A contactor is a type of electrical control equipment that is used for switching on and off an electrical circuit. The function of a contactor is just like a relay, however, relays are used for lower current and are for protection purposes, while contactors are used to control higher currents electrical equipment. That is the reason that contactor is rated for thousand-ampere currents.
Contactor parts and principle
Common parts of the contactor are Electromagnetic coil, Enclosure, and Contacts. The principle of operation of the contactor is simple. When current passes through the contactor, it energizes the coil or electromagnet. As a result, it produces a magnetic field. Which causes the armature core to move.
If it is Normally closed relay, the flow of current starts through the contacts. Now when the current supply disconnects, the coil de-energizes, and contact comes back to its original position.
Sometimes contactors fail to perform its function. There are different reasons behind this failure. In this article, I will study the causes of contactor failure and how to do maintenance.
For more details about contactor, read my other article here.
Causes of contactor Failure
Different types of contactors are available in the market. Therefore, the uses may vary from one device to another. Similarly, there are different causes of contactor failure. Common causes are mentioned below. I will list them and then I will go in details:
Contactor failure causes list:
- Voltage fluctuation
- Temperature rise
- Poor installation
- Product selection
Passing an excessive current through contactor causes overheating. Which sometimes melts the contacts in the Normally closed position. So, when the contacts de-energize it is still closed position. And the contacts stick with each other, and of course the current still passing.
Therefore, it is mandatory to use specific rating contactors for a specific load. If the installed contactor is below the rated current. It will surely fail.
I have came through this fault several times in my work. It causes us serious issues. Some times a contactor is connected to control a heater through a thermostat, If the contacts melt and stick contacting together, the heater still working even if the thermostat disconnect the control voltage of the contactor.
Coil overcurrent is another shape of overcurrent fault that can cause the burning of the electromagnetic coil. Which affects the overall performance of the contactor. This can happen if a short circuit inside the coil occur.
Variation in voltage is another cause of the failure of the contactor. Both types of fluctuations like over voltages and under voltages cause contactor failure.
As the operational limit of any contactor is between 85 Percent and 110 percent of the rated coil. So, in case of over-voltage speed of electromagnetic closing will increase. Over-voltage can also cause the burning of the electromagnetic coil.
Which affects the overall performance of the contactor. On the other hand, in the case of under voltages from the rated value, electromagnetic closing will decrease. Both cases cause a delay in the operation of a contactor, and hence the chances of contactor failure are more.
The aging of the contactors is another cause of contactor failure. With the passage of time, the efficiency of the contactor does not maintain at the same level due to the mechanical springs weakness, and contacts corrosion because of the spark during on and off operation.
Another cause of contactor failure is the temperature rise. Sometimes contactors are placed in an area, where there is no proper ventilation of air. Hence it increases the temperature of the contactor. And reduce the efficiency while sometimes causing failure.
Sometime contactors are installed at locations where there is a lot of vibration. Or it is a corrosive environment. Where vapors or chemicals can cause damage to contactor, and affect the efficiency of it.
In some dusty environments such as desert workplace, cement factories etc., the contactors suffer loosen connections, if installed in a poor IP rating panel. In such dusty environments the IP of the control panel should be suitable to provide protection for the contactor.
If the contactor has a loosen connection we simply use any kind of contact cleaner to clean it, or we can use an air jet to clean its contacts. But this process needs disconnecting power.
As mentioned earlier that each electrical device requires its own rated contactor. If contactor is installed at a wrong product, where the rated conditions of the contactor and product do not match. It will cause contactor to fail.
One of the important things to consider when rating a contactor for a load is the load type. Contactors have two types of current ratings, one for the thermal loads and the other is for other inductive loads.
I have written a detailed article about contactor thermal current, You can read it here.electrical4uonline
Other misuse of electrical Terminologies
Different electrical terminologies like connection and mountings mistakes, Selection of wrong cable, Transients, change over time between star and delta connection, Voltage drops, and current peaks are common causes of contactor failure.
To ensure safe operation and prolong the life of contactor it is mandatory to carry out proper maintenance of contactor.
Maintenance of contactor involves different steps including an inspection. Therefore, before going for maintenance, it is mandatory to perform a visual inspection of the contactor.
- In this type of inspection remove dust from the contactor.
- Tighten the connections to prevent losing.
- After this, it is time to check out the contact system of contactor.
- Check whether the auxiliary contact is flexible. The static contact is loose or tight. Whether contact and stroke distance are according to the standards or not. If there is an issue with distance or contact. Address the issue before moving forward.
- After all these now it’s time to check the electromagnetic coil. Address any issue if you find it abnormal in the coil or the overall core of contactor.
We are discussing some common maintenance steps of contactor in detail.
Contact System Maintenance
Contacts are important parts of contactor. The basic issue that comes in the contact system is inconsistency in a three-phase system, where all three phases do not close at a time.
If any of the contacts are affected, it causes disruption in operation. So always check for:
- Whether the auxiliary contact is flexible.
- The static contact is loose or tight.
- Whether contact and stroke distance are according to the standards or not. If there is an issue with distance or contact.
Arc chute maintenance
Sometimes contactors sparks during operation, it is due to a piece of metal or cracks in the arc shroud. If you find any of it during inspection remove or repair/replace it immediately
Iron core maintenance
Iron core maintenance includes clearing the contactor core from any type of dust particle. In iron core maintenance check for the following
- Whether short circuit ring is broken or detached?
- Whether any noise comes from the core?
- Whether there is any misalignment or electromagnetic coil suck well?
Electromagnetic coil maintenance
The electromagnetic coil can withstand 5% of tolerance in current or voltage. During this type of maintenance, check whether the coil heats up with voltage or current fluctuation.
Another factor is the quality of the coil if the coil is aged and does not perform well, replace it with a new one. Also, check any cracks or abnormal situations in the coil core. If it exists remove it at an earlier stage to get smooth operations.