A contactor is a specialized type of relay used for switching a large amount of electrical power through its contacts. Contactors are designed to handle high current ratings and are typically equipped with multiple contacts. These contacts are often (though not always) normally open, meaning that power to the load is disconnected when the coil of the contactor is de-energized.
The primary industrial application for contactors is the control of electric motors. Contactors play a crucial role in starting and stopping motors, particularly in situations where the motor current exceeds the capacity of standard relays. They provide a reliable means of controlling the power circuit that drives the motor.
Here are some key characteristics and distinctions of contactors:
High Current Ratings: Contactors are built to handle high levels of electrical current, making them suitable for heavy-duty applications where standard relays would be insufficient.
Controlled by Low-Power Circuits: Contactors are controlled by lower-power control circuits, such as those operated by switches, pushbuttons, or programmable logic controllers (PLCs). These control circuits have significantly lower power levels than the circuits they control.
Motor Control: One of the most common uses of contactors is in controlling the operation of electric motors. They allow for safe and efficient motor starting, stopping, and reversing.
Normally-Open Contacts: Contactors typically have normally open contacts, meaning that in their default state (when the coil is not energized), the contacts are open, and electrical power is not flowing through them.
Multiple Contacts: Contactors often have multiple sets of contacts, which can be used for various control and switching functions.
Contactors come in a variety of forms with different capacities and features. Unlike circuit breakers, contactors are not intended to interrupt short-circuit currents. They range in size from devices small enough to hold in one hand to large units approximately a meter (yard) in size. Contactors are used for controlling a wide range of electrical loads, including electric motors, lighting, heating systems, capacitor banks, thermal evaporators, and more.
Contactor Construction: A contactor consists of three main components:
Contacts: These are the current-carrying parts of the contactor. Contacts can include power contacts for controlling the main load, auxiliary contacts for additional control functions, and contact springs that provide the necessary mechanical force.
Electromagnet (Coil): The coil generates a magnetic field that exerts force on the contactor's moving core, closing the contacts when energized. It is the coil that controls the contactor's operation.
Enclosure: The enclosure is a frame that houses the contacts and the electromagnet. It is typically made of insulating materials like bakelite, nylon 6, or thermosetting plastics. The enclosure provides insulation to protect against electrical shock and offers some level of protection against personnel accidentally touching the contacts. In open-frame contactors, an additional enclosure may be used to safeguard against dust, oil, explosive environments, and adverse weather conditions.
Operating Principle: The operating principle of a contactor involves the interaction between the coil and the contacts:
When current flows through the coil (electromagnet), it generates a magnetic field that attracts the moving core of the contactor.
Initially, the coil draws more current, but as the metal core enters the coil, its inductance increases, limiting the current.
The magnetic force from the electromagnet holds the moving and fixed contacts together, closing the contacts.
When the coil is de-energized, gravity or a spring returns the core to its initial position, causing the contacts to open.
For contactors operating with alternating current (AC), a shading coil is often included around a portion of the core. This coil slightly delays the magnetic flux, helping to reduce buzzing or noise in the core caused by the alternating magnetic field.
To prevent arcing and contact damage during the rapid opening and closing of contacts, contactors are designed for fast action. They may incorporate mechanisms like internal tipping points or bifurcated contacts to minimize contact bounce and maintain a low-resistance connection. Additionally, some contactors feature multiple contacts that engage in rapid succession, with the last contact to break protecting the primary contacts from arcing.
Another technique to enhance contactor lifespan is contact wipe, where the contacts move past each other after initial contact to remove any contaminants and ensure a clean, low-resistance connection.
In summary, contactors are essential electrical switches designed for high-current applications, such as controlling motors and other heavy loads. Their construction and operation are optimized for reliability, rapid switching, and durability in various industrial and commercial settings.
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