🚀 go-pugleaf

Jordan wrote:
> 
> Can anyone please shed some light on a query that is puzzling me?
> When a magnet on a rotor passes a fixed coil, current is generated and
> this it's varying magnitude is usually represented as a sinewave.
> But, I've seen some where the waveform is not quite the regular sinewave
> shape.
> Namely, on a car's inductive pickup, the output is more like: starting
> at zero, slowly increasing, peaking and then steeply plunging to the
> other side of the zero line, before again slowly going back to zero.
> Why is this?

The sine wave assumption is a mathematical ideal.  If a rotor
consisted of a permanent magnet that produced a uniform field
strength, with the field being purely straight lines, and the stator
consisted of a single loop of wire that encircled this field passing
through the center of the shaft on each side, then the result would ne
a pure sine wave voltage in the rotor turned at a fixed speed.  A
car's inductive pickup is a coil wrapped around a bar magnet, and this
generates voltage as widely separated bumps pass near one pole of the
magnet.  For a large angle, there is effectively no change in the
field strength (between bumps) so no voltage is produced.  Then as the
bump approaches the pole, the field increases slowly, causing the
slowly increasing voltage.  Just as the bump passes under the pole,
the field stops increasing and the voltage falls to zero, since it is
proportional to the rate of change of field, not to the field
strength.  Then as the bump moves out from under the pole, the field
falls , fast at first, with a slow tail off.   This generates the time
and polarity reversed waveform that the approach caused.  Then nothing
happens for a while till the next bump approaches. 

-- 
John Popelish