D'ARSONVAL GALVANOMETER
CONSTRUCTION
• It is a permanent magnetic material.
• It consists of various diameters around the original metal, usually made of aluminu
m, removed from the pole of the permanent magnet.
• The coil is connected to a thin needle that moves on the measuring scale.
• A small torsion spring pulls the coil and points to the zero position.
• The steering wheel moves in a wide range. Generate a radial magnetic field up to 1.
5 mm.
• The coil is also connected to the mirror, which reflects light from a distance, eliminat
ing parallax and increasing sensitivity.
• Torque head is provided for coil position adjustment and zero adjustment.
PRINCIPLE
The working principle of the Darsenval galvanometer is based on the principle of the
motor running on the machine that carries the current in the magnet. When current
flows through the coil, it creates a magnetic field that interacts with the magnetic
field of the permanent magnet. This creates torque in the steering wheel, causing it
to turn. The amount of rotation is proportional to the current in the coil.
WORKING
When the measured current passes through the coil, the coil deviates from its zero
position. The torsion spring applies a restoring torque on the coil and thus measures
the deflection torque. A pointer connected to the coil indicates the amount of
deviation of the calibrated scale with respect to current or voltage. When the current
is interrupted, the coil returns to its zero position. The sensitivity of the galvanometer
can be increased by increasing the number of changes in the coil, the strength of the
permanent magnet, or the length of the air gap. By connecting a shunt or series
resistor, the D'Arsonval galvanometer can be converted into an ammeter or
voltmeter.
THREE WATTMETER METHOD OF
POWER MEASUREMENT
The three-wattmeter method is a technique for measuring power in a threephase system using three separate wattmeters. Three wattmeters are connected so that ea
ch one measures the power of one phase, and the readings of the three wattmeters are ad
ded to obtain the total power. This formula is based on Blondel's theorem, which states t
hat the number of wattmeters required to measure power in a Kirschner wire AC system is
k-1, regardless of whether the load is balanced or unbalanced.
The three-wattmeter method can be used for both four-wire and threewire systems, but is generally used on four-wire systems with starconnected loads. The circuit diagram and formula to calculate the total power using the thre
e wattmeter method are as follows:
P total = W1 + W2 + W3
Where,
W1 = Wattmeter reading 1 = V1 I1< br> > W2 = Wattmeter reading 2 = V2I2
W3 = Wattmeter reading 3 = V3I3
V1, V2, V3 = Line voltage
I1, I2, I3 = Current phase
Advantages of our wattmeter:
Balanced and can measure power in unbalanced products.
Can measure the power of four-wire and three-wire systems.
The energy used by each stage can be measured separately.
The disadvantages of the three wattmeter method are:
It requires three wattmeters, which is more expensive and more than one or two wattmeter
s.
Requires four cables, which some machines do not have.
If the midpoint is not zero potential, it will give incorrect results.
TWO WATTMETER METHOD OF
POWER MEASUREMENT
The two-wattmeter method is a technique that measures the power used by a threephase load. It uses two wattmeters, which are devices that measure voltage and current. Con
nect two wattmeters to two of the three wires of the threephase system and add their readings to get the total power.
The two wattmeter method is valid for both balanced and unbalanced and for both star an
d delta connections. The advantage of this method is that it does not require a neutral or thr
eephase wattmeter, which can be expensive or unavailable. Its disadvantage is that it may give
erroneous results if the load is inductive or capacitive or if the voltage is unbalanced.
The two wattmeter measurement method is based on the following formula:
𝑃 = √3(𝑉𝑅𝑌 𝐼𝑅 cos⁡ 𝜙𝑅 + 𝑉𝑌𝐵 𝐼𝑌 cos⁡ 𝜙𝑌 )
where P is the total power, VRY and VYB are the line voltages, IR and IY are the line currents,
and ϕR and ϕY are the phase angles between the line voltages and currents. The two
wattmeters measure the terms inside the parentheses, and their sum gives the total power.
The power factor of the load can be calculated from the ratio of the two wattmeter readings.
ONE WATTMETER METHOD OF
POWER MEASUREMENT
If the 3-phase load (Y or A) is balanced, then one wattmeter is sufficient to measure the
power drawn by the load. There are two methods in which one wattmeter may be applied.
In this method, two readings of the two-wattmeter method are taken with a single
wattmeter as shown in Fig. (b). The current coil of the wattmeter is connected in any one line
and the pressure coil is connected alternately between this and the other two lines. The
algebraic sum of the two readings gives the total power drawn by the balanced 3-phase
load.
A balanced load is a load that draws the same current from each phase of the three-phase
system, while an unbalanced load has at least one of those currents different from the rest.
In balanced 3-wire, 3-phase load circuit the power in each phase is equal. Therefore, the total
power of the circuit can be determined by multiplying the power measured in any one phase
by three.
Total power in balanced load = 3 x Power per Phase
= 3 x Wattmeter reading
MEGGER
Megger is a device that measures the insulation resistance of circuits and equipment. It appli
es high voltage to a circuit and measures the amount of current flowing through the insulato
r. This helps identify potential problems with the insulation, such as damage or deterioration.
Megger is also the name of a company that manufactures various electrical equipment such
as battery impedance testers, cable fault locators, circuit breaker analyzers and more. A meg
ohmmeter usually consists of a DC generator and an ohmmeter. The generator provides a co
nstant voltage for voltage measurement and the ohmmeter measures resistance by compari
ng voltage and current. Resistance is indicated by the needle on the scale and is measured in
megaohms. Meggers can be used in many applications such as testing cables, transformers,
motors, generators, switchgear and multimeters.
Working Principle of Megger
Megger works on the principle of electromagnetic attraction. The centrifugal clutch on the
generator is the mechanism that provides the constant for the insulation test. The insulation
with low resistance is tested using a steady voltage. The below figure shows the Megger
meter circuit diagram.
Structure of Megger
Megger consists of ohmmeter and DC generator. The megaohmmeter has two voltage coils
V1 and V2 and a current coil. The current drawn by the circuit under test will pass through th
e deflection yoke or current coil in series. The pressure coil, sometimes called the control coil,
is connected to the generator.
Battery or DC Generator Connection: Connect the generator to generate electricity. To pro
duce reverse torque, the deflection coil and the control coil must be placed at right angles to
each other and parallel to the generator.
Permanent Magnets: They use magnets to create a magnetic field to change hands.
Pointer: A pointer has two ends; one is where it is connected to the coil and the other is whe
re it is deflected from infinity to zero.
Scale: There is a scale on the front of the megaohmmeter that allows us to read the value fro
m "zero" to "infinity".
Uses of Megger
The following are the uses of megger,
o
o
o
o
o
It is used to measure an electrical wires insulation resistance
It is used to evaluate electrical components and systems.
Megger is also used for installing winding.
These are used in the testing of Battery, relay, ground connection, and other
tests
It is used as a Megger Earth tester.
LOADING EFFECT OF
OHMMETER
Loading Effect on Voltmeter:
Loading effect is the degree to which the meter affects the electrical properties of the circuit
such as voltage, current and resistance.
Generally speaking, the resistance of an ideal voltmeter is infinite, so the voltmeter does not
change the circuit current.
But in the real world, the resistance value of the voltmeter is not the final value, only the high
value.
If we use a voltmeter in a small circuit, there will not be much change in the number of circui
ts.
If we use a voltmeter in a high resistance circuit, the resistance of the voltmeter is less compa
red to the resistance of the circuit and this will act as a shunt path to the current and subseq
uently cause the voltage drop across the resistor. measurement. It will be less. Therefore the
voltmeter's reading will not be the actual voltage drop, but will be lower than the actual volta
ge drop before connecting the voltmeter. This is called the loading effect of the voltmeter. Si
nce the voltmeter produces deviation by measuring a portion of the load current drawn by t
he circuit under test.
In order to reduce the voltmeter load, the current required for the operation of the voltmeter
must be reduced.
Loading Effect of Ammeter:
We know that a good ammeter has its own flaws. Internal resistance indicates sensitivity. If th
e internal resistance of the ammeter is small, its sensitivity will be high, but if its internal resis
tance is large, its sensitivity will be low. Therefore, when a low-precision (highpressure) ammeter is placed in series between the source and the load, the total resistance o
f the circuit will increase and therefore the current flow will decrease. Therefore, the ammeter
cannot measure the actual current or the current drawn in the load.
For example, if we connect a 50 ohm ammeter in series to a 100 ohm circuit. The total resista
nce of the circuit is 150 ohms; This is 3 times the internal resistance of the ammeter. Therefor
e, it will definitely measure current and voltage.
AC BRIDGES : MAXWELLS BRIDGE
Maxwell bridge is an AC bridge in which four arms are connected together in a diamond or s
quare shape. This bridge has a resistor in two arms, one arm has a series of resistors and an i
nductor, and the other arm has an equivalent combination of resistors and capacitors.
AC detectors and AC voltage sources are used to find the value of unknown impedance. That
is, place one of them on one diagonal of the Maxwell bridge and the other on the other dia
gonal of the Maxwell bridge.
Maxwell bridge is used to measure the inductance value of the medium. The electrical diagra
m of the Maxwell bridge is shown in the figure below.
AC BRIDGES : HAY BRIDGE
The HAY bridge is a modification of the Maxwell bridge; We achieved this by transforming th
e arm into the arm of the Maxwell bridge with a connection consisting of resistors and capaci
tors.
HAY bridge is used to measure high inductance. The circuital diagram of the Hay bridge is sh
own in the figure below.
AC BRIDGES : SCHERING BRIDGE
This bridge is used to measure to the capacitance of the capacitor, dissipation factor and
measurement of relative permittivity. Let us consider the circuit of Schering bridge as shown
below:
Here, c1 is the unknown capacitance whose value is to be determined with series electrical
resistance r1.
c2 is a standard capacitor.
c4 is a variable capacitor.
r3 is a pure resistor (i.e. non inductive in nature).
And r4 is a variable non inductive resistor connected in parallel with variable capacitor c4.
Now the supply is given to the bridge between the points a and c. The detector is connected
between b and d. From the theory of ac bridges we have at balance condition,
AC BRIDGES : WEIN BRIDGE
The Wien’s bridge use in AC circuits for determining the value of unknown frequency. The
bridge measures the frequencies from 100Hz to 100kHz. The accuracy of the bridges lies
between 0.1 to 0.5 percent. The bridge is used for various other applications
like capacitance measurement, harmonic distortion analyser and in the HF frequency
oscillator.
When the bridge is in the balanced condition, the potential of the node B and C are equal,
i.e., the V1 = V2 and V3 = V4 The phase and the magnitude of V3 = I1R3 and V4 = I2R4 are
equal, and they are overlapping each other. The current I1 flowing through the arm BD and
the current I2 flowing through R4 is also in phase along with the I1R3 and I2R4.
The total voltage drop across the arm AC is equal to the sum of the voltage drop I2R2 across
the resistance R2 and the capacitive drop I2/wC2 across the capacitance C2. When the bridge
is in a balanced condition, the voltage V1 and V2 both are equals in magnitude and phase.
The phase of the voltage V1 and the voltage drop IRR1 across the arms R1 is also same. The
resistance R1 is in the same phase as that of the voltage V1. The phasor sum of V1 and
V3 or V2 and V4 will give the resultant supply voltage.
At balance condition,
On equating the real part,
On comparing the imaginary part,
By substituting the value of ω = 2πf,
The slider of the resistance R1 and R2 mechanically connect to each other. So that, the R1 =
R2 obtains.