2. MEGGER
The term megger or megohmmeter refers to an
instrument for measuring high voltage electrical
insulation.
“Megger” is the brand name of the first portable
insulation measuring instrument.
Also, insulation resistance tester known as megger
is small in size, portable and popular
Practically, it is used in operation of continuity,
short circuit and open circuit tests of electrical
installation.
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8. A Megger has a crank shaft with clutch mechanism.
We can vary the speed of cranking with a help of
clutch.
This arrangement is located between magnets. The
whole setup is called DC Generator.
On the left side of the DC generator, there is a scale
showing the resistance values from 0 to infinity.
The circuit consists of two coils Coil-A and Coil-B.
Both the coils are connected to the DC generator
9. Megger operates on the principle that current
flowing through a conductor placed in a magnetic
field experiences a torque. Mathematically, the force
is proportional to the current and the magnetic flux.
Where, Vector Force = force and direction of the
current and magnetic field.
Case (I): If the resistance has a higher value then the
pointer on the device coil will move towards infinity.
Case(II): If the resistance value is low then the
pointer will indicate zero resistance.
10. WORKING
When the current flows from the generator,
through the pressure coil, the coil tends to set
itself at right angles to the field of the
permanent magnet.
When the test terminals are open,
corresponding to infinite resistance, no current
flows through deflection coil.
The supply to the meter is usually given by a
hand driven permanent magnet dc generator.
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12. Thus the pressure coil governs the motion of the
moving element making it move to its extreme
anticlockwise position.
The pointer comes to rest at the infinity end of the
scale.
When the test terminals are short circuited (zero
resistance), the current from the generator flowing
through the current coil is large enough to produce
sufficient torque to overcome the counter-clockwise
torque of the pressure coil.
Due to this, pointer moves over a scale showing
zero resistance.
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14. When the high resistance to be tested is
connected between terminals T1 and T2 the
opposing torques of the coils balance each other
so that pointer attains a stationary position at
some intermediate point on scale.
The scale is calibrated in mega ohms .
15. Advantages of Hand Operated Megger
Still keeps important in such high-tech world as it’s an oldest
method for IR value determination.
No external source required to operate.
Cheaper available in market.
Disadvantages of Hand Operated Megger
At least 2 person required to operate i.e. one for rotation of
crank other to connect megger with electrical system to be
tested.
Accuracy is not up to the level as it’s varies with rotation of
crank.
Require very high care & safety during use of the same.
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18. LCR meter
LCR meters can be understood as a multimeter,
this is because it can measure resistance,
inductance, capacitance as per the
requirement.
The significant component of LCR meter is
the Wheatstone bridge and RC ratio arm
circuits.
The component whose value is to be measured is
connected in one of the arms of the bridge. There
are different provisions for the different type of
measurements.
19. If the value of resistance is to be measured, then
Wheatstone bridge comes into picture while the
value of inductance and capacitance can be
measured by comparing it with standard capacitor
present in RC ratio arm circuit.
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21. Working of LCR Meter
The bridge is adjusted in null position in order to
balance it completely.
Besides, the sensitivity of the meter should also be
adjusted along with balancing of the bridge.
The output from the bridge is fed to emitter follower
circuit.
The emitter follower is widely used as a buffer
amplifier to reduce the loading on the previous
stage and provide a lower impedance output for
any following circuits.
22. The output from emitter follower circuit is given as
an input to detector amplifier.
The significance of detector amplifier can be
understood by the fact that if the measuring
signal is low in magnitude, it will not be able to
move the indicator of PMMC meter.
Thus, in order to achieve the sustainable
indication, we need to have a high magnitude
measuring signal.
The rectifier is used in the circuit to convert the
AC signal into DC signal.
When the bridge is provided with AC excitation
then at the output end of the bridge the AC signal
needs transformation into DC signal.
