Specifically designed for the HVACR industry, definite purpose (DP) contactors are electrically operated switching devices. They are used for switching a power or control circuit, and are typically controlled by automatic (thermostat) control or manual (pushbutton) interfaces. Most commonly found in applications including refrigeration, a/c, and resistance heating, DP contactors are suitable for a wide range of other low-cycle duty applications that require low cost switching of single- and three-phase loads.


The concept for DP contactors and starters originated in the 1950s at the request of several large HVACR OEMs. Until that time, the OEMs had used National Electrical Manufacturers Association (NEMA)-rated contactors and starters in their equipment. These NEMA-rated devices were robust, expensive, and typically outlasted the equipment in which they were installed.

The HVACR OEMs called in several electrical control companies and asked them to design a line of contactors and starters that would have a much lower electrical life, and a reduced cost based on this performance, while maintaining the high quality of the NEMA-rated products. The new line became known as DP contactors and starters. They were designed for specific requirements and a specific market - North America. While North America is still the primary market for DP contactors and starters, outside North America, International Electrotechnical Commission (IEC)-rated contactors and starters are used for similar HVACR applications.


DP control products were originally designed for the HVACR industry, but over the years their use has expanded to a variety of other market segments. In general, if a customer has an application that heats, cools, refrigerates, controls, or moves air, then it is a likely candidate for these devices. Within the air conditioning OEM segment, DP contactors can be grouped into two main categories, depending on the application:

• Compact 1- and 2-pole DP contactors, which range from 20 A to 40 A, and

• Standard frame DP contactors, from 15 A to 360 A.

The first main category is often the prime choice for the unitary central air conditioning units installed in many homes. Standard ac coil voltages are 24 V, 120 V, and 240 V. The second main category is primarily used in the larger commercial/industrial air conditioning units that can be seen mounted on the rooftops of industrial plants, hotels, hospitals, office buildings, and other commercial and industrial installations. These air conditioning units carry ratings from a few tons to 500 tons.


One of the main complaints about DP contactors in general is noise. External contaminants on the surface of the contactor magnets can cause the contactor to buzz or hum at increased levels; this is due to the magnet’s inability to close completely or evenly. To eliminate noise, the housing shield design should prevent (as much as possible) external contaminants from getting into the magnet surfaces. Adding plastic side shields to the sides of the contactor, to cover the opening for side-mounted accessories, can further reduce contaminants and resulting noise.

A removable inspection cover enables maintenance personnel to inspect for contact wear and replace the contactor based on its maintenance schedule and a universal mounting plate assists in easy replacement. For those who prefer to wire from the bottom, it provides the ability to rotate coil terminals 180 degrees from the top of the contactor to the bottom. The reduced sealed coil VA and wattage ratings help lead to decreased energy consumption and may allow for the use of smaller, less expensive control transformers. In turn, this can enable customers to downsize their panels and save money.


When specifying DP contactors, consider the following features: there are a broad range of contactors from 15 to 360 amperes; single-, two-, and three-pole devices; sealed housing limits contaminants and reduces or eliminates noise; compact and efficient design with a low VA coil and straight-through wiring; low VA current ratings; universal mounting plate that eliminates changing hole patterns when replacing devices; a variety of terminal styles to fit specific application requirements; snap-on accessories that require no tools; simple coil change; convenient no-position sensitive mounting benefits; hassle-free installation and maintenance; devices furnished with pressure plates and (standard) quick connect terminals; and availability of snap-on mechanical interlock, which allows interlocking two contactors for reversing or two-speed applications.

Publication date:04/05/2010