5 May
2021
5 May
'21
3:24 p.m.
On May 5, 2021, at 11:07 AM, Grant Taylor via cctalk
<cctalk at classiccmp.org> wrote:
I have found the Motor Generator thread to be fascinating and enlightening. But it has
made many a reference to the 400 Hz or other frequency much higher than mains line
frequency. Despite the comments about the frequency, I'm still confused as to why the
higher than mains frequency was used.
Were the higher frequencies used because it directly effected the amount of time /
duration in (fractions of) seconds between peaks of rectified (but not yet smoothed)
power?
I ask because it seems to me like the percentage of time / duty cycle of raw rectified
but not yet smoothed) power would be the same at any and all frequencies. Is this
assumption / understanding correct or completely off the mark?
A few different people made references to the amount of capacitance needed at 400 Hz et
al. vs 50/60 Hz mains frequency. Someone even spoke about high power DC being produced by
polyphase converters and the possibility to tweak tweak winding voltages in order to
possibly do away with the need for capacitors.
There are a couple of considerations: transformers, filter capacitors, and ripple
amplitude.
Ripple amplitude is affected by the number of phases and by whether you use half wave or
(the normal) full wave rectifiers. For example, a full wave single phase rectifier
produces a abs(sin(2pi * f * t)) waveform. A multi-phase full wave rectifier produces the
max of these waveforms offset by the phase angles -- in other words, the
"valleys" are filled in by the "peaks" of some of the other phases.
The ripple filter then smooths that out into DC, or more precisely, makes the ripples a
lot smaller. What exactly those waveforms then look like depend on the filter used.
So rectifying 3 phase power produces much smaller ripple than rectifying single phase.
For exotic applications where you can afford to deal with more than 3 phases you can make
the amplitude smaller still.
The above is independent of frequency.
Now for transformers. As the operating frequency increases, the amount of iron or other
core material needed goes down. So a 400 Hz transformer for a given amount of power can
be much smaller than a 60 Hz transformer for the same job. This is why modern power
supplies are "switching supplies": they convert the mains voltage into high
frequency power -- sometimes as high as a megahertz or so -- which allows the power
transformer to be tiny.
Finally, capacitors. The ripple attenuation of a filter depends on the impedance of the
filter elements. The most common is a capacitor filter, so the filter capacitors (their
AC impedance) is in parallel with the load impedance. The ripple attentuation is
basically the ratio of filter capacitor impedance to load impedance. Capacitor impedance
is inversely proportional to frequency, so the use of a higher frequency allows the use of
smaller capacitors. This too is used in modern switching regulators, where the capacitors
are often just a few microfarads and are generally ceramic, not electrolytic.
In all this there are limits on how far you can usefully go. Motor generators above a few
hundred hertz are hard to build, though there exist alternators that produce
multiple-kilohertz output (look up Alexanderson machines). Switching regulators can to
into the MHz range, but if you go too high your benefits stop because the transformers and
capacitors are no longer close enough to "ideal components". For example, the
impedance of a capacitor stops dropping at some frequency that depends on its design, this
is the ESR (effective series resistance) of the part. Near or above that frequency is is
no longer a good filter capacitor, or for that matter a good capacitor for many other
purposes.
paul