7.1. Dynamometer Wattmeter. In Fig. 27, the dynamometer is connected as a wattmeter. This is one of the advantages of this type of-meter. If the coils are connected so that a value of current proportional to the load voltage flows in one, and a value of current proportional to the load current
Fig. 27. Connection of dynamometer for measuring power.
flows in the other, the meter may be calibrated directly in watts. This is true because the indication depends upon the product of the two magnetic field. The strength of the magnetic fields depends upon the values of currents flowing through the coils. If one current is proportional to load voltage and other current is the load current, then the meter can be calibrated in terms of watts or true power consumed by the load.
Let v = supply voltage,
i = load current, and
R = resistance of the moving coil circuit.
Current through fixed coils, if = i.
- For a D.C. circuit the deflecting torque is thus proportional to the power.
- For any circuit with fluctuating torque, the instantaneous torque is proportional instantaneous power. In this case due to inertia of moving parts the deflection will be proportional to the average torque i.e., the deflection will be proportional to the average power. For sinusoidal alternating quantities the average power is VI cos ɸ, where
V = r.m.s. value of voltage,
I= r.m.s. value of current, and
ɸ = phase angle between V and I.
Hence an electrodynamic instrument, when connected as shown in Fig. 27, indicates the power, irrespective of the fact it is connected in an A.C. or D.C. circuit.
- Scales of such wattmeters are more or less uniform because the deflection is proportional to the average power and for spring control, controlling torque is proportional to the deflection, hence e DC power. Damping is pneumatic.
- The error may .creep in due to the inductance of the moving or voltage coil. However, the high non-inductive resistance connected in series with coil swamps, to-a: great extent, the phasing effect of the voltage coil inductance.
- There may be error in the indicated power due to the following:
(i) Some voltage drop in the current circuit.
(ii) The current taken by the voltage coil.
This error, however, in standard wattmeters may be overcome by having an additional compensating winding connected in series with the voltage coil but is so placed that it produces a field in opposite direction to that of the fixed or current coils.
(i) Current circuit. 0.25 to 100 Awithout employing current transformers.
(ii) Potential circuit. 5 to 750 V without employing potential transformers.
(i) The scale of the instrument is uniform (because deflecting torque is proportional to true power in both the cases i.e., D.C. and A.C. and the instrument is spring controlled.)
(ii) High degree of accuracy can be obtained by careful design, hence these are used for calibration purposes.
(i) The error due to the inductance of pressure coil at low power factor is very serious (unless special features are incorporated to reduce its effect).
(ii) Stray field may effect the reading of the instrument. To reduce it, magnetic shielding is provided by enclosing the instrument in an iron case.