WESTINGHOUSE MM-421 (01) PDF MANUAL

The ratings for Type MM Contactors are as follows:

The operating coils are rated for continuous duty and will operate the contactors at 80 to 110 percent of their rated voltage. This contactor has Class H coils which operate at a temperature rise of 150 degrees C. The surface temperature of these coils will be higher than that of Class A coils.

The contactors are insulated for a maximum potential difference of 600 volts among parts.

The contactor is equipped with two operating coils. The upper coil is used to close the normally-open contacts, and the lower coil serves to produce a complete sealing of the normally-closed contacts. When neither operating coil is energized, the kickout spring holds the lower, or normally-closed contacts together. Because the kickout spring is not sufficiently strong to produce the full contact pressure, the lower coil should be energized at all times that the lower contacts are to be required to carry current.

Arc shields and blowout structures are always provided on the normally-open poles. They are optional on the normally-closed pole, and should always be provided when this pole is to be required to interrupt a circuit.

Each arc shield is held in place by the blowout pole pieces and a spring clip, and may be removed by pulling it directly forward. The blowout pole pieces act as fixed supports and guide the arc shield in its sliding movement.

Rear-connected contactors are held to the panel by eight mounting studs, five of which make provision for the electrical connections. Front-connected contactors are provided with pressure type solderless terminals and are held to the panel by two mounting studs.

The contactors are suitable for mounting only on insulating panels up to 2 inches thick. As the frame is at the same potential as the normally-closed moving contact, special insulating precautions must be taken if a contactor is to be mounted on a conducting surface.

The Type L-46 normally-open electrical interlock and the Type L-47 normally-closed electrical interlock may be applied to the contactors in numbers limited according to the following table:

Table No. 1 ELECTRICAL INTERLOCK APPLICATIONS

It should be noted that a contactor having magnetic blowouts on all poles can accommodate a total of four interlocks, whereas a contactor having magnetic blowouts on its normally-open poles only is limited to two interlocks.

The stationary contact assembly of either type of interlock is mounted on one of the molded bases of the contactor. If mounted on the upper base, it occupies a position between the arc shields. If mounted on the lower base, it is positioned beside the arc shield; in this case, an insulating member mounted on the contactor base at the factory must be removed and discarded before the interlock is mounted. The moving arm of the interlock is secured to the contactor armature by means of a single screw.

Care should be taken when installing an interlock to make certain that its moving parts function freely and that the contact gaps and over-travels conform to the information given in the interlock Instruction Leaflets 15-829-L46-1B and 15-829-L47-1A.

The sequence in which the various switching operations occur as the contactor magnet closes is as follows:

(1) the main normally-closed contacts open,

(2) the main normally-open contacts close,

(3) the Type L-46 normally-open interlock closes, and

(4) the Type L-47 normally-closed interlock opens.

A Type M-25 mechanical interlock, Style No. 1320 750, may be employed to safeguard a pair of contactors against the closing of one if the other is already closed. The contactors are mounted side-by-side on 6 1/4 inch centers.

Arc shields and their arc horns are essential to the performance of the contactor and these parts should always be in place. The flexible connections leading from the arc horns of the normally-open poles should always be securely fastened to the respective moving contacts. The flexible connection leading from the arc horn of the normally-closed pole should be firmly secured to the armature stop bracket.

The arc shields may be removed by pulling them directly forward, allowing the blowout pole pieces (which act as their fixed supports) to guide them in their sliding movement.

In replacing the arc shields, care must be taken that the guiding recesses in their sides locate themselves properly on and be gripped firmly by the supporting guides. If the guides are found to be spread apart so that they no longer grip the arc box firmly, remove the arc box and spring the guides together slightly by squeezing them with the fingers. The arc shields should be pushed back as far as they will go, until they are held securely against the molded base by the spring clips.

The armature hinges on a knife-edge bearing which has an extremely long life and requires little maintenance. The knife-edge bearing requires no maintenance other than the removal of accumulated dirt. Oil should not be used, as it hastens the collection of dust.

The upper magnet pole face is secured to the core by means of a bolt having a head that projects forward into a hole in the armature. Care should be taken that the parts maintain proper alignment, and particularly that the bolt head be accurately centered in the armature hole. Dirt should not be allowed to accumulate. Plates of nonmagnetic steel are secured to the underside of the armature to prevent the armature from sticking closed against either pole face due to residual magnetism following the interruption of voltage to an operating coil.

When a new operating coil is installed, the identification label should be examined to make certain that the voltage rating and the coil style number are correct for the application.

The more commonly used operating coils are listed below:

Table No. 2. OPERATING COILS

TYPES MM-321, 421 CONTACTOR COILS

The steps to be followed in removing either of the operating coils are:

1. Remove the arc shields, as described previously.

2. Remove the screw securing the right-hand moving contact assembly to the armature.

3. Loosen the two bolts securing the armature stop to the underside of the contactor frame until the armature stop can be lowered clear of the two dowel pins which locate it in position.

4. Allow the self-contained assembly comprising the armature, armature stop, moving contacts and springs to hang by the shunts.

5. Remove the screw holding the pole face to the magnet core on which the coil is mounted.

6. Disconnect the coil leads and slide the coil forward until it is clear of the contactor.

The moving and stationary contacts are faced with solid silver. Oil or other lubricant should not be used on the silver contacts. The discoloration which results from arcing and from exposure to the atmosphere usually is not harmful to their performance, and consequently no filing or dressing is necessary. The moving contacts are hinged on their supports in such a way that sliding or rubbing of the surfaces when the contacts meet is practically nonexistent, a provision essential to the long life of silver contacts.

When the contacts are new they should meet sufficiently in advance of the final sealing of the armature against either the upper or the lower pole face, that a gap of approximately 5/32 inch appears between the underside of each moving contact and the edge of its support. As the contacts wear, the amount of this overtravel diminishes, and when at length it decreases to 1/32 inch, the contacts should be replaced.

A moving contact can be removed for replacement by first disconnecting its shunt and then removing its contact spring. Each of the stationary contacts is slotted to facilitate removal, and it is necessary only to loosen the screw until the dowel projecting from the back of the contact is free of the locating hole in the stationary contact support.

When the contacts are new, the open gap of the normally-open contacts, with the armature sealed against the lower pole face, should be 9/32 to 11/32 inch. The open gap of the normally-closed contact, when the armature is sealed against the upper pole face, should be 1/4 to 5/16 inch.

If after applying new contacts it is found that the overtravels and contact gaps do not conform to the dimensions given, it may be necessary to bend the moving contact straps slightly.

The contact pressures of both the normally-open and normally-closed contacts, when they are new, should be as indicated in Table No. 3.

Table No. 3. CONTACT PRESSURES

To measure the final pressure of one of the contacts, first close the contactor mechanically in a direction producing full closing of the contact. Then by means of a spring scale hooked to a loop of fine wire, measure the force necessary to separate the moving from the stationary contact.

If, after new contacts are installed, the pressures are not correct, it may be necessary to replace the contact springs or to adjust the contact overtravels in the manner described previously.

Failure of the magnet to operate the contactor may result from an open-circuited operating coil, from a circuit condition in which the voltage is excessively low, or from the presence of friction between the parts.

Failure of the contactor to open may be caused by friction or by a defective kickout spring.

Renewal parts may be ordered from the nearest Westinghouse Sales Office. Be sure to describe the part or parts required and give complete nameplate reading on the contactor for positive identification.