Pulsating DC?

Question

I read how Tesla's AC won out over Edison's DC because AC can be reduced with transformers. When I look at an animation of a DC generator, it looks like it's output is pulsating DC. Don't transformers work with that, ala a car's spark coil and points? Why couldn't Edison use transformers with pulsating DC from generators?

Answer 1

Pulsating DC still has a DC RMS component, which will saturate the core of most transformers and cause great heat loss (as in transformers catching on fire). To design a transformer that is not prone to saturation, you need to put an air-gap in the core, and that reduces efficiency -- a LOT.

Ignition coils are designed to saturate, but they don't have too much power running through them, so they are still physically small and don't get hot.

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Answer 2

Pulsating DC does not really make sense. What you are describing is a rectified AC sinewave. A rectified sine-wave has a DC component, but it also has a strong second harmonic. (and other even harmonics). It is the energy in the harmonics (mostly the secondary harmonic) that can pass through a transformer. But, the DC component will cause the transformer core to go into saturation and so the transformer will end up absorbing most of the energy in the harmonics.

A car's ignition coil works by storing energy inside a ignition coil's (inductor) magnetic field, which is proportional to the square of the current passing through the coil (inductor). When the circuit is broken, the inductor tries to maintain it's current, and generates a very high voltage to accomplish this. However, because the circuit is switched on and off, it is not true DC. A step in applied voltage (from switching the inductor) has AC content. This can be seen by looking at the fourier transform of a step function.

Answer 3

DC generators actually put out a pretty decent dc wave shape. Pulsed DC will get energy through a transformer but only when the voltage is changing. That means the secondary only has output during the rise and fall time of the pulses.