Construction and Working Principle of Electrodynamometer Type Wattmeter

__Dynamo meter type watt meter__

In
general, a watt meter is used to measure the electric power of a circuit, or
sometime it also measures the rate of energy transferred from one circuit to
another circuit. When a moving coil (that is free to rotate) is kept under the
influence of a current carrying conductor, then automatically a mechanical
force will be applied to the moving coil, and this force will make a little
deflection of the moving coil. If a pointer is connected with the moving coil,
which will move of a scale, then the deflection can be easily measured by
connecting the moving coil with that pointer. This is the principle of
operation of all dynamo meter type instruments, and this principle is equally applicable
for dynamo meter type watt meter also.

This type
of watt meter consists of two types of coil, more specifically current coil and
voltage coil. There are two current coils which are kept at constant position
and the measurable current will flow through those current coils. A voltage
coil is placed inside those two current coils, and this voltage coil is totally
free to rotate. The current coils are arranged such a way, that they are
connected with the circuit in series. And the voltage coil is connected in
parallel with the circuit.

As simple
as other voltmeter and ammeter connection. In fact, a watt meter is a package
of an ammeter and a voltmeter, because the product of voltage and current is
the power, which is the measurable quantity of a watt meter

When
current flows through the current coils, then automatically a magnetic field is
developed around those coils. Under the influence of the electromagnetic field,
voltage coil also carries some amount of current as it is connected with the
circuit in parallel. In this way, the deflection of the pointer will
proportional to both current and voltage of the circuit. In this way, Watt =
Current × Voltage equation is satisfied and the deflection shows the value of
power inside the circuit. A dynamo meter type watt meter is used in various
applications where the power or energy transfer has to be measured.

**Construction and Working Principle of
Electrodynamometer Type Wattmeter**

Now let
us look at constructional details of **electrodynamometer**.
It consists of following parts There are two types of coils present in the
electrodynamometer.

They are
:

**(a) Moving coil : **Moving
coil moves the pointer with the help of spring control instrument. A limited
amount of current flows through the moving coil so as to avoid heating. So in
order to limit the current we have connect the high value resistor in series
with the moving coil. The moving is air cored and is mounted on a pivoted
spindle and can moves freely. In electrodynamometer type wattmeter, moving coil
works as pressure coil. Hence moving coil is connected across the voltage and
thus the current flowing through this coil is always proportional to the
voltage.

**(b) Fixed coil: **The fixed coil is divided into
two equal parts and these are connected in series with the load, therefore the
load current will flow through these coils. Now the reason is very obvious of using
two fixed coils instead of one, so that it can be constructed to carry
considerable amount of electric current. These coils are called the current
coils of electrodynamometer type wattmeter. Earlier these fixed coils are
designed to carry the current of about 100 amperes but now the modern wattmeter
are designed to carry current of about 20 amperes in order to save power.

**(c) Control system: **Out of
two controlling systems i.e.

(1).
Gravity control

(2)
Spring control, only spring controlled systems are used in these types of
wattmeter. Gravity controlled system cannot be employed because they will
appreciable amount of errors.

**(d) Damping system: **Air
friction damping is used, as eddy current damping will distort the weak
operating magnetic field and thus it may leads to error.

**(e) Scale:** There is uniform scale is used
in these types of instrument as moving coil moves linearly over a range of 40
degrees to 50 degrees on either sides.

Now let
us derive the expressions for the controlling torque and deflecting torques. In
order to derive these expressions let us consider the circuit diagram given
below:

We know
that instantaneous torque in electro dynamic type instruments is directly
proportional to product of instantaneous values of currents flowing through
both the coils and the rate of change of flux linked with the circuit.

Let I_{1}
and I_{2} be the instantaneous values of currents in pressure and
current coils respectively. So the expression for the torque can be written as:

T = I_{1}*I_{2}*(dM
/ dx)

Where x
is the angle

Now let
the applied value of voltage across the pressure coil be V= - V sin ωt

Assuming
the electrical resistance of the pressure coil be very high hence we can
neglect reactance with respect to its resistance. In this the impedance is
equal to its electrical resistance therefore it is purely resistive

The
expression for instantaneous current can be written as I_{2} = v / Rp
where Rp is the resistance of pressure coil.

I_{2}
= V sin ωt / Rp

If there
is phase difference between voltage and electric current, then expression for
instantaneous current through current coil can be written as

I_{1}
= I(t) = - I sin (ωt – Φ)

As
current through the pressure coil in very very small compare to current through
currentcoil hence current through the current coil can be considered as equal
to total load current.

Hence the
instantaneous value of torque can be written as - V sin ωt / Rp * - I sin (ωt –
Φ) * (dM / dx)

Average
value of deflecting torque can be obtained by integrating the instantaneous
torque from limit 0 to T where T is the time period of the cycle Td =
deflecting torque = VI cosΦ /Rp *(dM / dx)

Controlling
torque is given by Tc = Kx where K is spring constant and x is final steady
state value of deflection.

**Advantages of Electrodynamometer Type Wattmeter**

Following
are the advantages of electrodynamometer type wattmeters and they are written
as follows:

(a).
Scale is uniform up to certain limit

(b). They
can be used for both to measure AC as well as DC quantities as scale is
calibrated for both

**Errors in Electrodynamometer Type Wattmeter**

Following
are the errors in the electrodynamometer type watt meters:

(a) Errors in the pressure coil inductance.

(b) Errors may be due to pressure coil
capacitance.

(c) Errors may be due to mutual inductance
effects.

(d) Errors may be due connections.(i.e.
pressure coil is connected after current coil)

(e) Error due to Eddy currents.

(f) Errors caused by vibration of moving
system.

(g) Temperature error.

(h) Errors due to stray magnetic field.

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