Yes, many are starting capacitors that allow max current into the windings
to get the thing rotating. Pretty much a short on the grid for a short
duration.
Correct, reactance in itself can not produce harmonics. But the
interaction of the offset between voltage and current and reflected power
can.
Dave
On Thu, Aug 20, 2020 at 10:12 PM Jerry Gaffke via groups.io <jgaffke=
[email protected]> wrote:
Yes, PFC is an issue at 60hz.
Just one nit to pick:
Harmonic content is a signature of a reactive load - a
purely resistive load can not generate harmonics. here.
An inductive or capacitive load is still linear, it does not generate
harmonics.
All it does is cause the current through the device to be out of phase
with the voltage.
Small switching power supplies used to be just a bridge rectifier on the
AC
feeding a big cap, the high voltage from that was then brought down
to 12vdc or whatever using some switch mode topology, perhaps a flyback
converter.
But the bridge rectifier only has current flowing in spikes at the
positive and negative
voltage peaks of the incoming 60hz voltage sine wave, and that definitely
does
create harmonics. A number of power factor correction schemes have been
developed to smooth out the current draw from the mains for this
application.
In some other thread, we spoke of using a common mode choke in the antenna
feed line to keep household noise from sneaking into the receiver,
assuming the antenna itself is far from those noise sources. Those nasty
little wall warts without PFC would be a prime example of why this might
be needed.
I don't know a lot about the various kinds of electric motors, but many
have a "run capacitor" in there to balance out the inductive load for
some crude PFC. Not just to be nice to the utility company,
but to be able to get some torque out of the motor.
Jerry, KE7ER
On Thu, Aug 20, 2020 at 02:24 PM, David Eckhardt wrote:
Some excellent points, Jim!
Let me take this concept of reflected power to something we are all more
familiar with: The AC power sockets in our walls. Whether its 120 vrms,
220 vrms, 308 vrms, 408 vrms, or 480 vrms, at 60 Hz, the power source
assumes a purely resistive load. Where else have we run into this
concept? Definition of resonance: no reactive component to the load (or
source). Why does the power provider prefer resistive-only loads? Their
generators run cooler and the whole grid distribution system operates
more
efficiently. They are a resonant system. Now what happens if your
appliance, not a filamented light bulb, your toaster, your electric oven
which are purely resistive, happens to not resonate at 60 Hz. Heavy
inductive motor start-up is a good example. Note how your lights dim when
these items are first started? Reactive loads reflect power back onto the
power grid and ultimately the generators. Those non-resistive appliances
reflect some complex portion of their load onto the power grid and
ultimately the generators. These reactive loads many times require what
is
called power factor correction (PFC). This is nothing more than a
matching
network to cancel the complex portion of the load which those reactive
loads present to the power grid and generators to keep the whole system
cooler.
By law, appliances less than 70-watts power consumption are exempted from
PFC (Power Factor Correction). Above that, laws driven into the systems
by
the power utilities, the FCC, and the EU require PFC. The EU requires (by
law) limits on harmonic output from appliances and power consuming
installations. Harmonic content is a signature of a reactive load - a
purely resistive load can not generate harmonics. Germany is a stickler
for this. However, large consumers of power like large factories (think
Detroit auto manufacturers) are exempted from PFC. If California required
PFC on all power consuming installations, I'm willing to bet there might
be
no need for rolling power outages. I can't prove that statement, but I'm
sure requiring PFC would alleviate much of the problem Cal. has every
summer. Of course, the power providers make more money in supplying more
power to deal with the reflected waves from complex loads, so, they may
prefer no PFC (California power lobby?). Money talks.
These PFC circuits are nothing more than matching circuits. They are
designed to provide to the power grid as much as possible a purely
resistive load from a complex power consuming appliance. Since the power
grid is pretty much zero-ohms, non-reactive, any resistive load can be
handled with grace and no need for generating extra power to compensate
for
reflections caused by reactive loads to the grid.
Matching loads to sources doesn't apply just to RF energy. It's
everywhere. Bet you weren't aware of the PFC and power grid matching?
--
*Dave - W?LEV*
*Just Let Darwin Work*
