Electromagnetic contactors explained
An essential guide for efficient power control
Contactors are present in various industries. They are used in battery management systems, motor control, HVAC systems, lighting control, etc. Do you know how it works and what I should look for while buying a contractor? This comprehensive guide will answer all your questions associated with contractors and help you select the right contractor for all your requirements.
Contactor- What is it?
A contactor, also known as an electromagnetic contactor, is an electromagnetically controlled device that switches on and off an electrical circuit. Its basic construction contains a coil and contacts, similar to what you see in a relay. The major difference between a contactor and a relay is the current rating of the contacts. Relays have contacts with low current carrying capacities, while contactors have contacts that can switch much higher currents.
How a contactor works?
The operation of a contactor is similar to an electromechanical relay. It consists of an operating coil, a magnetic circuit and one or more set of contacts. When voltage is applied to the coils of a contactor, the excitation current starts flowing. This current produces a magnetic field in the ferromagnetic iron core. This magnetization generates a force, which causes the physical movement of the armature. The movable contact support connected to the movable iron core by a pin contains a movable contact. This movable contact moves when the movable core connects the fixed contact attached to the frame and closes the circuit. When the voltage applied to the coil is removed, the core loses the excitation, and the back spring pushes the movable core back to the right. Simultaneously, the movable core moves apart from the movable contact and causes the circuit to open.
Figure 1: Working of a contactor (Source)
What are the applications of a contactor?
Electromagnetic contactors are present in most industrial and domestic applications. Here are some significant applications of contactors, but they are not limited to:
- Battery management system (BMS): Contactors are used in the battery disconnect unit (BDU) of electric vehicles (EVs). The BDU switches the battery on/off during different operating modes of an EV. It also distributes and protects power within the EV system. In the event of a failure, the contactor receives command from the BMS to disconnect the battery.
- Motor control: Contactors control the electric current to the motor. Their function is to repeatedly make or break power in an electrical circuit. Depending on the requirements, they can work as standalone devices or be integrated with a controller like a PLC, etc.
- Lighting control: Contactors are used in commercial and industrial lighting systems to switch large lighting loads on and off. They can be used individually for simple control schemes or as integral components in full-featured lighting control systems.
- HVAC control: In an HVAC system, a contactor controls the flow of electricity to the compressor and blower motor. It receives a command from the controller and turns on and off the compressor and blower motor, thereby regulating the temperature.
- Applications in industrial safety: The contactor acts like an actuator where the former controls electrical power flow to machinery or equipment. In emergency or hazardous situations, the machine is set to a safe state.
How to wire a contactor?
Figure 2 shows some examples of mechanically contacting elements.
Figure 2: Types of contactors (Source)
Figure 3 shows the operation of a 3-phase motor induction using a LC1D09BD contactor from Schneider Electric controlled by a M-Duino PLC. Here, 24V DC voltage is applied to the Vdc pin of the power terminal of the PLC. Terminals L1, L2, and L3 of the contactor are connected to the 3-phase AC supply, and terminals T1, T2, and T3 of the contactor are connected to the motor. Now, to provide the command to the contactor from PLC, the relay output terminal R 0. X (X = 1..8) of the PLC is connected to the A1 terminal of the contactor, and the A2 terminal is connected to the ground.
Figure 3: Connection diagram of a 3-phase motor with a contactor and a PLC (Source)
How are relay and contactor different from each other?
Although, according to the principle of working and basic installation, relay and contactors share certain similarities, they also differ in:
| Parameters | Relay | Contactor |
|---|---|---|
| Load Capacity | Generally used with low current loads, such as 10 amperes or below. | Designed for high-power applications with loads greater than 10 amperes |
| Operation modes | Can operate in both modes i.e., normally open, or normally closed as per the requirement | Primarily designed to work in a normally open configuration but can also connect the circuit in continuous mode or only when required. |
| Contact Material | Commonly made up of Silver Nickel, Silver Cadmium Oxide and Silver Tin Oxide. Silver Nickel | Commonly made up of Silver Nickel, Silver Cadmium Oxide and Silver Tin Oxide. Silver Nickel Made of alloy epitaxial metals like platinum and palladium which reduce contact resistance and ensure good current flow |
| Arc Suppression Techniques | Generally, don't have a built-in arc suppression system | Contains chutes and blowout magnets to prevent arc formation |
| Contact Configuration | It can be designed with normally closed or normally open contacts and can be configured as single-pole double-throw (SPDT) or double-pole double-throw (DPDT). | Primarily designed to work as a normally open configuration. |
What are the parameters to consider while choosing a contactor?
Choosing a suitable contactor is essential for the smooth, efficient, and safe operation of the process. The following points must be considered while choosing the right contractor:
- Load current: It is the maximum amount of current drawn by the device connected to the contactor at full load. It also represents the maximum current a contactor can handle without damage.
- Coil voltage: It is the minimum voltage required by the contactor's coil to produce the magnetic field to displace contactor's contact. It can be rated in AC or DC depending on the AC or DC contactor.
- Operating voltage: It is the maximum voltage that is applied on the power poles of a contactor. For 3 pole contactors, it can be measured with respect to terminals L1, L2 and L3.
- No. of poles: The number of poles indicates the number of separate connection or circuits a contactor can control.
- Contact configuration: It represents the number of output connections a contactor's pole can handle, such as SPST-NO, DPST-NC, etc.
- Mounting: Depending on the application, contactors are available with multiple mount options, such as DIN rail mount, panel mount, bracket mount, or flange mount.
Frequently asked questions (FAQs) about contactors
- Can the contactor be actuated with AC supply?
Yes, it can be actuated by AC as well as DC supply.
- How residual magnetism affects the performance of a contactor?
If an AC relay is contactor from AC voltage, residual magnetism has no problem holding the armature seated after the release of coil power. But when an AC contactor is operated from DC voltage, there is a danger that residual magnetism may hold the armature seated. At the very least, the presence of residual magnetism in the core causes a reduction in the dropout voltage of the contactor. Also, it should be ensured that the DC voltage used is less than the AC voltage rating of the coil. To use an AC coil on DC requires lowering the amount of DC voltage to that value where coil power is within maximum limits.
- What are the common industry standards of contactors? How they are different from each other?
In industry, there are two standards for contactors: NEMA and IEC. NEMA-rated contactors cover a broader range of applications and provide flexibility during commissioning when the actual load is not known. IEC-rated contactors are more application-specific, which allows them to perform at a higher level within a narrower application range. Similarly, NEMA contactors are physically more robust and have longer mechanical life than IEC contactors. The mechanical life of NEMA, which is 0 to 3 contactors, may be up to 30 million operations. Equivalently rated IEC contactors (9A to 100A) typically provide a mechanical life of 10 million to 15 million operations. Finally, for a given rating, an IEC contactor is generally less expensive (up to 40%) than a NEMA contactor. IEC contactors are more suitable for DIN mount applications, whereas NEMA contactors are suitable for panel mounting designs.
- What are the common failures of a contractor?
Some of the common failures of a contactor are as follows:
Overheating: It is caused by factors like increased load on the system, short circuits, or sudden voltage spikes. It may lead to permanent damage to the contacts.
Coil failures occur due to insulation breakdown, overheating, or coil winding defects.
Buzzing, chattering, and humming noise: Caused by insufficient current from the control source to actuate the electromagnetic coil, incorrect voltage supply to the coil, or contamination of the moving part.
Contact wear: Due to frequent movement, the contacts may wear over time. They are also subjected to corrosion due to moisture or the presence of corrosive vapors in the environment where contactors are installed.
- What does utilisation categories AC1 and AC3 of a contactor conforming to IEC 60947-1 mean?
The utilisation categories are dependent on the switching requirements of a contractor.
AC-1 category applies to all AC devices (loads) with a power factor equal to or greater than 0.95. Application examples are non-inductive or slightly inductive loads like heaters, resistive furnaces, distribution, etc.
AC-3 category applies to squirrel cage motors where breaking occurs during normal operation. Application examples include all standard squirrel cage motors, lifts, escalators, conveyors, bucket elevators, compressors, pumps, mixers, air conditioning units, etc.
- Where can I buy contactors at best price?
Farnell/Newark/Element 14 offers wide variety wide range of contactors from established suppliers along with the associated accessories components tailored to meet the requirements of all your domestic and industrial application.
