A Complete Guide to Contactors: What It Is and How It Works

A contactor is an electrically controlled switch designed to switch power circuits on or off. It is essentially a heavy-duty relay designed to handle high currents, making it ideal for applications involving large electric motors, lighting systems, HVAC equipment, and industrial machinery.

Although an electrical contactor falls under the broader category of relays, it is different from standard relays. The difference between contactors vs relays lies primarily in their current-carrying capacity and application. Contactors are built to handle much higher currents, typically ranging from 10A up to several hundred amps, and are commonly in three-phase power systems. In contrast, relays are generally used for lower-current applications in electronics and control circuits.

To understand how a contactor works, it's helpful to break down its operation into three key stages:

1. Energising the Coil: When an electrical signal is applied to the contactor's coil, it generates a magnetic field. This magnetic field acts on a movable contact, pulling it towards a stationary contact.
2. Closing the Contacts: As the movable contact is pulled in by the magnetic field, it closes the circuit, allowing a large current to flow through the contactor. This action effectively turns the connected load "ON".
3. Opening the Contacts: When the electrical signal to the coil is removed, the magnetic field collapses. This allows a spring mechanism to pull the movable contact back to its original position, opening the circuit and turning the load "OFF".

Proper contactor wiring is crucial for safe and efficient operation of electrical systems. Incorrect wiring can lead to malfunctions, damage to equipment, or even electrical hazards.

Key Components for Wiring a Contactor

To wire a contactor, you’ll need the following components:

• Suitable Wires: These are the conductors that carry the electrical current. The wire gauge should be selected based on the current rating of the contactor and the connected load. Ensure the wires have proper insulation to prevent short circuits.
• Wire Strippers: Wire strippers are essential for removing the insulation from the wires to expose the conductive core for connection to the contactor terminals.
• Screwdrivers: Use appropriate screwdrivers (flat-head or Phillips) to tighten the terminal screws and secure the wires to the contactor.
• Crimping Tool (if using crimp connectors): If you prefer using crimp connectors instead of directly connecting wires to the terminals, a crimping tool will be needed to secure the connectors to the wires. Read our guide on what is the best crimper to buy.
• Connectors (optional): These can include ring terminals, spade connectors, or other types of connectors that facilitate easier connection to the contactor terminals.
• Multimeter (for testing): A multimeter is useful for verifying the voltage at the contactor coil and checking for continuity in the wiring.
• Labelling Materials: Use labels or markers to identify the different wires and terminals, ensuring proper connections and easier troubleshooting in the future.

Basic Contactor Wiring Procedure

Here's a simplified procedure for basic contactor wiring:

1. Safety First: Always disconnect the power supply before working on any electrical equipment. Use appropriate personal protective equipment (PPE) such as insulated gloves and safety glasses.
2. Connect the Coil: Refer to the contactor wiring diagram. Connect the control voltage supply to the coil terminals. Ensure the voltage and current ratings match the coil specifications.
3. Connect the Power Circuit: Connect the incoming power supply lines to the contactor's main power terminals.
4. Connect the Load: Connect the outgoing power lines to the load terminals, which will be connected to the motor or other electrical equipment.
5. Connect Auxiliary Contacts (if applicable): If your application requires auxiliary contacts, connect them according to your specific control circuit requirements. Remember to refer to your contactor connection diagram for this step.
6. Verify Connections: Double-check all connections against the contactor diagram to ensure they are correct and secure.
7. Power On and Test: Turn on the power supply and test the contactor operation. Ensure the contactor energises and de-energises correctly when the control signal is applied and removed.

Safety Considerations:

• Always follow Australian electrical standards and regulations (e.g., AS/NZS 3000) to ensure the contactor wiring is compliant.
• Use properly rated wires and cables to prevent overheating and short circuits. The wire gauge should be appropriate for the current carrying capacity of the circuit.
• Ensure the current and voltage rating is compatible with the connected load, wires, and the contactor's specifications.
• Follow the electrical wire colour code in Australia to make it easier to identify and troubleshoot any wiring issues in the future.
• If you are unsure about any aspect of the wiring, consult a qualified electrician.

Current Handling Capacity & Voltage Rating

• Contactors: Designed for high-current applications, typically handling loads from 10A up to several hundred amps. They can also handle higher voltages, often used in three-phase power systems.
• Relays: Generally used for lower-current applications, typically below 10A. They are more common in single-phase systems with lower voltage ratings.

Mechanical Durability

• Contactors: Built robustly suited towards heavy-duty usage, capable of enduring frequent switching cycles even under harsh environmental conditions encountered across industrial settings.
• Relays: Generally smaller and less robust than contactors, making them suitable for lighter-duty applications.

Overload Protection

• Contactors: Often equipped with built-in overload protection mechanisms to prevent damage to the connected equipment in case of excessive current. Electrical contactors are specifically designed for this purpose.
• Relays: Typically lack integrated overload protection, instead relying upon external fuses or circuit breakers safeguarding against overcurrent conditions experienced during operation cycles.

Size & Cost

• Contactors: Larger and more expensive than relays due to their higher current handling capacity and robust construction.
• Relays: Smaller and more cost-effective, making them suitable for applications where space and budget are constraints.

Applications

• Contactors: Commonly used in industrial settings for controlling motors, pumps, lighting systems, and other heavy loads.
• Relays: Widely used in electronics, control circuits, and automotive applications for switching smaller loads and signals.

Contactors come in various types designed for specific applications and operating conditions. Here are some common types:

AC Contactor

What is it: An AC contactor is designed to switch alternating current (AC) circuits. It's the most common type of contactor, widely used in residential, commercial, and industrial settings.

Key Characteristics:

• Operates on AC voltage (typically 110V, 220V, or higher).
• Can handle high currents and frequent switching.
• Often includes features like arc suppression and overload protection.

Common Applications:

• Controlling AC motors in HVAC systems, pumps, and industrial machinery.
• Switching lighting circuits in large buildings.
• Powering heating elements in industrial ovens and furnaces.

DC Contactor

What is it: A DC contactor is designed to switch direct current (DC) circuits. It's commonly used in applications where DC power is prevalent.

Key Characteristics:

• Operates on DC voltage (typically 12V, 24V, or higher).
• Can handle high DC currents.
• May require different contact materials to handle DC arcing.

Common Applications:

• Controlling DC motors in electric vehicles and battery-powered equipment.
• Switching DC power in renewable energy systems (solar, wind).
• Telecommunications and battery backup systems.

Magnetic Contactors

What is it: While technically both AC and DC contactors utilise magnetic fields, the term "magnetic contactor" often refers to contactors that rely solely on electromagnetic principles for their operation.

Key Characteristics:

• Uses an electromagnetic coil to create a magnetic field that closes the contacts.
• Offers fast and reliable switching.
• Can be designed for both AC and DC applications.

Common Applications:

• General purpose motor control.
• Lighting control.
• Industrial automation.

Reversing Contactors

What is it: A reversing contactor is a specialised type of contactor that allows for the reversal of motor direction. It consists of two contactors mechanically interlocked to prevent simultaneous closure.

Key Characteristics:

• Changes the direction of motor rotation by switching the polarity of the power supply to the motor.
• Ensures safe and reliable motor reversal.

Common Applications:

• Controlling motors that require bidirectional operation (e.g., conveyor belts, cranes, elevators).
• Industrial automation systems where precise motor control is needed.

Here's what you should consider when choosing a contactor in Australia:  

1. Voltage and Current Rating:

• Ensure the contactor's coil voltage matches your system voltage. Australian systems typically use 240V for single-phase residential circuits and 415V for three-phase industrial systems.
• The contactor's current rating should exceed the load current. Consider the inrush current for motor loads, which can be significantly higher than the running current. Oversizing the contactor slightly provides a safety margin.
• Applying a higher voltage than the contactor is rated for can damage the coil, while a lower voltage may not properly energise the coil, leading to malfunction.

2. Contact Configuration:

• Determine whether you need the contacts to be closed or open in their default state (Normally Open - NO, or Normally Closed - NC).
• Choose the appropriate number of poles based on your application (single-phase, three-phase, etc.).

3. Contact Material and Type:

• Different materials offer varying levels of conductivity, durability, and resistance to arcing.
• Consider the type of load being switched. For example, motors with high inrush currents may require heavy-duty contacts.

4. Coil Characteristics:

• Choose a coil voltage that matches your control circuit voltage.
• Consider the coil current, as it determines the power consumption of the coil.
• Select the coil type (AC or DC) based on your control circuit.

5. Auxiliary Contacts:

• Determine if you need auxiliary contacts for control or indication purposes.
• Specify the required number of NO and NC auxiliary contacts.

6. Terminal Type:

• Choose the terminal type that best suits your wiring preferences and the size of the connecting wires. Options include screw terminals, spring clamp terminals, or ring terminals.

7. Enclosure and Overload Protection:

• Select an IP rating appropriate for the environment where the contactor will be installed. Australian conditions often require higher IP ratings due to dust, humidity, and potential for water exposure. In Australia's harsh environments, especially in mining or coastal areas, ensure the contactor is built to withstand temperature extremes, humidity, and dust.
• Consider whether the contactor has sufficient overload protection for your use case to prevent motor damage.

8. Operating Temperature:

• Ensure the contactor can operate reliably within the expected temperature range of the installation location.

9. Mounting and Size:

• Select the mounting method that best suits your installation requirements (DIN rail, panel mount, or direct mount).
• Consider the available space for installation and choose a contactor that fits.

10. Compliance and Standards:

• Ensure the contactor complies with Australian wiring rules and standards (AS/NZS 3000).
• Consider any industry-specific standards or regulations that may apply to your application.