3-15
full-scale value. Because electrodynamic wattmeter errors increase with frequency, they are used
primarily for measuring 60-hertz line power. Unshielded electrodynamic wattmeters should not be placed
in the vicinity of stray magnetic fields. A wattmeter has current, voltage, and power ratings; therefore,
damage may result when any of these ratings is exceeded.
The electrodynamic wattmeter may be converted into an instrument for measuring reactive power by
replacing the resistance normally in series with the voltage coil with a large inductance. A 90-degree
current lag within the voltage coil provides a direct reading proportional to the reactive power in the
circuit. Compensating networks must be used to cause the phase shift to be exactly 90º.
Q-11.
What type of device is used to extend the current-measuring capability of electrodynamic
wattmeters?
IRON-CORE, COMPOSITE-COIL, AND TORSION-HEAD WATTMETERS
Iron-core wattmeters are primarily used as switchboard instruments and employ the induction
principle. Voltage and current coils are wound around a laminated iron core shaped to produce a mutually
perpendicular magnetic field across an air gap. Eddy currents induced in a thin metal cylinder rotating in
this air gap interact with the magnetic field to produce a torque proportional to the instantaneous power.
This type of construction provides the advantages of increased operating torque, larger angles of rotation,
ruggedness, compactness, and freedom from errors caused by stray fields. It has the disadvantage of a
very narrow frequency range.
The composite-coil wattmeter uses the upscale torque, produced by the ac power being measured, in
opposition to the torque produced by an adjustable dc current in a set of windings intermingled or wound
within the ac windings. Greater reading precision is obtained with this method than is possible with
straightforward wattmeters, and errors caused by elasticity of the spring suspension carrying the moving-
coil system are avoided. The torsion-head wattmeter is used to restore the movable coil to its original
position after deflection and to remove the mutual inductance error.
ELECTRONIC WATTMETER
Electronic wattmeters are used for direct, small power measurements or for power measurements at
frequencies beyond the range of electrodynamometer-type instruments. A simplified electronic wattmeter
circuit is shown in figure 3-13. The matched triodes are operated in the nonlinear portion of their
characteristic grid-voltage, plate-current curves. The symmetrical resistive T network between the
generator and load will provide V1 and V2 voltages proportional to, and in phase with, the load current
and voltage, respectively. A source of ac power is connected to the load through the series resistors R1
and R2. These two resistors are of equal value and are made small to prevent the voltage drop across them
from reducing the load voltage appreciably. R3 is made large enough to have negligible power
consumption. Therefore, the R3 voltage is equal to the load voltage, and the voltage across either series
resistor is proportional to the difference in the output currents of the tubes. The average value of the
difference could be measured by a dc meter connected to read the voltage potential between the grids of
V1 and V2. This method is adequate only at low frequencies. As the frequency increases, the stray
capacitances and inductances also increase. The frequency range of the electronic wattmeter can be
extended up to 20 megahertz by using pentodes instead of triode tubes. The operating conditions in a
pentode are adjusted so that plate current is proportional to the product of a linear function of plate
voltage and an exponential function of grid voltage.
