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-11 (Edited)
Instructor:
Mohd. Umar Rehman
EES, University Polytechnic, AMU
May 27, 2021
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2. Unit-IV: Electrical Measurements & Instruments
Essential Features of Instruments—Deflecting Torque, Controlling Torque & Damp-
ing Torque, Moving Coil & Moving Iron Instruments, Advantages, Disadvantages &
Applications. Energy Meter & Megger.
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3. Essential Features of Indicating Instruments
An indicating instrument essentially consists of a pointer that moves over a
graduated scale. It is known as Moving System of an instrument.
To ensure proper operation of the instrument, the following three torques are
required:
1. Deflecting (or Operational) Torque, TD
2. Controlling Torque, TC
3. Damping Torque, Td
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4. 1. Deflecting Torque
When an instrument is connected in a circuit to measure an electrical quantity,
a reading is obtained when the pointer moves over the scale
To move the pointer, a deflecting torque is required.
This is achieved by utilizing the various effects of current/voltage.
The deflecting torque causes the pointer to move from zero position to the
value of reading of the measured quantity.
The actual method of producing deflection torque depends upon the type of
the instrument.
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5. 2. Controlling Torque
If only deflection torque is present in the instrument, pointer would move indef-
initely and would swing over to the maximum deflection position and will not
stop at the intended position.
Therefore, in order to stop the pointer at the correct position, there should be a
controlling torque present that balances the deflecting torque.
The pointer will stop at the reading value when the two torques are equal, i.e.
TD = TC
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6. 2. Controlling Torque...Contd
There are two functions of the controlling torque:
1. It increases with the deflection of the moving system, so that the final position
of the pointer on the scale is according to the magnitude of the quantity under
measurement.
2. It brings the pointer back to the zero position when the deflecting torque is
removed. In absence of controlling torque, pointer would not return to the zero
position.
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7. 3. Damping Torque
Suppose there are only deflection and controlling torque in the instrument.
Then due to inertia, the pointer will oscillate about its final deflecting position
for quite some time before coming to rest
This is an undesirable phenomenon, and makes reading the value difficult.
Damping torque acts only when the pointer is moving and always opposes the
motion.
Damping is of three types:
(a) Critical Damping
(b) Over-damping
(c) Under-damping
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8. 3. Damping Torque
The deflection v/s time graph for three damping cases is shown below:
Damping can be provided by the following methods:
(i) Air friction damping
(ii) Fluid friction damping
(iii) Eddy current damping
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9. Permanent Magnet Moving Coil (PMMC) Instruments
These instruments are suitable for DC measurements only.
Can be used both as voltmeter and ammeter.
Principle: When a current carrying conductor is placed in a magnetic field, it
experiences a force.
In these instruments, a coil is placed in a magnetic field that carries the oper-
ating current.
This coil is attached to the moving pointer, when current flows through the coil,
it moves and along with it the pointer also moves indicating the reading.
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10. Construction of PMMC Instruments
It consists of light rectangular coil of fine wire wound on an Aluminium former,
inside which is an Iron core.
The coil is delicately pivoted on jewel bearings and is mounted between the
poles of a permanent magnet.
To these poles, two soft iron pieces are attached which concentrate the mag-
netic field.
The current passes through the coil through two control springs as shown.
These springs also provide controlling torque.
Eddy current damping is provided.
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11. Construction...Contd
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12. Construction...Contd
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13. Working of PMMC Instruments
When the PMMC instrument is connected in the circuit to measure voltage or
current, the operating current flows through the coil.
The coil is placed in the magnetic field and due to the current flowing in it, a
force (torque) is exerted on it.
As a result, the pointer attached to the moving system also moves in the clock-
wise direction over a scale to give an indication of the reading.
If the current in the coil is reversed, the deflecting torque also reverses.
Since, TD reverses with the reversal of current and also a mechanical pointer
cannot respond to high frequency of AC (50 Hz), therefore a PMMC instrument
cannot be used for measurement of AC.
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14. Expression for deflecting torque
Let
B = Flux Density, Wb/m2
l = Length of coil, m
b = breadth of coil, m
N = No. or turns of coil
If a current I amperes flows through the coil, then the force (in newtons) acting on
each side is given by
F = NIlB
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15. Expression for deflecting torque...Contd
Deflecting Torque,
TD = Force ×Perpendicular distance
= NIlB×b
= NIBA
= (NBA)I
= KI
TD ∝ I
Since, Deflection is proportional to deflecting torque, hence,
θ ∝ TD
θ ∝ I
Hence, deflection is proportional to current, and consequently the scale of MC
instrument is uniform.
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16. Advantages of PMMC Instruments
(i) Uniform scale, easier to read.
(ii) Effective damping
(iii) Very low effect of external magnet
(iv) High efficiency and reliability.
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17. Disadvantages of PMMC Instruments
(i) These instruments cannot measure AC quantities.
(ii) Cost of these instruments is high as compared to moving iron instruments.
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18. Moving Iron (MI) Type Instruments
This type of instrument is mainly used for the measurement of AC quantities, though
it can also be used for DC measurements.
There are two types of moving-iron instruments:
1. Attraction type: in which a single soft-iron vane (or moving iron) is mounted
on the spindle and is attracted towards the coil when operating current flows
through it.
2. Repulsion type: in which two soft-iron vanes are used; one fixed and attached
to the stationary coil while the other is movable (i.e. moving iron) and mounted
on the spindle of the instrument. When operating current flows through the coil,
the two vanes are magnetized, developing similar polarity at the same ends.
Consequently, repulsion takes place between the vanes and the movable vane
causes the pointer to move over the scale.
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19. Construction of MI Instrument
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20. Expression for Deflecting Torque (Attracting Type MI
Instruments)
The force F that pulls a soft Fe piece towards the coil is directly proportional to:
(i) Magnetic field strength H produced by the coil.
(ii) pole strength m developed in the iron piece.
Thus,
F ∝ mH
But, the pole strength developed is itself proportional to field strength.
m ∝ H2
Therefore,
F ∝ H2
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21. Expression for Deflecting Torque...Contd
Field strength is proportional to current H ∝ i, Thus,
F ∝ i2
And, Torque is proportional to force, so that
TD ∝ i2
Deflection,
θ ∝ TD ∝ i2
As can be observed from the above proportionality, that the deflection is
proportional to the square of the current, the scale of MI instruments is
non-uniform, crowded in the beginning and then spreads out.
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22. Advantages of MI Instruments
(i) These are less expensive, robust and simple in construction.
(ii) These can be used for both DC and AC measurements. However, when used
with AC, they are liable to small errors due to residual magnetism.
(iii) These instruments have high operating torque.
(iv) These instruments are reasonably accurate.
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23. Disadvantages of MI Instruments
(i) These instruments have non-linear scales.
(ii) These instruments are not as sensitive as the PMMC instruments.
(iii) Errors are introduced due to change in frequency in case of AC
measurements.
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