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 I1 and I2 be the instantaneous values of currents in pressure and current coils respectively. So the expression for the torque can be written as:
T = I1*I2*(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 I2 = v / Rp where Rp is the resistance of pressure coil.
I2 = 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
I1 = 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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