A Complete Guide to Contactors

This introductory guide to contactors aims to provide clear and comprehensive answers to common questions about contactors. We will include a brief look at how electrical contactors work, how they are used, and which types are sold online.

We will also cover some of the more popular types and brands of contactors available and identify how to choose the best for your needs.

A contactor is an electrical switch designed to handle high-current loads in industrial and large-scale electrical systems throughout Malaysia. The function of a contactor is to act as heavy-duty relays, opening and closing circuits to control the flow of electricity to demanding loads such as motors, pumps and HVAC systems.

Contactors differ from standard switches due to their robust construction and ability to withstand the stresses of high currents. This resilience is achieved through several key features:

• Sturdy Contacts: Contactors utilise large contact surfaces made of durable materials like silver-cadmium oxide or tungsten, which can withstand the heat and arcing generated during high-current switching.
• Arc Suppression: They incorporate arc chutes or other suppression mechanisms to quickly extinguish the electrical arc that forms when the contacts open, preventing damage to the contacts and ensuring a long lifespan.
• Magnetic Blowouts: Some contactors employ magnetic blowouts, which use magnetic fields to stretch and cool the arc, further minimising wear and tear on the contacts.

A contactor operates through electromagnetic principles. When an electrical current is passed through the contactor's coil, it generates a magnetic field. This magnetic field attracts a movable contact, closing the circuit and allowing electricity to flow to the load. When the current to the coil is interrupted, the magnetic field collapses, and the contact returns to its open position, breaking the circuit. This simple yet effective mechanism allows for reliable and safe switching of high-current loads in countless applications across Malaysia.

Contactor Capacities

Contactors come in a wide range of capacities to handle diverse electrical loads. In Malaysia, you'll commonly find contactors with the following capacities and typical applications:

Low-capacity contactors (9A - 25A): These are often used for controlling lighting systems, small motors in household appliances, and HVAC systems in residential buildings.

Medium-capacity contactors (40A - 100A): These are commonly found in industrial settings, powering machinery like pumps, conveyors, and small machine tools. They are also used in commercial buildings for controlling larger HVAC systems and elevators.

High-capacity contactors (150A - 800A and above): These heavy-duty contactors are essential for large-scale industrial applications, such as controlling large motors in manufacturing plants, cranes, and power distribution systems. They are also vital for infrastructure projects, powering pumps in water treatment plants and ventilation systems in tunnels.

What Is the Main Function of a Contactor?

• To safely and efficiently control high-current electrical loads.
• To open and close circuits, allowing or interrupting the flow of electricity to a load.
• To provide remote operation of electrical equipment.
• To protect expensive equipment from damage due to overloads or short circuits (when combined with appropriate protective devices).

Typically, a contactor will be used in situations where power loads need to be turned on and off frequently or rapidly. However, they can also be configured either to power on a circuit when activated (normally open, or NO contacts), or to shut down power to a circuit when activated (normally closed, or NC contacts).

The two classic applications for a contactor are as an electric motor starter, such as those that use auxiliary contacts and connectors for use in electrical vehicles, and in high-powered lighting control systems.

When a contactor is used as a magnetic starter for an electric motor, it will usually also provide a range of other safety features such as power-cutoff, short circuit protection, overload protection, and under-voltage protection.

Contactors being used to control high-power lighting installations will often be arranged in a latching configuration, to lower overall power consumption. This arrangement involves two electromagnetic coils working in tandem. One coil will close the circuit contacts when briefly energised and hold them closed magnetically. The second coil will open them again when powered.

With Malaysia's target to achieve 23 percent energy savings from industrial and commercial sectors by 2050, contactors play a key role in reducing power consumption. For example, in large-scale commercial buildings and infrastructure projects, latching configurations are employed to minimise energy usage in lighting systems.

As contactors are intended specifically for these sorts of high-voltage applications, they tend to be physically larger and more robust than standard relay switching devices. However, most electrical contactors are still designed to be easily portable and mountable and are generally considered highly suitable for use in the field.

To better understand how a contactor works, it is helpful to know about the three core components of any electrical contactor device when assembled. These are normally the coil, the contacts, and the device enclosure.

Contactor Components

• The coil, or electromagnet, is the key component of a contactor. Depending on how the device is set up, it will perform a specific action on the switch contacts (opening or closing them) when it receives power.
• The contacts are the components of the device that carry power across the circuit being switched. There are various types of contacts found in most contactors, including springs and power contacts. Each type performs a specific function in transferring current and voltage.
• The contactor enclosure is another important part of the device. This is the housing that surrounds the coil and contacts, helping to insulate the contactor’s key components. The enclosure protects users against accidentally touching any conductive parts of the switch, as well as offering robust protection against risks such as overheating, explosion, and environmental hazards like dirt and moisture ingress.

The operating principle of an electrical contactor is straightforward. When the electromagnetic coil has a current passed through it, a magnetic field is created. This causes the armature within the contactor to move in a certain way regarding the electrical contacts.

Depending on how the specific device has been designed and the role it is intended for, this will normally be to either open or close the contacts.

• If the contactor is designed as normally open (NO), exciting the coil with voltage will push the contacts together, establish the circuit, and allow power to flow around the circuit. When the coil is de-energised, the contacts will be open, and the circuit will be off. This is how most contactors are designed.
• A normally closed (NC) contactor works the opposite way. The circuit is complete (contacts closed) while the contactor is de-energised but interrupted (contacts open) whenever current is supplied to the electromagnet. This is a less common configuration for contactors, although it is a relatively common alternative setup for standard relay switches.

Contactors can rapidly perform this switching task over many thousands (or indeed millions) of cycles during their full working lives.

Wiring a contactor correctly is crucial for proper function and to ensure safe and reliable operation in electrical systems. Here’s how to wire a contactor:

1. Identify the Terminals:

• Coil Terminals (A1 and A2): These terminals are for connecting the control circuit, which energises the contactor's coil.
• Power Terminals (L1, L2, L3 for three-phase; L1 and L2 for single-phase): These are the incoming power supply terminals.
• Load Terminals (T1, T2, T3 for three-phase; T1 and T2 for single-phase): These terminals connect to the load you want to control (e.g., motor, pump).

2. Wiring the Coil Circuit:

• Connect the control circuit's power supply to terminal A1.
• Connect the other side of the control circuit (often a switch or relay contact) to terminal A2.

3. Wiring the Power Circuit:

• Connect the incoming power supply lines to the corresponding L terminals (L1, L2, L3 for three-phase or L1 and L2 for single-phase). Ensure proper phase sequencing if it's a three-phase system.
• Connect the wires leading to the load to the corresponding T terminals (T1, T2, T3 for three-phase or T1 and T2 for single-phase).

4. Secure All Connections:

• Tighten all terminal screws securely to ensure good electrical contact and prevent loosening due to vibrations.
• Double-check all connections against the contactor wiring diagram to ensure accuracy.

5. Test the Contactor:

• Turn on the power supply and activate the control circuit.
• The contactor should energise, and you should hear an audible click as the contacts close.
• Use a multimeter to verify that the voltage is present at the load terminals.

What Is the T1 and T2 on a Contactor?

The T1 and T2 terminals on a contactor are the load terminals. These are the connection points where the wires leading to the electrical load (e.g., motor, pump, or other equipment) are connected.

• In a single-phase contactor, you'll have T1 and T2.
• In a three-phase contactor, you'll have T1, T2, and T3.

When the contactor is energised, the contacts close, and electricity flows from the power terminals (L1, L2, etc.) through the contactor to the load terminals (T1, T2, etc.), powering the connected equipment.

Although contactors are generally used for higher power applications compared to relays, the full technical differences between the two are slightly more complex.

A more complete list of the differences between a contactor and a relay would include the following:

• Load Capacity: Contactors are designed and built to handle much higher power-switching applications than control relays. Relays are typically reserved for use with loads of around 5A-15A, and they are most often rated for 10A or less.
• Contact Standards: Contactors are almost always set up in a normally open (NO) configuration. This means that the circuit will only be established while the electromagnet in the contactor is receiving current. Relays are easy to find with both NO and NC contacts.
• Protections and Safety Features: Contactors typically offer a much wider range of safety cut-offs and protections, reflecting the fact that they are designed for higher-power applications. Indeed, a specific type of switch known as contactor overload relays is specifically designed for use in preventing machinery and power circuits from overheating.

Elsewhere, common examples of standard contactor safety features include:

• Spring-loaded contacts, for interrupting an electrical circuit if the contactor is powered off.
• Overload protection that kicks in if the circuit receives a current surge for a defined period.
• Magnetic arc suppression, forcing any current arcs to travel a greater distance than the energy they carry can sustain.

What Happens When You Use a Contactor in Place of a Relay, and Vice Versa?

• Using a Relay for a Contactor Application: If you use a relay to switch a high-current load that exceeds its capacity, it can lead to overheating, contact welding, and even fire hazards. The relay's contacts are not designed to handle the high currents and may fail prematurely, causing damage to the connected equipment.
• Using a Contactor for a Relay Application: While a contactor can technically switch a low-current load, it's not always the best choice. Contactors are generally bulkier, more expensive, and consume more power than relays. Using a contactor for a small load can be overkill and inefficient.

Due to contactors being intended for heavy-duty, higher-power applications, they tend to be physically bigger and heavier than relays, and their switching speed is considerably slower. They are also more expensive than relays in most cases and consume more power due to their larger electromagnetic coils.

1. Load and Power Ratings:

• Voltage: Ensure the contactor's voltage rating matches your electrical system (240V in Malaysia). Using a contactor with a lower voltage rating can lead to overheating and damage.
• Current: The contactor's current rating should be equal to or greater than the full load current of the equipment it will control. This prevents overloading and ensures safe operation.
• Power: Consider the power rating (HP or kW) of the contactor to ensure it can handle the demands of the load. For heavy-duty applications common in Malaysian industries like palm oil processing or manufacturing, higher power ratings are necessary.

2. Coil Voltage vs. Load Voltage:

• Coil Voltage: This is the voltage required to energize the contactor's electromagnetic coil. It can be different from the load voltage.
• Load Voltage: This is the voltage that the contactor switches.
• Important: While the coil and load voltages can differ, they must be compatible with your control circuit and power supply.

3. Contact Configuration:

• Normally Open (NO): The contacts are open in their de-energized state. When the coil is energized, the contacts close, completing the circuit. This is the most common configuration for contactors.
• Normally Closed (NC): The contacts are closed in their de-energized state. When the coil is energised, the contacts open, interrupting the circuit. NC contactors are less common and typically used for specific safety applications.

4. Safety Features:

• Overload Protection: Essential for preventing motor damage due to excessive current. Thermal overload relays are commonly used in conjunction with contactors to provide this protection, complying with Malaysian safety standards.
• Short Circuit Protection: Fuses or circuit breakers are used in the power circuit to protect the contactor and connected equipment from damage caused by short circuits.
• Under-Voltage Protection: This feature prevents unexpected motor restarts after a power outage, ensuring safety in industrial settings.
• Compliance with Standards: Ensure the contactor complies with relevant Malaysian safety standards, such as MS IEC 60947-4-1, which specifies requirements for contactors and motor starters.

5. Environmental Suitability:

• IP Ratings: Consider the IP (Ingress Protection) rating of the contactor, which indicates its level of protection against dust and moisture. In Malaysia's tropical climate, a higher IP rating is often desirable to ensure reliable operation in humid and dusty conditions.
• Temperature Tolerance: Choose a contactor with a suitable operating temperature range for the environment it will be installed in.
• Shock Resistance: For applications with vibrations or potential impacts, select a contactor with adequate shock resistance to ensure durability and prevent malfunctions.

Magnetic Contactor

A magnetic switch contactor operates entirely via electromagnetism and therefore does not need any direct intervention to perform its role consistently. This makes it one of the more efficient and reliable designs since electromagnetic switching only requires a small amount of power. It also enables the full remote operation of the contactor. Almost all electrical contactors work on this basis today.

Solid State Contactors

Solid state contactors use semiconductor devices to switch loads, offering advantages like increased speed, longer lifespan, silent operation, and improved efficiency. However, they can be more expensive, have a higher voltage drop, and be sensitive to temperature. In Malaysia, they are used in renewable energy systems, HVAC, and industrial automation.

Single Phase & 3-Phase Contactors

Single-phase contactors are used in residential and small-scale applications with single-phase power supply (two wires). They have one set of contacts to switch the live wire, while the neutral wire remains uninterrupted. Three-phase contactors, on the other hand, are designed for industrial settings with three-phase power supply (three wires carrying alternating current). They have three sets of contacts to switch all three phases simultaneously, enabling them to handle much higher power loads compared to single-phase contactors. This makes them ideal for controlling large motors, heavy machinery, and other power-hungry equipment commonly used in Malaysian industries like manufacturing and palm oil processing.

AC vs DC Contactors

Contactors are designed to operate on either alternating current (AC) or direct current (DC) power supplies, and the type you choose depends on the specific application and electrical system. AC contactors are more prevalent, especially in Malaysia where the standard power supply is 240V AC. They utilise an AC coil to generate the magnetic field that closes the contacts. These contactors are commonly found in industrial settings, controlling motors, pumps, and lighting systems.

DC contactors, on the other hand, have a DC coil and are typically used in applications with DC power sources, such as battery-powered systems, renewable energy systems, and electric vehicles. They are also employed in specific industrial processes where DC power is preferred. The key distinction lies in the coil design and how the magnetic field is generated to activate the contactor.

Auxiliary Contactors

Auxiliary contactors are smaller contactors with lower current ratings than the main contactors they are associated with. They are primarily used for control and signalling purposes, enhancing the functionality and safety of electrical control systems. For instance, in a motor control circuit, an auxiliary contact can be used to activate an indicator light, signalling that the motor is running. They can also be used to create interlocking circuits, ensuring that certain safety conditions are met before a machine can operate.

In complex industrial settings in Malaysia, auxiliary contactors play a crucial role in automating processes, improving safety, and providing valuable feedback on the status of equipment.

Switch Ratings and Coil Ratings (Contact Voltage and Current)

Contactor switch ratings are usually given as two separate metrics - maximum switching voltage and maximum switching current. The upper limits of both voltage and current that a design, brand, or model of a switch can handle must always be directly assessed in terms of the requirements for the circuit or motor where it is being used.

While a product may be listed as a 230V contactor, 240V contactor, or 1000V DC contactor, more detailed manufacturer specifications will usually make direct reference to the maximum coil voltage, contact current rating, contact voltage rating, and overall power rating of a device. They will also list the number of auxiliary contacts, terminal type, normal state configuration, and minimum and maximum operating temperatures. Contactors generate more heat than relays, and this must be factored in when choosing a suitable unit for installation.

Various electrical ratings for contactors will often be given as either resistive or inductive, depending on the intended use of the module. Resistive ratings are more common for contactors being used with heating elements or lighting control installations, whereas inductive load ratings tend to be more common for motors, transformers, and solenoids.

It is also worth bearing in mind that the contactor coil voltage (control circuit voltage) does not necessarily have to be the same as the load voltage being switched on and off. For example, the coil voltage could be 24VDC, but the motor being switched on and off could be 400VAC. Typical coil voltages available include 12, 24, 48, 110, 230, and 400V.