What Does it Mean that a Technician Megged out my Compressor?
What is a megohmmeter?

Megging is a Good Thing

megohmmeterFirst of all, you have to understand that “megging” or using a megohmmeter is a necessary step in determining if the compressor in your AC unit is functioning correctly. A megohmmeter (megger) is a type of ohmmeter used to measure the electrical resistance of insulators. A compressor’s winding insulation must be checked periodically to determine if electricity is traveling over or through the insulation surrounding the windings.


Three Kinds of Electrical Current

When the technician measures current in this case he is determining the capacitance charging current. This is a type of current that starts high and drops after the insulation has been charged to full voltage. Secondly, the absorption current must be determined and measured. This current is caused by moisture buildup or the presence of contaminants in the insulation. Oil levels, leakage through terminal plates and electrical fusites or contaminated refrigerant can also cause a buildup of current. Thirdly, the megohmmeter will measure the conduction or leakage current in the insulation.

Two Kinds of Methods

The goal of the measurements of current is to determine if the compressor is good or bad. Dielectric Absorption Ratio will be determined by the technician. This is a fairly sophisticated mathematical calculation requiring specialized knowledge and skill. A 60 second reading of each compressor terminal to ground is taken and Megohms at 30 second intervals are taken. Between each reading the terminal to ground is shunted with a jumper. The 60 second readings are then divided by the 30 second readings. This yields a calculation known as the DAR. An absorption ratio between 1.0 to 1.35 is a questionable range indicating system contamination. A reading between 1.4 to 1.6 is a good reading.

Polarization Index

A polarization index is used also and requires the technician to “Megg” the unit for 10 minutes. The common terminal on the compressor is used to take the reading using the megohmeter. After 1 minute a reading is taken. At the end of 10 minutes another reading is taken. The 10 minute reading is divided by the 1 minute reading and a polarization index is determined. If the reading is less than 1.0, the compressor must be replaced. Between 1.0 to 2.0 the condition of the compressor is questionable and maintenance procedures must be followed. The unit must be tested again within 48 hours to determine its condition and usability. 2.0 to 4.0 indicates a well running compressor.




Don JohnsonDon Johnson is the President of Freedom Heating and Cooling in Birmingham, Alabama which offers home owners tools including:
9 Things to Check Before Calling for Service on Your Air Conditioner or Furnace”,  a resource to help home owners save on HVAC problems
The Home Owner’s Guide to Hiring a Heating and Air Conditioning Company“, a quick read on how to guarantee you never suffer by hiring the wrong contractor.
The Ultimate Home Owner’s Guide to Designing an HVAC System , a 59-page eBook covering the 9 steps to building a Complete Home Comfort System.   Contact him at 205-444-4444 or connect on facebook or Google +