What Every Design Engineer Should Know About Motor Run Capacitors

By Anthony Colone, Genteq

When it comes to capacitor products and the multiple manufacturers in the world, is there a difference in quality? The short answer is yes. A capacitor is an electric component that temporarily stores an electrical charge. The simplest form of a capacitor is two conductive plates separated by an insulating material, or dielectric. As voltage is applied across the conductive plates, the capacitor begins to build up charge for eventual release of the energy.

Many motors in the HVACR segment are furnished with a run capacitor. Designed for continuous duty, the metalized film run capacitor lets a single-phase AC electric motor operate with high efficiency by always remaining energized and connected in the motor’s electrical circuit. A typical run capacitor rating ranges from 2µF to 80µF and is either rated at 370VAC or 440VAC. A properly sized run capacitor will increase the efficiency of the motor operation by providing the proper “phase angle” between voltage and current to create the rotational electrical field needed by the motor.

Why quality is so important
 

The keys to a quality capacitor, aside from using quality materials in its production, are the design, quality control systems and performance testing throughout the production process that will ensure a capacitor that fulfills industry standard requirements for long-term performance. Most, if not all capacitors, will test the same off the shelf, but over the life of the capacitor there will be a difference in performance between manufacturers. This is where an industry standard can help provide guidance on how to assess the quality and long-term reliability of the capacitor being evaluated or qualified.

Industry standards
 

Several industry standards have been developed over the years, but the most stringent, thorough and widely accepted is the EIA-456-A. It is the basis for most OEM reliability standards for capacitors.

The EIA-456-A was created by the Electronics Industries Alliance (EIA). This standard is mainly used in the U.S.A., and is an all-encompassing standard for metalized film capacitors for AC applications. It not only covers motor run applications, but also includes the capacitors used in high intensity discharge lighting applications and general purpose applications such as power supplies and power factor correction banks.

The EIA-456-A has established a reliability standard involving a Highly Accelerated Life Test (HALT), in which capacitors are subjected to 125 percent of their rated voltage and 10oC above their rated temperature for 2,000 hours. This test simulates 60,000 applied hours of field life.

For example, a capacitor that is rated at 5uf/440VAC, with an operating temperature of 70°C, is tested at 550VAC and 80°C for 2,000 hours. If you estimate 5,000 hours of capacitor operational time per year, a 60,000-hour capacitor could last approximately 12 years in the field. EIA-456-A calls for a first year failure rate of no more than 0.50 percent, and no less than a 94 percent survival rating at the end of 60,000 hours of field life.

Figure 1 illustrates the amount of test time, and its correlated field life.

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Total cost of ownership
 

Two key components of a business’s total cost of ownership for a purchased product are initial purchase price and warranty cost. The initial purchase price simply consists of the upfront cost of obtaining the product, while the warranty cost is the associated cost of premature failures in the field after installation where a company would have to correct the issue.

Figure 2 demonstrates several capacitor manufacturers’ products that were randomly selected and tested using a TPI 135 Digital Multimeter. It should be noted that all 3 capacitors provide similar readings. It is typical to see a manufacturer express a capacitor’s capacitance on its product label with a microfarad rating together with +/- percent tolerance. The most common tolerance provided in the HVACR segment for capacitors is +/-6 percent. All three readings fall between 45µF +/-6 percent tolerance. A capacitor is considered passing if it has a microfarad reading within the tolerance band – in this case between 42.3µF and 47.7µF. As shown in Figure 2, all of the capacitors meet the criteria.

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Unfortunately, the initial reading does not reflect the product’s long-term reliability. The EIA-456-A HALT test is how we determine reliability. The following example assumes that the warranty period for the product using the capacitor provides coverage for both parts and labor in the first year. After the first year, only the parts are covered under warranty. The three capacitors shown in Figure 2 were tested to the EIA-456-A standard. Ten parts of the same rating were tested for each of the three manufacturers. Listed below are the test results for each capacitor for one, five, and ten years of simulated field reliability. As previously stated, a capacitor’s estimated operation time per year is 5,000 hours.

Figure 3 shows a simulation of one, five and ten years of capacitor operation and demonstrates that, over time, the capacitor failure rate begins to increase depending on the manufacturer. Testing results demonstrate that after an estimated 12 months in the field (assumed 5,000 hours of applied hours per year), one manufacturer had zero failures, one had a 40 percent failure rate, and the third had a 10 percent failure rate.

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Although Mfg C had only one failure, Figure 4 shows the very real effects of a 10 percent failure rate, as well as the true total cost of ownership on one failed capacitor for a business. An analysis of the results in Figure 4 shows that a seemingly low failure rate of 10 percent will cost a business roughly $3,500 in warranty costs alone.

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As Figure 5 illustrates, higher-priced capacitors will cost more up-front, but what is being purchased is quality and reliability. When capacitors do not meet the performance and reliability ratings specified, the entire system can be affected. A failed capacitor will cause the motor to run hotter, the bearings and insulation to wear out and the noise level to increase. And, it eventually will cause the motor to fail.

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Products, like capacitors, may seem like a logical place to save a few dollars by switching to the lowest priced product in the HVACR segment. Although you may be able to save a few dollars on the up-front cost, the warranty costs associated will eventually create the highest total cost capacitor. In addition, to these costs, there are the intangible long-term effects that are associated with field failures such as: a company’s reputation in the industry, an increased focus on the failures of a supplier’s products versus improvements to an OEM’s design, and lost sales due to potential field failures.

Genteq's Anthony Colo?ne authored this article for Appliance Design.

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