No matter what the application or electrical system in question, electromagnetic interference (EMI) tends to be a nuisance. Because EMI can be directed back into the power network or radiate through the air with the potential to cause disturbances and malfunctions to other sensitive electrical equipment, it needs to be mitigated to avoid system failure or disruption to the operation of electrical equipment. Filters designed specifically to prevent electrical noise from electrical and electronic equipment are often used to prevent these disturbances caused by EMI.

The European Union (EU) and many other countries have enforced stringent regulations limiting the level of interference an electrical apparatus may generate or radiate. The EU’s electromagnetic compatibility (EMC) regulations cover all electrical apparatus, however the United States—through the Federal Communication Commission (FCC)—limits the type of apparatus covered and does not include general household appliances, for example.

Typical Filter Design

Passive EMI filters are traditionally designed with single or multiple stages of coils and capacitors. These capacitors can be subjected to excessive and repetitive voltage spikes from the power line. Voltage spikes can cause a degradation of the capacitor dielectric, especially over long periods of time and could cause the capacitor to fail or rupture. Therefore, capacitors used in EMI filters and other “across the line” applications are designed to meet more stringent international standards.

EMI filters for typical laundry appliances tend to be treated differently than many other household appliances permanently connected to a wall socket such as microwave ovens, refrigerators and televisions, in that laundry-appliance manufacturers typically subject those EMI filters to more stringent safety and life test requirements. Research has not really shown the reason why, but historically, EMI filters for laundry equipment were constructed utilizing capacitors with a three-layer oil impregnated paper dielectric, since it was likely the most reliable technology available when these machines were invented.

Three Layer Capacitors

EMI filters using metalized capacitors provide reliability and safety, at similar price points of those with paper capacitors. Photo: LCR Electronics Inc.

Being porous, paper has random microscopic holes throughout so multiple sheets of paper were used in three-layer paper capacitors to misalign the holes across the length of the stacked sheets. Since paper absorbs moisture, the capacitor windings were heat and vacuum treated to remove moisture and air that was trapped between the dielectric sheets, while the paper was impregnated with oil or another impregnating material to fill any voids and keep the paper moist to prevent it from becoming brittle. .

Some studies found that thermal stress or over-voltage in certain applications could cause degradation of the dielectric during long periods of operation—possibly resulting in a dielectric break down and a short circuit between the electrodes as well as a rupture of oil impregnated paper capacitors, subsequently causing a fire or explosion. .

Originally, standard paper capacitors were commonly used in antique radio equipment. Impregnated paper capacitors started to find their way into motor-start or high-voltage power applications. While such capacitors fit the applications at the time, paper capacitors haven’t evolved along with the rest of the electronic components industry. Radios themselves don’t even use this technology any longer, so why would other equipment that has increased in sophistication over the past several decades use it?.

Sure enough, eventually many suppliers stopped offering three-layer oil-impregnated paper capacitors making it difficult and expensive for laundry equipment manufacturers to include this type of capacitor in their machines to stay within the compliance laws of their respective countries. .

Even with the introduction of newer dielectric materials, paper (or film) foil-capacitor construction has the distinct disadvantage of a larger size and—more importantly—a lack of self-healing capabilities that could prevent a short-circuit failure. .

At one time, the use of paper dielectric was acceptable because other dielectric material was either very costly or simply unavailable. If an appliance manufacturer was working on a cost-sensitive project, these types of capacitors were overlooked. But with today’s available technology, cost and accessibility are no longer factors in the production of EMI filters..

Capacitors utilizing metalized film technology are smaller and safer than paper-based capacitors, at relatively the same cost. Should these capacitors fail, they do so in an open-circuit mode, eliminating the risk of fire or explosion. The metallization over the film evaporates should a failure occur, eliminating the chance of electrodes of opposite polarity coming in contact with each other and causing a short-circuit condition. .

For some white goods manufacturers, metalized polypropylene is the capacitor of choice, because of its extremely low dissipation factor, high dielectric strength, low moisture absorption and high insulation resistance. In recent years, EMI filters using this type of construction have been replacing the older, oil impregnated paper capacitors at an accelerated rate. .

In the past, there were some concerns that metalized polypropylene capacitors were more susceptible to transient over voltages than oil impregnated paper types. However, by applying specific, enhanced manufacturing techniques to the metalized polypropylene capacitors and EMI filter construction, these filters now comply with the most stringent laundry equipment manufacturers’ accelerated and enhanced safety specifications, including high-voltage combined ring wave tests. In addition, these EMI filters have passed tests in other related industries.

With only a limited number of companies still producing multi-layer impregnated paper capacitors, the product is quickly becoming costly and hard to find. EMI filters featuring metalized capacitors are effectively used as direct-form, fit-and-function replacements for the impregnated paper capacitors of the past.