Lecture Slides for the course Fundamentals of Electrical Engineering delivered by me to First Semester Diploma in Mechanical Engineering.

1. PEE-102A
Fundamentals of Electrical Engineering
Lecture-12
Instructor:
Mohd. Umar Rehman
EES, University Polytechnic, AMU
May 27, 2021
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2. Energy Meter
(a) Induction type (b) Digital Energy Meter
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3. Introduction
Single-phase induction type energy meters are extensively used for the mea-
surement of electrical energy in AC circuits. One can find such meters installed
in homes.
An energy meter is basically a wattmeter (instrument that measures electrical
power) fitted with some recording mechanism. An energy meter is also called
a kWh meter, i.e., kilo Watt hour meter.
The meter which is installed at the main distribution board of power supply at
residences is a kWh meter.
The energy consumed over a period of time, say two months, is recorded in the
form of digits. The electricity supply authority prepares the energy consumption
bill according to the consumption made.
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4. Construction of Induction Type Energy Meter
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5. Construction...Contd I
The above figure shows the various parts of a single-phase induction type energy
meter.
(i) It consists of (a) two AC electromagnets ; the series magnet and shunt magnet
(b) an aluminium disc or rotor placed between the two electromagnets (c) brake
magnet and (d) counting mechanism.
(ii) The shunt magnet is wound with a fine wire of many turns and is connected
across the supply so that it carries current proportional to the supply voltage.
Since the coil of shunt magnet is highly inductive, the current (and hence the
flux) in it lags the supply voltage by 90◦.
The series magnet is wound with a heavy wire of few turns and is connected in
series with the load so that it carries the load current. The coil of this magnet
is highly non-inductive so that angle of lag or lead is determined wholly by the
load.
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6. Construction...Contd II
(iii) A thin aluminium disc mounted on the spindle is placed between the shunt and
series magnets so that it cuts the fluxes of both the magnets.
(iv) The braking torque is obtained by placing a permanent magnet near the ro-
tating disc so that the disc rotates in the field established by the permanent
magnet. Eddy currents induced in disc produce a braking or retarding torque
that is proportional to the disc speed.
(v) A short-circuited copper loop (also known as power factor compensator) is
provided on the central limb of the shunt magnet. By adjusting the position of
this loop, the shunt magnet flux can be made to lag behind the supply voltage
exactly by 90◦.
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7. Theory
When induction type energy meter is connected in the circuit to measure en-
ergy, the shunt magnet carries current proportional to the supply voltage and
the series magnet carries the load current.
Average deflecting torque is proportional to the average power i. e.
TD ∝ Average Power
∝ VI cosφ
The braking torque is due to the eddy currents induced in the aluminium disc.
Since the magnitude of eddy currents is proportional to the disc speed, the
braking torque will also be proportional to the disc speed n i.e.,
Tb ∝ n
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8. Theory...Contd
For steady speed of rotation, TD = Tb
∴ Power ∝ n
. Multiplying both sides by t, the time for which power is supplied,
Power ×t ∝ nt
or Energy ∝ N
where, N = nt is the total number of revolutions in time t.
Since the product Nt represents the number of revolutions of the disc in time t
and the product Pt represents the energy passing through the meter in time t,
therefore, number of revolutions of the disc is proportional to the energy
passing through the meter.
The counting mechanism is so arranged that the meter indicates kilowatthours
(kWh) directly and not the revolutions.
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9. Course is Complete!
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