WEAR ALLOWANCE.- A contact is service as long as its wear allowance, and its associated contacts, exceeds the minimum value specified by the manufacturer. (Usually the minimum value is 0.015 to 0.030 inch). The "wear allowance" of contacts is defined as the total thickness of contact material which may be worn away before the contact of two associated surfaces becomes inadequate to carry rated current. In an electric-motor contactor, the wear allowance of the power pole contacts is usually related to the closed position of the magnetic operator. The wear allowance of the power pole contacts of a magnetic contactor is the amount of silver that can be worn away without resulting in failure of the contacts to touch when the magnetic operator is at its closed position.

Blowout Coils Blowout coils seldom wear out or give trouble when used within their rating. However, if they are required to carry excessive currents, the insulation becomes charred and fails, causing flashovers and failure of the device. Arc shields are constantly subjected to the intense heat of arcing and may eventually burn away, allowing the arc to short-circuit to the metal blowout pole pieces. Therefore, arc shields should be inspected regularly and renewed before they burn through. Arc barriers provide insulation between electrical circuits and must be replaced if broken or burned to a degree where short circuits are likely to occur. The importance of having clean, tight electrical connections must be emphasized. Where practical, it is a good idea and common practice to solder electrical connections.

Excessive slamming on closing, particularly on ac magnetic-operated devices, will eventually damage the laminated face of the magnetic armature and may damage the shading coil. Magnetic coils should be kept dry. Wet coils should always be dried out before using. They may be dried by baking them in a well-vented oven at not more than 194 F to prevent water from boiling in the insulation. The length of time in the oven depends on the size of the coil. If an oven isn't available, place the unit under a canvas cover roomy enough for hot air to be circulated within. Another alternative is to direct infrared lamps on the windings.

The closed pressure of contacts is an important factor in their ability to carry current. A small contact with proper contact pressure carries more current than a large one with little pressure. Contact springs must be kept in condition. Replace them when they have been damaged or have lost temper by exposure to high temperatures.

Connections should always be clean and tight. Loose connections result in overheated parts that eventually need replacing. Periodic inspections are necessary because temperature changes, vibration, and carelessness may loosen the connections.

Inspect the movable core of a controller for cleanliness. Accumulated dirt causes sluggish mechanical action which impairs the opening and closing of the contact.

Noise results if the movable and stationary pole pieces don't fit together well when the contactor is closed or when dirt or rust prevents proper closure. The most prominent noise produced in a controller comes from a broken shaded pole, which is a single turn of wire or strap, imbedded in part of the laminated magnetic structure.

Check the cabinet which houses the controller for cleanliness. Make sure the cover fits properly to keep moisture, dirt, and dust from entering. Check for corrosion of all metal parts. Table 7-5 is a guide for troubleshooting ac controllers.

Table 7-5.- Troubleshooting chart lor alternating-current controllers

Table 7-5.- Troubleshooting chart for alternating-current controllers- Continued

Table 7-5.- Troubleshooting chart for alternating-current controllers- Continued

Table 7-5.- Troubleshooting chart for alternating-current controllers- Continued