Perhaps
@Brew12 can correct me if I am wrong.
Thanks for the invite! I love these conversations!
More head height on an AC pump will slow the motor (greater load), causing it to draw less amperage.
current (I) = voltage (E) / resistance (R)
R goes up, I goes down.
This isn't quite what happens. In Ohms Law (I=E/R) the resistance refers to the electrical resistance of the motor, not the opposition to flow. The only way to change resistance would be to mechanically alter the motor. Ohms law also only applies to DC. In AC we also have impedance that goes along with resistance. In the case of an AC return pump as the rotor slows down due to increased back pressure the relative speed between the rotor and stator increases. As the relative speed increases the counter generator action in the stator increases and this reduces current, which reduces power.
The exact same thing applies to a DC motor. You increase head height WITHOUT changing motor speed, and you'll use less current.
This happens with a true DC motor, but it doesn't happen with a what hobbyists call DC motors. Since the DC motor has a controller, the controller dictates the power to the motor. The motor will achieve the speed and subsequent flow rate that match the provided power.
As you turn up the speed on your DC pump, you're increasing volts, thus increasing watts.
There are several ways to increase speed to a DC pump other than just increase voltage. You can either increase the frequency of the AC waveform you send to it (normally a square wave or filtered square wave). You can use PWM control. Or you can control which point in the AC sine wave the thyristors fire to control the power output to the motor.
Nope, they're really DC motors. The controller varies the voltage. On the cheap ones, at least, they lower voltage by increasing resistance... meaning they do not actually use less voltage when you run them at lower speeds, they're just shunting off some through a resistor (hopefully, a linear regulator).
They really aren't DC motors. A DC motor needs to have some form of commutation which would require brushes between the rotor and the stator.
If you took a brushless DC motor, and applied DC to it, it would not turn more than 1/2 of a rotation.
I know that the cheap Chinese DC pumps that I've bought were, in fact, DC motors... I put a meter on the output of one I had and tested it.
I don't think you were getting the result that you think you were. I'm not sure what kind of meter you were using but if it wasn't listed as "True RMS" you won't get an accurate reading. Even then, the best way to see what is really going on is with an oscilloscope. This way you can see exactly how the waveform is being manipulated to create relative motion.
One point often left out in the AC vs DC pump conversation is the inefficiency of the DC power supply bricks we use. You've got a DC pump that is, at the speed you want to run it, say, 15% more efficient that an AC pump of the same output... but you've got it plugged into a cheap DC wall wart that runs at 70% efficiency... you're not gaining anything.
Now this comment is something I completely agree with and, from a practical point of view is more important than any of the theory discussed above. Manufacturers love to talk pump efficiency with DC pumps. They don't like to mention total system efficiency. The heat generated in a DC controller is all electrical loses. It is common to have 10% to 30% total loses in a small motor controller like these. Yes, the pumps are more efficient (largely due to impeller design) but when you add the controller? Not so much. And the further you run them from 100% speed the less efficient they get as a system.
Hope that helps, and let me know if I can explain something better.
