|
Click on the cover to read the latest issue of PSB
Affiliate Links
|
Interpreting contact wear01 May 2003ABB has issued Guidelines for inspection and maintenance of main circuit contactsThe number of mechanical operations has a low impact on the life of the main contacts and their overall
electrical life. 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. If so the contactor needs to be exchanged.
Another reason for AC hum could be corrosion caused by environmental conditions that exceed the contactor specifications. Contactors need to be protected from condensation in order to keep the pole surfaces of the magnet free from corrosion.
The operational limit of the contactor is 85-110% of the rated coil voltage. A voltage variation of ±5% will minimise the 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 level.
A contact is not necessarily damaged or worn out just because the surface is rough and discoloured. The contacts in the figure below may look rather inferior but experience has shown that they are better than a new set of contacts. This is because they are “electrically seated” and make good contact over the entire surface. Based on this it would be completely wrong to change a contact only by judging from the appearance of the surface.
Today, with highly developed contact materials and high performance contactors, filing, grinding, or other
attempts to re-store contacts or contact surfaces should be avoided. This increases the risk of causing other problems. For example, the risk of increasing the contact’s resistance is high due to remnants from grinding paper.
Also lack of cleanliness when attempting to restore the contacts can influence the contactor performance.
Maintenance should be limited to inspection of the contacts for the level of contact wear. This is to ensure a
trouble free operation of the contactor until the next service is required.
At the same time observations can be made to judge if the contactor operates well in the application and that no signs of abnormal wear or damage are present on the contacts.
Determining the level of wear on contactors in use is essential from both the functional and economical point of view. Premature service on contactors will increase the overall cost. However, service at the right time to avoid interruptions or breakdowns is more cost-effective.
The rate of wear is different depending of the utilisation category or the application of the contactor. For motor control it also depends on the selected starting method. The following main utilisation categories are the most common.
AC-1. (General purpose) Non-inductive or slightly inductive loads like resistance furnaces. The closing of the main contacts is relatively easy as the starting current is equivalent to the rated current of the load. Breaking is made at full voltage, the arc has a rather low level of energy.
AC-2. Slip-ring motors. Starting, switching off. Typical making and breaking is with approximately 2.5 times the rated current of the motor the voltage is corresponding to the nominal voltage.
AC-3. Cage motors. Starting, switching off running motors. Closing of the main contacts with approximately
6-8 times the rated motor current, depending on motor characteristics and type of load. Breaking is easier, as the current equals the rated motor current, and voltage is reduced to 17 % of the rated voltage. Typical
electrical wear comes from the contact making operation. Visual inspection of an AC-3 application normally shows none or very little material from the contact tips thrown out into the arcextinction packages.
AC 4. Cage motors. Starting, plugging, inching. Making and breaking the motor starting current at approximately 6-8 times the rated motor current. Voltage is not reduced and equals the rated voltage of the
motor. Both making and breaking are contributing to the contact wear. The arc-extinction packages have an
important role. Therefore material from the contact tips is in most cases thrown out in the packages. Contact Details and Archive...Most Viewed Articles... |
|