What is a Definite Purpose Contactor? How Does it Work?

A definite purpose contactor, also known as a specialized contactor, is a type of electrical switch designed to control specific loads or equipment. It is commonly used in industrial settings to control motors, pumps, and other high-power electrical devices. The working principle of a definite purpose contactor is based on the use of electromagnetism to control the flow of electrical energy to a specific load. The contactor is controlled by an external power source, such as a control circuit, and can be used to start, stop, or reverse the direction of a motor or other load. In this article, we will delve deeper into the definition, working principle, and applications of definite purpose contactors.

What is a Definite Purpose Contactor?

The definite purpose contactor (DP Contactor) is a special type of switching device that is designed for heating, ventilation, air conditioning, and refrigeration (HVACR) applications.

It is designed to carry out numerous close/open cycles and is commonly controlled by on/off push buttons. It is generally held closed by a reduced current through the closing solenoid. It may have auxiliary contacts, normal-close (N.C.), and normal-open (N.O.) contacts, to be used for control functions.

Definite-purpose contactors are available in single-, two-, three- and four-pole configurations. The characteristics of contactors are speciﬁed by the operating duration, the application in which to be used, the number of start/stop cycles per hour, and mechanical and electric endurance.

In HVACR applications, the definite purpose contactor provides excellent performance. Besides, it is cost-effective. It is selected based on the full load amps (FLA) or inductive amp load, as well as the coil voltage.

DP contactors have also completely encased design. But the front covers can be removed and the internal parts can be inspected. DP contactors are generally easily interchangeable. If you are using a specific manufacturer you can most likely use any others without any issues. DP contactors are compact in size and offer three convenient methods of wire connection: quick-connect terminals, screws, or box lugs.

Difference between IEC contactor, NEMA contactor, and definite purpose contactor?

The main difference between a normal contactor and a definite purpose contactor is the definition. NEMA contactor is considered a general-purpose contactor and IEC contactor is considered a normal contactor or power contactor.

Let me give you more detail about the history.

Manufacturers design their products to the durability and performance standards established by the National Electrical Manufacturers Association (NEMA), and International Electrotechnical Commission (IEC).

NEMA contactors were considered general-purpose contactors up to the 1950s. They could be used in a variety of applications with high efficiencies. Although they were very robust, their construction was very large and their prices were so high.

To make the equipment smaller in size so that it would fit into more areas, as well as make it more affordable, the new definite purpose contactor was designed. This resulted in a smaller, quieter, less costly contactor, because the lighter load of an HVACR application did not require the same demands that a harsh industrial application might. Although their cost is less, these contactors must still meet harsh conditions, such as rapid cycling, sustained overloads, and low system voltages.

The IEC components were primarily focused on the European market. Products coming into the U.S. from Europe would typically be designed to the IEC standards. Today, the IEC standards are seen globally. More and more IEC products are being used in the U.S. in both HVACR manufacturing and the aftermarket. The IEC contactor is typically matched more specifically to the application. Like the definite purpose contactor, this could result in the IEC contactor being significantly smaller than the NEMA contactor, and consequently less costly, particularly for contactors below 100 Amps. IEC contactors are allowed to have a smaller creepage path and higher temperature rise than NEMA contactors which results in a smaller physical size.

How Does a Definite Purpose Contactor Work?

DP (definite purpose) contactor has two separate circuits. Main circuit and control circuit. The opening and closing of the main circuit are accomplished by an electromagnetic control circuit (Coil). The magnetic field is created in the core by the current in the coil. This attracts the armature. The movement of the armature closes and opens the main circuit.

The control circuit supplies the power to the coil. The voltage for the main load circuit and control load circuit is often different. The DP contactors are mainly available with 24VAC, 120VAC, 208/240VAC, 277VAC, 480VAC, and 575V AC coils. DC voltage coils are available for special applications. The DC coils have an economizer coil design (called three lead DC as these have dual winding coils). Some of the DP 3 pole and 4 pole contactors are available with 12VDC, 24VDC, 48VDC, and 115VDC.

Applications of Definite Purpose Contactors

Definite-purpose contactors are ideal for heating, air conditioning, refrigeration, data processing, and food service equipment. The life expectancy, both electrical and mechanical, will meet or exceed industry performance requirements for definite-purpose devices.

While they are called definite purpose contactors, their purposes span a wide variety of applications. They provide high performance with flexibility and reliability, designed to match numerous applications including:

Motors
Power supplies
Foodservice equipment
Compressors
Business machines
Resistive heating
Air conditioning
Refrigeration equipment
Welding

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

There is 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; a 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.