Application Note

22 Fluke Corporation Power Quality Troubleshooting
About two-thirds of the electric
power in the U.S. is consumed
by motors, with industrial
three-phase motors above 5 HP
(7 kW) being by far the bulk of
that load. They are linear loads
and therefore don’t contribute
to harmonics. They are, how-
ever, the major contributor to
reduced Displacement Power
Factor, which is a measurement
of the effective use of system
capacity.
Measurements
1. Voltage unbalance
Voltage unbalance should not
exceed 1-2% (unless the motor
is lightly loaded). Why such a
small number? Voltage unbal-
ance has a very large effect on
current unbalance, in the neigh-
borhood of 8:1. In other words,
a voltage unbalance of 1% can
cause current unbalance of 8%.
Current unbalance will cause
the motor to draw more current
than it otherwise would. This in
turn causes more heat and heat
is the enemy of motor life, since
it deteriorates the winding insu-
lation.
2. Voltage %THD and
harmonic spectrum
Voltage THD should not exceed
5% on any phase. If the voltage
distortion on any phase is ex-
cessive, it can cause current un-
balance. The usual culprit is the
5th harmonic and therefore the
harmonic spectrum should be
examined for the 5th in particu-
lar. The 5th is a negative se-
quence harmonic which creates
counter-torque in the motor. A
motor fed by a voltage with
high 5th harmonic content will
tend to draw more current than
otherwise. This is a major prob-
lem when across-the-line or
soft-start motors share the same
bus with ASDs.
Section 6
Polyphase Induction Motors
3. Current unbalance
To find current unbalance, mea-
sure amps in all three phases.
Do the same calculation as for
voltage unbalance. In general,
current unbalance should not
exceed 10%. However, unbal-
ance can usually be tolerated if
the high leg reading doesn’t ex-
ceed the nameplate FLA (Full
Load Amps) and SF (Service
Factor). The FLA and Service
Factor are available on the mo-
tor nameplate. If the voltage un-
balance and the voltage THD
are within limits, high current
unbalance can be an indication
of motor problems, such as
damaged winding insulation or
uneven air gaps.
Current measurement will
also find single-phasing. If a
three-phase motor loses a
phase (perhaps caused by a
blown fuse or loose connection),
it may still try to run single
phase off the remaining two
phases. Since the motor acts
like a constant power device, it
will simply draw additional cur-
rent in an attempt to provide
sufficient torque. A voltage mea-
surement alone will not neces-
sarily find this condition, since
voltage is induced by the two
powered windings into the
non-powered winding.
Part II: Three-Phase Loads
Example of voltage unbalance calculation:
%V
UNBALANCE
= Max deviation from average x 100%
Average (of three phases)
1. Make three phase-to-phase measurements:
A-B = 475V A-C = 471V B-C = 470V
2. Find the average: (475+471+470) ÷ 3 = 472V
3. Find the maximum deviation from the average:
This occurs on the A-B phase: 475V-472V = 3V
4. Divide maximum deviation by average to find
% unbalance: 3V/472V <1%
Voltage unbalance can be caused by severe load unbalance
but it could just as easily be caused by loose connections
and worn contacts.
Table 6.1 Measurements at the motor.
Measurement Look for Instrument
1. Voltage unbalance Unbalance <1% 43, 41B, 87
2. Voltage %THD %THD <5% 43, 41B
3. Current unbalance Unbalance <10% 43, 41B
87 w/80i-400
Single phasing No current on one phase.
(extreme current
unbalance)
4. Loading
•
Nameplate data on FLA
1
and SF
2
: Same
Current < (FLA x SF).
•
Overloading or extreme underloading.
80% of rated load is optimal.
5. Inrush current
•
Inrush causing voltage sag. 43
•
Inrush causing nuisance trips.
6. Power factor
•
Low Displacement PF. 43, 41B
•
Large difference between DPF and PF
(Total Power Factor) indicating harmonics.
1
FLA = Full Load Amps
2
SF = Service Factor
(If the FLA = 100A and the SF=1.15, the motor can be run at 115A continuously.)