What is a contactor? How is it different from a relay?

The contactor has the advantages of rapid operation, safe and convenient operation, ease of remote control, and protection against undervoltage and zero voltage. It is widely used in electric motors, electric welding machines, small generators, and machine tool control. Therefore, mastering the rapid judgment and handling of contactor faults is also an essential knowledge for electricians. Contactors are divided into AC (alternating current) contactors and DC (direct current) contactors. They are used in power distribution systems and can be used for remote control and frequent operation of AC and DC circuits. By allowing the coil to pass current to generate a magnetic field, the contacts close, thereby achieving the control of load components. Contactors have the advantages of high operation frequency, long service life, reliable operation, stable performance, low cost, and easy maintenance.
1. What is the function of the contactor?
The function of a contactor is similar to that of a knife switch. However, a contactor not only can connect and disconnect circuits, but also has under-voltage release protection, zero-voltage protection, large control capacity, suitability for frequent operation and long-distance control, reliable operation, and long service life. In contrast, a knife switch has no under-voltage protection and can only be operated at close range. Therefore, the contactor is the most widely used type of electrical appliance in automatic control circuit systems.
A contactor is an electrical device that operates by the combined action of an electromagnet’s attraction force and the counteracting force of a spring, causing the contacts to open or close. Depending on whether it controls alternating current or direct current, it is classified as an AC contactor or a DC contactor. The main difference between the two is the method of arc suppression.
II. Differences between Contactors and Relays
The main function of relays is to serve as a means for signal detection, transmission, transformation or processing. The current passing through the circuit that they control is usually relatively small, and they are generally used in control circuits (in contrast to the “main circuit”).
The main function of a contactor is to connect or disconnect the main circuit. The main circuit refers to a circuit whose operation is determined by whether it is connected or not. The concept of the main circuit is opposite to that of the control circuit. Generally, the current passing through the main circuit is larger than that of the control circuit. Therefore, as a friend on the first floor said, contactors with larger capacity usually have an arc extinguishing cover (because when a large current is disconnected, an arc will be generated. Without using an arc extinguishing cover to extinguish the arc, the contacts will be burned).
Electromagnetic relays and contactors have the same working principle. Sometimes the same component is used as a contactor in one circuit and as a relay in another circuit.
How to distinguish them? The way to do it is to look at their specific purposes.
If the working current of a certain main circuit is relatively small, then it is entirely possible to use the electrical appliances usually used as relays as the devices for switching the main circuit. That is, to use the relay as a contactor. However, if the working current of a certain main circuit is extremely large, to the extent that the capacity of the contactor used for switching the main circuit is very large, and to make the current of such a contactor also very large, that is, the current flowing through its control circuit is very large, it is difficult to switch its control circuit using an ordinary relay. At this time, a certain electrical appliance originally used as a contactor can be selected to be used as the switching device for this control circuit. The role of this contactor in this situation is now that of a relay.
III. How to Deal with the 6 Common Faults
Too much noise
Reason: The short-circuit ring of the contactor is broken or the end faces of the moving and static cores are uneven, skewed, or have dirt, etc.
Solution: Remove the coil and flatten or smooth the iron core pole surface, or clean the oil stains with gasoline. If it is a short circuit or breakage, replace the iron filings or contactor. If the iron core is skewed, it should be corrected or tightened.
2. The contactor core fails to engage.
Reason: The working voltage of the contactor coil is too low, the control circuit is incorrect, the coil is broken or burned out, the mechanical part of the armature is not flexible or the moving contact is stuck, the auxiliary contact is loose or the slot is stuck, etc.
Solution:
If the voltage is too low, the contactor will frequently experience the process of closing and opening. In this case, it is necessary to check the control power supply or the control circuit, and adjust the voltage to a normal level.
First, check the voltage at both ends of the contactor coil. If there is no voltage, check the control circuit or replace the damaged electrical components.
Replace the coil or contactor. If the problem is caused by an external factor, it is also necessary to identify and resolve the external fault.
Repair the mechanical faults of the contactor, remove the rust, and add some lubricating oil at the mechanical operation points.
The auxiliary contacts need to be checked to ensure they are not loose and have been properly installed. If the auxiliary contacts are worn out due to the wear of the movable card slot, they should be replaced.
3. The contactor core fails to release slowly or does not release at all.
The residual magnetism effect or the dirt on the end face of the iron core causes the moving and stationary cores to stick together. Faults such as the damage of the non-magnetic spacer in the DC electrical appliance, which reduces the minimum air gap after the armature is closed, occur.
Solution: If there is dirt on the end face of the iron core, it should be cleaned with gasoline. For the non-magnetic pads of the DC electromagnet that are damaged, replace the pads or the contactor directly.
4. Contact overheating and burn injury
Reason: This is caused by factors such as the contactor starting too frequently, insufficient contact capacity, failure of the three-phase contacts to make synchronous contact, presence of impurities or unevenness on the contact surface, or uneven wear of the contact surfaces.
Solution: If the contacts become overheated and damaged, they need to be ground smooth or replaced with new contacts (if the cost allows, the contactor can be replaced directly). Only then can it be used again. Otherwise, it is prone to contact sticking or overheating and melting, causing a short circuit in the main circuit or phase loss problems. If the contactor capacity is insufficient, a larger one or a contactor with a larger capacity needs to be replaced.
5. Contact welding
Reason: The extremely high voltage generated by the severe arc causes the contact surface of the moving and static contacts to melt and fuse together, making it impossible to separate. The contact welding phenomenon is usually caused by insufficient contact capacity, damaged contact spring, reduced initial pressure, or short circuit on the load side.
Solution: Replace the severely damaged contacts or directly replace the contactor.
6. The contactor coil is severely overheated or smoking and has been burned out.
Reason: Inter-turn short circuit of the coil, insulation aging, or the power supply voltage being lower or higher than the operating voltage of the coil.
Solution: If there is a short circuit between the coils or the insulation is aging, the coils or the contactors should be replaced. If the working voltage of the coil does not match the power supply voltage, the coil with a matching working voltage and power supply voltage should be replaced.