Can someone help me understand why one shouldn't wire a variable resistor...

...power switch to a submersible water pump?


Pardon my ignorance, but considering the nature of electricity, isn't there some reason why one shouldn't take this...

**broken link removed**

...and wire it to the power cable of this...

**broken link removed**



Seems like this guy was trying to explain why, but it's so beyond me, I don't get it...

https://www.420magazine.com/forums/...i-use-dimmer-switch-control-speed-my-fan.html


Also, here's some info that can help explain different pump motors...

https://pets.thenest.com/aquarium-pump-work-8082.html




Wow, look at this weird crap...

https://www.axair-fans.co.uk/technical/fan-control.php

I wish I understood this stuff.

It might work but I would doubt it.

What it is trying to explain is: the speed of an AC motor, such as used in the small pumps, isn't proportional to the voltage you connect them to. They work by spinning a fixed magnet in an alternating magnetic field, the speed at which it spins is directly related to the rate at which the current alternates. I'm guessing from the web sites you list that you are in the USA so the speed is fixed at 60Hz (60 cycles per second = 3600 revolutions per minute). The lighting dimmer circuits do not change that frequency, to dim a light they chop out part of each cycle, making the voltage appear to be lower. The more they chop out, the dimmer the light becomes but the frequency stays the same. It works on lighting because although chopping the cycles would tend to make the light flicker, the time it takes for the filament in the lamp to cool down and re-heat evens out the light it produces. In a motor, when you chop the incoming waveform, the speed does not change, what happens instead is you starve the motor of power. It's speed tries to remain constant because it is locked to the AC line frequency but the power 'pushing' it is reduced. The effect is that the motor will slow down but erratically, an analogy would be running out of fuel in a car. You might for example find the pump still runs at the same speed out of water but stops when submerged and has the added volume of water to pass through it.

The only viable solution is to use variable frequency drive (VFD). One of the pages you listed shows a VFD controller. These convert the incoming AC line voltage to DC then back to AC again but in a way that allows the new AC to change frequency. The effect is that the voltage, and hence motor power stays fairly constant but the frequency making the motor rotate is controllable. The motor maintains full power while the speed is adjustable. The only caveat with a VFD is that the motor design will be optimized for 60Hz operation and as you drop or increase the speed it will become slightly less efficient. It should work over a fairly wide speed range though and be far more controllable.

Brian.

Wow, thank you so much for helping me further understand the nature of electricity. That actually made sense. Now I get the use of that 60 Hz.