Download Analogue meters

Contents
Introduction
3
Analogue meters
4
Moving coil instruments
4
Moving iron instruments
8
Digital meters
11
The ohmmeter
13
The wattmeter
14
The kilowatt-hour meter
16
Summary
19
Answers
22
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
1
2
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
Introduction
The most common electrical measuring instruments are the voltmeter and
the ammeter. These devices can be constructed using a sensitive electromechanical ‘movement’ which responds to current. These so-called
analogue meters are still in widespread use today. More recently, electrical
meters can be constructed using an electronic module with no moving parts,
which responds to voltage, and drives a numerical (digital) display. These
are known as digital meters.
Power and energy also need to be measured, so that residences and
commercial premises can be billed for their use of electricity. In this section
you will learn about the wattmeter and the kilowatt-hour meter, and how to
use those instruments to measure power and energy consumption
respectively.
After completing this topic, you should be able to:

describe the construction of the two types of analogue meter movement

compare the operating characteristics of analogue and digital meters

connect instruments into a circuit to measure power or energy
consumption

calculate the costs of power consumption.
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
3
Analogue meters
An analogue meter indicates electrical values using a pointer moving across
a scale. These meters use the magnetic effect of electric current flowing
through a coil to move the pointer.
The analogue meter responds to dc current. However we can construct other
instruments including ac and dc ammeters or voltmeters, and ohmmeters,
from this basic meter movement by the addition of other simple
components.
There are two main types of analogue meter for general use: the moving coil
meter and the moving iron meter.
Moving coil instruments
The essential feature of this type of instrument is a small, light coil of
insulated copper wire pivoted or suspended between the poles of a
permanent magnet. This combination forms the most sensitive current
measuring instrument available, because very few refinements are needed to
enable it to indicate currents of one microampere or less.
The form used in most commercial types of permanent-magnet moving-coil
ammeters and voltmeters is shown in Figure 1. Alnico or similar permanent
magnet alloy is used to obtain a strong permanent magnet of small size.
Figure 1: Permanent-magnet moving-coil instrument
4
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
The permanent magnet
The permanent magnet, Figure 2(a) is fitted with accurately machined pole
pieces of soft iron between which a cylindrical-shaped core of soft iron is
inserted. The coil moves in the narrow air gap between the poles and the
core. Because of this construction, the magnetic field in the air gap is
uniform and the scale divisions can be equal.
The coil
The coil consists of a number of turns of fine-insulated copper wire wound
on a rectangular aluminium former. The complete unit is suspended so it can
rotate in the airgaps.
There are two methods used for suspending the moving coil:

jewel and pivot suspension (Figure 2(c))

taut-band suspension (Figure 3).
Figure 2: Jewel and pivot suspension
Figure 3: Taut–band suspension
In the jewel and pivot suspension, the coil former is supported at each end
by two hardened steel pivots which turn in jewel bearings. The hair springs,
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
5
which are attached to each pivot, provide the controlling torque and the
electrical connections to the coil. The jewel and pivot method of suspension
is shown in Figure 2(c).
In the taut band method of suspension, the moving-coil assembly is attached
at either end to a ribbon of platinum-nickel that is lightly tensioned. These
taut bands serve three functions. They:

support the moving coil assembly

provide the controlling torque

provide the electrical connections to the coil.
The taut-band method of suspension is much more shock and vibration
resistant than the jewel and pivot suspension.
In both types, the pointer is generally a long thin aluminium tube rigidly
attached to the moving system. The complete moving system is accurately
balanced.
Principle of operation
The action of the permanent-magnet moving-coil type instrument depends
on the magnetic effect of an electric current, the principle being similar to
that which is used in an electric motor, and is illustrated in Figure 4.
Figure 4: Action of permanent-magnet moving-coil type instrument
An electric current in the moving coil sets up a magnetic reaction with the
field of the permanent magnet. This reaction produces a deflecting torque
and the pointer is caused to move. As the strength of the magnetic field
throughout the air gaps is constant and uniform, the magnitude of the
deflecting torque then varies directly with the current in the moving coil.
If the direction of current in the moving coil is reversed, the coil will turn in
the opposite direction. Most meter scales are designed to display only
positive readings, and so will display a reading only when current is in the
6
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
correct direction. To ensure this correct direction, one terminal is marked ‘+’
(and/or coloured red) and the other is marked ‘– ’ (and/or coloured black).
An instrument which operates only with current in a particular direction is
referred to as ‘polarised.’
Because the direction of current is continually reversing in alternatingcurrent circuits (there are 100 reversals every second in a 50-hertz system),
an instrument of this type remains on zero if connected into an alternatingcurrent circuit. A rectifier can be incorporated into the meter to measure ac
current or voltage.
Two important things you should remember about the moving coil meter are
that:

the scale is linear,

the connections to the meter must be made with the correct polarity.
Deflecting torque and controlling torque
When a current passes through an analogue instrument, it produces a
deflecting torque which rotates the pointer upscale. This torque is dependent
upon the current value and it is generally produced by the magnetic effect of
current. If the moving coil instrument did not have springs acting in
opposition to the deflecting torque, the pointer would simply rotate to full
scale for any applied current.
A measurement is achieved because the springs provide a known controlling
torque for any pointer position. The controlling torque acts downscale
(tending to push the pointer toward zero), and increases in magnitude as the
pointer moves upscale. When a current is applied, the deflecting torque
moves the pointer to that position where the two torques are exactly
balanced (are ‘in equilibrium’). When the current ceases, the controlling
torque returns the moving system to its zero position. The principle is the
same as when you weigh yourself – the springs in the bathroom scales are
calibrated so that they are compressed to different extents by different
weights.
Damping
To achieve satisfactory operation, meters movements are damped, or
‘smoothed out’ to prevent overshooting or oscillation of the pointer.
This is accomplished by applying a damping torque, which always acts in
opposition to the direction of pointer movement, whether upscale or
downscale. This torque can be obtained in a few different ways. For
example a vane can be attached to the moving coil such that its movement is
impeded by the viscosity of the air.
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
7
Full scale deflection
The current required to cause the pointer of the permanent-magnet movingcoil type instrument to move to the limit of the scale is the full-scale
deflection current. As the resistance of the moving-coil circuit is usually
very low, a relatively small potential difference applied to its terminals will
produce full-scale deflection current.
Under no circumstances should the current in the moving coil exceed that
required for full-scale deflection. Otherwise, the coil will overheat and
‘burn-out’. When connecting an instrument in a circuit, care should be
exercised to avoid any incorrect connection which may possibly damage the
instrument.
Typical values of resistance and current for a moving-coil instrument are:
resistance of the moving coil circuit 5 ohm and full-scale deflection current
15 milliampere. The potential difference required across the moving coil for
full-scale deflection is then:
V  IR
15
=
5
1000
 0.075 V
=75 mV
Such an instrument could be used either as a milliammeter capable of
indicating currents to 15 milliampere or as a millivoltmeter capable of
indicating potential differences to 75 millivolt.
To extend the range of a moving-coil instrument, that is, to use it to measure
currents or voltages greater than the initial range, resistors must be added to
prevent damage to the moving coil by excessive current.
To measure higher currents than the initial range, a resistor is added across
(in parallel with) the meter movement. The resistor in this case is known as
a shunt.
If the meter is to operate as a voltmeter, a resistor must be added in series
with the meter. In this case, the resistor is known as a multiplier.
Moving iron instruments
Moving iron instruments are the second broad type of analogue meter. They
have this name because instead of a coil, they have a piece of iron attached
to the pointer, which moves in response to the current. There are two types
of moving iron instrument:

8
the attraction type
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669

the repulsion type.
Both types have a non-linear scale because the deflection of the pointer is
proportional to the square of the current flowing through the coil.
Both types can be used for ac or dc without any change in internal circuitry,
however the meter is most useful for ac measurements, because of its ability
to read true rms (root mean squared) values.
Note: you will encounter rms values later in your course of study, but briefly
an rms value is the ‘effective’ value of an ac voltage or current which can be
used for power calculations.
Attraction type moving iron meter
The principle of operation is shown in Figure 5.
The meter essentially consists of a:

soft iron vane to which a pointer is attached and the assembly supported
at a pivot point

spring which supplies the retarding or controlling torque

coil in the form of a solenoid through which the sample current flows

non linear scale.
When a current flows through the coil it creates a magnetic field which
attracts the iron vane. The amount of magnetic pull being exerted on the
vane is proportional to the square of the current flowing in the coil. For
example, if the current in the coil is doubled then the deflecting torque
is increased by a factor of four.
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
9
Figure 5: Attraction type moving iron meter
Repulsion type moving iron meter
The principle of operation is shown in Figure 6.
The meter operates by two similarly magnetised pieces of soft iron repelling
each other.
A piece of soft iron is attached to the spindle carrying the pointer, and
another piece is fixed to the coil bobbin. When current flows in the coil,
the two the two pieces of iron are magnetised in the same direction and
therefore repel each other. The pointer is moved by an amount which is
proportional to the square of the current flowing in the coil.
Figure 6: Repulsion type moving iron meter
10
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
Digital meters
The basic operating principle of a digital meter is shown in Figure 7. Unlike
the analogue meter movement which is current operated, the digital meter
responds to voltage. Also unlike the analogue meter movement which
instantaneously converts an input current into a pointer movement responds
almost immediately to any change in input voltage. Another major
difference is that a digital meter will not be damaged if the wrong polarity is
applied to the probes. This is sometimes known as the ‘autopolarity feature’
and the meter merely displays a negative value.
In operation, the digital meter continually samples the input voltage and
converts each value to a digital quantity (that is, a number) before displaying
it on the numerical display. This sampling and conversion process takes
perhaps 100 milliseconds to complete.
The heart of the digital voltmeter is the analogue to digital converter or
ADC. These devices are integrated circuit chips. The operation of a very
simple type of ADC is as follows:

During the sampling period a series of pulses is generated, where the
number of pulses per second is proportional to the magnitude of the
voltage being measured. For example, a 200 mV signal might result in
2000 pulses per second.

A counter counts these pulses for the duration of the sampling period. So
for if the sampling period is 0.1 second, there would be 200 pulses
counted for this 200 mV signal. This accumulated count is displayed on
the digital display.

The counter is then reset to zero and another sampling period started.

While sampling is in progress the meter displays the count for the
previous sampling period—it is for this reason that digital meters do not
respond quickly to a changing input voltage.
This description is for conceptual purposes only. In practice, more complex
types of ADC are used where the input can be compared to a known
reference voltage.
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
11
Figure 7: Digital sampling in a digital meter
The digital meter module described above forms the core of digital
voltmeters, ammeters and multimeters.
If you have Hampson, see pages 41 and 42 for a discussion of analogue and
digital meters.
If you have Jenneson, refer to Section 7.8.4, ‘The complete multimeter’ and
figure 7.20 for interesting extension material.
12
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
The ohmmeter
The Ohmmeter measures resistance, and this can be accomplished in several
ways.
By Ohm’s Law, we can calculate the resistance if we know the voltage
across the resistance and the current through the resistance. This fact allows
us to measure resistance by:

Applying a known voltage, and measuring the current that results, or

Using a voltmeter and an ammeter and combining the results.
The first method is used in multimeters, where the battery within the
instrument provides known voltage source. The ammeter scale is marked to
indicate resistance. Note that zero resistance gives maximum current, so
zero ohms will appear on the right side of the scale. This method suffers
from inaccuracies caused by variations in battery voltage, and from
interpolating a non-linear scale, but is usually accurate enough for
measurements in the field.
The second method can be used to obtain a more accurate measurement if a
voltmeter and an ammeter are available.
The most accurate method of resistance measurement uses a bridge circuit
(the Wheatstone Bridge) that allows the unknown resistance to be directly
compared to a an accurately known resistance.
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
13
The wattmeter
A wattmeter combines voltage and current measurements to achieve a
measurement of the power delivered to a circuit.
The electrodynamometer type wattmeter consists of a moving coil and a
fixed coil. The moving coil is the voltage or potential coil, and like a
voltmeter it is connected across the supply. The fixed coil is the ammeter, or
current coil, and is connected in series with the load.
Read page 78 of Hampson for a brief coverage of wattmeters.
Refer to Appendix 1 of Jenneson on page 433 for power and energy meters.
Figure A1.1 shows the basic construction of a moving coil wattmeter. You
will notice that the pointer is directly connected to the moving potential coil,
and the heavier wound current coil is fixed to the meter frame. The current
coil is in two sections that are normally connected in series with each other.
The electromagnetic fields produced by the two coils (current and voltage)
will cause the potential coil, and hence the pointer, to move. This
movement, or ‘deflection’, indicates the amount of true power consumed.
The amount of deflection depends on the strength of the two fields, so that
the total magnetic effect is the product of the two fields.
A wattmeter indicates the product of circuit voltage and current.
The terminals of a wattmeter are usually labelled as:
14

V1 and V2 for the potential coil

M and L for the current coil. (The ‘M’ represents the mains terminal,
and the ‘L’ represents the load terminal.)
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
Figure 1 shows a wattmeter connected into a circuit to measure the power
consumed. Note the symbols used to represent the high resistance potential
coil and the low resistance current coil.
Figure 1: Wattmeter connection in a circuit
You can see from Figure 1 how the:

current coil is connected in series with the load to measure circuit
current

potential coil is connected across the supply to measure the supply
voltage. This will indicate the power consumed by the single-phase load
shown here.
It is most important that the coils have the correct connections, otherwise the
pointer will deflect downscale.
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
15
The kilowatt-hour meter
The energy used in residential or commercial premises is recorded on
kilowatt-hour meters. Usually the people who pay the account are referred to
as consumers and the organisation that supplies the energy is referred to as
the supply authority.
A kilowatt-hour meter is referred to as an integrating instrument, because it
adds up, or ‘integrates’ the usage over time. The symbol is shown in
Figure 2. Note that the connections to the instrument are exactly the same as
the connections to a wattmeter.
Figure 2: A kilowatt-hour energy meter connected to determine the energy supplied
to the resistor load
Energy is supplied at a certain cost per kilowatt-hour. The cost may vary
greatly, depending upon the type of consumer and the use to which the
energy is applied. In addition, most supply authorities have a sliding rate
of charges where a certain amount of energy is supplied at the highest rate,
another amount of energy at a lower rate and the remainder at a lower rate
still. Energy for special purposes, for example off-peak heating of hot water,
is charged for at a very low tariff.
Example 1
A supply authority had a scale of charges for domestic consumers as
follows. For each two months of supply:
16

first 133 kilowatt-hours of general use—12 cents per kilowatt-hour

remainder of kilowatt-hours of general use—8.7 cents per kilowatt-hour

off-peak hot water heating—3.9 cents per kilowatt-hour.
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
A consumer’s energy reading for two months is found to be:

general use—708 kWh

off-peak hot water heating—456 kWh.
How much is the consumer charged for the two-month period?
For the first 133 kWh of general use:
cost  rate  kWH
 0.12 133
(rate is converted from cents to
dollars by dividing by 100)
 $15.96
For the remainder of general use:
708  133  575 kWH
cost  rate  kWH
 0.087  575
 $50.03
(rate is converted from cents to
dollars)
For the hot water heating:
cost  rate  kWH
 0.039  456
(rate is converted from cents to
dollars)
 $17.78
Total account for the two-month period:
First133kilowatt-hours:
$15.96
Remainder of general kilowatt-hours: $50.03
Hot water heating:
$17.78
$83.77
The total cost of energy over the two-month period is $83.77.
Note: The charges for energy will vary from those shown in the above
example so you should check the tariff rates of your local supply authority if
calculating energy charges.
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
17
Activity 1
1
Write down the names by which the user and supplier of electrical energy are generally
known.
_____________________________________________________________________
_____________________________________________________________________
2
A domestic consumer has an energy consumption of 1085 kWh for a period of 12
weeks. Energy supplied by the local supply authority is charged for at the following
rates:

first 200 kWh—11.5 cents per kWh

next 400 kWh—8 cents per kWh

remainder of kWh—6.42 cents per kWh
What will the energy cost the consumer?
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
Check your answers with those given at the end of the section.
If you have Hampson, read the ‘Watt –hour meter’ section on pages 207 and
208.
If you have Janneson, refer to Section 7.6 on pages 137 for interesting
extension material.
18
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
Summary

Analogue reading meters are in two general categories—moving coil and
moving iron.

Moving coil meters will work only on DC.

Moving coil meters have a linear scale.

Moving iron meters will work on either DC or AC.

Moving iron meters generally have a non-linear scale.

Both moving iron and moving coil meter movements respond to current
(they are low resistance instruments).

Digital meter movements respond to voltage (they are high resistance
instruments).

Moving iron, moving coil and digital meter movements can be adapted
to read voltage or current.

Digital voltmeter modules have a high input resistance usually 10 M.

Analogue voltmeters have an input resistance that is dependent on the
voltage range and is specified in kilo Ohm per volt.

A wattmeter is an instrument used for measuring power in a circuit.

A kilowatt-hour meter is an instrument used for measuring energy.
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
19
Check your progress
In Questions 1–10 only one of the suggested answers is current. Place the letter
corresponding to your answer in the brackets provided.
1
An ideal voltmeter has a resistance of:
(a) infinity
(b) 10  to 100 
(c) 1000  to 10 000 
(d) 1 M  to 5 M.
4
( )
The input resistance of a typical digital multimeter set to voltage range is:
(a) 1 M 
(b) 10 M 
(c) zero
(d) infinity.
5
( )
The ‘autopolarity’ feature of a digital multimeter means that the:
(a) battery powering the meter can be connected either way
(b) meter will display the correct reading regardless of the probe polarity
(c) meter will be destroyed if the input has the wrong polarity
(d) meter can read AC current when set to its DC current range.
6
20
( )
Complete the following circuit to show a wattmeter measuring load power
consumption.
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
7
A domestic consumer uses 850 kWh over the billing period of 13 weeks. Calculate the
cost, given the following rates:
first 250 kWh
16 cents per kWh
next 500 kWh 11.5 cents per kWh
remainder
9.2 cents per kWh
_____________________________________________________________________
_____________________________________________________________________
8
In an analogue wattmeter the current coil is internally connected between terminals:
(a) M and V1
(b) M and L
(c) M and V2
(d) L and V1
9
What does a kilowatt-hour meter measure?
(a) power
(b) energy
(c) torque
(d) force
Check your answers with those given at the end of the section.
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
21
Answers
Activity 1
1
The user is called the consumer; the supplier is called the supply
authority.
2
200 11.5
100
 $23
400  8
=
100
= $32
First 200 kWh @ 11.5 cents/kWh =
next 400 kWh @ 8 cents/kWh
remainder
= 1085  600
 485 kWh @ 6.42 cents/kWh
485  6.42
100
 $31.14

The total energy cost for 12 weeks is $86.14.
Check your progress
22
1
(a)
2
(c)
3
(b)
4
(b)
5
(b)
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
6
7
$10.67
8
(b)
9
(b)
EEE042A: 14 Describe types of electrical meters
 NSW DET 2017 2006/060/05/2017 LRR 3669
23