This is a bit of a vague question as I'm not 100% sure what I'm asking so I apologise in advance, I'm just hoping to better understand something I noticed the other day.
In my endless tinkering I recently dismantled a cordless power drill to cannibalise the DV motor, as well as the variable speed control trigger assembly to be used on another project which requires a variable speed motor with good torque. The battery power source for the drill is rated at 18V (measured closer to 19V with a voltmeter) and this works perfectly. However, I would like to wire it up directly to a mains plug so that I can run the motor for long periods of time (on the order of a few hours). I happened to have a mains power supply laying around with a measured output of ~21V (rated 24V). I assumed this was probably "close enough" and hooked up the circuit and what I found surprised me.
Under normal circumstances, if the motor/speed controller are connected to the 19V battery supply that it came with, and I gradually depress the speed controller (which I assume is just a potentiometer?), the speed of the motor gradually increases expected as expected. However, when I connect it up to the 21V supply (also DC) and gradually depress the speed controller just as before, the motor doesn't turn at all until the button is almost completely depressed, then suddenly the motor whirls into life at its maximum speed! So when the source voltage is only 2 volts higher, the potentiometer(?) starts behaving more like an on/off switch.
I was not expecting this. Does anyone have some explanation as to why this would be the case? I was expecting little to no significant difference in behaviour, maybe a slight speed increase due to the higher voltage, but for the speed control to just stop working is strange to me.
To be clear, the speed controller still works fine, it's not broken. If I hook it back up to the 19V supply, everything works as normal.