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 TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
brated portable standard meter. The test connec-
adjustment is generally made at 50 percent power
tions are shown in figures 12-1 and 12-2.
factor, with rated current and voltage applied. Fifty
(1) Full load adjustment. The full load adjust-
percent power factor is generally used, because it
ment in most meters is accomplished by changing
can be readily obtained from a polyphase circuit
damping magnet positions. This involves changing
without auxiliary equipment.
the position of the magnetic shunt to vary the
(4) Creep adjustment. Creep is the continuing
amount of flux passing through the disk. The
rotation of a meter disk at no load for at least one
change produced in the registration, expressed in
complete revolution. Creep may occur either as
percentage is practically the same on all loads.
backward or forward motion. Although only a rapid
Thus, if the registration is 98 percent at both full
rate of creeping will result in appreciable registra-
load and light load, shifting the full load adjustment
tion of watthours, no meter should be allowed to
to increase the speed 2 percent will make the meter
creep `continuously. When load is removed, a meter
correct at both loads. The duration of the test at full
will generally rotate for a part of a revolution before
load should be 10 revolutions of the meter disk.
coming to rest. This is not creep. Leakage or
(2) Light load adjustment. The light load ad-
grounding in the load circuit may also cause a turn-
justment is made by varying the amount of friction
ing of the rotation element, which may be mistaken
compensating torque. The effect of this is inversely
for creeping. Most induction meters are designed
proportional to the load, so twice as much torque is
with holes or slots cut in opposite sides of the disk to
required at 5 percent load as at 10 percent load, or
prevent creeping. To locate the problem causing
one-tenth as much at full load as at 10 percent load.
creeping, check the meter for evidence of:
Generally, when a meter is inaccurate at light load,
more than compensation is required to correct the
(b) Vibration.
problem. In such cases, the tester should locate and
(c) Stray fields (internal or external).
correct the problem, and again check the accuracy
(d) High volt age, that acts to overcompen-
before attempting adjustment. The light load test
sate.
should be made at 10 percent of full load for a
(e) Incorrect connection of the potential cir-
duration equivalent to two revolutions of the meter
cuit. For example, the potential circuit is connected
disk.
--
on the load side of the meter, when the adjustment
(3) Lag or phase adjustment. The lag or phase
has been made for connection to the service side of
adjustment is ordinarily made in the shop as part of
the meter.
the routine test of all ac meters. Facilities for this
(f) Short circuits in the current coils.
test are readily available in the shop, and once the
(g) Mechanical damage or disarrangement of
adjustment is made, it will not change significantly
in service. The test to determine the lag or phase
the electromagnetic circuit.
Section IV-REPAIRS
12-13. Field repairs of instruments and
equipped with proper tools. The manufacturer's in-
meters.
struction book should be consulted when making
major repairs and when overhauling instruments.
Minor replacement of parts, such as dial faces,
After the work is completed, the instrument should
pointers, bearings and pivots, chart paper, and
be adjusted and checked for accuracy.
meter registers, may be made in the field. If exten-
sive repairs are required, they should be made in a
(2) Meters. The following steps should be taken
shop. When meter bearings or registers are re-
when a meter is brought to a shop for a complete
placed, recalibration of the meter is required.
overhaul:
(a) Take an initial reading, known as an "as
12-14. Shop repairs of instruments and
found" reading as an accuracy check, and record the
meters.
data.
Reference should be made to the manufacturer's
(b) Clean the meter thoroughly, removing
instruction books for methods of assembly and ad-
any dust or dirt with special attention to the mag-
justment. After parts have been replaced, meters or
net poles.
instruments should be recalibrated. The methods to
(c) Remove and examine the register to de-
be followed are given in section III.
tect any defects that may prevent its correct regis-
a. Overhauling. Major repair of a meter or instru-
tration. The worm or pinion on the shaft should be
ment should be performed in a shop.
examined to see that it matches properly with the
(1) Instruments. Repair of instruments should
register wheel, which it drives. A slight amount of
not be undertaken, except by qualified personnel
12-4
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