I think in the latest home air conditioners compressor motor design, the manufacturers have adopted starting capacitor less design as the refrigerant pressure has been allowed to bleed down.
capacitor
- Thread starter domnic
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Mgraw
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- Opelousas, Louisiana
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I think it has to do with evaporator coil metering device design. When capillary tubes are used as the metering device start capacitors are not needed. These bleed down very quickly. Thermal expansion valves(TXV) bleed down very slowly. Even TXV's with bleed ports bleed slowly. Start capacitors are generally required for these. A fixed orifice bleeds down slower than a cap tube but faster than a TXV. Usually a start cap is not needed. In all cases a time delay will generally allow enough time for a unit to equalize pressures.
kwired
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They all bleed down, until there is equal differential across the compressor. It doesn't take much torque to start the compressor if there is no pressure against it.
junkhound
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To help visualize, using number similar to #27 for consistency (except on 2nd graph, 1 ohm vs. 10 for more realistic starting torque graph)
1st graph is how a 141mH winding and 50uF cap change the current (and winding and capacitor voltage) with frequency
2nd graph is for a 1 ohm winding motor of 141 mH and design start cap of 50 uF and shows percentage of starting torque as capacitor value changes for 60 Hz applied.
1st graph is how a 141mH winding and 50uF cap change the current (and winding and capacitor voltage) with frequency
2nd graph is for a 1 ohm winding motor of 141 mH and design start cap of 50 uF and shows percentage of starting torque as capacitor value changes for 60 Hz applied.
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Mgraw
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That is not always true, especially in refrigeration.
kwired
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Refrigeration often puts a solenoid valve in the refrigeration line and the unit is shut down by closing that valve and when pressure builds up to a predetermined level, a pressure switch is used to open control power for compressor contactor/starter. The unit is started by opening the solenoid valve which allows the refrigerant pressure to equalize, pressure switch likely operated before it is completely equalized but it would completely equalize if valve stayed open and for some reason the compressor didn't run.
The typical household AC unit doesn't have such a valve and pressure is equal in the system when not running, or at least within a couple minutes or so after compressor stops running.
gar
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160918-2344 EDT
junkhound:
Very nice plots.
The reader should note that as frequency goes below resonance that in the limit the capacitor voltage equals the source voltage and the pure inductive voltage drops to zero. But in a real circuit you can not put your voltmeter only across the inductive component.
Above resonance the roles are reversed.
Both above and below resonance the resistive component voltage goes to zero in the limit because in both cases current goes to zero.
Below resonance capacitance is dominate, and above resonance inductance is dominate.
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junkhound:
Very nice plots.
The reader should note that as frequency goes below resonance that in the limit the capacitor voltage equals the source voltage and the pure inductive voltage drops to zero. But in a real circuit you can not put your voltmeter only across the inductive component.
Above resonance the roles are reversed.
Both above and below resonance the resistive component voltage goes to zero in the limit because in both cases current goes to zero.
Below resonance capacitance is dominate, and above resonance inductance is dominate.
.
David Goodman
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That shouldn't happen with most res. and comm. A/C systems that use compressors. The circuitry in the thermostat, air-handler or the condensor units have time delays to prevent a rapid restart. Sometimes all three have time delays that have to satisfy sequentially when powering up a system. That is a real pain for the technician.
If a breaker trips, then something else is wrong. A rapid restart would more likely cause the motor to trip on the overload device and not the breaker. The breaker protects the wiring, but the overload protects the motor.
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Not sure what is most common these days, but the older A/Cs, refrigerators, etc. that I have will indeed trip the OL after a few seconds when a quick restart is attempted. There will then be delay, typically around 5 minutes, before the OL lets it try to start again.
Mgraw
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- Opelousas, Louisiana
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On "pump down" systems as you described the compressor cycles on low pressure not high. The refrigerant doesn't get close to being "equalized" in a normally operating system.
There are some systems that operate on pressure control to regulate temperature. These units do not have thermostats. They also never equalize in a properly operating system.
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The OL will trip but not the breaker.
It is for this reason I always install and recommend using the largest breaker listed on the label.
Mgraw
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- Opelousas, Louisiana
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I agree with that except for DC inverter units. Many give a "recommended fuse size" and that is what I would use.
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Come on, you must know me enough by now that if the labeling required a fuse I would use a fuse.
Mgraw
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Come on, you must know me enough by now that if the labeling required a fuse I would use a fuse.
Of course, that is not what I am saying. On many DC inverter units the label will state MOCP and MCA(as required) but they will also give a "recommended fuse size" that is much lower than the MOCP.
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OK, well then we are different. I am going to use the largest size OCPD allowed.
My goal is no call backs on the electrical system. The company I work for does not pay me to help the manufacturer protect a poorly designed product. (If the inverter requires special fusing the manufacturer should provide it internally IMO)
Mgraw
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- Opelousas, Louisiana
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- Electrician
Actually it is more about a standard not keeping up with technology. DC inverter units don't have the same issues AC units have with huge locked rotor amps. The compressors and motors start slowly then ramp up speed as required by the demand.
kwired
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Correct, pressure will equalize though if compressor is not running and any valves in the lines are open. Systems set up this way usually are not PSC compressor motors with low starting torque either, they have a start capacitor or are three phase.
Fuse will also have a different trip curve then a breaker.
Also true, and such units probably have lower MOCP marked on them then a similar sized unit that starts across the line as well.
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Are you saying the standards would not allow the manufacturer to choose to include internal fuse protection for any component they feel needs it?
Mgraw
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- Opelousas, Louisiana
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- Electrician
I am sure they could but why? The standards could be changed to modify the way mca and mocp are calculated on these units and there would be no added cost to the equipment. Likewise under current standards the least expensive solution would be to follow the manufactures recommendations.
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