How To Tell If A Contactor Is Bad

Contactors have a very high electrical life & normally don't need so much maintenance. But it's always meliorate to take some noesis about the failures of contactors. Therefore, special attention should exist paid to the following points.

Reasons for contactor failure are:

Overcurrent in the chief contacts

Loftier current carrying loads create heat at the primary contacts of the contactor, which causes welding in the airtight position. Please choose your contactor correctly based on the full load electric current and utilization category. If you are at the operation limit, select a bigger contactor.

On the other hand, pole wear is normal during the contactor's lifetime. Contacts tin can be changed with a set of spare contact kits when the end of life is reached. Only if this habiliment appears very early on after commissioning, it could be due to a non-adequation between the contactor's selection and application.

Overvoltage and undervoltage in the roll

The operational limit of the contactor is between 85 and 110% of the rated coil. A voltage variation of ±5% of the higher up limits will increase the contact bounce which leads to increased contact wearable. The reason for this is that college voltages will increase the speed of the electromagnet at closing. Lower voltages volition subtract the speed at endmost. Both these factors can pb to a higher level of contact bounce at closing. High voltage and the increased speed at closing as well contribute to some increased sound.

The control voltage of less than 85% during option upwardly may not be enough to safely close the contactor and can crusade the coil to cook due to overheating. This is because the coil is designed to withstand the inrush current for a limited time only. 1 common reason is when the coil supply is taken from the main supply where a high electric current is causing the voltage to drop. Too high continuous control voltage could as well crusade the whorl to melt.

Overvoltage and undervoltage in the coil

Wrong product option

Wrong product selection is some other cause of contactor failure. Contactor should be selected according to the blazon of load. The standard utilization categories ascertain the electric current values which the contactor must be able to brand or interruption. These values depend on:

The type of load beingness switched: squirrel cage or slip ring motor, resistors.
The conditions nether which making or breaking takes identify: motor stalled, starting or running, reversing, plugging.

Category Air-conditioning-i

This category applies to all types of a.c. load with a power gene equal to or greater than 0.95 (cos Φ 0.95). Awarding examples: heating, distribution.

Category Air conditioning-2

This category applies to starting, plugging, and inching of skid ring motors. On closing, the contactor makes the starting current, which is well-nigh ii.five 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 electric current, which is most 5 to 7 times the rated current of the motor. On opening, information technology breaks the rated current drawn past the motor; at this point, the voltage at the contactor terminals is about 20 % of the mains supply voltage. Breaking is low-cal. Application examples: all standard squirrel cage motors: lifts, escalators, conveyor belts, bucket elevators, compressors, pumps, mixers, air-conditioning units, etc.

Category Air conditioning-iv

This category covers applications with plugging and inching of squirrel cage and slip ring motors. The contactor closes at a current peak which may exist equally loftier as v or seven times the rated motor current. On opening, it breaks this same electric current at a voltage that is higher, the lower the motor speed. This voltage can be the same equally the mains voltage. Breaking is astringent. Awarding examples: press machines, wire drawing machines, cranes and hoists, metallurgy industry.

Voltage drop during showtime-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. Besides, other disturbances in the control voltage such equally poor relay contacts or too small command transformer/power supply could be responsible for this problem. The upshot of these voltage problems can exist increased contact wear and also atomic number 82 to welding of main contacts.

Current peaks used past heavy-duty starting

If the awarding requires a high level of torque during first-up, for example, a big fan or a pump, information technology is important that the contactor'southward making chapters and short-time withstand currents are capable of treatment the starting electric current and electric current superlative.

Restarting with motor idling

Attempting to restart the motor earlier it has come to a full stop will crusade 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 brusk will not let the motor to reach 80-ninety% 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 menstruation; this will increase the stress of the star contactor because the star contactor is mainly sized for the short time current rating.

The alter over fourth dimension 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 electric current top than expected. This can result in welded contacts. Mostly, the idle fourth dimension should not exceed 50 ms.

Product coordination

Lack of product coordination with fuse or breaker missing or wrongly sized can cause excessive contact article of clothing or atomic number 82 to welded contacts.

Transients affecting coils

Voltage peaks/transients are not uncommon, especially in poor quality or weak networks. Also loftier voltage peaks/transients may cause damage to the coil. Yous 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 volition have to sum the ambience temperature and the temperature raise allowed. When concerned nearly loftier temperatures on and around the contactor please check the post-obit:

Make sure the connections are properly torqued.
Make certain the ventilation of the electrical console is correct.
Make sure the cable size is what is recommended.
Make sure the correct coil is selected for the applications.

Wrong cable choice

An wrong cable section pick in regard to the current level creates cable overheating. The event is contactor overheated.

Vibration

If the contactor vibrates for a long time, it will crusade the internal whorl to burn. Yous should check the vibration and shock parameters of the contactors from the manuals of the manufacturers. You can also prefer special contactors produced for loftier-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 tin can also occur if the pole surfaces are deformed. A louder hum will occur if the shading coil is cleaved because the magnet will cause the contactor to chatter.

Contactors need to exist protected from condensation to keep the pole surfaces of the magnet gratuitous from corrosion. Besides, the contactor should exist mounted within an enclosure that protects the unit of measurement from dust and water. Unclean terminals increase connexion resistance and, in this instance, create an overheating outcome.

Connectedness 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 connexion on the terminal creates an overheating condition. On the other paw, the terminal screw tin can be damaged when assembly torque is excessive.

Loftier 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 functioning at these altitudes. To compensate for increased arcing at loftier altitudes, users must derate the current ratings given by the manufacturer.

Mounting position

Some contactors are position-sensitive. The mounting positions and distances should exist checked properly.

Lack of maintenance

Generally, contactors do not need so much maintenance. Just to be able to stay on the safety side, a maintenance plan should exist established as before long as the contactor is installed and put into operation. After the contactor has been inspected several times at monthly intervals, and the status noted, the frequency of inspection can be increased or decreased to adapt the conditions establish since this volition depend upon the severity of the contactor duty.

Source: http://www.electricalterminology.com/what-causes-contactor-failure/