Following figure shows the arrangement of thermal wattmeter for measurement of power:
This wattmeter uses two similar thermocouples (1 and 2) whose outputs are connected in opposition with a galvanometer in between. Rh is the resistance of each thermocouple heating element. R is high series resistance, and between C and D is a low resistance R2 capable of carrying the load current i. the resistance R2 develops a potential difference which depends upon the load current, together with the current of one heater, and the series resistance R carries the current of both heaters.
If v be the instantaneous voltage at the load, then assuming identical thermocouples, we have:
From above two equations, we get:
e.m.f. of thermocouple 1,
If Rh+R is not very different from Rh, the C2*i^2 may be neglected and T(inst.) is directly proportional to v*i or instantaneous power.
Thus, galvanometer may be calibrated to read the power.
The commercial thermal wattmeters employ a number of thermocouples connected in the form of a chain in order to increase the output. C.T.s and P.T.s are also used with these instruments.
For high frequency measurements careful shielding is required.
The thermal wattmeters can be used for measurement of power is several circuits and the sum of their outputs can be applied to a recording potentiometer which records the total power.
These wattmeters are used for measurement of small amount of power, practically when the voltage is high and power factor is low. This type of wattmeter is also used for measurement of dielectric loss of cables on alternating voltage and for calibration of wattmeters and energy meters.
Electrostatic wattmeter consists of a quadrant electrometer used with a non-inductive resistor R as shown in figure below.
i.e. instantaneous torque is proportional to the instantaneous power in the load plus half of the power lost in noninductive resistance.
Advantages and Disadvantages:
Electrostatic wattmeter is a precision instrument and should be used as such.
It is free from errors on account of waveforms, frequency and eddy currents.
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Hello friends, in this post we will see construction and working principle & classification of dynamometer type wattmeter. We will also see the advantages and disadvantages of dynamometer type wattmeter.
Construction of Dynamometer Type Wattmeter:
The following figure shows the dynamometer wattmeter for measuring the power. If two coils are connected such that, current proportional to the load voltage, flows through one coil and current proportional to the load current, flows through another coil, the meter can be calibrated directly in watts. This is true because the indication depends upon the product of the two magnetic fields. The strength of the magnetic fields depends upon the values of the current flowing through the coils.
Working of Dynamometer Type Wattmeter:
Let us consider
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)=v/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 the inertia of moving parts, the deflection will be proportional to the average power. For sinusoidal alternating quantities, the average power is VI COSθ where
V = r.m.s. value of voltage,
I = r.m.s. value of current, and
θ = phase angle between V and I
Hence an electrodynamic instrument, when connected as shown in the figure, indicates the power, irrespective of the fact it is connected in an AC or DC circuit.
Current circuit: 0.25 A to 100 A with employing current transformers (CTs).
Potential circuit: 5V to 750 V without employing potential transformers (PTs).
Types of Dynamometer Wattmeter:
Dynamometer wattmeters may be divided into two classes:
Suspended-coil torsion instruments.
Pivoted-coil, direct indicating instruments.
1. Suspended-coil Torsion Wattmeters:
These instruments are used largely as standard wattmeters.
The moving, or voltage, the coil is suspended from a torsion head by a metallic suspension which serves as a lead to the coil. This coil is situated entirely inside the current or fixed coils and the winding in such that the system is static. Errors due to external magnetic fields are thus avoided.
The torsion heads carry a scale, and when in use, the moving coil is bought back to the zero position by turning this head; the number of divisions turned through when multiplied by a constant for the instrument gives the power.
Eddy currents are eliminated as far as possible by winding the current coils of standard wire and by using no metal parts within the region of the magnetic field of the instrument.
The mutual inductance errors are completely eliminated by making zero position of the coil such that the angle between the planes of moving coil and the fixed coil is 90 degree. i.e. the mutual inductance between the fixed and moving coil is zero.
The elimination of pivot friction makes possible the construction of extremely sensitive and accurate electrodynamic instruments of this pattern.
2. Pivoted-coil Direct-indicating Wattmeters:
These instruments are commonly used as a switchboard or portable instruments.
In these instruments, the fixed coil is wound in two halves, which are placed in parallel to another at such a distance, that uniform field is obtained. The moving coil is wound of such a size and pivoted centrally so that it does not project outside the field coils at its maximum deflection position.
The springs are pivoted for controlling the movement of the moving coil, which also serves as currents lead to the moving coil.
The damping is provided by using the damping vane attached to the moving system and moving in a sector-shaped box.
The reading is indicated directly by the pointer attached to the moving system and moving over the calibrated scale.
The eddy current errors, within the region of the magnetic field of the instrument, are minimized by the use of non-metallic parts of high resistivity material.
Advantages and disadvantages of dynamometer type wattmeter:
The advantages and disadvantages of dynamometer type wattmeters are as under:
1) In dynamometer type wattmeter, the scale of the instrument is uniform (because deflecting torque is proportional to the true power in both DC as well as AC and the instrument is spring controlled.)
2) High degree of accuracy can be obtained by careful design; hence these are used for calibration purposes.
1) The error due to the inductance of the pressure coil at low power factor is very serious (unless special features are incorporated to reduce its effect)
2) In dynamometer type wattmeter, a stray field may affect the reading of the instrument. To reduce it, magnetic shielding is provided by enclosing the instrument in an iron case.