Measuring Insulation Resistance of Capacitors

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Mar 14,

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A common use of high resistance measuring instruments (often called megohmmeters or insulation resistance testers) is measuring the insulation resistance of capacitors. Such tests are useful to quality engineers in the production of capacitive components, by design engineers to determine suitability for a particular application or at incoming inspection. By the proper application of a megohmmeter type instrument a capacitor&#;s dielectric material can be tested and evaluated in two ways.

First, the DC value of its impedance (resistance) can be determined. This is an important parameter in some types of capacitors such as ceramic or film, where a high value of insulation resistance is a primary reason in choosing them for an application. It may be that a design engineer has determined that his circuit will not work well below a certain value of insulation resistance. In addition, the DC resistance of a capacitor tells something about its quality. Wide variations from unit to unit or consistently low values may indicate a quality problem.

Second, the measurement of the capacitor&#;s insulation resistance with high voltage is an excellent way of detecting flaws in the dielectric material, which might not otherwise make themselves known until long after installation in the user&#;s equipment. Ceramic dielectrics are subject to cracking (as are all ceramic materials), and often these cracks will not be noticed at normal voltages. However, with the application of 500 or volts, breakdown along the crack edges often occurs resulting in an abnormally low value for DC resistance.

Why are Capacitors hard to Test? When a capacitor with high insulation resistance is attached to the measurement terminals of the typical high resistance meter, the user may notice some very strange behavior of the instrument. Resistance readings will fluctuate widely in continuous mode, and never settle down. If a pure resistor of similar value is substituted for the capacitor, the readings usually settle down and the instrument measures the device perfectly. This situation can be exhibited on most megohmmeters, whether the instrument has the older analog display or the newer digital display. What is interesting is that, although the variation of readings may be greater on the analog meter, it may be less evident. This is because the resolution on these older instruments is less than those designed today. To the casual user the analog readout that covers one or two decades from zero to full scale will not appear to waiver much for values changing by 2:1. On a digital display even though the readout may be more representative of the actual results in many cases it can be annoying and make readings unreliable to difficult. This phenomenon is caused by the way that megohmmeters measure resistance.

If the unknown is a capacitor, however, things get a little more complex

A &#;real&#; capacitor consists of an ideal capacitor in parallel with its insulation resistance. This ideal capacitor has infinite resistance at DC. As frequency goes up, however, its reactance decreases according to:

where f is the frequency in hertz, and C is the capacitance in farads.

Notice that we use the symbol Xc for the reactance of the pure capacitor, to distinguish from its insulation resistance, R.

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In this example we&#;ll consider a ceramic capacitor of 2.2uf (2.2x10&#;6 farads) with a typical minimum insulation resistance of 2G&#;. If a capacitor is tested at 200V and measures a dielectric leakage current of 10nA the insulation resistance must be 20G&#;. For 10nA the instrument would be on the 100nA full-scale range with a feedback resistor of 20M&#;. In this case the gain of the detector is 20M&#;/20G&#;, or .001. The output voltage would then be [(.001) &#; (200V)], or 200mV.

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This all assumes DC. As soon as we consider AC, things change. At a frequency of 1Hz, the &#;ideal&#; capacitor in this unit will have a reactance, Xc, equal to:

The AC gain of the detector would be:

The source of 1Hz voltage on the input could be any one of several things. It could be room noise picked up by the meter leads, or small fluctuations in the instrument power supply. Even moving the leads (as on an automatic handler) can induce low frequency AC on them. Generally, the induced AC voltage will be small perhaps 1mV, but the detector would multiply this by 270 to give an output of 270mV. This is higher than the &#;true&#; reading of 200mV. The noise from this 1mV source can cause the result to vary by 135%.

So What Do You Do?

This situation could be improved by shielding the device under test (DUT) and the test leads from the AC noise. Indeed, at high values of resistance (above 1G&#;) low noise shielded cables are highly recommended. The real solution to the problem of capacitor testing comes from remembering the cause &#; high AC gain. If we can reduce the AC gain, we can eliminate the problem.

AC Gain Compensation Recall that the AC gain is Rf/Xc. If we were to add a compensation resistor, Rc of 1M&#; in series with the DUT, the AC gain would become

This is a lot better than 270! With the same 1mV of noise in, we get only 18.65mV out compared to the &#;true&#; signal of 200mV. The series resistor, Rc, has reduced the AC gain to 9.35% of the DC gain, a tolerable level.

DC Errors

At this point, one might worry that, although we&#;ve solved the instability problem, we&#;ve introduced an error term of 1M&#;. We have! But, so what? Remember, we&#;re in the business of trying to measure 2G&#; versus an induced error of 1M&#; (=.001G&#;). The error is .05%. At lower values of DC resistance, a 1M&#; resistor may be significant, so a smaller compensation resistor should be used. Capacitors with a dielectric leakage current greater than 10uA are simply too &#;leaky&#; to exhibit the fluctuations of readings in the first place. Lastly, the addition of series Rc will increase the charge time somewhat. This is unavoidable, but in most cases will be inconsequential.

Procedure Using Megohmmeter/IR Tester

The is supplied with two quieting resistors that can be placed in series with the DUT when measuring low leakage capacitors.

Conclusion

To determine the proper adapter to use, simply measure several devices on the with Auto Range selected. Generally, no resistor is required for the 100uA and 1mA ranges. This can be confirmed by looking for instability in the readings over a measure time of, say, ten seconds. Note that, during this time, the readings on a non-charged capacitor will change as the DUT charges up. But the reading should increase steadily, not fluctuate up and down. If the readings seem to fluctuate, a quieting resistor may be required. Refer to Table 1 to help select the proper resistor for the measurement range being used.

There may be cases (hard to predict) where a higher value compensating-resistor is required on the low current range (1nA). In general, the two resistors supplied with the unit should suffice. The Current Range is calculated by dividing the Test Voltage by the Resistance Limit. IRANGE = VTEST / RLIMIT

Humber888

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Location: Bromley, London, UK.

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Re: Measuring Insulation Resistance of Motor Run Capacitors

I got a very comprehensive reply from Peter Mony of Nagravox, on the motor capacitor question. He did not specifically mention built-in discharge components, but he has given me permission to precis his reply below.

Mike

Firstly let me assure you that you are not installing an inferior motor capacitor. Indeed quite the contrary and I have tried my level best to achieve that, not fit a compromise "what's available".

1. The motor capacitors in Revox Studer tape movement systems are a little different to a standard PSC motor and one of the main reasons why I have been particularly careful in my specs to the manufacturer. The capstan system for example operates like a pseudo 3 phase motor with a 50 Hz AC and a chopped DC 800 / Hz. A complex situation for analysis considering the mixed capacitive/ inductive load. On things you can measure and tolerance / stability considerations - capacitance tolerance and variation is usually less than 5% and in most cases 2% or better. Operating voltage and dissipation factor are conservative.

2. Within reason, the important considerations are not DC leakage but rather capacitance, dissipation factor and tolerances over a wide range of frequencies. Simple DC insulation or leakage test do not tell us much. Obviously if you were measuring 1Kohm there is an obvious problem but 1M - 10M is seriously not important and of curiosity rather than something that could affect a capacitors performance. If you really want to measure the leakage it would have to be done at about 100 V and the variables used in calculations would involve current, voltage and most importantly time then "resistance" calculated. But its really not relevant to this application. Sure - DC polar capacitors have things like ESR, DC leakage that are important and more easily assessed.

3. The construction of a metallised polypropylene motor capacitors for our application means they are self healing - a bit of a misnomer. Sure they self heal but each "event" decreases the capacitance AND increases the insulation resistance making it appear "better" as far as DC resistance / leakage is concerned. Taking cognisance that at the Mohm level all of this becomes rather academic really.

4. My spec I have these motor capacitors made to is tight and specific. I have been dealing with Chiefcon (Taiwan) and their various sister companies for upwards of 12 years now. We are talking many millions of capacitors and frankly because they are the best, most cost effective and in many instances far better than more well known brands which WWW "aficionados" and "self style guru's" wax lyrical over. Over the last 6 years I estimate (from orders at least 1 - 2x per year) that we have supplied the 3 motor capacitors for a wide range of Studer, Revox and a few other makes in the region of capacitors ~ machines. We have never had a failure or a problem.

5. The construction and "ingredients" between the original and our motor capacitors are different for sure.

6. I dont believe you have missed anything just what is relevant to measure and important. Good on ya for being inquisitive and wanting to understand.

7. One can always make a better capacitor but within MOQ, cost and tolerances considerations my motor caps are pretty good. I could I suppose have pure gold lettering and natural silk shrink fit.;-)

Kind regards,

Peter

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