air conditioner

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motobeart50

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Hello
A quick question. My a/c unit has failed. The fan and compressor motors aren't starting ,with that distinctive
humming sound. There is a 25/5 uf capacitor and a 30uf capacitor in parallel. Is there any reason I can't use a single
55/5 uf capacitor instead of the two caps.I would think if the single dual capacitor will fit in the space provided that that would be okay.
I would hate to damage the unit further, so that's why I'm asking the question.
Thank you
Barry
 
Are you just assuming that the cause of the failure is an open circuited cap? You can make a rough test of the cap with an ohmmeter if you know the procedure.
Are all three capacitor sections really in parallel? More common would be separate start and run capacitors.
What about the voltage ratings?
An alternative cause for your symptom would be an open centrifugal start switch inside the motor is the capacitor is only used for starting.
If you get a wiring diagram for the unit, it will let you confirm the roles of the two capacitors (three capacitor sections?)

Even at the same nominal capacitance and voltage, some AC (not A/C) capacitors are intended for constant use and others are for intermittent starting use only and will not live as long when used as run capacitors.
 
two capacitors

two capacitors

GoldDigger said:
Are you just assuming that the cause of the failure is an open circuited cap? You can make a rough test of the cap with an ohmmeter if you know the procedure.
Are all three capacitor sections really in parallel? More common would be separate start and run capacitors.
What about the voltage ratings?
An alternative cause for your symptom would be an open centrifugal start switch inside the motor is the capacitor is only used for starting.
If you get a wiring diagram for the unit, it will let you confirm the roles of the two capacitors (three capacitor sections?)

Even at the same nominal capacitance and voltage, some AC (not A/C) capacitors are intended for constant use and others are for intermittent starting use only and will not live as long when used as run capacitors.
Click to expand...
The two capacitors have the same ratings for voltage and freq.
I can visually verify they are in parallel.that is the 25 uf section is in parallel with the single 30 uf capacitor,with the 5 uf section connected to the fan.I did check them with a meter and both read open with no ramping up or down of the display
 
CSR and PSC

CSR and PSC

Single phase condensing units are set up one of 2 ways. 'They are always PSC motors and you will always have a RUN capacitor and a separate capacitor for the condenser fan, OR a dualized capacitor that has a common connection to deal with FAN and COMPRESSOR.
If you look at the original unit wiring diagram it will show exactly how they are wired in.
You may then have a start capacitor and potential relay [CSR configuration ] or a PTC device for start assist. The start capacitor will generally not be metal, but black hard plastic or more modern ultra cheap plastic.

Often times with a failed RUN capacitor the compressor will start but will go off on internal overload shortly due to overcurrent.

Sometime a service Tech may parallel 2 capacitors to get the MFD value of one he does not have on his truck, but generally you will not see PARALLEL capacitors, but one for compressor and one for ODF motor as stated above. The smaller value capacitor is always the fan.

You symptom sounds like a locked rotor failure.
This can mean the compressor is mechanically locked and will attempt to start then open the internal protector.
In this case the compressor will draw locked rotor amps and it can be read
You must verify the integrity of the wire all the way to the compressor to test accurately.

You may:
verify good capacitors and check the wiring all the way to the compressor terminals being sure nothing is burned or loose
remove the compressor terminals and check the motor windings C-S S-R R-C and each to ground with your meter set for OHMs, and then attempt re-start with a recording ammeter on the R or C leg and see if you read LRA
 
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motobeart50 said:
The two capacitors have the same ratings for voltage and freq.
I can visually verify they are in parallel.that is the 25 uf section is in parallel with the single 30 uf capacitor,with the 5 uf section connected to the fan.I did check them with a meter and both read open with no ramping up or down of the display
Click to expand...
Sounds like open circuited capacitors. Does fan run or is the 5uf portion open as well? (it likely is as these capacitors typically have a fuse link that opens the circuit when pressure builds up in a failed capacitor and the case begins to expand from the pressure - this protects from having that capacitor explode.

Without the capacitor to create necessary phase shift in the aux winding the motor doesn't develop any torque on the rotor. An open capacitor doesn't even allow current to flow in the aux winding.

I have paralleled capacitors many times before to get something running in a pinch, all you do is add the values of each cap for a total value for the bank. In AC units there is often room to leave the extra capacitors in place, but with many other motors I usually have to rig up something temporary until I can return with the capacitor that fits the intended space.
 
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