Westinghouse DK-15, Manual

Page: 75 / 96

Type
DK-15
and
DK-2 5
Air Circuit
Breaker
Characteristics of Thermal-Magnetic Dual Overload
The general arrangement of thi s attachment i s shown in Fig .
3
with invers e t ime delay charac ter i s t i c s in Fig .
7 .
The
100%
calibrati on
s e tt ing of the pointer on the s cale pla t e i s the normal rat ing of the trip
uni t and that value of current will j u s t trip the breaker in approx imately
3 0 minutes or l onger in a normal amb ient t emperatur e . Adjus tment of the
thumb s crew t ownrd the
150%
calibrat i on s et t ing rai s e s the minimum tripping
current in minm:;e s tep s as desired .
Ambi ent c orre c t i on i s explained in Fig .
3
as 1% increas e in cali ­
bra t i on for each 2 oc . r i s e in t emperature . The attachment i s cal ibrated
for 2 4oc . amb i ent , but if it i s des ired to mee t the calibra t i on character­
i s t i c s o f Fig .
5
at 4ooc . , for ins tan c e , the calibrati on ad j u s tment should
be s e t at approx imat ely 108% for m inimum tripping at normal current rat i ng .
The calibra t i on adjus tment can therefore b e s e t a s c l o s e t o the
normal running current of the motor as i s de s ired, depending on amb i ent
t emperature , frequency of s tarting , e t c . C omple t e pro t e c t i on i s thus af­
forded under all c ondi ti on s of opera t i on .
Full deta i l s f or adjus tment and maint enanc e of the thermal-mag­
ne t i c trip attachment are explained in Fig .
3 .
Replacement of Overload Attachments
B o th types of overload trip unit s may be easi ly r emoved . From the
rear , take out two s crews that fas t en the magne t a s s embly to the main pane l
and di s c onne c t the trip uni t upper t erminal where i t i s b ol ted t o . the main
pole uni t shunt s e ct i on . In the case of drawout s tuds the f inger c lu s t er
and large nut mus t al s o b e removed . The trip uni t may now b e removed . R e ­
plac ement i s s imply the revers e o f thi s pro cedure .
After the uni t i s s ecurely mounted t o the panel , only one ad jus t­
men t is n e c e s s ary f or c orre c t operat i on with the breaker . Thi s adjus tment
c on s i s t s of s e t t ing the s crew in the out er end of the trip armature lever
to properly trip the breaker with a thin spa c er b etween the armature and
magne t , as exp lained in Figs . 2 and
3 .
Electric Closing Mechanism
, Ele ctri cally operated
DK
c ircui t breaker s are equ ipped with an
ent i re ly s el f - c ontained motor mechan i sm ( fi g . 11 ) mounted on the right
s ide of the breaker frame . Two of the t oggle linkage pins s erve as dowel s
t o a ccurately l ocate the mechani sm ins�lating bas e . B y r em oving three b o l t s
thi s c omplete a s s embly can be taken o f f the breaker without di s engaging any
par t s except the wiring .
The c l o s ing motor i s a s ingle pha s e c ommu ta t or type , v erti cal
shaft mot or wi th ball b earings s ealed wi th lubri cant that will normally
las t the l i fe of the apparatus . A s ingl e redu c t i on of speed between the
motor shaft and the mechan i sm crank i s a c c ompli shed with a worm mounted on
the upper end of the motor shaft engaging a worm g ear on the crank shaft .
Thi s gear a s s embly i s lubri cated on a s s embly suffi c i ent for s everal year s
of normal s ervi c e . An oil s eal on the motor shaft preven t s any oil or
grea s e from running down int o the motor winding s .
The mot or i s de s i gned t o develop an exc ep t i onal ly high t orque f or
i t s s i z e , and i s therefor e rated f or intermittent s ervi c e . If the breaker
i s c l o s ed more frequently than onc e every
3
minutes for l ong p er iods , the
motor may overheat . A c entrifugal speed l imi t ing dev i c e i s bui l t into the
l ower end-bell of the motor to pr event exc e s s i ve speed at normal and over ­
voltage, and t o prevent c oa s t ing o f the me chani sm after the l imit swi t ch
has cut off the motor curren t .
3
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