Tuesday, October 21, 2008

CLEAN-UP AFTER A BURNOUT ON COMPRESSOR

1. How do you clean up a refrigerant system after a compressor motor burnout?

If you ask ten mechanics, you’ll probably get ten different answers. Every mechanic has his own way. One thing is for sure, unless the system is cleaned properly and you correct the problem which caused the burnout, you’ll have another compressor failure.

2. In this training lesson, we’ll answer the following questions about a burnout:

What is it?
What are the different types?
What contaminants are created?
What causes them? And,
What is the proper procedure for cleaning a system?

3. What is a Burnout?

A burnout is the breakdown of the motor winding insulation, which causes the motor to short out and fail.

There are two types of burnouts – mild and severe.

What is important to you is that each type required its own special procedure for clean-up.

4. A mild burnout is when the motor stops before the contaminants created by the burnout leave the compressor.

5. Here is a compressor motor. Notice that when a mild burnout occurs, only one spot on the windings is burned.

6. A severe burnout is when the contaminants are pumped through the refrigerant system before the motor stops.

7. This type of burnout is not confined to one small spot on the windings, as you can see here.

8. What contaminants are created by a burnout?

The contaminants can include moisture, acid, soot, varnish and hard carbon, and copper plating.

9. Moisture in a refrigerant system can cause oil sludge, which reduces the lubrication properties of the oil and plugs oil passages and screens.

The screen on the left is from a compressor that’s operating normally. On the right is an oil pump suction screen that is plugged with sludge.


10. Moisture can also react with the refrigerant to form hydrochloric and hydrofluoric acid.

These acids will cause corrosion of metals and breakdown of the insulation on the motor windings. And, in this case, the acid ate through the insulation on the terminal wires in the compressor, which caused the motor to short out.

11. Hydrofluoric acid can also damage a sight glass. That’s what this used to be.

If acid is strong enough to “eat” through metal and glass, think of what it can do to your skin. So, if you have to go into a system where a burnout has occurred, DON’T FORGET to wear safety glasses and acid-resistant gloves.

12. Soot, another contaminant, is generally a soft carbon caused by charring of the insulation and oil. It is usually confined to the compressor unless the burnout has been going on for a period of time while the compressor continued to run. Soot is easily loosened and filtered out.

13. Varnish and hard carbon are caused by excessive heat and are the toughest of all contaminants to remove. Because the compressor is the warmest spot in the system at the time of the burnout, most of the varnish and carbon deposits occur in the compressor.

14. Here’s an example of copper plating. It is the result of a combination of factors: the contaminants, the type of oil used, and the high temperature.

The gradual buildup of copper plating on bearing surfaces reduces the clearances and results in increased friction and wear.

15. To prevent a repeat failure, the contaminants created by a burnout must be removed from the system before putting it back into operation. Before we see how to this, let’s see what problems in a system can cause a burnout.

16. A mild burnout is usually caused by defects or damage in the winding insulation; winding insulation is actually a thin layer of varnish. Too thin a spot in the varnish can cause a short, especially in the end turns. Each time the motor starts, the locked rotor inrush current actually flexes the winding end turns. Flexing will wear off the varnish and eventually cause a short. This will happen quicker if the compressor is allowed to short cycle.

Scratches or nicks in the varnish can also cause a mild burnout. Even with extreme care a motor winding can be nicked slightly at the time the motor is installed in the compressor. This damage may not be serious enough to prevent the motor from passing the factory start-up test, but when put into actual operation, it may fail.

17. Severe burnouts are caused by a combination of two things:

1. High motor winding temperature, and,
2. The motor protection system fails to shut down the motor before it overheats.

18. Check to see why the protection system failed.

This system includes all the safety devices in the electrical circuit, such as overcurrent protection; and switches that sense high and low pressure, oil pressure, and temperature.

19. To check the safeties, you’ll have to wait until the unit is back in operation. Then, check the current, pressures, and temperatures to make sure they don’t exceed the trip limits of the safety devices.

For example, with the unit operating, check the high and low side pressures. If the high side pressure exceeds the trip limit of high pressure switch, this switch is bad and must be replaced.

Also, make sure that a safety device hasn’t been jumped or that a contactor isn’t welded shut.

20. Once you’ve determined why the protection system failed and have corrected the problem, the next step is to determine why the motor windings overheated. To do this, you’ll have to wait until the system is back in operation; just like you had to do when checking out the protection system.

21. The reason for overheating can be grouped into two categories – refrigeration and electrical.

22. On the refrigeration side, overheating can be caused by the following:

Poor installation procedures, which will result in moisture and air in the system and cause the insulation to break down. And,

High head or low suction pressure or a combination of the two, which will increase the compression ratio and the current draw of the compressor.

23. On the electrical side, overheating can be caused by the following:

High or low voltage.

Bad starting gear for single-phase compressors, which includes the start or run capacitor or the start relay. And,

Short cycling on controls or safety devices which causes a buildup of heat because the heat from the previous start has not been removed.

24. Another problem which can cause overheating in an R-22 system falls under both the refrigeration and electrical categories; it’s called a flooded start. This is when the compressor starts with liquid refrigerant in the crankcase.

If there’s enough liquid, it will reach the compressor motor. This liquid will reduce the resistance between the motor windings and the compressor shell or casting. And, if a winding is leaking current through a nick or thin spot in the varnish, the refrigerant may allow the current to arc to the shell and cause a direct short to ground. It can also cause a short between windings or a turn-to-turn short.

25. Flooding can be caused if a crankcase heater isn’t working or the compressor isn’t equipped with one.

26. Now that we know what a burnout is, what the two types are, the contaminants they create, and what causes them, let’s see how to clean up a system.

Before you start, there are two things that you should do:

1. Make sure there is a burnout, and
2. If there is, determine the type – mild or severe.

27. When checking out a system, there are safety rules that you should follow:

Make sure the electrical disconnect is off and properly tagged or locked.

Wear rubber gloves and safety glasses. The refrigerant oil mixture is acidic and can cause severe burns.

Before opening the system, make sure the refrigerant has been removed.

And, cut the refrigerant piping with a tube cutter. Do not unsweat it because oil in the piping may ignite.

28. To check for a burnout, measure the resistance of the compressor motor windings and the resistance to ground. Do this at the control box.

DO NOT measure the resistance at the terminal on the compressor. If there has been a burnout, the terminals could be loose; and if you remove the wires from them, the refrigerant pressure could cause a terminal to “blow out.” This will cause the refrigerant and oil to be discharged, which can cause severe burns if it gets on your skin or in your eyes.

To determine where to disconnect the wires and place the meter probes, refer to the unit-wiring diagram. Also, don’t forget to check the condition of the wires back to the compressor. If you don’t, and you have a broken wire, for example, the meter will show an open and you may think that the winding is bad.

29. When checking a single-phase compressor, the resistance of the start winding should be greater than that of the run winding.

On three-phase compressors, the resistance of each winding should be the same.

To determine what the exact resistances should be, refer to the manufacturer’s specifications.

And when you’re checking the resistance from a winding to ground on a single-phase compressor, it should not be less than 1,000 ohms per volt. For example, on a 230-volt motor, the resistance should not be less than 230,000 ohms, that’s 230 times 1,000.

Also when checking a compressor with an internal line break thermostat, make sure that the compressor is cold when checking the resistances. If you check a compressor when it’s hot, this thermostat may be open, giving you a false indication that the windings are bad.

30. Once you’ve determined that the compressor motor is bad, the next step is to determine the type of burnout.

To do this on hermetic compressors or semi-hermetics without service valves, first remove the refrigerant charge from the system.

If it’s a semi-hermetic with service valves, you can valve off and bleed the refrigerant from the compressor.

31. On a semi-hermetic the simplest way to tell the type of burnout is to remove a cylinder head.

If there is carbon, it’s a good indication that the contaminants have left the compressor, which means it’s a severe burnout. In this case, you’ll have to remove the entire refrigerant charge before cleaning the system.

If there isn’t any carbon in the head, and the motor checked out bad, it could be either a mild or a severe burnout.

32. To determine which type of burnout it is, you’ll have to check the acid level in the system. Use Totaltest to check the system. With this instrument, you don’t have to bother with the mess of taking an oil sample. Complete instructions are provided in the kit. Your distributor can also provide additional training on Totaltest.

33. To check out the type of burnout in a hermetic compressor, cut the discharge line and check for carbon. Also, like a semi-hermetic, the lack of carbon doesn’t necessarily mean it’s a mild burnout. You’ll still have to check for acid with Totaltest before you bleed off the charge.

34. Whether it’s a mild or severe burnout, the refrigerant can be reused. However, whether or not you save and reuse it or vent it to the atmosphere is a matter of economics. If it’s going to cost more for the man-hours to save the refrigerant than it’s worth, vent it.

35. Now, we’re ready to do the clean-up procedures for a mild and a severe burnout. Let’s start with a mild one.

We will only cover in detail the steps that are actually used to clean-up or check the condition of the system.

36. For a mild burnout in a system with a hermetic or semi-hermetic, here’s what you should do:

After removing the charge and the compressor, add a liquid line filter-drier or, if there was one already in the system, replace it. In must be one size larger in capacity than the unit being worked on. If it’s a heat pump, use a bi-flow type.

If you’re working on a semi-hermetic and you can valve off filter-drier, you won’t have to remove the charge.

37. Next, before installing the new compressor, check the windings to make sure they are OK.

On systems with semi-hermetics, transfer the necessary components from the old compressor to the new one. For example, the crankcase heater and the unloaders.

38. Also, if it’s a semi-hermetic, add an angle valve to the crankcase. This will allow you to easily change the compressor oil.

39. Now, triple evacuate the system. To reduce the amount of time for evacuation, the hoses should be as short and as large in diameter as possible.

The first two times the system is evacuated, it should be pulled down to 5,000 microns or 29.72 inches of mercury. Then, each vacuum should be broken with the same type of refrigerant that’s used in the system.

The third evacuation should be a deep vacuum of 500 microns or 29.90 inches. We recommend using a vacuum gauge because it’s more accurate than a suction pressure gauge. Break the deep vacuum by recharging the system.

40. If you’re working on a unit with a semi-hermetic compressor, operate the system for two hours. Then, check the acid level with Totaltest. We recommend this test as an added precaution to make sure that there weren’t any contaminants pumped out of the compressor during the burnout. If the system still contains acid, change the compressor oil and the liquid line filter-drier.

On a hermetic system, also use Totaltest. If the system still contains acid, change the liquid line filter-drier. If you have a hermetic system that has contamination that’s on the borderline between mild and severe, treat as if it were severe.

41. Now, let’s check out the clean-up procedure for a severe burnout. Part of the procedure includes the steps we’ve just covered for a mild burnout. So, we’ll only go into detail for the steps that are different.

42. After removing the refrigerant charge, the compressor, and the liquid line filter-drier (if there is one) flush the piping with clean refrigerant. Flush it in the direction opposite the normal refrigerant flow. The refrigerant should be the same type that’s used in the system.

Depending on the carbon buildup in the piping, the refrigerant components may need to be removed and cleaned separately or replaced. These components include metering devices, accumulators, and reversing valves.

43. Next, add or replace the liquid line filter-drier. Don’t forget, it must be one size larger in capacity than the unit and if it’s a heat pump, use a bi-flow type. If it’s a replaceable core filter-drier, then just replace the core with the same size that you remove.

Also, add a suction line filter-drier. It must be installed vertically to prevent oil entrapment and it should have pressure taps on the inlet and outlet.

Both filter-driers should be the acid core type for a severe burnout.

44. Now, replace the compressor, triple evacuate and recharge the system. If it’s a semi-hermetic, add an angle valve to the crankcase so you can change the oil.

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