Guidelines For Performing Inspections Of Vacuum Contactors

Starters are made from two building blocks, vacuum contactors and overload protection. Contactors control the electric current flow to the motor. Their function is to repeatedly establish and interrupt an electrical power circuit. A vacuum contactor can stand on its own as a power control device, or as part of a starter.Vacuum contactors operate electromechanically and use a small control current to open and close the circuit. The electromechanical components do the work, not the human hand, as is the case with a knife blade switch or a manual controller. The sequence of operation of a vacuum contactor is as follows: first, a control current is applied to the coil; next, current flow into the coil creates a magnetic field that magnetizes the E-frame making it an electromagnet; finally, the electromagnet draws the armature towards it, closing the contacts.

 

A vacuum contactor has a life expectancy. If the vacuum contactor contacts are frequently opened and closed, it will shorten the life of the unit. As the contacts are exercised, an electrical arc is created between the contacts. Arcs produce heat, which can damage the contacts. Contacts eventually become oxidized with a black deposit. This black deposit may actually improve the electrical connection between the contacts by improving the seat, but burn marks, pitting, as well as corrosion indicate it's time to replace the contacts. The following thermal patterns are associated with contactors. The coil of the vacuum contactor is usually the warmest part of the unit. High temperatures may indicate a breakdown of the coil. Line side and load side lug connections may show high resistance heating from poor connections. Heating from burned and pitted contacts may be thermally "visible" on the body of the contactor.

 

Typical thermal problems in overloads are found in the connections to vacuum contactors

 

Starters are the combination of a controller, usually a vacuum contactor as well as an overload relay. The above descriptions of the individual components apply to the starter systems. Reduced voltage starters are used in applications that involve large horsepower motors. They are used to reduce the in-rush current as well as limit the torque, and thus the mechanical stress on the load. The components of this type of starter ought to be inspected as the motor steps up to speed. A separate low-voltage starter circuit is used to step the motor up to speed. Once at operating speed, these components are de-energized.

 

Completing Inspections

 

Remember that primary anomalies are the problems that readily stand out while secondary anomalies may require that main anomalies be adjusted into saturation to allow for the identification of a secondary anomaly. For example, different fuse types and sizes will cause different thermal signatures as will overload relays that are sized differently within the same circuit. Anomalies like this should be identified and reported. Also note that when evaluating the severity of a problem, temperature is just one variable. All of the parameters involved with the severity of the anomaly ought to be considered. To improve temperature measurements, avoid low emissive surfaces. Look for cavity radiators or highly emissive insulation on conductors. Measure loads where component sizing, overloading, or load imbalances are observed. Beware of the effects of wind or convection on components. Note ambient temperatures, large thermal gradients, and the source of heating. Safety should be the top consideration.