Microscopic investigations give important contributions to electrical failure analyses. The following picture shows an enameled wire after 336 h of thermal aging with an oil-refrigerant mixture.

Electrical components as for example stators are tested for partial discharges or with surge pulses before and after aging. In case aging takes place in oil the stators have to be prepared for the initial measurements by vacuum impregnation of refrigeration machine oil. This ensures equal test conditions for the before-and-after-comparison regarding oil wetting.

One form of electrical aging is the application of partial discharges (PD). The isolation system is stressed by accelerated particles, ozone and UV irradiation in this case. If these discharges occur over longer periods of time an electrical brake down may be the result. The following picture shows the local application of corona discharges to magnet wires of a stator prior to a thermal chemical aging.

How behave pin-holes in wire isolations under typical refrigeration cycle conditions? Preparing magnet wires and expose them to thermal chemical aging gives first indications. The picture shows an “artificial”  pin-hole prepared prior to an aging with pulsed voltage and simultaneous contact to an refrigerant-oil-mixture.

Manufacturers provide a great variety of magnet wires types. Most of these wires are advertised as partial discharge resistant. But which one is “more” resistant than the other? And, is there a change of this property after aging?

The resistivity against electrical stress including partial discharges of enameled wires can be compared with reasonable statistics by means of a special test set-up in a short time. 60 twisted pairs are simultaneously exposed to 1500 V peak, 16 kHz short rise time pulses at a temperature of up to 180 °C. Every twist is monitored separately for breakdown. Result of this procedure are times to failure of the different wire types which are compared to each other.

Weibull plots give indications on weather observed differences of electrical stress resistivities are statistically relevant or not, the characteristics of the failures or the mean time to failure.

Microscopic investigations show different failure modes at the end of an aging period.

The electrical stress tests are usually carried out in an air atmosphere. But what happens when a refrigerant is the circumfluent gas? To answer this question special autoclaves with high voltage feedthroughs were designed and set up to stress twisted pairs under refrigerant atmosphere.

From its natur an oil refrigerant mixture is a dielectric. It has a high volume resistivity. But what happens inside a compressor when this mixture is enriched with water and further impurities and an applied voltage fosters electrochemical processes? Does this influence the isolation properties of the compressor?

A test set-up was developed to investigate this process at 18 specimen simultaneously. High voltage pulses of the required polarity are applied to them. From time to time this procedure is interrupted and the isolation resistivity is measured for 60 s at one compressor after the other. All this happens automatically and the measuring device fills a database. This test can be combined, for example, with the methods of defined pre-damage mentioned above.

Operation voltages of MAC compressors rise higher and higher. What about partial discharges (PD) inside these components? Do they occur?

This question is difficult to answer because the housing of the compressor is tight for electromagnetic radiation and modern compressors come with flanged variable frequency drives (VFD) so that the stator connectors are not directly assessable. There are two ways to detect PD in this case: removal of the VFD so that the connectors can be enclosed by a wire sling. This sling is the sensing element of a high sensitive UHF PD detector. An other way to look inside of a compressor would be the installation of a dielectric window into the housing. Leaving radiation which originates from PD impulses can be detected by a UHF narrow bandwidth antenna which is also part of the PD detection system.