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numbers.
For such instruments, the connection dia
(
grams in this book may be followed by observing the
pictorial positioning of the terminals, rather than the
stud markings.
Warning: Care must be exercised in handling high
tension wires when installing instruments on such
circuits.
POLARITY MARKINGS OF TRANSFORMERS
r
Transformers of present manufacture have polarity
markings of white paint or markers, H 1
and X1
for secondary, on or near the proper terminal.
These markings denote the relative polarity and facili
tate the making of proper connections for correct di
rection of deflection of instruments.
the marked leads is such that instantaneous direction
of the current in them is the same; namely, toward the
transforrr.er in the marked primary lead and from the
transformer in the marked secondary lead, or vice
versa.
These polarity markings are indicated in the
connection diagrams and should be followed irrespec
tive of their physical location on the transformers.
GROUNDING CASES OF A-C INSTRUMENTS
It is frequently desirable to use transk>rmers on
circuits of over 150 volts.
which are used with current and potential transform
ers should be connected to the grounded side of the
secondary circuits of such transformers.
copper wire is suitable for this purpose.
Grounding connections from the grounded side of
the secondary circuits to earth should be made in
accordance with the provisions of the National Electric
Code.
FRONT
Fig. 3.
for primary
The relation of
The cases of instruments
No. 12 Awg
MOUNTING
I"
4 -28THD.
Dimensions of Types DB-12 and -13 ammeters with rheostat for lead-length compensation.
PRINCIPLES OF OPERATION
The d-e instruments utilize a permanent-magnet,
moving-coil mechanism adapted for the longscale in
strument design.
Current passing through the mov
ing coil produces a magnetic field which interacts
with the field produced by the permanent magnet,
thus supplying the deflecting torque.
system deflects until counterbalanced by the instru
ment control springs.
The a-c instruments (except expanded-scale volt
meters) employ a moving iron mechanism of the at
traction-repulsion type.
soft-iron strips of carefully determined configuration
are mounted inside the field coil.
When this coil is energized, it magnetizes the three
iron strips and also a vane attached to the pivoted
shaft.
The center iron strip exerts a force of repul
sion, and the other two strips a force of attraction,
on the moving vane, in such a manner that the pointer
moves upscale until the torque is counterbalanced by
the instrument control spring.
The expanded-scale, a-c voltmeters utilize a stand
ard electrodynamometer mechanism with a saturable
reactor placed in series with the moving coil.
moving-coil circuit is connected in parallel with a
circuit consisting of the field and frequency compen
sating network.
When the voltage source is applied across the satur
able reactor, the exciting current varies in a nonlinear
fashion with the voltage.
responds to this nonlinear variation, and the instru
ment produces an accurate, readable indication in the
area of the reference voltage.
SIDE
STUD
TERMINALS
Installation
GEH-15398
The moving
In these instruments, three
The
The scale distribution cor
BACK
8
31"
32
5
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