HVACR(Heating, Ventilation, Air Conditioning/Refrigeration) Training Board: How To Troubleshoot A Contactor (Electrical Training Board/2 Pole Contactors) How To Check A HVAC Contactor With A Multimeter

A contactor is an electrical device which is used for switching an electrical circuit on or off. It is considered to be a special type of relay. However, the basic difference between the relay and contactor is that the contactor is used in applications with higher current carrying capacity, whereas the relay is used for lower current applications. Contactors can be field mounted easily and are compact in size. Generally, these electrical devices feature multiple contacts. These contacts are in most cases normally open and provide operating power to the load when the contactor coil is energized. Contactors are most commonly used for controlling electric motors.

There are various types of contactors, and each type has its own set of features, capabilities, and applications. Contactors can break current over a wide range of currents, from a few amperes to thousands of amperes, and voltages from 24 VDC to thousands of volts. In addition, these electrical devices come in varying sizes, from hand-held dimensions to sizes measuring a meter or yard on one side (approximately).

The most common application area of the contactor is high-current load. Contactors are known for their capability to handle currents of over 5000 amperes and high power over 100 kW. Heavy motor currents produce arcs when being interrupted. These arcs can be reduced and controlled using a contactor.

The following three are crucial components of the contactor:

Coil or Electromagnet: This is the most crucial component of a contactor. The driving force that is required to close the contacts is provided by the coil or electromagnet of the contactor. The coil or electromagnet and contacts are protected by an enclosure.

Enclosure: Just like the enclosures used in any other application, contactors also feature an enclosure, which provides insulation and protection from personnel touching the contacts. The protective enclosure is made from different materials, such as polycarbonate, polyester, Nylon 6, Bakelite, thermosetting plastics, and others. Generally, the open-frame contactor features an additional enclosure, which protects the device from bad weather, hazards of explosion, dust, and oil.

Contacts: This is yet another important component of this electrical device. The current carrying task of the contactor is done by the contacts. There are different types of contacts in a contactor namely, contact springs, auxiliary contacts, and power contacts. Each type of contact has an individual role to play.

How the Contactor Operates:

Operating Principle of a Contactor: The current passing through the contactor excites the electromagnet. The excited electromagnet produces a magnetic field, causing the contactor core to move the armature. A normally closed (NC) contact completes the circuit between the fixed contacts and the moving contacts. This permits the current to pass through these contacts to the load. When current is removed, the coil is de-energized and opens the circuit. The contacts of the contactors are known for their rapid open and close action.

The contactor in your air conditioner is a small electrical
relay that starts or stops your compressor after a small signal is
received from your thermostat; much like the ignition circuit of
your car. Over its life your contactor can be asked to start and
stop your air conditioning system thousands of times. After
thousands of on and off cycles your faulty contactor can cause
major problems. Your contactor can fail in one of two ways,
electrically or mechanically.

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