Application Note

2 Fluke Corporation How do you measure energy consumption?
Figure 1. Fluke 435 energy screen. The Energy tab allows you to
track accumulated kWh, kVAh, and kVARh.
Figure 2. Fluke 435 power screen. This is a real time view of all the
common power calculations. The kW, kVA, kVAR, PF, dPF, along with
Vrms are displayed for each phase and total. The symbol to the right
of kVAR indicates whether the load inductive or capacitive.
phase loads such as computers
and lighting. Three phases make
it harder to measure power or
energy usage, particularly if you
plan to use improved efficiency
to reduce energy use.
Imaginary power: Volt-Amps
Reactive (VAR) is a strange
type of current flow that pro-
duces no work, but is present
on your electrical distribution
system. It’s part of the difference
between power and demand
and thus contributes to lowering
power factor. It’s usually caused
by motor inductance, and is
greater when those motors are
not loaded to their full capacity.
A constant speed motor driv-
ing a large air movement fan
is an example where mechani-
cal dampers have been used to
regulate air flow, making a fan
less efficient. This also reduces
the load on the drive motor and
increases imaginary power in
the electrical supply system.
Many facilities opt to change
their motor supply from direct
line drive to an adjustable speed
motor drive, so that they can
optimize the motor’s opera-
tion and speed to its load. That
optimization uses energy more
efficiently in the fan and motor
and increases power factor.
Harmonic currents, reflected
back into the supply system, are
produced by the input rectifier
loads of adjustable speed motor
drives, computers, and similar
electronic devices. Harmonics
also reduce power factor.
How to measure power
To measure real power, we need
a meter that can simultaneously
measure voltage, current, and
all the stuff mentioned above
that lies in-between, over a one
second period. A digital multim-
eter can’t do that. The solution
lies in a power quality tool.
Depending on the make and
model you select, you can test
single phase, split-phase, three
phase (3 wire or 4 wire) mea-
surement configurations, and
measure or record, V, A, W, VA,
VAR, PF and Harmonics. Some
of the recording models also
provide the means to record
measurements over time to
report the energy readings used
by the utility—kWh, kVAh and
kVARh (see Figures 1 and 2).
The good news here is that
these tools will account for all
of the issues mentioned above
and accurately report energy use
when it happens, as a function
of instantaneous voltage and
current measurements over time.
Setting up logging intervals.