Power Measurement Equipment 69
age windings and the current windings. The field developed in the volt-
age windings causes a current to be induced into the aluminum disk.
The torque produced is proportional to the voltage and the in-phase cur-
rent of the system. Therefore, the watt-hour meter will monitor the true
power converted in a system.MEASURING THREE-PHASE ELECTRICAL ENERGY
For industrial and commercial applications, it is usually necessary to
monitor the three-phase energy that is utilized. It is possible to use a combi-
nation of single-phase wattmeters to measure the total three-phase power,
as shown in Figure 3-8. The methods shown are ordinarily not very practi-
cal, since the sum of the meter readings would have to be found in order
to calculate the total power of a three-phase system. Three-phase power ana-
lyzers are designed to monitor the true power of a three-phase system.Measuring Power Factor
Power factor is the ratio of the true power of a system to the apparent
power (volts × amperes). To determine power factor, we could use a re-
lationship of pf = W/VA. However, it would be more convenient to use
a power factor meter in situations where the power factor must be moni-
tored.
The principle of a power factor meter is shown in Figure 3-9. The pow-
er factor meter is similar to a wattmeter, except that it has two armature
coils that rotate because of their electromagnetic field strengths. The arma-
ture coils are mounted on the same shaft so that their alignment is about
90° apart. One coil is connected across the AC line (in series with a resis-
tance), while the other coil is connected across the line through an induc-
tance. The resistive path through the coil reacts to produce a flux propor-
tional to the in-phase component of the power. The inductive path reacts in
proportion to the out-of-phase component of the power.
If a unity (1.0) power factor load is connected to the meter, the current
in the resistive path through coil A will develop full torque. Since there is
no out-of-phase component, no torque will be developed through the in-
ductive path. The meter movement will now indicate full-scale or unity
power factor. As the power factor decreases below 1.0, the torque devel-
oped by the inductive path through coil B becomes greater. This torque
will be in opposition to the torque developed by the resistive path. There-