Re: Induction kWh meter theory

On Sun, 09 Jul 2006 22:43:08 +0000, Christopher Tidy
<cdt22NOSPAM@xxxxxxxxxxxxxx> wrote:

Hi all,

I'm wondering if I can draw on the knowledge of some of the electrical
engineering experts here. This coming week I'm going to be talking to a
group of high school students about engineering. They will mostly be 17
year olds who study physics and mathematics and have a pretty good
understanding of science. People who might become future scientists or
engineers, or who might be drawn into IT, management consultancy or
whatever. My job is to offer them a brief insight into engineering and
to take them through a few questions of the kind that might be asked by
university interviewers.

Usually I take along a few small machines for us examine and discuss.
Last time I took a box of small electric motors in various states (some
complete and working, some disassembled) and this worked well. This time
I was thinking of taking a few domestic induction meters. I know that
energy measurement is on their syllabus, but that the exact theory
behind the meters is a little above it.

I have hunted for the oldest meter I can find, which offers a good view
of the magnets and coils. As I understand it the conductive disc, which
is connected to the counter via a gear train, is acted upon by three
magnetic fields, all of which act perpendicular to the disc. One field
is produced by a coil wound with a few turns of thick wire. This coil
carries the line current and shows little inductance, so the magnetic
field it produces is in phase with the line current. Another field is
produced by a coil wound with many turns of thin wire. This is connected
between the live and neutral of the supply, and is highly inductive, so
that the magnetic field it produces is almost in quadrature with the
supply voltage. I think I'm right in saying that for the meter to work,
this field must be at least as strong as the field produced by the
maximum allowable line current through the first coil - perhaps someone
can confirm this for me? A third field is produced by strong permanent
magnets. This produces a retarding torque which is proportional to the
rotational speed of the disc. When the load is purely resistive, the two
alternating magnetic fields are in quadrature, and the disc experiences
a moving magnetic field which drags it around. When the load is purely
inductive, the two fields are in phase, and the disc experiences a
pulsating magnetic field which does not drag it around. Of course the
usual situation is for the load to be partially resistive and partially
inductive, and in this case the meter only registers the consumption of
real power.

So that's my understanding of how an induction meter works. Do correct
me if I've got anything wrong. Now (at last) on to my questions. The
oldest meter I have has an arrangement of coils like this (the more
modern meters have encapsulated coils which are small and hard to see):

|----------------|
-----| |-----
| | High L | |
| -| |- |
| | |----------------| | |
| | | |
| -------------------- |
| Core |
| ------- ------- |
| | | | | |
| | ---------- | |
| | | | | |
| | | Low | | |
| | | L | | |
| | | | | |
| | ---------- | |
| | | | | |
--- ---- ---
---------------------------------- <- Disc
----------------------------
| Core |
----------------------------

Here are some pictures of the meter:

http://www.mythic-beasts.com/~cdt22/elec_meter1.jpg
http://www.mythic-beasts.com/~cdt22/elec_meter2.jpg
http://www.mythic-beasts.com/~cdt22/elec_meter3.jpg

So here are a few questions:

1. Why does this meter use the arrangement of coils and core shown
above? Someone is bound to ask me.

2. Does anyone know of any eloquent, succinct explanations of induction
meter theory online which I can read over?

3. Does anyone know who invented the induction meter? I was under the
impression that it was Elihu Thomson, but I'm not sure of this.

I'm not firmly decided on using the induction meter as an example for
discussion, but these are bright students and the meters can be found in
almost every home in England, so it seems like a good topic.

I'd be interested to hear your thoughts.

Best wishes,

Chris


From: www.radianresearch.com/PDF/Bul_102.PDF

BULLETIN 102
Copyright © UTEC – 1999. All rights reserved. 1 043099
Document Revision 1.0
INTRODUCTION TO WATTHOUR METER TESTING
The information presented in this bulletin has been compiled from
several sources by Utility Test
Equipment Company (UTEC), in an effort to provide a general
description of the functioning of
watthour meters and test and calibration techniques.
Of necessity, the material is very general in its nature as it applies
to all makes and types of
meters and testing equipment. Care should be taken in the application
of this general
information to specific types of meters and testing equipment and the
information given in this
bulletin should be carefully checked for correctness with the
manufacturer’s information and
instructions for the particular make and type of meter or testing
equipment used.
THEORY OF OPERATION OF WATTHOUR METERS
Basically, the watthour meter consists of a motor whose torque is
proportional to the power
flowing through it, a magnetic brake to retard the speed of the motor
in such a way that it is
proportional to power, and a register to count the numbers of
revolutions the motor makes.
There are three principle torques involved in the operation of a
watthour meter; first, the
propelling torque of the motor element; second, the retarding torque
of the magnetic brake; and
third, the retarding torque due to friction.
The motor is made up of a stator with electrical connections as shown
in Fig. 1, and a disk. The
stator has two windings. One of them, the Current Coil, is connected
in series with the load and
the other, the Potential Coil, is connected across the line and
carries a current proportional to
the voltage of the circuit. The split phase effect causing rotation is
developed by winding the
current coil with few turns and by winding the potential coil with
many turns of fine wire making
its magnetic circuit of low reluctance and high reactance. As a
result, the current in the potential
coil is made to lag almost 90º behind the line voltage. The potential
coil with its core is
commonly referred to as the Voltage Electromagnet and the current coil
with its core as the
Current Electromagnet.
The magnetic flux set up by the voltage electromagnet extends across
the air gap over to the
iron core of the current electromagnet. Similarly, the magnetic flux
set up by the current
electromagnet extends across the air gap over to the iron core of the
voltage electromagnet.
The resultant flux of the voltage and current electromagnets then
passes through the disk of the
meter, and since there is a difference in phase between the two
separate fluxes, the resultant
flux undergoes a continual shift or “sweep” from one side to the
other, always in the same
direction. The eddy currents set up in the disk as a result of the
magnetic flux penetration, react
with this shifting flux pattern and cause the disk to rotate.
BULLETIN 102
Copyright © UTEC – 1999. All rights reserved. 2 043099
Document Revision 1.0
Basic Single Stator Electromagnet
Figure 1
The torque on the disk caused by the interaction of fluxes tends to
cause constant acceleration.
Without a brake the speed of rotation would be limited by the supply
frequency, by friction, and
by certain counter torques at higher speeds but the speed of rotation
would be very high.
Therefore, some method of making the speed proportional to power and
also of reducing it to a
usable value is needed. A permanent magnet performs these functions.
When the disk is
rotated in the field of the permanent magnets the eddy currents set up
in the disk react with the
magnetic flux from the permanent magnets in such a manner that there
is a retarding torque or
“drag” applied to the disk which is always directly proportional to
the speed. For this reason, the
permanent magnets are referred to as “Drag Magnets”.
The retarding torque due to friction does not vary with the speed, and
increases only as the
bearings and register become worn. Minor amounts of friction can be
compensated for, as long
as they remain constant, by means of the ‘Light Load’ adjustment.
To register the amount of energy measured by the meter mechanism, a
register is geared to the
meter disk shaft. The reduction gearing in the register is designed to
make the register read
directly in units of kilowatt hours. It is therefore necessary to
determine not only that the meter
element has the correct speed when a known load is applied, but also
that the gear ratio and
register constant bear the proper relation to each other to correctly
register the energy passing
through the meter.
Multi-stator watthour meters, usually referred to as “Polyphase”
watthour meters are essentially
a combination of single-stator meters on a common disk. Therefore, we
can rely on the basic
meter theory of the single-stator meter for an understanding. The
differences are mainly in a few
special features and in the various applications to polyphase power
circuits. The theory of
polyphase metering was set forth on a scientific basis in 1893 by
Andre E. Blondel, engineer
BULLETIN 102
Copyright © UTEC – 1999. All rights reserved. 3 043099
Document Revision 1.0
and mathematician. His theorem, known as “Blondel’ s Theorem”, applies
to the measurement
of power in a polyphase system of any number of wires. The theorem is
as follows:
If energy be supplied to any system of conductors through ‘N’ wires,
the total
power in the system is given by the algebraic sum of the readings of
‘N’
wattmeters, so arranged that each of the ‘N’ wires contains one
current coil, the
corresponding potential coil being connected between that wire and
some
common point. If this common point is one of ‘N’ wires, the
measurement may be
made by the use of N- 1 wattmeter.
From this theorem it follows that basically a meter containing two
stators is necessary for a
three-wire, three-phase circuit and a meter with three stators for a
four-wire, three-phase circuit.
HISTORY OF WATTHOUR METERS AND TESTING EQUIPMENT
Since the first “Thomson Recording Wattmeter” manufactured by the
Thomson-Houston Electric
Company in 1899, manufacturers have made many improvements in the
accuracy and reliability
of the watthour meter. With these improvements has developed the
necessity for faster, more
reliable, and more accurate testing and calibration equipment.
In the early days, calibration of the watthour meter was a major
problem because suitable
standards of comparison were not available. At first, only indicating
instruments were used for
calibrating purposes. In order to make a complete calibration it was
necessary to measure time
along with voltage and current or power. Prior to 1900, voltage was
measured with a Cardew
hotwire voltmeter. Current was measured by means of a Siemens
dynamometer or by a Kelvin
balance and power was measured by the Siemens watt-dynamometer or by a
Kelvin wattbalance.
The portable rotating standard watthour meter was introduced by
Westinghouse in
1899.
M. Mowbray designed and built the first multiple-range portable
standard watthour meter. In
1904, Westinghouse developed a “Precision Wattmeter” which was an
improvement of the
dynamometer type Kelvin bridge. The development of these reference
standards made possible
the testing method commonly used today where the standard meter and
the meter under test
are connected in series with a suitable load. This method of testing
greatly speeded up the
calibration procedure since any fluctuations in load affected the
meter and standard alike and
therefore did not alter the result.
In the late 1920’s it was realized that the tremendous increase in the
number of meters in
service necessitated more efficient methods of testing to maintain the
high standards set for
metering electric energy. Testing of meters on the customer’s premises
was a slow and costly
process. In many sections of the United States, it became increasingly
apparent that more
satisfactory results could be obtained by testing large quantities of
meters in centralized shops
where automatic test equipment could be used.
By 1925, development in electronic devices had progressed to the point
where their use as
auxiliaries in the testing process gave promise of both greater speed
and accuracy. The first
electronic development was in 1925 by A.R. Rutter of Westinghouse.
This development used a
photoelectric device cut by holes in a meter disk to produce marks on
a printed tape that were
compared to master clock marks to determine the speed of the meter. In
1927, H. P. Sparks,
also of Westinghouse developed the use of the stroboscopic principle
which allowed the meter
BULLETIN 102
Copyright © UTEC – 1999. All rights reserved. 4 043099
Document Revision 1.0
to be adjusted visually, without the necessity of counting
revolutions.
By 1940, some utilities had developed watthour meter test boards that
were essentially fully
automatic. In 1960, Weston Instruments developed the inductronic
wattmeter which was the first
electronic wattmeter. Also in the ‘60’s the single revolution method
of calibrating watthour
meters and the application of using digital counters was. introduced.
In 1968 a method for
computer-controlled meter calibration was patented by Duncan Electric
(Landis & Gyr). By 1969
photoelectric test equipment utilizing programming for sequence of
tests, computers to calculate
meter accuracy, digital readout and printout of meter accuracies, and
solid-state circuitry was
being widely used in test equipment designs.
In the early 1980’s a method of generating the precision voltage and
current using solid state
amplifiers was introduced by test equipment manufacturers. This method
eliminated the testing
problems associated with watthour meter burden affecting the accuracy
of the test system and
allowed precise control of the phase angle between the current and
voltage.
Further improvements in the reference standard were made by Radian
Research by their
introduction of a fully auto-ranging ‘summing’ reference standard with
three current circuits
making possible the testing of watthour meters with the potential
clips closed.
All of these improvements have made testing equipment faster, more
accurate, more
dependable, and more versatile.
METHODS OF TESTING
There are basically two methods of testing watthour meters. One is
where the load during the
test is controlled and the disk is timed and the other is where the
meter being tested is
compared with a known precision reference standard.
There are times when a simple quick method of checking watthour meters
for accuracy is
needed. The method outlined here is used for various purposes by many
companies, both large
and small, for making an approximate check with a fair degree of
accuracy. The accuracy of this
method of checking should not be expected to be better than ±2%
consequently it should not be
used for calibrating watthour meters. This method is most commonly
used in field testing, when
a load box is not available, for determining approximate service load,
suspected cases of meter
tampering etc.
The method consists merely of connecting a known load to the watthour
meter in the
conventional manner and timing the disk for a desired number of
revolutions. One of the most
consistent and readily available loads is a standard incandescent
lamp. The accuracy of the
field check can be substantially improved by measuring the service
voltage in each case and
adjusting the “known” wattage accordingly. For voltages within ±10
volts of lamp rating, the watt
load of the lamp will increase or decrease 1.5% for each 1% of voltage
above or below their
rating.
The disk of the meter should be timed for a convenient number of
revolutions depending on the
rating of the meter and the load used. It is usually desirable to run
meters for about one minute
or more to minimize errors in reading time. It is preferable to use a
stop watch or a synchronous
timer, however, any digital watch may be used with good accuracy.
BULLETIN 102
Copyright © UTEC – 1999. All rights reserved. 5 043099
Document Revision 1.0
The required number of seconds with a known watt load for a given
number of revolutions of the
disk in an accurately calibrated meter is given by the equation:
= t
W
Kh x 3600 x R
where: Kh = Watthour (or Disk) Constant (Wh per revolution)
3600 = 60min. x60 sec. = 1 hour (needed to convert Wh to
wattseconds)
R = Revolutions of meter disk for time of test
W = Watt load on meter (E x I x Cosq)
t = Time of run in seconds
The watthour or disk constant, Kh, will be found on the nameplate of
all modem meters. On
some older types it was marked on the disk. They may also be found in
the Handbook for
Electricity Metering or from the manufacturer’s data sheets.
EXAMPLE: Assume a 15 ampere, 240 volt, 3 wire meter, Kh = 2, connected
to a
nominal 240 volt service with voltage actually 240 volts and a check
is made
using lamps having a total rating of 600 watts at 120 volts; the meter
is run for 5
revolutions, the required time for the run in seconds is as follows:
If the observed time is 62 seconds, it would show the meter to be slow
approximately 3.33% or
if 59 seconds, about 1.67% fast. Since the accuracy of this method
should not be expected to
be better than ±2%, in either case the meter is in all probability
within commercial accuracy and
a big part of the small apparent error is in the method of testing.
( )
Theoretical Time
Theoretical Time Actual Time x 100
Percent Error
-
=
( )
3.33%
60
60 62 x 100
Percent Error = -
-
=
( ) 1.67%
60
60 59 x 100
Percent Error = +
-
=
When testing 3 wire meters, the load should be applied to both current
coils. This may be done
by dividing the load between the two line conductors and the neutral.
When using this method, be sure all loads, other than lamps, to be
considered are turned off
before making the check. Look out for hidden load such as lamps in
closets, basements, etc.,
that may be on or go on with switches for other lamps. It is desirable
in all cases to see that the
meter is not running before applying the test load.
60 seconds
600
2 x 3600 x 5
time = =
BULLETIN 102
Copyright © UTEC – 1999. All rights reserved. 6 043099
Document Revision 1.0
Time-Load Testing - Polyphase
The time load method may also be used for testing polyphase meters
although it may be less
convenient. The important thing to remember is that watt load (W) on
the meter is the sum of
the watts in all elements of the meter.
a a a b b b c c c W = E I Cosq + E I Cosq + E I Cosq
The formula therefore becomes:
a a a b b b c c c E I Cos E I Cos E I Cos
Kh x 3600 x R
q + q + q
= t
where: Kh = Watthour (or disk) Constant (watthours per revolution)
3600 = 60 minutes x 60 seconds = 1 hour (needed to convert
watthours to wattseconds)
R = Revolutions of meter disk during test
W = Watt load on the meter (sums of EI Cosq)
t = Time of test in seconds
EXAMPLES: Assume we desire to test a form 16 meter (3 stator, 3 phase,
4 wire, wye)
that is rated at 120 volts, 30 amperes and has a Kh of 21.6. We
examine the connected
load and find it to be resistive in nature which would make the power
factor unity (1). We
next measure the current and voltage of each phase and compute the
watts.
A Phase: E = 120 Volts
I = 5 Amperes
Load = Resistive (PF=1)
B Phase: E = 119 Volts
I = 2.5 Amperes
Load = Resistive (PF=1)
C Phase: E = 121 Volts
I = 10 Amperes
Load = Resistive (PF=1)
How many seconds should it take for the meter to make two revolutions
under this load if it is
100% accurate?
W
Kh x 3600 x R
= t
( ) ( ) ( ) t
120 x 5 x 1 119 x 2.5 x 1 121 x 10 x 1
21.6 x 3600 x 2
=
+ +
BULLETIN 102
Copyright © UTEC – 1999. All rights reserved. 7 043099
Document Revision 1.0
t
2107.5
155520 =
73.8 = t (seconds)
Remember, when using this method for polyphase, you must calculate the
watts in each phase.
Some meters such as a 2 stator, 3 phase, 4 wire, wye are very tricky.
This meter, because of its
design, looks like it has four elements. Therefore, in calculating
watts you must consider all four
current coils.
Comparison Testing - Single Phase
Probably the best way to test watthour meters is the comparison
method. In this method, the
meter under test is compared to a highly accurate meter, commonly
called a reference
standard. This method applies, the same power, or watts, is to the
test meter and the reference
standard for the same length of time, and the rotating time of the
test meter is compared to that
of the reference standard.
When older style ‘tap’ standards are used, this comparison is based on
revolutions of both the
meter under test and the standard. The newer style ‘summing’ standards
display in watthours
which simplifies testing procedures by not having to compute the
revolutions ratio between the
meter under test and the standard. ‘Summing’ standards always have a
Kh value of 1.0 for all
voltages, currents and power factors. If the same number of current
inputs are used for the
meter under test and the reference standard, the ‘summing’ type
reference standard will display
the meter under test Kh for every revolution tested.
In order to apply the same power, regardless of whether you are
testing a simple single phase
meter or one of the more complex polyphase meters, there are two
things that must be done.
First, the potential coils of the test meter and the reference
standard must be connected in
parallel with the same voltage source. Secondly, the current coils of
the test meter must be
connected in series with the current coils of the reference standard
and with a source of known
current.
Because the same voltage and current are applied to the test meter and
reference standard,
both have the same power (watts = voltage x amps) applied; and
therefore, any variations in
voltage and/or current during the test will have an equal effect on
both the test meter and the
reference standard and will not effect the accuracy of the test.
Consequently, it is not necessary
to apply precise values of voltage and current, nor is it necessary to
maintain the voltage and
current at exact values. It is however, important to use test sources
that are free of noise,
distortion and harmonics.
It is usually very easy to obtain the desired test voltage, since it
can readily be obtained from the
power line. However, a more complex arrangement is necessary to obtain
the desired test
current because it must be adjusted to different values depending on
the type of meter being
tested.
When using the comparison method, the accuracy, or more properly, the
percent registration of
the test meter is given by the following formula:
BULLETIN 102
Copyright © UTEC – 1999. All rights reserved. 8 043099
Document Revision 1.0
Percent Registraton
R x KH
r x kh
100 x =
where: r = revolutions of test meter
kh = watthour constant of test meter
R = revolutions of reference standard
KH = watthour constant of reference standard
Watthour Constant
Before continuing, some discussion of Basic and Test or Nameplate Kh
is necessary. The Basic
Kh is the Watthour Constant defined at 5 amperes and 120 volts. ‘Tap’
reference standards,
such as the Scientific Columbus SC-10 and SC-10V, are specified in
Basic Kh since they can
be configured to many combinations of voltage and current. Before a
comparison test can be
made the Kh value for the ‘tap’ standard in its test configuration
must be computed for
standards that readout in revolutions. Manufacturers of ‘tap’
standards usually provide a chart
in the lid of the standard with the correction factors and the Test Kh
values listed.
For ‘summing’ standards that readout in watthours, such as the Radian
RM-10, 11, 15 and the
Scientific Columbus SC-30, this calculation is not necessary because
they are designed to have
a Kh of 1.0 for all values of voltage, current and power factor.
5
Test Current *
x
120
Test Voltage
Test Kh = Basic Kh x
* This is the ‘tap’ value not the test current value. For example: if
the test current was 30A and
the 50A tap on the standard was used for testing; Test Current = 50A.
The Basic Kh of a watthour meter can be computed by working backwards
from the Nameplate
Kh. For example, a common residential 2-3 wire, form 2 meter rated at
240V and 30 Amps has
a nameplate Kh of 7.2. If we compute the factor for voltage and
current (last two fractions of
above formula) we get 2 for voltage and 6 for current making a
multiplication of 12. If we now
divide the nameplate Kh by 12 we obtain the basic Kh of 0.6. Note that
the basic Kh for the
form 2 meter is the same as the normal ‘tap’ standards basic Kh.
The following example will show how the testing formula is applied.
Assume you are required to
test a meter rated at 240 volts, 30 amperes, and having a Kh of 7.2. A
standard with a basic Kh
of 0.6 will be used. Because 30 amperes will be used to test the
meter, the 50 ampere coil of
the standard must be used. The value of the Test Kh for the standard
using the 50 ampere coil
is 0.6 x 2 x 10 [twice the basic voltage (120 volts) and 10 times the
basic current (5 amperes)] =
12.0. Substituting these values in the formula we have:
R x 12
r x 7.2
Percent Registration = 100 x
It can be seen that the ratio of r to R is as 0.6 to 1. When using
‘tap’ standards, it has been
determined that the revolutions of the standard should always be 10 or
more, and since r must
always be a whole number of revolutions the nearest value of r that
will make R 10 or more is
BULLETIN 102
Copyright © UTEC – 1999. All rights reserved. 9 043099
Document Revision 1.0
20, making R= 12. Using these values then:
R x KH
r x kh
Percent Registration = 100 x
100%
12 x 12
20 x 7.2
Percent Registration = 100 x =
If the meter runs fast or slow, then the value of R in the formula
will be less or greater than 12.
Suppose that for 20 revolutions of the meter, R of the reference
standard is 12.12 then:
99%
12.12 x 12
20 x 7.2
Percent Registration = 100 x =
or the meter is 1% slow. Now suppose that R = 11.76 then:
102%
11.76 x 12
20 x 7.2
Percent Registration = 100 x =
or the meter is 2% fast.
A quick way to find the ratio of meter revolutions to standard
revolutions is to find the ratio of the
product of the current rating of the standard and its basic Kh (120V
at 5A) and the product of the
current rating of the meter and its basic Kh.
m m
s s
s
m
Kh x C
Kh x C
R
R
=
where: Rm = Revolutions of meter
Rs = Revolutions of standard
Khm = Basic Kh of meter
Khs = Basic Kh of standard
Cm = Current rating of meter
Cs = Current rating of standard
Using the same meter example as above:
12
20
3
5
0.6 x 30
0.6 x 50
R
R
s
m = = =
If you are using a standard that reads out in watthours, such as the
Radian RM10, 11 and 15, or
Scientific Columbus SC-30 the calculation of percent registration is
much simpler. The formula
for this type of reference standard is as follows:
BULLETIN 102
Copyright © UTEC – 1999. All rights reserved. 10 043099
Document Revision 1.0
Display Number x ME
100 x Kh x R x SE
Percent Registration =
where: Kh = Disk constant of meter under test (MUT)
R = Number of revolutions of MUT
SE = Number of standard elements included in test
Display Number = Display reading of standard in watthours
ME = Number of MUT elements included in test
Comparison Testing – Polyphase
When testing polyphase meters with two or three current coils, they
must be connected in series
aiding; thereby driving the disk in the same direction and with the
same force that the coils
would produce under normal operating conditions.
Because of the series connection of the current coils when testing
polyphase meters, the
accuracy formula must be modified to account for the number of current
coils of the meter under
test through which current is passed. This is necessary since the test
current passes through a
multi-tap standard only once, but goes through the meter being tested
as many times as there
are current coils. The following formula takes the current circuits
into consideration:
R x KH x C
r x kh
Percent Registration = 100 x
where: r = Revolutions of meter under test (MUT)
kh = Watthour constant of MUT
R = Revolutions of reference standard
Kh = Watthour constant of reference standard
C = Number of current coils energized in MUT
as given in the list below
SINGLE STATOR METERS
2-wire
All tests C = 1
3-wire
Testing all current windings series C = 1
Testing individual current windings C = ½
TWO STATOR METERS
3-wire, 3-phase
Testing individual stators C = 1
Testing stators in series C = 2
BULLETIN 102
Copyright © UTEC – 1999. All rights reserved. 11 043099
Document Revision 1.0
4-wire Y, 3-phase
Testing individual circuits, single coil C = 1
Testing double coil (Z-coil) or all circuits in
series with only one potential coil
energized C = 2
Testing all circuits in series C = 4
4-wire delta, 3-phase
Testing individual circuits, 2-wire coil C = 1
3-wire coil, windings separately C = ½
3-wire coil, windings in series C = 1
Testing all circuits in series C = 2
THREE STATOR METERS
4-wire Y, 3 phase
Testing individual stators C=1
Testing two stators in series C=2
Testing three stators in series C=3
The following example will show how the testing formula is applied.
Assume we desire to test a
form 16 meter (3 stator, 3 phase, 4 wire, wye) that is rated at 120
volts, 15 amperes and has a
Kh of 5.4. A standard having a basic Kh of 0.6 will be used. This
particular standard does not
have a 15 ampere range but it does have a 12.5 ampere range. The value
of the Kh of the
standard is 1.5.
We must first determine the number of revolutions the standard rotates
to each revolution of the
test meter using the formula:
m m
s s
s
m
Kh x C
Kh x C x ME
R
R
=
This is the same formula used in the single phase section that has
been modified to account for
the number of current coils of the meter under test ME through which
current is passed. In our
example:
R x KH x C
r x kh
Percent Registration = 100 x
100%
12 x 1.5 x 3
10 x 5.4
Percent Registration = 100 x =
BULLETIN 102
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As in previous examples given for single phase meter testing, if the
standard rotates less than
the expected revolutions for 100%, the test meter is greater than
100%: conversely, if the
standard rotates more than the expected revolutions for 100%, the test
meter is less than 100%.
Suppose in the example above the standard rotated 11.8 times for 10
times of the test meter,
then:
101.7%
11.8 x 1.5 x 3
10 x 5.4
Percent Registration = 100 x =
or the meter is 1.7% fast.
Now, suppose that R = 12.2, then:
98.3%
12.2 x 1.5 x 3
10 x 5.4
Percent Registration = 100 x =
or the meter is 1.7% slow.
TYPES OF TEST EQUIPMENT
There are three basic methods for developing a calibrated current for
meter testing. They are
Resistance Loading, Phantom Loading, and Solid State Loading. Testing
equipment is most
commonly called by the name of its loading method...such as a Phantom
Load Box etc.
Resistive Load
In the resistance loading method the current coils of the test meter
and the standard are
connected in series with the loading resistance across a voltage
source. Because of this
connection, the current which is permitted to flow by the selected
resistance passes through
both the test meter and the standard. As illustrated in the figure
below, a Resistance Load
usually consists of several fixed resistances of various values which
can be selected to obtain
different current values needed for testing. The resistors are
calibrated for specific voltages and
the switches are generally marked to indicate the current each allows
to flow.
The major disadvantage to Resistance Loads is the problem of
dissipating the energy
consumed by the I²R loss at high currents since the current being
supplied to the test meter is
also taken from the line. Resistance Loads are primarily used when
non-inductive loading is
required such as in protective relay testing.
BULLETIN 102
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Phantom Load
Through years of use, it has been found that the safest and the best
method of producing the
test current is by using a Phantom Load. As illustrated in the figure
below, a Phantom Load
basically consists of a special loading transformer that reduces the
line voltage to a lower
voltage which is applied through loading resistors to the meter under
test; thereby, producing
current. Phantom loading reduces the power dissipation in the current
circuit because of the
reduced voltage across which the load is connected and therefore,
requires less line current by
the ratio of the transformer. Let us assume that it requires 5 volts
to cause 50 amperes to flow
through a selected resistor. If the phantom load is powered by 120V
only 2.08 amperes is
required from the line source to produce 50 amperes in the loading
circuit... thus the name
‘phantom load’.
Even though there appears to be magic in this method in its capability
of being able to supply 50
amperes to the meter while taking only 2.08 amperes from the service,
the laws of physics
prevail. The VA (Volt Amp) of the primary is equal to the VA of the
secondary.
Primary = 120V x 2.08A = 250VA
Secondary = 5V x 50A = 250VA
BULLETIN 102
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Solid State Loading
New technology coupling solid state amplifiers and the phantom loading
method is now being
used to generate test voltages and currents. This new method
eliminates several problems that
exist with the standard phantom loading method. Because many
transformers were needed to
generate matching currents and the required voltages; test circuits
were sensitive to the
different burdens presented them by different types of meters. These
burden errors caused
changes in power factor and many times changes in the values of
voltage and current from one
test to another generating errors in the tests results.
While these errors were small (usually less than .2%) they have been a
concern to Utility
Companies, Meter Manufacturers, and Test Equipment Manufacturers
alike. The new
technology basically consists of an amplifier, much like your stereo
amplifier, that generates a
line frequency signal to drive an output transformer like the one in
the phantom load. The
difference is that a computer monitors the amplitude of the output
signal and its phase
relationship and makes adjustments to the input signal of the
amplifier to compensate for any
changes due to changing load etc. It is a closed loop system which
corrects itself so as to be
exact at all loading points. This new technology lends itself to easy
and exact control of the
power factor for the test. Conventional methods of attaining 0.5 power
factor were to select two
phases of a three phase delta service or use a calibrated gaped core
inductor in the current
circuit to shift phase. These methods limit the selection of power
factors available (namely 0.5)
and result in very approximate phase shifts. The new technology can
provide power factors of
from 0 to 1 lead or lag easily and with great precision making
possible the testing of VAR and QHour
meters as well as Watthour meters on the same test system.
Meter Shop Test Tables
All modern meter shop test tables use solid state loading for
producing test voltage, current and
phase angle. The primary difference between a meter shop test table
and the phantom load box
is that it generates the test voltage, current and phase angle from
digital synthesizers and not
the service line; which eliminates the noise and harmonics from the
service. It is, of course,
more sophisticated which increases the efficiency and accuracy of
testing meters.
BULLETIN 102
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Some manual test tables can, with a series of selector switches, offer
the tester the flexibility of
easily and quickly matching the meter current coils to test hookups
without tearing down each
set up. This is especially convenient when a large number and variety
of meters must be tested.
Some manual test table manufacturers use what is called a cookbook
approach to meter
testing. By simply looking up the form number or type of meter in
their instruction manual, the
hookup and control switch settings are clearly shown. The meter tester
has little problem
hooking up even the more complex meters.
Other more sophisticated test tables are controlled by computers. In
this case every function is
precisely timed and monitored so that each second of test time is used
to its fullest extent;
thereby performing tests in the fastest possible times. The operator
initializes the test by placing
the meter into the test jack, aligns the pick-up, enters the meter
information and test sequences
to be tested and gives the command to test. The test system performs
the test automatically,
printing the results after each test or sending the test results to
the main frame computers
history file. Many of these systems use bar code to automatically set
up the meter test and
program the test system.
CONTRIBUTING ERRORS TO METER TESTING
There are several things that can contribute error when testing
watthour meters. Some of the
sources of error apply only to the conventional phantom loading method
and some apply to both
the conventional phantom loading and solid state loading method.
The first contributing error to meter testing is resistance in the
potential circuit. While a small
amount of resistance does not appreciably effect full load unity power
factor test, it is of major
concern when testing 0.5 power factor contributing as much as .2% for
0.2 ohms of resistance.
Resistance is introduced most commonly from dirty contacts on
connector jacks, loose or
corroded connections and either too long or too small a wire gauge for
potential leads. The
Phantom Loading method is subject to all of the possible problem areas
while the Solid State
Loading is subject only to dirty contacts on the connector. Because
this method senses the
voltage and phase angle at the meter socket, dirty meter blades etc.
are outside the control loop
and therefore will cause errors in testing.
The second contributing error to meter testing is wave form
distortion. Improperly designed
transformers, oscillating amplifiers, and voltage regulators coupled
with poor transformer
designs are the major reasons for wave form distortion. This type of
error applies to both
phantom and solid state loading methods. This error can be eliminated
by choosing a solid
state test system that monitors the wave form for distortion and stop
the test should distortion
appear.
The third contributing error to meter testing is timing errors from
the photoelectric counter. The
relay used to control the standard potential becomes worn and dirty
with use which causes
unreliable and unpredictable drop-out and pull-in times. Also changing
lighting conditions and
power supply voltages change the threshold point at which the relay is
instructed close and
open. Errors produced from the photoelectric counter can contribute as
much as ±0.2%. This
error applies only to testing systems using switched voltage standards
such as the Scientific
Columbus SC-10. This error can be eliminated by using only gated
display solid state reference
standards, such as the Radian RM-10, 11, 15 or Scientific Columbus
SC-10V, 20, 30 and UTEC
711 or 712 electronic revolutions counters.
BULLETIN 102
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The fourth contributing error to meter testing is a rotating standard.
Rotating standards suffer
from a variety of possible problems; among them are dirt, friction,
worn bearing, tilt errors,
temperature drift, and coasting; all of which contribute errors to
meter testing. These errors can
be eliminated by using a solid state standard.
The fifth contributing error to meter testing is the resolution to
which the reference standard is
capable of reading. Most rotating standards have resolving abilities
of only 1%. In order to get
increased resolving ability, multiple revolutions of the standard are
necessary. This error is
eliminated when the test system uses a solid state standard.
The sixth contributing error to meter testing is magnetic offset both
in the meter under test and
the testing equipment’s standard meter and transformers. Magnetic
offset is most commonly
caused from switching the load on and off at points other than zero on
the sine wave. This error
is eliminated with test systems that use zero crossing switching or
use a ramp function to start
and stop the test.
The seventh contributing error to meter testing is related to solid
state meters. Most solid state
meter designs require that the load be applied a few seconds before a
measurement of
accuracy is taken. This time delay ranges from about 3 to 7 seconds.
To eliminate this source of
error, energize the MUT with potential and current for at least 10
seconds before beginning a
test.
The eighth contributing error to meter testing is low service voltage.
The problem primarily
affects induction (disk type) meters since they typically do not have
linear or flat voltage
response curves. This condition of low voltage is usually created by
the additional load drawn by
the phantom load when connected to the PT secondary and can cause test
error as much as
15%. This error does not exist when using solid state test devices or
when testing selfcontained
meter installations or transformer-rated installations that do not use
PT
The last and probably the largest contributing error to meter testing
is the human error factor.
Improper load adjustments, improper test sequences, improper
application of correction factors,
improper connections, improper recording of test data, and improper
selection of testing
parameters are among the most common human errors. These errors apply
to any test method
and are the most difficult to control. Fully automatic solid state
test systems minimize these
errors.
TESTING SAFETY
Safety should be on the mind of every meter tester. When performing
field tests, the voltage
levels of the service and the fault current capabilities are very
dangerous and should not be
dealt with lightly. Safety glasses, hard hats, low voltage insulated
gloves, and long sleeve fire
retardant protective clothing should be worn at all times when a
service connecting device such
as a meter socket is exposed. The dangers of testing are increased
when the tests involve a
transformer rated service. These services contain Current Transformers
which reduce the high
primary currents to lower currents (usually 5 amperes) so that a
electricity meter may be used.
The Current Transformer is a device that has a low voltage secondary
as long as the secondary
connection is a continuos connection. If however, the secondary
connection is opened and
there is current flowing in the primary, the current transformer
becomes a step up voltage
transformer and the secondary voltage can rise to many thousands of
volts.
The high voltage that is present on the open secondary of an energized
current transformer
generates two great hazards. The first hazard is ELECTRICAL SHOCK TO
THE TESTING
BULLETIN 102
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PERSONNEL. The second hazard is THE BREAKDOWN OF THE CURRENT
TRANSFORMER INSULATION resulting in the destruction of the current
transformer. Both
hazards can be avoided provided that the secondary of the current
transformer is never opened.
The safest current transformer installations for testing are those
that have as part of the
secondary loop a Test Switch. A Test Switch is a device that will
facilitate shunting of the
current transformer secondary loop without the danger of opening the
circuit. This device
provides a make-before-break connection to prevent accidental opening
of the current
transformer secondary when isolating it from the metering circuit. In
addition, the test switch
provides for the safe insertion of other instruments in the CT
secondary loop such as ammeters
using a test switch safety plug sometimes referred to as a duck bill
plug. The test switch safety
plug, like the knife switches, provides a make-before-brake connection
so as to guarantee that
the CT secondary is never opened.
On installations that do not have a Test Switch included in the
current transformer secondary
loop, THE SECONDARY TERMINALS OF THE CURRENT TRANSFORMER MUST BE
SHORTED BEFORE THE LOOP IS OPENED! The shunt or short connected across
the CT
secondary should be a BOLT-ON or CAPTURED type of connection. Test
clips, or any spring
type connection, should never be used for shorting a CT secondary.
When the CT secondary
has been shorted with a bolt-on connection, the electricity meter may
be isolated from the
secondary circuit for testing. The CT secondary shunt must remain in
place until the electricity
meter is again wired back into the CT secondary loop so as to complete
the circuit. Do not
forget to remove the CT secondary shunt before leaving the service
site. Leaving the CT
secondary shunt ON will, of course, cause the electricity meter not to
register energy for that CT
and cause a decrease in the customer billing.
PROCEDURE FOR TESTING WATTHOUR METERS
In the case of single stator meters there are two adjustments to be
made in calibrating a
watthour meter; one, the “Full Load” adjustment, which involves
changing the drag torque
developed by the permanent magnets (drag Magnets), the other, the
“Light Load” adjustment
which compensates for friction. In single stator meters, the power
factor adjustment is made at
the factory and can not be easily changed in the field. Multi-stator
meters (polyphase), however,
have power factor adjustments. Typically there is one FL adjustment
for the meter, and one LL
and PF adjustment for each electromagnet assembly in a multi-stator
meter.
To properly adjust a watthour meter, its present accuracy must first
be determined. This is
known as the “As Found” test. After the percentage registration of the
meter has been
determined by the ‘as found’ test, the necessary adjustments to the
FL, PF and LL adjustments
are made to bring the meter within the desired accuracy. The final
test made on the meter after
adjustments are made is known as the “As Left” test, because it is
that test made on the
accuracy of the meter in the condition in which it was left by the
tester.
The procedures for “As Found” and “As Left” tests are identically the
same as far as meter
connections and readings are concerned.
Because of the different procedures for testing, depending on what
type and manufacturer of
equipment you are using, only the method for using a field load box
will be discussed in this
paper. However, the basic testing principles can be applied to any
piece of testing equipment.
Before using any piece of testing equipment, be sure to always consult
the manufacturer’s
operation manual for complete and correct operation.
BULLETIN 102
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Connect the meter under test to the portable load box and to the
reference standard in such a
way that the potential coil of the meter under test is in parallel
with the potential coil of the
reference standard. The current coil or coils of the meter under test
should be connected in
series with the current coil or coils of the reference standard and in
series with the current circuit
of the portable load. The particular current coil of the reference
standard should in every case
be one which will easily carry as a minimum at least 50% of the
testing current but one where
the testing current will never be greater than 150% of the reference
standard current tap values.
In general, two different reference standard current coils must be
used for the FL and LL test
when using a ‘tap’ type standard.
Apply voltage and current to see that the meter under test (MUT) and
the reference standard
rotate in the proper direction. In those cases where the testing
facility consist of a load box and
a reference standard which must be connected to the meter by means of
removable leads, it is
easily possible to accidentally reverse the polarity on either meter
or reference standard. In case
of reverse rotation, the following should be done:
1. If both meter and standard rotate backward, interchange either the
current potential
connections or the current connections of the load box. Note, solid
state standards
will not run backwards, they simply will not run at all if either the
voltage and current
is wired backwards. In this case, the meter will rotate backwards and
the standard
will not operate.
2. If the meter only rotates backward, interchange current feed
connections to the
meter.
3. If the reference standard only rotates backward or does not operate
at all,
interchange either the potential connections or the current
connections of the
reference standard.
4. If a solid state standard is used and either the potential or
current polarity is
reversed, the standard will not run at all. Reversing either the
potential or current
connections to make the standard run.
In certain phantom load designs such as the UTEC 402, 403, 404, 406,
407, 408, 440, 441,
443, 452, 453 and 454 meter test kits, permanent connections between
the load box and the
reference standard eliminate all guesswork because the proper
combinations of current and
reference standard current coil selection are made automatically.
Apply full load to the meter and standard. FL is taken to mean the
rating of the meter in
amperes, as noted on the nameplate of the meter (TA). Proceed with the
full load test, allowing
the meter under test to rotate a sufficient number of revolutions so
as to give at least 10*
revolutions for the reference standard. This is in accordance with the
general practice to obtain
a sufficient number of revolutions on the rotating reference standard
to be able to detect meter
inaccuracies of a fraction of a percent. For instance, the large dial
of a rotating reference
standard is always subdivided into 100 divisions. When the sweep hand
pointer of the
reference standard has completed 10 revolutions, it has passed over 10
x 100 = 1000 readable
divisions, so that the meter accuracy can be read to 1 part in 1000,
or 1/10 of 1 percent (0.1%).
The check on meter accuracy is accomplished by allowing the reference
standard to rotate only
during that interval of time which is needed for the meter under test
to complete a given number
BULLETIN 102
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Document Revision 1.0
of revolutions. For example, assume that 20 revolutions of the meter
under test are required to
make a check. With load turned on and the MUT rotating, set the
reference standard to zero,
with the click switch off. Then snap the click switch on at the
instant the black mark on the edge
of the test meter disk passes a conveniently visible stationary
reference point, which is usually
taken as the space between the front edges of the drag magnets or the
mark usually provided
above the disk opening of the meter nameplate. Then count test meter
disk revolutions until 20
revolutions have been completed and snap the click switch off when the
black mark has passed
the reference point for the 20th time after the start.
In cases where photoelectric counting equipment is used, the
photoelectric counter is used in
place of the snap switch.
The accuracy, or more properly the percent registration of the meter,
is given by the following
formula:
R x KH x C
r x kh
Percent Registration = 100 x
where: r = Revolutions of meter under test (MUT)
kh = Watthour constant of MUT
R = Revolutions of reference standard
Kh = Watthour constant of reference standard
C = Number of current coils energized in MUT
as given in the list below
After the full load test, apply light load to the meter. The usual
light load value is 10% of the TA
value.
The procedure for obtaining a light load check is exactly the same as
the procedure outlined
above for a full load check, except that the meter under test now runs
at a speed which is only
10% of its speed at full load. So as not to make the time required for
testing too long, 2*
revolutions of the meter under test will usually suffice.
The procedure for obtaining a power factor check is exactly the same
as the procedure outlined
for a full load check, except the power factor must be set to 0.5
which will cause the meter to
run at 50% of its speed at full load.
It is important that an “As Found” test be made on the meter without
any cleaning or adjustment,
since in case of a dispute with a customer it is essential for the
power company to know exactly
what condition the meter was received in before adjustments were made.
For this reason, an
“As Found” test is made without touching the meter more than is
absolutely necessary in order
to make the electrical connections required for test.
When the “As Found” test has been completed, worn and defected parts
of the meter should be
replaced.
The procedure for the “As Left” test, both full load, power factor,
and light load, is the same as
for the “As Found” test, as far as connections and calculations are
concerned.
BULLETIN 102
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Document Revision 1.0
The meter is run at full load current (TA), and the necessary
adjustment made to the drag
magnets to bring the percent registration of the meter at full load
within the prescribed limits.
After the full load check is completed, the meter is tested on light
load and power factor and the
necessary adjustments are made to bring the loads within the
prescribed limits.
The “As Left” test is made with the register in place on the meter.
Detailed procedures for meter testing can be found in the Handbook for
Electricity Metering, 8th
edition, chapter 15 or the 9th edition, chapter 14.
( * Less revolutions are possible when using solid state reference
standards.)
INTERPRETATION OF “AS FOUND” TEST RESULTS
Of course, once the “As Found” tests are made they must be interpreted
as to how to proceed.
If the results are within acceptable limits the test may be complete.
If, however, the results are
outside the acceptable limits further action is necessary. Below are
listed some common
conditions that are found in ferrous meters and their possible causes.
FOR FERROUS (DISK TYPE) METERS ONLY
CONDITIONS FOUND POSSIBLE CAUSES
1. Meter slow principally at light load.
2. Meter slow full load and light load.
3. Meter fast principally at light load.
4. Meter fast full load and light load.
5. Meter creeps but is correct on full and light
loads.
6. Meter creeps either forward or backward and
is either fast or slow on light load.
7. Meter slow on full and light loads, much
faster on loads of low power factor.
8. Disk revolves but meter does not register.
1. Inaccurate previous adjustment: friction or dirt in
register, or on magnet, worm, worm wheel, or
upper or lower bearing.
2. Inaccurate previous adjustments, iron filings in
magnet gaps; ground or short circuit in current
electromagnet.
3. Inaccurate previous adjustment; disappearance
of friction which has formerly been compensated
for by light load adjustment.
4. Inaccurate previous adjustment; weakened
permanent magnet.
5. Presence of excessive friction which has been
compensated for by changing the light load
adjustment instead of removing the friction.
6. Short circuit in voltage electro-magnet; inaccurate
previous adjustment.
7. Short circuit in current coil.
8. Worm or worm wheel out of mesh; dogs at rear of
register out of mesh, defective register.
Bulletin 102
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Document Revision 1.0
METER ADJUSTMENTS IN SOLID STATE METERS
Solid state meters are typically not adjustable for calibration by the
utility. If a solid state meter
test falls outside of acceptable limits, first make sure the meter was
put into the test mode. Next
the meters programming should be checked for accuracy, particularly
the test pulse value. If
the programming is ok, the testing equipment should be checked for
proper programming and
accuracy. If the test equipment is found to be programmed correctly
and in calibration; and the
meter continues to test out of acceptable limits, it should be
returned to the manufacture for
repair or replacement.
Solid state meters will some day be self adjusting. The meter will
control the test via a solid
state test bench and will make the necessary calibration adjustments
in its own software.
METER ADJUSTMENTS IN FERROUS METERS
Full Load Adjustment... The full load adjustment is made by varying
the amount of damping flux
passing through the disk. In modern meters this is done by a steel
screw mounted between the
pole faces of the damping magnet which, depending on its position,
shunts more or less flux
resulting in the speeding or the slowing of the disk. In older meter
designs, the magnet position
was changed to accomplish the same result.
Light Load Adjustment... The light load adjustment is made by varying
the amount of light load
compensating torque. These adjustments are most commonly screws or
wheels which, when
turned, shifts a coil so that its position with respect to the element
potential coil pole is changed.
When this coil is shifted, torque is produced in the meter disk which
will turn the disk in the
direction of the shift. Over adjustment of the light load may result
in “creep” which is a condition
where the meter disk rotates with applied voltage and no applied
current.
Lag or Power Factor Adjustment... The lag adjustments are normally
made only in the meter
shop. These adjustments establish the flux produced by the potential
coil to lag the flux
produced by the current coil by exactly 90°. Some older meter designs
used a coil with exposed
pigtail ends that were soldered so as to lengthen or shorten the
overall length of the coil,
thereby changing its resistance. Other designs used a lag plate which
was adjusted by a screw.
In most modern single phase meters the lag adjustment is made by
punching a lag plate during
the manufacturer’s testing. This type cannot be easily adjusted in the
field.
Element Balance... A polyphase meter is actually two or three single
phase meters sharing the
same disk. The result shown by the register of a polyphase meter is,
of course, the polyphase
watthours or the sum of the individual phase energies. It is therefore
necessary to make certain
that all phase elements are calibrated correctly.
This is accomplished by testing and calibrating each element of a
polyphase meter so that the
individual elements are as equal as possible for each load point.
After the elements are balanced, a series element test is made to
verify the summing of the
watthour meter.
A more complete description of these adjustments can be found in the
Handbook for Electricity
Metering, 8th edition or 9th edition, chapter 7.
Bulletin 102
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Document Revision 1.0
MECHANICAL REGISTER INSPECTION
The register is the counting or totalizing device which ultimately
translates the revolutions of the
meter into kilowatt-hour readings. It is important for the register to
function properly as for the
meter to rotate at the proper speed. The following points concerning
the register should be
noted:
Friction... The projecting dog or gear which meshes with the meter
disk drive should be spun
around rapidly with one finger. It should move smoothly and easily.
Meshing... With the register in place on the meter, there should be a
slight amount of “wiggle” or
backlash between the first register gear and the worm or pinion on the
meter disk shaft, so that
there will be no friction due to binding.
Register Ratio... All registers are marked with a number which is
known as the “register ratio”.
This ratio is the speed reduction between the first or meshing gear of
the register and the units
dial of the register. To actually check the ratio of the gearing
within a register, special laboratory
facilities are required. The tester in the field must necessarily
assume that the register ratio has
the value which is marked on the register, and it is the function of
the tester to see that the value
agrees with the rated capacity of the meter.
RECORDING TEST RESULTS
After the meter has been cleaned, calibrated, and the register
inspected, an “as left” test should
be run. The result should be recorded together with the serial number,
date tested, tester, test
equipment number, and in many cases the “as found” data for the meter
history file. In the more
sophisticated testing equipment this may be done automatically. In
other cases the procedure
outlined by your meter shop foreman should be used.
The meter cover should now be replaced and sealed so that the meter is
ready for installation
on a customer’s service.

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