Before we study the internal construction of electrodynamometer wattmeter, it very essential to know the principle of working of electrodynamometer type wattmeter. Dynamometer type wattmeter works on a very simple principle which is stated as "when any current carrying conductor is placed inside a magnetic field, it experiences a mechanical force and due to this mechanical force, deflection of conductor takes place."
Construction –
It consists of the following parts:
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 connected 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.
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.
Control system - Out of two controlling systems i.e. gravity control and spring control, only spring controlled systems are used in these types of wattmeter. Gravity controlled system cannot be employed because they will contain appreciable amount of errors.
Damping system - Air friction damping is used, as eddy current damping will distort the weak operating magnetic field and thus it may lead to error.
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.
Working –
Let
v=supply voltage
i=load current and
R=resistance of the moving coil circuit
Current through fixed coils, $i_f=I$
Current through the moving coil, $i_m= \frac{v}R$
Deflecting torque,
$$T_d∝(i_f * i_m)∝\frac{iv}{R}$$
For a DC circuit the deflecting torque is thus proportional to the power.
For any circuit with fluctuating torque, the instantaneous torque is proportional to instantaneous power. In this case due to inertia of moving parts, the deflection will be proportional to the average power. For sinusoidal alternating quantities the average power isV.I.cosϕ, where
V= RMS value of voltage
I= RMS value of current, and
ϕ= phase angle between V and I
Hence an electrodynamic instrument, when connected as shown in figure, indicates the power, irrespective of the fact it is connected in an AC or DC circuit.
Merits –
Scale is uniform up to certain limit.
- They can be used for both to measure ac as well dc quantities as scale is calibrated for both.
Errors/Demerits –
Errors in the pressure coil inductance
Errors may be due to pressure coil capacitance
Errors may be due to mutual inductance effects
Errors may be due connections (i.e. pressure coil is connected after current coil)
Error due to Eddy currents
Errors caused by vibration of moving system
Temperature error
Errors due to stray magnetic field.
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