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INTENANCE:

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DYNAMIC BRAKE GRID SHORTING CONTACTORS

8416136 8436326 8459697 AND 9521384

INTRODUCTION

The data contained in this Maintenance Instructionwill apply to contactors 8416136 8436326 8459697and 9521384 unless specifically identified. For easeof reference where procedural differences occur, theinformation will be presented by subject followed bythe applicable part number(s).

DESCRIPTION

The dynamic brake grid shorting contactors,Fig. 1, are normally open, single-pole devices,capable of connecting and interrupting DC currentup to 2250 amperes at 700 volts.

Contactors 84 6 36 8436326 and 8459697 areelectrically polarized devices equipped with a

permanent magnet which provides the magneticfield necessary for proper directional arc transfer.Because the magnet is permanently polarized,reverse polar it y w il l damage t hese contacrors.

Contactor9521384, however, is anon-polarizeddevice equipped with an intermittent duty blowout

coil in place of a permanent magnet. Thisarrangement desensitizes this contactor to currentdirection (polarity), when properly installed.

Interlocks are enclosed in two molded, dustproofhousings, two single-pole switches to a unit. Circuitconnections for the interlocks are provided byexternal quick-disconnect type terminals. Theinterlocks are identified by letters stamped on the

brackets immediately next to the terminals, or onthe interlocks themselves.

The contactor operates when 74 VDC is applied tothe magnet (operating) coil. Energizing the magnetcoil generates a magnetic flux which closes the maincontacts and mechanically actuates the two pair ofsingle-pole control interlocks. The main contactsare held in a closed position by magnetic force andare opened by a return spring when the magnet coilis de-energized.

All of these contactors use two sets of contacts: themain current carrying contacts and the arcingcontacts. The arcing contacts close before and open

after the main contacts.In contactors 8416136, 8436326, and 8459697, thearc first appears at the arcing contacts and thentransfers onto runners in the arc chute. A magnethoused in the arc chute generates a magnetic fieldbetween the pole pieces of the chute which forces thearc to transfer. Because t he magnet i s permanentl ypolarized, current must alw ays flow int o the

contactor terminal marked (+)for t ransfer to occur.If current flows in the reverse direction, themagnetic field forces the arc down into thecontactor (possibly destroying it.)

In contactor 9521384, the intermittent duty blowoutcoil replaces the permanent magnet. Current flowsthrough the coil generating a magnetic field betweenthe pole pieces. h dir ecti on of t he current l owdet ermi nes t he polar it y of t he magnet ic fi eld.

Therefore, the arc is forced to transfer to the arcrunners whenever the intermittent duty blowout coilis energized.

After the arc expands sufficiently, it can no longerbe sustained and the current flow stops. At all other

*This bulletin is revised and supersedes previous issues of this number.

Areas of change are indicated by vertical bars.

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M.I. 5406

times of power flow, the current flows through aparallel circuit composed of the main contacts andthe arcing contacts.

SAFETY PRECAUTIONS

WARNING

This contactor was designed for specificapplication to electric power circuits of diesel

electric equipment where the circuits anddevices are enclosed in suitable protectivecabinets. Care has been taken in the design ofthe equipment to provide for the safety ofoperating and service personnel, providedreasonable care is exercised in the per-formance of operating and service functions.

POLARIZED CONTACTOR

NON-POLARIZED CONTACTORS

Fig.1 Dynamic Brake Grid Shorting Contactors

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