The electrodynamometer wattmeter
was introduced in Section 3.3 along
with its movement and terminal connections. The same meter can be
used to measure the power in a dc or an ac network using the same connection strategy; in fact, it can be used to measure the wattage of any
network with a periodic or a nonperiodic input.
Fig. 1: Digital wattmeter.
The digital display wattmeter of Fig. 1 employs a sophisticated
electronic package to sense the voltage and current levels and, through
the use of an analog-to-digital conversion unit, display the proper digits
on the display. It is capable of providing a digital readout for distorted
non-sinusoidal waveforms, and it can provide the phase power, total
power, apparent power, reactive power, and power factor.
When using a wattmeter, the operator must take care not to exceed
the current, voltage, or wattage rating. The product of the voltage and
current ratings may or may not equal the wattage rating. In the highpower-factor wattmeter, the product of the voltage and current ratings is
usually equal to the wattage rating, or at least 80% of it. For a lowpower-factor wattmeter, the product of the current and voltage ratings is
much greater than the wattage rating. For obvious reasons, the lowpower-factor meter is used only in circuits with low power factors (total
impedance highly reactive). Typical ratings for high-power-factor
(HPF) and low-power-factor (LPF) meters are shown in Table 1.
Meters of both high and low power factors have an accuracy of 0.5% to
1% of full scale.
As the name implies, power-factor meters are designed to read the
power factor of a load under operating conditions. Most are designed to
be used on single- or three-phase systems. Both the voltage and the current are typically measured using nonintrusive methods; that is, connections are made directly to the terminals for the voltage measurements, whereas clamp-on current transformers are used to sense the
Once the power factor is known, most power-factor meters come
with a set of tables that will help define the power-factor capacitor that
should be used to improve the power factor. Power-factor capacitors are
typically rated in kVAR, with typical ratings extending from 1 to
25 kVAR at 240 V and 1 to 50 kVAR at 480 V or 600 V.