Contactor Has Continuity All the Time
Contactors have a very high electrical life & normally don't need so much maintenance. But it's always better to have some knowledge about the failures of contactors. Therefore, special attention should be paid to the following points.
Reasons for contactor failure are:
Overcurrent in the main contacts
High current carrying loads create heat at the main contacts of the contactor, which causes welding in the closed position. Please choose your contactor correctly based on the full load current and utilization category. If you are at the performance limit, select a bigger contactor.
On the other hand, pole wear is normal during the contactor's lifetime. Contacts can be changed with a set of spare contact kits when the end of life is reached. But if this wear appears very early after commissioning, it could be due to a non-adequation between the contactor's selection and application.
Overvoltage and undervoltage in the coil
The operational limit of the contactor is between 85 and 110% of the rated coil. A voltage variation of ±5% of the above limits will increase the contact bounce which leads to increased contact wear. The reason for this is that higher voltages will increase the speed of the electromagnet at closing. Lower voltages will decrease the speed at closing. Both these factors can lead to a higher level of contact bounce at closing. High voltage and the increased speed at closing also contribute to some increased sound.
The control voltage of less than 85% during pick up may not be enough to safely close the contactor and can cause the coil to melt due to overheating. This is because the coil is designed to withstand the inrush current for a limited time only. One common reason is when the coil supply is taken from the main supply where a high current is causing the voltage to drop. Too high continuous control voltage could also cause the coil to melt.
Wrong product selection
Wrong product selection is another cause of contactor failure. Contactor should be selected according to the type of load. The standard utilization categories define the current values which the contactor must be able to make or break. These values depend on:
- The type of load being switched: squirrel cage or slip ring motor, resistors.
- The conditions under which making or breaking takes place: motor stalled, starting or running, reversing, plugging.
Category AC-1
This category applies to all types of a.c. load with a power factor equal to or greater than 0.95 (cos Φ 0.95). Application examples: heating, distribution.
Category AC-2
This category applies to starting, plugging, and inching of slip ring motors. On closing, the contactor makes the starting current, which is about 2.5 times the rated current of the motor. On opening, it must break the starting current, at a voltage less than or equal to the mains supply voltage.
Category AC-3
This category applies to squirrel cage motors with breaking during normal running of the motor. On closing, the contactor makes the starting current, which is about 5 to 7 times the rated current of the motor. On opening, it breaks the rated current drawn by the motor; at this point, the voltage at the contactor terminals is about 20 % of the mains supply voltage. Breaking is light. Application examples: all standard squirrel cage motors: lifts, escalators, conveyor belts, bucket elevators, compressors, pumps, mixers, air conditioning units, etc.
Category AC-4
This category covers applications with plugging and inching of squirrel cage and slip ring motors. The contactor closes at a current peak which may be as high as 5 or 7 times the rated motor current. On opening, it breaks this same current at a voltage that is higher, the lower the motor speed. This voltage can be the same as the mains voltage. Breaking is severe. Application examples: printing machines, wire drawing machines, cranes and hoists, metallurgy industry.
Voltage drop during start-up of the motor
A voltage drop to less than 85 % of nominal voltage for 5 to 10 ms can be enough for the contactor to start opening. Also, other disturbances in the control voltage such as poor relay contacts or too small control transformer/power supply could be responsible for this problem. The result of these voltage problems can be increased contact wear and also lead to welding of main contacts.
Current peaks used by heavy-duty starting
If the application requires a high level of torque during start-up, for example, a big fan or a pump, it is important that the contactor's making capacity and short-time withstand currents are capable of handling the starting current and current peak.
Restarting with motor idling
Attempting to restart the motor before it has come to a full stop will cause a current peak that theoretically can reach twice the current compared to starting the motor from a standstill. This can result in welded contacts.
Change over time between star and delta
Setting the change over time between star and delta too short will not allow the motor to reach 80-90% of its nominal speed. This will cause the star contactor to break a higher current than expected. If the starter stays in the star position for too long of a period; this will increase the stress of the star contactor because the star contactor is mainly sized for the short time current rating.
The change over time between the star and delta is important. If no timer or other device is used to get an idle time between the opening of the star contactor and the closing of the delta contactor, there is a risk that the arc inside the star contactor could remain and create a short circuit. If the elapsed time is too long, the motor starts to decelerate and the delta closing will create a higher current peak than expected. This can result in welded contacts. Generally, the idle time should not exceed 50 ms.
Product coordination
Lack of product coordination with fuse or breaker missing or wrongly sized can cause excessive contact wear or lead to welded contacts.
Transients affecting coils
Voltage peaks/transients are not uncommon, especially in poor quality or weak networks. Too high voltage peaks/transients may cause damage to the coil. You can use RC filters or varistors to protect the coil from transients.
High temperature
High temperature causes contact welding and coil melting. The standards define temperature raise regarding the ambient temperature. In case the contactor is overheating, you will have to sum the ambient temperature and the temperature raise allowed. When concerned about high temperatures on and around the contactor please check the following:
- Make sure the connections are properly torqued.
- Make sure the ventilation of the electrical panel is correct.
- Make sure the cable size is what is recommended.
- Make sure the correct coil is selected for the applications.
Wrong cable selection
An incorrect cable section choice in regard to the current level creates cable overheating. The consequence is contactor overheated.
Vibration
If the contactor vibrates for a long time, it will cause the internal coil to burn. You should check the vibration and shock parameters of the contactors from the manuals of the manufacturers. You can also prefer special contactors produced for high-vibrated applications such as the railway.
Environmental factors
Pollution of the magnet pole surfaces will create a slight hum coming from the contactor in the closed position. Hum can also occur if the pole surfaces are deformed. A louder hum will occur if the shading coil is broken because the magnet will cause the contactor to chatter.
Contactors need to be protected from condensation to keep the pole surfaces of the magnet free from corrosion. Besides, the contactor should be mounted inside an enclosure that protects the unit from dust and water. Unclean terminals increase connection resistance and, in this case, create an overheating effect.
Connection and mounting mistakes
Make sure the recommended torque values given in the instructions are followed and that proper torque checks depending on the application are done. A loose connection on the terminal creates an overheating condition. On the other hand, the terminal screw can be damaged when assembly torque is excessive.
High altitudes
With a decrease in atmospheric pressure, the spacing required to prevent flashover increases substantially. Since arc-over occurs more readily at higher altitudes, contact life decreases substantially with operation at these altitudes. To compensate for increased arcing at high altitudes, users must derate the current ratings given by the manufacturer.
Mounting position
Some contactors are position-sensitive. The mounting positions and distances should be checked properly.
Lack of maintenance
Generally, contactors do not need so much maintenance. But to be able to stay on the safe side, a maintenance program should be established as soon as the contactor is installed and put into operation. After the contactor has been inspected several times at monthly intervals, and the condition noted, the frequency of inspection can be increased or decreased to suit the conditions found since this will depend upon the severity of the contactor duty.
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Source: http://www.electricalterminology.com/what-causes-contactor-failure/