Application Note

24 Fluke Corporation Power Quality Troubleshooting
Measuring Displacement Power Factor
on 3-phase Induction Motors
Select which of the two methods to use based on
the transformer configuration supplying the mo-
tor. Either method will give the same results.
Method #1 is for the grounded-Y source. It is
simple and can be applied in most situations,
since virtually all of the low voltage motors in
commercial and light industrial facilities are fed
from a grounded-Y source. Method #2 is for float-
ing sources sometimes found in heavy industrial
facilities.
Method #1: Grounded-Y source
To check if the source is grounded-Y, measure
voltage of each phase to ground. If the readings
are equal, you can use this measurement method.
Set-up:
1. Clamp the current probe on any phase (with
arrow on clamp pointing towards the motor).
2. Attach the red voltage probe to the same
phase and the black probe to ground (not to
another phase).
Active Power: Read kW and multiply by 3:
kW
MOTOR
= 3 * kW
Displacement PF: Read DPF.
(Not necessary to measure kVA)
Method #2: Three-wire Source
With floating-Y, floating-delta or grounded-delta,
the voltage will be different for at least one of
the phase-to-ground readings. (For the floating
source, the phase-to-ground voltage is unpredictable,
since it depends on phase to ground capacitance).
Method #2 is known as the Two-Wattmeter method.
Set-up:
1. Connect the black voltage probe to any phase.
2. Connect the red voltage probe and the clamp
(arrow towards the load) together on a second
phase. Record kW
1
.
3. Move red probe and clamp to the third phase
(do not move the black probe). Record kW
2
.
Record kVA (kVA of either phase will be more
or less equal if the current unbalance is within
limits).
Active Power: Kw
MOTOR
= kW
1
+ kW
2
(If either
kW reading is negative, as might happen on a
very lightly loaded motor, it would be subtracted
instead of added)
Apparent Power: kVA
MOTOR
= kVA * 1.73.
DPF = kW
MOTOR
/ kVA
MOTOR
Example (Two-Wattmeter):
Measurements:
kW
1
= + 1.52
kW
2
= + 1.74
kVA = 2.41
Calculations:
Kw
MOTOR
= kW
1
+ kW
2
= (+ 1.52) + (+1.74) = 3.26 kW
kVA
MOTOR
= kVA * 1.73. = (2.41) (1.73) = 4.17 kVA
DPF = Kw
MOTOR
/ kVA
MOTOR
= 3.26 / 4.17 = 0.78
black
red
Load
kW
TOTAL
= kW
1
+ kW
2
kVA
TOTAL
= kVA
1
• 3
PF =
kW
TOTAL
kVA
TOTAL
kVA
kW
1
Load
kW
2
Power Factor Using the Two-Wattmeter Method:
red
red
blackblack