Definite Purpose Contactors - Everything you need to know

Introduction

Let’s start with the basics of a Contactor. A Contactor is an electromechanical device that controls the switching of a load connected across it. When the coil is energized, the contactor will change its contact state from open to closed, making the equipment turn ON. Between the two main ratings of contactors, IEC and NEMA, there are many applications and contactor types a person can use. A common contactor in the HVAC industry is the Definite Purpose Contactor. As the name suggests, it was designed for a specific or “definite” purpose — HVAC applications (Heating, Ventilation, Air Conditioning and Refrigeration). Let’s find out why designing a Definite Purpose Contactor was needed, and why a general contactor wasn’t used.

During the mid 20th Century, leaders from the HVAC industry identified that this industry does not require expensive general purpose contactors. HVAC equipment, such as hermetic compressors have light duty applications, and using high-duty general purpose contactors for their switching was actually a mismatch. Out of this mismatch, a new line of contactors were designed and known as “Definite Purpose” , or DP contactors.

Major differences between this new Definite Purpose contactor and a general purpose contactor are:

• Definite Purpose contactors are UL recognized, not listed.
• General purpose contactors have a smaller footprint when compared to a Definite Purpose contactor of a similar current rating.
• Apart from these differences, many of the IEC general purpose contactors available in the market have less coil noise and are easily removable and replaceable. The service life of a Definite Purpose contactor is shorter than that of a general purpose contactor because they are primarily designed to have much more mechanical, electrical, and recycle life.

Still, the DP contactor is a perfect choice for specific applications due to its greater compatibility and lower price.

So, what applications prefer DP contactors?

A thumb rule says that if a customer has an application that “heats, cools, refrigerates, controls, or moves air” then it is a suitable candidate for DP contactors. In the HVAC industry, these DP contactors are further divided into two segments:

1. Compact 1 and 2 Pole DP Contactors, up to 40A: Majorly used for residential and unitary air conditioning installation.
2. Standard Frame 2 to 3 Pole DP Contactors, up to a few hundred amps: Majorly used for commercial or industrial establishments which require centralized air conditioning.

As seen below, DP contactors have attracted several other industries as well.

How Definite Purpose Contactors are selected?

• Unlike General purpose contactors, which are rated typically based on HorsePower (HP) of the motor, Definite Purpose contactors are rated on Motor Full Load Amps (FLA).
• DP contactors often have limitations on the maximum Locked Rotor Amps (LRA) it can handle. Generally, locked rotor amp ratings are 600% for 240 volts, 500% for 480 volts and 400% for 600 volts. Because of this limitation, IEC General purpose contactors rated for similar FLA can control a motor of higher HP compared to a DP contactor. Commonly available DP contactors are rated for 20, 25, 30, 40, 50, 60, 75, 90A (FLA).
• Coil voltage: Available voltages include 24V AC, 120V AC, 208/240V AC, 480V AC, 575V AC and some DC voltages as well.
• No. of poles: Manufacturers give a choice of 1, 2 and 3 poles. Often the manufacturer will give a 1 pole unit with shunt. What that means is that 1 line switching is controlled by a coil voltage and the other line (neutral) is always shorted.
• Further, two frames of 3 Pole DP contactors can be configured for reversing applications as well.

c3controls offers a wide range of IEC General purpose contactors (Series 300, 310) as well as Definite Purpose contactors (Series 280, 290). Both product lines include reversing and non-reversing configurations. Contact c3controls for help in selecting the appropriate contactor for your application.

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