Potentiometer DC motor and current draw question

[Stephen Durr]

Hello Everyone,

This is my first post on the forum so I hope it won't be too ridiculous.  I'm not an engineer, just an old fart that likes to play with old cars so I don't know much electronics.  Even though I am old Dave would call me a "young playa" in electronics.    Most of the stuff I work on is very simple DC electrical circuits on old cars, but I am interested in learning more about electronics.  I have probably watched over a hundred of Dave's videos so far and I really enjoy them!  Perhaps electronics will become my second hobby   

For my very first question on the beginner forum I hope someone can teach me a little something about the current draw of a DC motor controlled by a potentiometer.  Unfortunately I do not have a schematic of the circuit but maybe the answer is so simple that one is not needed.  Anyway, I have this old car (1970 Dodge Challenger) that is equipped with variable speed windshield wipers.  Long story short, I no longer have the OEM/factory dashboard or any of the original switches so I ordered a new switch and I wanted to bench test it with the wiper motor before installing it.  I wanted to test the current draw so I would know what size fuse to use to protect the circuit.  So I hooked up a DMM and tested it.  This is where my confusion comes in.  On the lowest speed setting (potentiometer turned to lowest setting) the current draw was about 2.5 amps.  As I turn the potentiometer clockwise the motor speeds up as expected, but the current draw decreases down to about 2.0 amps on the fastest speed.  Why does it draw less current to run the motor faster?   

Here is a short video of my bench test... about as long as a bee's dick   



Edit 1: can someone please tell me how to properly embed a youtube video in a post?  I just pasted the URL directly into the body of the post since I did not see any buttons for "embed video".  But it just shows up as a hyperlink.  How do you get it to display as a youtube video?

Edit 2: Ok, I guess the forum doesn't like the "embed url" that has youtu.be in it.  So I deleted that and pasted in the full "video url" that has in it and then changed the https to http.  I'm not sure if that's the official technique, but anyway it seems to have worked.

[SeanB]

2-3A unloaded is fairly typical for these motors. You will need a 10 or 15A fuse to handle the full load current and yet still blow when the motor is jammed or tries to move heavy snow or ice.

The reduced current is because as the motor turns faster the back EMF generated by the rotor increases, and this reduces the current draw. The control either works by dropping power in a resistor, or by PWM of the voltage to the motor, likely in your case it simply adds a variable resistance to the circuit as it is fairly old.