Can you IR test a capacitor?

TYPE OF DIELECTRIC

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Each dielectric medium has its own inherent in­sulation resistance characteristic which largely de­pends on the chemical and molecular structure composition of the material.

TEMPERATURE

Insulation resistance properties of all dielectrics will decrease with increased temperature. This in­crease in temperature causes an increase in the orbital velocity of the electrons which, in turn, results in a higher flow of electrons through the di­electric.

CAPACITANCE RATING

Inasmuch as the capacitance rating in effect reflects the total area (square inches) of dielectric in the capacitor, it can (within design limits) be used as a direct measure for insulation resistance. In general, if we double the area of dielectric, we also double the number of paths for electrons to flow through the dielectric, and the final result is double the leakage current (one-half the insulation resis­tance).

Now, however, this inverse ratio between the ca­pacitance and insulation resistance for any given dielectric provides the capacitor manufacturer with a handy tool for designating a single value of insulation resistance as a guarantee to cover all capacitance values for that line. This is done by multiplying the insulation resistance (ohms) times the capacitance (farads) to arrive at a constant val­ue of (ohms x farads) or, more commonly (meg­ohms x microfarads).

This use of a limiting value became necessary as a convenience when plastic films made their appear­ance as capacitor dielectrics. These plastic films have such high inherent insulation resistance that very small values of capacitance ratings would re­quire instruments that could measure in the mill­ions of megohms area. Since present standard measuring equipment is not capable of reasonable accuracy above approximately 500,000 megohms, this limitation is used.

Note: It is "megohms times microfarad" not "meg­ohms per microfarad."

TIME OF ELECTRIFICATION

If we were to designate any single factor in this discussion of insulation resistance that is the most neglected and subject to the highest error probability, "time of electrification" would certainly haven to be considered. What must be recognized here is the fact that all dielectrics have some degree of chemical polarity, and therefore are subject to "in­terfacial polarization." A highly polar dielectric (such as mylar) would have a high degree of "interfacial polarization" which, as we measure it would manifest itself as a fairly high value of "dielectric absorption." On the other hand, a non-polar dielectric (such as polystyrene) would show its low degree of "interfacial polarization" as a low value of dielectric absorption.

How long it takes to reach this steady state condition will vary not only with the dielectric, but also with many other factors. However, all units will allow the general pattern shown in Figure 3. For illustration purposes only, the curves approximate the typical curves for non-impregnated mylar dielectric capacitors. The effect of temperature on both the insulation resistance value itself and the time to reach a steady state condition is shown. The variance of the insulation resistance value between "identical" units from the same lot is also depicted by the dotted lines on the +25°C (only) curve.

The "time of electrification" should always be no­ted when specifying insulation resistance value whether it be a user's specification or a manufact­urer's catalog sheet involved. Two minutes electrifi­cation time is most common and in general usage. Figure 4 shows a comparative analysis of the typi­cal curves of insulation resistance vs. temperature for various dielectrics (two minute electrification time). What must be kept in mind when analyzing Figure 4 is that curves are average figures only and it is entirely possible to get individual capacitance that will vary as much as 1 O or 20 to 1 from the average value.

The effect of the insulation resistance value, both in magnitude and how it varies with time and temperature, is quite critical in circuitry where leakage of current through the capacitor can cause malfunction or undesirable results to occur. Prime ex­amples of this type of application can occur in most coupling or decoupling circuits, and some blocking, timing, or signal pickup situations.

Tom Lasek is a field engineer with ASI Robicon, a major manufacturer of high-powered solid-state variable frequency drives for controlling industrial AC motors up to HP and fed by voltages up to 13,800 volts. Lasek&#;s responsibilities include installation/commissioning, general preventive maintenance and troubleshooting these high-powered drive systems. He uses a Fluke B Megohmmeter* almost daily for:

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• Insulation resistance test and documentation for commissioning and startups on company systems.
• Troubleshooting high voltage components in high powered drives.

This case study details the high voltage component and capacitor testing methodology Lasek has developed over his 30 years of experience.

By using a B as a high voltage component tester in regular preventive maintenance procedures, Lasek finds faulty components before they fail. Preventing premature failures saves both ASI Robicon and its customers time and money.

SCR voltage testing

The Fluke B Megohmmeter can quickly determine the relative quality/weakness of nonlinear devices in high-power solid-state power supplies: SCRs (Silicon Controlled Rectifier semiconductor) and IGBT high power devices. Normal solid-state component testers utilize low voltage, low current test sources and won&#;t always pinpoint bad devices, especially if they may breakdown under load.

Methodology

1. Isolate the device from all its connections to ensure accurate readings.
2. Set a high voltage on the B, apply it and note reading.

Results

• Damaged SCR&#;s exhibit large differential readings when tested forward biased and reversed biased (anode to cathode).
• New SCR&#;s generally have almost equal readings in both directions (roughly 100 MΩ).
• Any high-power SCR reading under 5 MΩ is damaged and should be discarded.
• If the SCR reads 50 MΩ to 200 MΩ in 1 direction and 10 MΩ to 50 MΩ in the other (4-to-1 difference), it will likely self-gate under load (instead of firing at the desired trigger point in time, it will randomly gate itself on). This is very bad. It causes extreme current pulses in the power supply and can cause commutator flash over in dc motors operated by these supplies.

SCR heat testing

If the component is physically damaged, it may need to be physically removed for a heat test.

Methodology

1. Set the Fluke B to the actual voltage rating of the device. Most ASI Robicon SCRs used on 480 V VFDs (variable frequency drives) are rated at V. They also use V devices stacked in series on medium voltage ( V and V) drives.
2. Take two readings, forward and reverse while the device is cold.
3. Warm the component to 180 °F in an oven and repeat the test.

Results

If the leakage is up 50 % or more, the device must be discarded.

Conventional testing of SCR&#;s says that they are OK if they are not dead shorted. Lasek says this assumption is false. When ASI Robicon repairs SCR power supplies, their goal is to minimize further failures and down time, not minimize the parts cost of the repair.

Any device that exhibits changing, varying, non-stable readings when raised to its rated voltage should be suspected as near failure and isolated. Unstable readings indicate internal arc damage or semiconductor self-gating/ conduction.

IGBT switching device testing

IGBT switching devices can be heat tested as well but they all have a back diode so they can only be checked one way(forward). High power diodes can be heat checked (Cold and hot readings compared for change) as well.

Generally, diodes are much higher resistance, lower leakage than SCRs (readings 200 MΩ to 700 MΩ normal).

An SCR that reads 20 MΩ both ways is OK, but one that reads 80 MΩ one way and 20 MΩ the other is a suspect for failure. Out of the box new SCRs usually read 100 MΩ to 200 MΩ both directions and are within 50 % of being equal both directions.

Capacitor testing

Lasek also uses the megohmmeter adjustable voltage function to test high voltage capacitors.

Methodology

Charge several identical capacitors and compare the time it takes them to charge to like readings.

Results

• If a capacitor charges extremely quickly, it may be open-circuited.
• If the readings go up and down repeatedly, the capacitor may be arcing internally.

Remove and replace the device.

*Notice: Since the original publishing of this case study, the Fluke B has been replaced by the Fluke C Insulation Tester.

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