Megging a Scroll – Short #166

In this short podcast episode, Bryan talks about using a megohmmeter on a scroll compressor (or “megging” a scroll).

Scroll compressors are among the most common compressor types nowadays, and they come with their unique needs and best practices. You can't pump them down into vacuums (in many cases, you can't do that anyway due to internal protections), run them in a vacuum, or run a high-voltage megohmmeter or hipot test.

Scroll compressors differ from reciprocating compressors. A scroll compressor's motor is located at the bottom of the compressor, meaning it is immersed in refrigerant and oil when the system is operating AND when it is off; when the compressor is off and cold, there is a chance that there will be liquid refrigerant at the bottom. Compared to reciprocating compressors, scrolls tend to have a more compact and balanced design, and there could be a higher risk of internal arcing due to the tighter electrical tolerances associated with the design.

Many inexpensive megohmmeters will say that any measurement below 10-20 megohms indicates a short, but some scrolls will have acceptable readings as low as 0.5 megohms to ground; these readings will typically show up on the smaller kilohm scale. You must only use a megohmmeter to ground, not from winding to winding.

Moisture contamination, metallic debris, and lubrication issues can also cause a lower ohm reading than acceptable, so it's best to have historic data and track readings over time to make a diagnosis. Many modern multimeters can help you determine if a compressor is shorted to ground; you don't necessarily need a megohmmeter.

You may also read the following tech tip to learn more: https://hvacrschool.com/scroll-compressor-pump-down-megohm-test-fusite-plugs/

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In this short episode, Bryan clears up the confusion behind pumping vs. compression.

Pumping is usually associated with liquid. Liquid, like vapor, is a fluid, but it’s a lot denser and is essentially incompressible because the molecules are close together. Liquid volume varies based on temperature and density, but we can’t squish the molecules together. That’s why compressors with very little axial and radial compliance fail when flooded; they can’t compress the liquid. A circulator pump in a hydronic system is an example of a pump that pushes liquid along, but the pressure difference across the pump is minimal compared to that of a compressor; the same is true of liquid vs. vapor CO2.

However, vapor molecules are far apart and can be squished together to take up a smaller volume. That’s why vapor can go through compressors, which take a fluid of one volume and apply pressure to shrink it to a smaller volume, raising the temperature in the process. Pneumatics, as opposed to hydraulics, utilize compression to move vapor around in a circuit. Recovery machines need to be able to compress vapor refrigerant and pump liquid into the recovery tank.

Compressors and pumps can either be positive displacement or non-positive displacement systems. A positive displacement system creates a trap and forces a fixed amount of fluid through a system, like a scroll compressor or a circulator pump. Refrigerant volumes may vary because vapor density varies with temperature and pressure (the volume flow is the same, but the mass flow varies). A condensing fan motor, on the other hand, is dynamic and varies the volume of air it moves in response to pressure, as the mechanism relies on velocity and kinetic energy to move air.

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