high current on start winding while running

[jmdurell1]

Installed new scroll compressor in 3 ton heat pump, single phase. Starts and runs good, all pressures, SH, Subcooling and indoor cooling are within diserable range. However amps using an amp-clamp on start conductor and common conductor are the same while running and run conductor is half that amount. I have tried 3 new capacitors 45\5 MFD 440 volts. Shouldn't the Run amps be same as common amps and start amps be approx half that amount?

[Yuri B.]

It seems you are confusing "start" and "run" windings and everything is working fine (run winding should be connected directly to line - isn't so done yours?).
As long as the current being consumed does not overpass the rated current of the compressor's motor - and pressures and temperatures are good, all should be right.

[Brian_UK]

I'm not an electrical expert so will not comment but I remembered a bulletin from Bristol compressors I had seen which detailed the electrical checks to be made on first start up.

This may be of help I hope, see page 5...

http://www.bristolcompressors.com/InfoDocs/200026.pdf

[jmdurell1]

Compressor is wired correctly, Common Black lead from contactor line 1, Run Red lead from contactor line 2, Start Blue lead from capacitor Herm terminal.

2nd Red wire from contactor line 2 connected to capacitor Common terminal. The compressor is fitted with a keyed moulded connector furnished with new compressor that will only fit on way. Ohming the compressor terminals + ohoming the connecting wires gives same results. C-R is 1.3 ohms, C-S is 2.7 ohms and S-R is 4 ohms. While compressor is running the amps with amp-clamp on conductors Black Com-6.37 amps, Red Run 3.78 amps, Blue Start-6.6 (Old start wire was yellow). Checking voltage while running, S-C 290 volts, S-R 370 volts, R-C 240 volts.

Using this formular, Amps X 2,650 ÷ Voltage = Actual microfarads, I come up with 46.7 MFD while capacitor is in circuit running. SH & SC are in normal range for the load. Thanks for reading.

[Yuri B.]

I would not do experiments selecting capacitors - would put that one which manufacturer of the compressor (the motor) demands.

Now yours compressor consumes 6.4 A - does it has a name plate stating the rated current ? (in the formula

Amps X 2,650 ÷ Voltage = Actual microfarads

would you say, where the "Amps" are taken from?)

[Yuri B.]

With mixed capacitive / inductive impedance one cannot at all rely on indications of a clamp-meter.

[jmdurell1]

Amps taken from start winding lead, from an article in an A\C journal as per:

How to check a run capacitor’s “actual microfarads

Unfortunately, some capacitor checkers we use in the field are ineffective at testing for a bleeding/leaking capacitor because there is no load on the capacitor at the time that it’s being tested.
The good news is that there is a fairly reliable way to test a capacitor under load.

Name plate on compressor says Rated Load Amps 14.7, LRA 83

While the compressor is running, measure the ac voltage across the run capacitor. You will be reading the voltage that the compressor is generating. The term for this is “back electromotive force.” Measure the amperage being drawn through the start wire between the capacitor and the compressor start terminal.
Be sure to keep your amp meter away from the components in the control box - that could distort your reading. Use the voltage and amperage readings you’ve obtained in the following formula:
Amps X 2,650 ÷ Voltage = Actual microfarads.

If the solution to your test gives you a microfarad rating that is 5% below the capacitor’s labeled rating, be suspicious. If the results of your test show the capacitor to be 10% or more below the labeled rating on the capacitor, replace it.


The most important amp draw reading to take is of the start winding (the wire that is connected to the run cap). If it is approx. half of the run winding then the run cap is good. For example if the run winding is 14 and the start winding is 3 then we have a problem. This is one of the biggest call back producers in our industry. Techs think that the run caps are either bad or good (not true) many many are just weak.
See if no current is going thru the start winding (run cap weak/bad) then all the current will go thru the run winding and the amps will show much higher than normal. This is the reason you will find melted/loose connection on the common or run terminal of a compressor. You will never see it on the start terminal.

[jmdurell1]

Re: high current on start winding while running
With mixed capacitive / inductive impedance one cannot at all rely on indications of a clamp-meter.
-----------------------
You are probably correct, but I have measured current on 3 other compressors, same model (but at least 5 years older compressors) and they each have near equal amps on Common & Run conductors and approx half on start conductor.

This one has 6.2 amps on common, 6.6 amps on start, 3.6 amps on run, continuely while running. conductors are keyed to compressor and checking the resistance on each terminal, C-R, C-S, S-R indicates they are wired correctly as the C-R Resistance is lowest, C-S is mid, and S-R equals the sum of the C-R & C-S.

This may just be the characteristics of new generation compressor even though same model as the one replaced.

[nike123]

AC currents on inductive and capacitive loads cannot simply be summed. They are vectors and you must use vector algebra to calculate these currents.
http://www.allaboutcircuits.com/vol_2/chpt_2/8.html

[GHAZ]

Hi is 3 ton a 3hp compressor , i would use 30 mfd for run and 260 mfd for start winding with a potenial relay

[GHAZ]

ive just read your last post and it says that while running the amps on start are higher then run, the runs amps should be higher then the start amps, because the relay should take out the start capacitors, when the compressor is running after a second or so , if not then the wiring up of the compressor is wrong. after the compressor is running you will get an ampage reading on the clamp meter on the start winding but it will will be very low, i think the start and run are mixed up or if you are using a potential relay its the wrong size .

[jmdurell1]

This unit uses only a Run capacitor, specified on equipment and in manufactors website as being a 45\5 MFD. There is no relay or other type starter, only a run capacitor. Wiring diagram supports this.

It is a 32K BTU model, I believe it is installed as a 3 ton model as the condensing unit is a 3 ton and I read several references to it being a 3 ton.

It is installed with a 42K indoor unit (3½ ton). First compressor was also a 32K, same model, just 5 years older. Orfice piston installed originally was a .074, I had to replace it with a .071 to get the super heat up. However that does (or should not) affect the electrical incedent as that problem was occuring before orfice was changed. Again, iit starts smooth, cools good, cycles on & off and is very quiet.

[jmdurell1]

Update on this units abnormalty. It has now ran for 90 days in both heating and cooling. It still runs quite with normal superheat and subcooling. Pressures and evaporator Temp differences are right on target. Still have the high amps on start winding and low amps on run winding.

[jmdurell1]

This unit has now ran for over a year and still has the same high current on the start winding. Runs quite, cools & heats good with correct superheat & subcooling! Have had no complaints with it, nor have I ran into any other unit that has these characteristics.