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.)