HOW IS LIFE OF CAPACITOR DETERMINED AND VERIFIED?

Reliability of an equipment or device depends on the construction and components going into it. The components have to withstand all weather conditions, electrical over voltages /surges, transients, shocks etc. Capacitors have to be sufficiently reliable over the life of equipment they are used in. Safety factors are built in design stage of capacitors and care has to be taken during manufacture to ensure that they are suitable for their intended application. Number of factors affect their performance in service, and it becomes necessary to ensure that their design and construction is suitable for intended performance over expected lifetime.

Type Tests

A manufacturer must validate that the capacitors will stand the service conditions required over the expected life of capacitor. While there are tests for major checks like high voltage test, capacitance, insulation test etc., which are carried out on every piece, life expectancy in service cannot be directly verified. Empirical methods are designed and developed over several decades to reasonably assess their performance life by subjecting components to conditions of high severity in laboratories, and if components withstand these tests, they can be inferred to last their intended lifetime without failures. These are now a standardized procedure all over the world, covered by national and international standards.

A life expectancy test has to be done on representative sample lot from a production batch. The tests are done at design stage as validation, before final approval for production. The test has to be done at factory, or at user end as validation as per design standard, and ways are devised to create conditions in which accelerated tests may be done with accepted or standardized norms. These tests are called Type Tests, designed for the type and application of component.

While the tests and procedures vary with design and applications, and there could be several of them for different types, we consider major common tests applicable to capacitors.

Endurance Test

Designers and companies develop their own accelerated tests, or follow established norms, as validation of component performance over expected life span. A series of tests are performed on a set of components to this end, before a component design is accepted and finalized. During production these tests are done time to time on sample lots to check conformity of products to design standards.

 The test is done at design stage as validation, before final approval for production. The test has to be done at factory, or at user end as validation as per design standard, and ways are devised to create conditions in which accelerated tests may be done with accepted or standardized norms. These tests are called endurance tests, designed for the type and application of component. Accelerated life test (endurance test) is usually designed with higher than rated voltage at highest possible temperature as per rating (or sometimes even higher). There could be different or additional conditions for different types of capacitors, depending on design and application of capacitors. The conditions are held for predertimined period of time. If no failure is observed during this period, it can be reasonably presumed that capacitor of the design will withstand the requisite service life period at rated conditions without failure.

Often the capacitors are checked by manufacturers to look for any structural or internal changes. This helps in improvements in design, if necessary, for a new product. Record of endurance tests on sample components from lots are maintained by manufacturers as a standard statutory requirement. This ensures conformity and consistency of manufacturing process as per design factors.

The components used up in life test cannot be passed on to clients, since they have reasonably lived their life during test. They are disposed in safe ways and not passed on for use.

Factors to be considered for endurance tests of capacitors are:

• Capacitor rated voltage
• Capacitance
• Dielectric loss factor
• Dielectric strength
• Rates service temperature
• Life expectancy
• Safety class

A voltage above rated temperature is applied when capacitor is kept at temperature above the rated temperature. Voltage and temperature are monitored closely. At the end of specified period (number of hours or days), component is tested for technical and physical parameters. Sometimes the standards nowadays mention alternative test conditions in these respects.

For fan and motor capacitors, usually temperature of 70⁰C and 85⁰C are the most common, while power factor correction capacitors may be tested at 50/55⁰C. Capacitors intended for high temperature uses may be tested at 100⁰C or even higher.

Voltage for accelerated life test may be 25% or 35% of rated voltage (for motor capacitors) depending on severity needed. This is decided by the period for which the test is scheduled. Higher the voltage, smaller the time period. A test for 500 hours duration was specified for most power factor as also fan, motor and lighting capacitors till few decades back. It has been revised to 6000 hours at 25% overvoltage, while 35% higher voltage may be used for 3000 hours test for fan and motor capacitors as per Indian Standards.

Following parameters are essentially checked before and after endurance test on capacitors:

1. Capacitance value – It must not drop appreciably below a small specified limit.
2. Loss angle must not rise beyond acceptable limit
3. Capacitor should not show appreciable degradation.
4. There should be no physical damage to container.

Safety class P0, P1 and P2 are specified in some capacitors, and endurance test conditions are modified to suit different requirements. Standards may specify say, 500 hours, 3000 hours or 6000 hours for endurance tests. If a component passes test for specified period, it may be presumed that the entire production or supply batch may withstand required life expectation.

Vibration Test

Components face vibrations of different severities in service due to natural causes or due to equipment or conditions in which they operate. These could be continuous or intermittent. Capacitors are often tested for vibrations as per standards set for different types or intended applications. These are standardized in different countries, and tests are conducted accordingly.

Humidity Test

Tests are carried out in extreme humidity and temperature in a humidity chamber for specified period (commonly 21 days) to see the effect of atmospheric humidity on a component. After the test period, it should not undergo any appreciable change. Humidity test withstand capacity under test conditions is often a part of component specification.

Conformity

If capacitors pass all these tests satisfactorily, it reasonably confirms the reliability and performance of capacitor over the entire design life. Consistency in performance of endurance test by manufacturer helps maintain reliability and create brand value for manufacturer and also satisfies the end user about performance of their end product.

Just as capacitors, all components like resistors, inductors and others are also subjected to endurance tests, which an essential part of quality assurance for any manufacturer.