Application Note

Power Quality Troubleshooting Fluke Corporation 23
4. Loading
Measure current draw of the
motor. If the motor is at or near
its FLA rating (times the Service
Factor multiplier), it will be
more sensitive to the additional
heating from harmonics, as well
as current unbalance. A motor
that is only lightly loaded is
usually safe from overheating.
On the other hand, its efficiency
and DPF are both less than op-
timal. Most motors reach maxi-
mum efficiency at 60%-80% of
full load rating. Displacement
Power Factor is maximum at
rated load (including S.F.) and
drops off, especially at less than
80% of rated load. This leads to
the conclusion that, to the de-
gree a motor load is constant
and predictable, 80% of rated
load is the most efficient oper-
ating range.
5. Inrush
Motors which are started
across-the-line (as opposed to
those using soft-starts or drives)
draw a current inrush, also
called locked rotor current. This
inrush tapers off to normal run-
ning current as the motor comes
up to speed.
•
Older motors draw an inrush
of typically 500-600% of the
running current. Newer en-
ergy efficient designs draw
brief inrushes as high as
1200% of running current, a
direct result of the lower im-
pedances which help make
them more energy efficient
in the first place.
•
High torque, high HP motor
loads require proportionally
higher inrush.
•
Motor loads started at the
same time will have a cumu-
lative inrush.
Another source of inrush is UPS
and ASD systems with diode
converters. They draw inrush
current as their cap banks first
charge.
Effects of inrush current
1. Inrush causes voltage sags if
the source voltage is not stiff
enough:
•
Relays and contactor coils
might drop out (typically,
the sag would have to get
as bad as about 70% of
normal line voltage); or, if
they hold in, their contacts
might chatter (especially if
the additional load causes
a long-term undervoltage).
•
Control circuits might reset
or lock up (at 90% and
below).
•
Drives might trip off-line
(undervoltage trip).
2. High peak demand periods,
which may cause higher
utility bills.
3. Cycling loads can cause peri-
odic sags, which might show
up as flickering lights.
4. If the motor is required to
start up a high torque load,
the inrush can be relatively
prolonged (e.g., 10 to 20
seconds or more) and this
can cause nuisance tripping
as the overload heaters trip
the motor starter.
6. Power Factor
To size PF correction capacitors,
it is necessary to measure the
DPF (Displacement PF) and
Active Power consumption (kW)
of the motor load. Measurement
of the DPF and kW of a three-
phase induction motor is ex-
plained in the sidebar on the
next page. These measurements
assume that the motor voltage
and current is balanced. There-
fore, before undertaking PF cor-
rection, first make sure that
voltage and current unbalance
are within limits. Either problem
can shorten motor life and
should take priority over DPF
correction.
Flattopping
Total harmonic distortion < 5%
Figure 6.1. Voltage distortion.
One
Open
Fuse
L1 L2 L3
T1 T3
T2
M
Single-phasing
Figure 6.2. Single-phasing.
Figure 6.3. Inrush current (Model 43).