Megohmmeters

Chapter 8

Megohmmeters

The proper use of a megohmmeter indicates many things to an experienceduser. These instruments are used to check the very high electrical resistanceof a component in millions (mega) of ohms. Service technicians have usedthem with great success on large electric motors for many years, and theseuses have led to their use in many other areas of the air conditioning andrefrigeration industry. However, most technicians have not had much expe-rience with megohmmeters. This chapter presents the importance ofmegohmmeters in service work.

Megohmmeters are currently used to check the electrical resistance of mo-tor oil. This oil acts as an insulator in centrifugal and hermetic compressormotors. As the contaminants in the oil increase, the electrical resistancedecreases. Thus, as the oil becomes contaminated, the resistance readingdrops. When the contaminants are metallic particles or moisture, the resis-tance drops drastically, causing a much lower resistance measurement.Because of this, when regular resistance checks on a system are made andan accurate record is kept, a contaminated condition may be detected andcorrected before any serious damage occurs to the system.

Because of the very high insulating effect of the winding insulation, theordinary ohmmeter cannot detect moisture and other contaminants in thesystem. Ohmmeters do not generate enough voltage to detect high resis-tance problems that might cause problems in the very near future. On theother hand, the megohmmeter produces very high voltages, usually around500 volts dc (vdc) and measures from 1 to 1000 ohms. An ohmmeter witha higher voltage output is not recommended for the inexperienced user,because a higher voltage may cause a weak winding to fail under test con-ditions, resulting in a burned out motor.

The 500 vdc capacity of the megohmmeter allows a circuit to ground to bedetected if the winding insulation is weak. In this manner, a breakdown ofthe winding insulation can be detected and precautions taken before anyreal damage occurs. The megohmmeter is used to locate weak motor wind-ing insulation and to detect moisture accumulation and acid before they

©2002 by The Fairmont Press. All rights reserved.

have the opportunity to cause more damage. This is not to say thatmegohmmeters will or should replace acid test kits or moisture indicators,which are also used for this purpose.

Megohmmeters are especially useful in a preventive maintenance program.Megohmmeters are also very useful in checking refrigeration systems be-fore signing a maintenance contract for the unit.

MEASURING WINDING RESISTANCE

The readings taken with a megohmmeter indicate the winding resistance toground. When the insulation is in good condition, the resistance is normallyvery high. The 500 volt megohmmeter uses 500 vdc across the windinginsulation to ground in order to measure the resistance of the insulation.

Readings should be taken immediately after the system is shut down, afteroperating for at least one hour. When all of the readings are taken under thesame basic conditions and with virtually the same winding temperature,the readings can be properly interpreted. If any corrections are needed forvarious reasons, the instrument manufacturer usually provides them for theconditions encountered, such as temperature differences.

To measure the resistance, turn of f all electrical power to the unit and re-move the electrical wires from the terminals being tested. Themegohmmeter will measure all electrical paths in the circuit being tested.When the wiring is left attached, the megohmmeter will measure the resis-tance of the complete circuit and will indicate the lowest resistance toground in the circuit.

When using a megohmmeter be sure to follow the instrumentmanufacturer’s instructions to prevent damage or electrical shock. Neveruse an ohmmeter on a motor winding that is under a vacuum. Be sure theinstrument test leads are in proper working condition.

Equipment should be tested regularly, and these readings should be re-corded. This record will indicate any changes in the condition of the system.

SIGNIFICANCE OF THE READING

Most industry experts agree that the motor winding should have a mini-mum of 100 megohms resistance. W indings that have a resistance of 100

©2002 by The Fairmont Press. All rights reserved.

megohms to infinity are in very good condition. Windings that have lessthan 100 megohms are a cause for concern. Table 8-1 is a listing of thecommon resistance reading levels of a system with varying degrees of con-tamination.

It has been found through experience that changing the filter-drier, perhapsseveral times, causes an increase in the resistance. Many times this bringsthe resistance to above 100 megohms. When a resistance of 100 megohmsor less is found, be sure to check for electrical problems in the other partsof the system, including the hermetic compressor motor terminal block.

Figure 8-2 is an example of a megohmmeter check log. You may want tomake some changes to include some of the readings that are preferred forthe specific installation on which you are working.

Table 8-1. Megohmmeter reading significance

Required Condition Required Percent ofreading indicated preventive windings(megohms) maintenance in field

Over 100 Excellent None 30%

100-50 Some Changemoisture filter-drier 35%present

50-20 Severe Severalmoisture and/or filter-driercontaminated changes;oil change oil if

acid is present 20%

20-0 Severe Check entirecontamination system and make

corrections. Consideran oversized filter-drier,refrigerant and oil change,and re-evacuation. 15%

©2002 by The Fairmont Press. All rights reserved.

Figure 8-2. Example of a megohmmeter check log

INSULATION RESISTANCE RECORD

Date Meter Device Air temp. Humidity Commentsreading temperature (°F) (%)

( ) Hot Cold

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©2002 by The Fairmont Press. All rights reserved.