I need a sanity check - I think I understand, but there's doubt.
If I rectify AC with a bridge and use the full-wave output to power a DC motor, it's not the same as using "pure" DC. There is an AC component to the full wave. (The motor is a brushed PM if it matters.)
If I take the Fourier series of the 1/2-sinusoid and consider each component separately & then superimpose them, I should get the behavior of the motor on the full-wave source. The Fourier series consists of a DC component and the even harmonics of 120Hz. The DC will simply drive the motor as would a battery. The AC, however, will not have a net affect on motor's output: for its positive 1/2 cycle it will contribute to the output and on the negative 1/2 it will oppose it. So the superimposed result is that the useful motor output is due to the DC component only and the AC components only produce a modulation (240,
480, ... Hz "buzz") on the output.I long ago lost any ability to do the Fourier calculation, but somewhere on the web (source lost), I found that the DC component (a0) is 88% of the RMS AC input to the bridge. (If it's not too much trouble, could someone confirm this?)
Now here's the problem: reality contradicts theory (I hate when that happens!). The theory is that if I apply 20v AC, for example, to a bridge & use the output to drive a DC motor, that motor will run at 88% of the speed which it would if it was driven a regulated DC source of
20v. (DC motor speed is linearly proportional to voltage.)In a test, it doesn't - it actually runs faster on the rectified AC than on DC!!! That's impossible! What's wrong - my understanding of the theory, or my test? Or both? Or ...?
Thanks, Bob