A slightly differnt A/C system

[pbreed]

Upfront this is not in a CAR, its in an Airplane.
Its my Airplane and I have my Aircraft Mechanics license so its legal for me to work on it.

When I comes to A/C system I'm lost.

The system is 1980's vintage. (Both the A/C and the plane)

It has a York EFR210 piston compressor driven with a a large 28V DC motor via a belt drive.

Its an R-12 system that has not been converted.

The problem is that there is too much load on the compressor.
If I charge to the point of the no/few bubbles in the sight glass it works ok, but after a few minutes it blows the breaker for the compresor motor.

The compressor motor is supposed to draw 45A it draws 60+

If I drain refrigerant to the point the compressor current is in spec, the A/C basically does not get cold.

I've replaced the motor with a rebuilt unit.
I've replaced the compressor with with a new unit.
Old compressor had the correct amount of Mineral OIl in it.
New compressor came preloaded with PAG46, so that is what is currently in the system.
I'd expect that it also has residual mineral oil.

The Low side pressure is 20 to 40 PSI and the high side is 150 to 180.
Both in spec via the service manual for the aircraft.

I have no leaks... (it holds a 30" vacuum for 3 days with ZERO gauge movement.)

I added an extra valve so I could evacuate the line connecting to the refrigerant to the gauge set before adding
refrigerant, I'd expect is has NO air in it.

It has a sight glass on the filter dryer and when I fill to the point of very few/no bubbles the compressor draws too much current and pops the breaker.
I've instrumented the load current on the compressor and the overload is real (IE the breaker is tripping at the correct load.)

I'd suspect that the expansion valve might be stuck open, but on shutdown the high/low side pressure equalizes slowly, indicating the expansion valve closes.

I've been messing with this for more than a month and its driving me crazy.

Whens its running... there is about a 20F drop in air temp across the evaporator.

The inlet side of the compressor fittings have condensation on them and are very cold, maybe even below freezing..
(IE I've seen frost)

The hot side outlet from the Comrpessor is 180 to 190 Deg F/
The input to the filter dryer (after the condenser) is 120F or so.

These temperatures are not real precise. In the next day or so I'll get better more accurate readings.


My current plan is to order a new expansion valve and new filter dryer, flush the entire system.
Since this is a certified aircraft I can't just toss in any random parts, so I expect the new old stock
expansion valve and filter dryer to be in the several K$ range.
So If I could know whats wrong before changing everything it would be really helpful.

When I swap these I may switch to R134A as R-12 is hard to get.

I was thinking its an expansion valve problem, alas after reading some of the threads on here,
the fact that after shutdown it takes a couple of minutes for the hi/low pressures to equalize makes me think this is not the issue.

Thoughts?

[Dougflas]

It may require less refrigerant because of the fact that when you're flying, you are so much higher and the altitude effects the refrigerant charge. Do you have the system's specs and did you weigh the charge? Also, I have 30 pounder drums of virgin R12.

[pbreed]

System is sealed so should be no pressure change inside the system.
So far all testing described has been on the ground at sea level.

At Altitude one might expect the condenser to be less effective, but outside air will be colder.
I reality at crusing altitude don't need A/C. A/C is 90% for taxing on the ground.

[bohica2xo]

My first thought is that the refrigerant loop may be just fine. York units are not prone to sudden changes in performance if they have oil. You have cooling, pressures are in spec (and very conservative).

I would look at the motor. DC motors in that HP & voltage range tend to be shunt wound beasts. Just because a motor runs does not mean it is ok. Commutator issues, shorted field windings etc can drive the current up.

Start by checking the motor with the belt off. Does it spin easily? No unusual noises? Bearings need lube or replacement? There should be a spec for no-load RPM & current draw. Check that.

You mention 134a - my first question Is there an STC for that?

134a will just exacerbate the load issue.

[pbreed]

I have three motors, one origional, one from an airplane scrap yard and one from Ebay with the correct model number.
All three have no bearing issues. The original seems to have some serious brush wear.
The other two have good brushes.

Can't find the exact motor specs to test properly.

Io (ie idle current no load) is 8 to 10 A.
Its possible that all three motors have similar faults issues as none of them are new.

If it was an experimental aircraft I'd just replace the motor with a big brush less motor.

[bohica2xo]

Post some pics of the motor if you can, and any data you have.

It worked once, and your head pressures are not that high.

How is the pulley bearing in the York? Is the clutch still being used?

[pbreed]

Can't seem to get the motor picture to attach even shrunken to the point its unreadable the upload says too big.

Here is a link to the motor picture...
https://goo.gl/photos/k4dwi6fEz7ejyDBb6

The pulley bearing is fine. Clutch not used at all.

[bohica2xo]

Oh, one of those...

I was hoping for a more mainstream motor I might have data for. In the 1970's there were a bunch of little aerospace shops scattered across Burbank, Sun Valley etc. That was one of them. I have seen the belt drive compressor / 28v motor before, Beech Super 18. Which aircraft is this in?

Hard to say from the pic whether it is series wound or shunt wound. The 4k rpm rating makes me wonder if it is series wound. The data plate indicates 52 amps @ full load - so blowing a 60 amp breaker is a problem.

Series wound motors have a very steep amperage curve under load, so let's see if something changed.

You have 3 motors with similar no-load draw, but the one having issues has badly worn brushes. Doubtful they all have the same winding failure. I would build a BOB motor from all 3 if I could, or at least swap in the new brushes.

Have you checked the oil level on the York? With the system recovered, try turning the pump by hand to see if there are tight spots or drag with no load.

[pbreed]

This is in a aerostar.

The compresor turns freely and has oil.
No tight spots or other issues....


I've tried ALL three motors in place all behave the same.
Given that I went to measure the no-load RPM at 28V using a laser tach.
I aborted the test when the motor hit 14,000 RPM

This may be a shunt wound motor with issues....
I'm going to get a commercial motor repair place to look at one of the motors and
see if they can test/rebuild. If that works on the ground it will tell me whats wrong.

(So far 90% of the testing has been on a APU on the ground).

[bohica2xo]

Ah, T.R.'s last bird.

14k rpm sounds like a series motor to me. They have ski jump curves on load vs current...

What is the GPU output voltage? Series motors have the same sort of curves on voltage. It might be just fine on a 24v battery system with voltage sag under load - like a battery cart, but a generator making 30 volts might push the current too high...