but the overall current will drop as you add resistance.
If you are pumping a fluid (gas or liquid) the load itself decreases as the rotor speed drops. If you were attached to a direct mechanical load you would start approaching locked rotor a lot faster and that would be the effect you are talking about.
DC motors are not going to react to voltage decreases like an (AC) induction motor tho.

Thanks Greg, so you are saying that the DC motor will not draw more current to keep power even as voltage drops or load increases? It will instead just slow down?

The efficiency might be lower but the current will be going down if this is that blower in the car you used as an example. .