Wacky Weekend: NERC to relax power grid frequency strictures

Jussi Peltola pelzi at pelzi.net
Sun Jun 26 18:50:41 CDT 2011


On Sun, Jun 26, 2011 at 06:46:09PM -0400, Pete Carah wrote:
> HVAC compressors have their own problems; once fully stopped you have
> to wait for the liquid to clear the compressor before restarting, or
> have LOTS of torque (like a car unit) available (and a supply of new
> belts :-)

[begin OT lecture about refrigeration]

If you have liquid in your compressor you are very screwed, it will
dilute the oil and cause the surfaces of the compressor to wear out very
quick (and if enough liquid gets far enough to actually get compressed,
the compressor will very likely self destruct because liquids will not
compress)

To avoid this, many larger systems actually have a solenoid valve before
the metering device which completely stops the refrigerant from moving
the instant the compressor stops - the liquid refrigerant will then stay
in the condenser, and the machine will cool instantly on the next
start-up as an added benefit. These machines start against their normal
working pressure.

Some bigger systems have automatic pump-down which will actually run the
compressor after shut-down against the closed valve with a pressure
switch that makes sure all the refrigerant is in the condenser (and
receiver) - this is required if the charge is sufficiently large that
the amount of liquid in the evaporator (that hasn't evaporated yet) is
enough to flood the compressor after shutdown. These systems can start
against more than the normal operating pressure.

The only ACs that have problems with restarting due to backpressure are
single phase units - my window rattler has a bad thermostat that
sometimes bounces the compressor off for a second. The result is a very
loud hum and dim lights until the circuit breaker blows. But even single
phase compressors with a proper start relay and capacitor can start
against pressure, and these are fitted (called a hard start kit) if the
system has a solenoid valve, or if the compressor is not starting due to
wear or weak mains power (but these uses are band-aids.)

All single phase ACs with electronic controls that I've seen have either
a start up delay (often a few minutes, which I think is mostly to avoid
excessive inrush when the power comes back) or they will not start at
all without pressing the button manually. This usually works very well,
but in some the logic is in the indoor unit, which in some cases can be
powered separately from the outdoor unit.

> What I mean by kind-of capacitive is a bit odd; it looks capacitive on
> the voltage rise but resistive and/or a little inductive on the peak and
> fall, and the current is 0 when the voltage is below some threshold. 
> Actually these days the feds (and I believe EC also) spec power-factor
> correction for switching power supplies so this effect is less.  The
> main way this is arranged is to make sure the input-rectifier filter
> capacitors are SMALL enough; then have the switcher waveform compensate
> for the voltage droop.  Bigger VF motor controllers do this  also.
 
This is true, but if you are feeding them through on-line UPSes they
should look pretty resistive again. The EU has pretty strict
specifications for PFC, and practically all modern servers I have tested
are visually indistinguishable from a light bulb when viewed on a scope
(I know that is not a very good measure of distortion.)




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