Application Note

2 Fluke Corporation Optimizing electric-motor efficiency – Part II
Measuring off-design voltage from supply
to motor terminals can be accomplished using
some combination of a digital multimeter (DMM),
clamp meter and a power quality analyzer. If the
voltage anomalies are sporadic—spikes and/or
sags—the best way to pinpoint their source may
be to connect an electronic meter with waveform
capture capabilities. This allows you to correlate
off-design voltage events with other events in the
facility. However, the first indication that there is
excessive under- or over-voltage might show up in
an inspection of motors using an infrared ther-
mometer or imager.
Correcting off-design voltage entails trying
the following remedies:
•
When voltage is constantly and consistently
over- or under-voltage by the same amount,
change the main service transformer’s tap
setting, and adjust branch or secondary trans-
former settings.
•
To correct daily voltage variations at the service
entrance, install a transformer with an auto-
matic tap changer.
•
For voltage variations throughout a facility but
not at the service entrance, replace or augment
existing conductors and replace transformers as
necessary.
•
Use capacitors to correct power factor at points
of use.
Potential savings and ROI
To calculate the potential savings and ROI result-
ing from actions to correct off-design voltages, you
need to know the following (sample values are in
parentheses):
•
number of hours per year a stalled motor
stopped the process (30 hr),
•
annual income from the process ($2,500,000),
•
number of days per year the process runs
(365 days/yr),
•
number of hours per day the process runs
(24 hr/day),
•
cost of the corrective device or component
($7,000),
•
labor rates for corrective action or installation
($50/hr),
•
number or electricians, etc. required (2), and
•
estimated hours to do the job (10 hr).
Using the sample numbers, the annual lost income
(L$) from downtime would be:
L$ = 30 hr x [$2,500,000 ÷
(365 days/yr x 24 hr/day)] = $8,562,
and the cost of the corrective action (C$) would
be:
C$ = $7,000 + [2 x ($50/hr x 10 hr)] = $8,000.
So, the payback period for the investment is
expected to be less than a year.
Bad connections and
grounding problems
Regularly inspecting your electrical distribution
systems for bad connections, poor grounding and
shorts to ground will help improve equipment per-
formance and avoid system outages and premature
equipment failure.
Detecting bad connections, poor grounding
and shorts to ground should be carried out by an
experienced electrician or power specialist. These
conditions can often be found by a visual inspec-
tion for looseness, corrosion or conductive paths
to ground. Another telltale method for finding
problem connections is to look for overheating at
connections using an infrared thermometer or a
thermal imager. Look, too, for voltage drops across
connections. Compare all three phases. A voltage
variation of two or three percent among connec-
tions suggests a need for corrective action.
Corrections can often be accomplished by
regular cleaning and tightening of connections.
You can also use an insulation multimeter to
check starter and control contacts and to mea-
sure the insulation resistance of the line and load
circuits to ground. These latter tests are important
because you may find that you can get a motor
back into operation with a fix as simple as replac-
ing a cable.