This document presents a method to estimate the operating power factor (PF) of an induction motor using only the measured motor current and manufacturer data. The method involves estimating the reactive current component from nameplate data and then calculating PF based on the measured current and assumed constant reactive current. Experimental results on a test motor show the PF estimated using this current-only method has an error of around 0.04 compared to methods using voltage and current measurements, with higher error only at no-load condition. This low-cost method provides an alternative to monitor PF without voltage sensors.
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Estimation of induction motor operating power factor.
1. Estimation of Induction Motor
Operating Power Factor From
Measured Current and
Manufacturer Data
2. Estimation of IM Operating PF From Measured Current and Manufacturer Data
index
Sl
No
Tittle Page
No
1 Introduction 02
2 Background Information 04
3 Current –Only, Pf Estimation 06
4 Power Factor: From Voltage, Current ZC 11
5 Power Factor: From Instantaneous Power 14
6 Discussion Of Results 16
7 Conclusion 20
8 References 21
1
3. Estimation of IM Operating PF From Measured Current and Manufacturer Data
introduction
“ THREE-PHASE induction motors (IM) are industrial work-horses,
responsible for consumption of 40–50% of generated
electrical power. ”
2
4. Estimation of IM Operating PF From Measured Current and Manufacturer Data
Recently there has been a lot of focus on IM
protection at LV, &MV levels.
These protection devices typically monitor the
motor current and/or voltage to provide the motor
protection functionalities
One of the interesting parameters to monitor is the
operating power factor (PF)
Traditionally, to monitor the operating PF of the IM,
one would require both the voltage and the current
A low cost method of determining the operating PF
of the IM using only the measured current and the
manufacturer data is developed.
3
5. Estimation of IM Operating PF From Measured Current and Manufacturer Data
BACKGROUND INFORMATION
IM equivalent circuit.
Leakage reactance
I1 = Im+I1 ’ Load
v1
I1
r1 X1
rC Xm
I1’
X2’
r2’
s
component
Exciting
component
Im
core-loss
resistance
magnetizing
reactance
stator
Rotor
4
6. Estimation of IM Operating PF From Measured Current and Manufacturer Data
Power Factor and Motor Load
휽ퟎ in the range 75 –85
ie, stator PF at no load may
be as low as 0.1–0.3.
Typically, stator PF of
about 0.8–0.9 at 80–100% of
the full-load
I1’ N1= I2’ N2
5
7. Estimation of IM Operating PF From Measured Current and Manufacturer Data
CURRENT –ONLY, PF ESTIMATION
Total input electrical apparent power , 푷 = ퟑ푽푰
Active power, for supplying the load ,푷 = ퟑ푽푰 cos ∅
PF =
푷
푷
= cos ∅ Eqn 1
I= 푰풂풄풕풊풗풆
ퟐ + 푰풓풆풂풄풕풊풗풆
ퟐ
푰풂풄풕풊풗풆 = I cos ∅ , 푰풓풆풂풄풕풊풗풆= I sin ∅
푰풓풆풂풄풕풊풗풆=I sin cos−ퟏ(푷푭) Eqn 2
PF = 푐표푠 ∅ = 1 − 푠푖푛∅2 = 1 − (
퐼푟푒푎푐푡푖푣푒
퐼
)2 Eqn 3
6
8. Estimation of IM Operating PF From Measured Current and Manufacturer Data
푰풓풆풂풄풕풊풗풆 remains constant, it can be estimated from
nameplate data using 푰풓풆풂풄풕풊풗풆= I sin cos−ퟏ(푷푭)
At no-load condition, there is no active current flow.
So, at no-load, I= 푰풓풆풂풄풕풊풗풆 PF = 0 (
퐼PF= 1 − (
푟푒푎푐푡푖푣푒
퐼
)2 )
As motor load increases the PF will increase toward
unity.
Motor
load
increases
Total
motor I
increase
푰풓풆풂풄풕풊풗풆
remains
constant
(
푰풓풆풂풄풕풊풗풆
푰
)ퟐ
Decreases
Physically, at no-load, there is not much mechanical
resistance, so the whole circuit is mostly inductive due to
the stator coils, causing low PF.
7
9. Estimation of IM Operating PF From Measured Current and Manufacturer Data
Basic Algorithm
Estimate the 푰풓풆풂풄풕풊풗풆 from the nominal PF out of
the nameplate data. ( 푰풓풆풂풄풕풊풗풆=I sin cos−ퟏ(푷푭))
Estimate the operating PF from measured motor
current & constant 푰풓풆풂풄풕풊풗풆
“ It would not require synchronized voltage and
current measurement like in the displacement PF
measurement principle.”
8
10. Estimation of IM Operating PF From Measured Current and Manufacturer Data
experimental setup
Load
supply
Motor
supply
ABB
Lv drive
IM 3 ph
Load
Motor
(7.5kw)
Test Motor
(2.2 kw)
two pole
pairs IM,
M2AA LA4
Motor
current
measureme
nt
Main supply (50 Hz)
IM 3 ph
9
11. Estimation of IM Operating PF From Measured Current and Manufacturer Data
NAME PLATE RATING OF THE
TEST MOTOR TYPE ABB M2AALA4.
NOMINAL VOLTAGE 4 00V
NOMINAL CURRENT 4.9A
RATED POWER 2.2KW
RATED PF 0.81
RATED SPEED 1430 RPM
푰풓풆풂풄풕풊풗풆=I sin cos−ퟏ(푷푭)
= 4.9* Sin (cos−1 0.81)
=2.87
MEASURED I,CALCULATED PF
FROM
CURRENT-ONLY METHOD AT
DIFFERENT LOAD
Measured
CURRENT
I(A)
I/푰풏풐풎풊풏풂풍 (%) PF
3.01 61.43 .30
3.23 65.92 .46
4.37 89.18 .75
5.21 106.33 .83
6.21 126.73 .89
PF= 1 − (
퐼푟푒푎푐푡푖푣푒
퐼
)2=
2.87
3.01
1 − (
)2 = 0.3
10
12. Estimation of IM Operating PF From Measured Current and Manufacturer Data
Power Factor: From Voltage, Current ZC
Basic Algorithm
Synchronized measurement of the supply voltage and
the motor current are done.
Displacements in the ZC timings between the voltage
(taken as reference) and the current signals are
estimated.
Estimate the PF using the equation ±x ms : Deviation between the current
ZC w.r.t. the voltage ZC,
풇ퟎ ∶ 퐒upply frequency.
PF=풄풐풔(
풙
(
ퟏ
풇ퟎ
)
X ퟑퟔퟎퟎ)
The current ZC deviation is positive or negative w.r.t.
The voltage would decide whether the PF is lagging
or leading.
11
13. Estimation of IM Operating PF From Measured Current and Manufacturer Data
experimental setup
Load
supply
Motor
supply
ABB
Lv drive
IM 3 ph
Load
Motor
(7.5kw)
Tektronix Voltage probe
Current hall-sensor probe
Test Motor
(2.2 kw)
two pole pairs IM,
M2AA LA4
Main supply (50 Hz)
oscilloscope
Agilent
IM 3 ph
12
14. Estimation of IM Operating PF From Measured Current and Manufacturer Data
PF MEASUREMENT USING
DISPLACEMENT IN THE ZC OF THE
SYNCHRONIZED SUPPLY VOLTAGE
AND MOTOR CURRENT AT 66%
CURRENT LOAD.
±x ms : 3.4 ms ,풇ퟎ =
ퟓퟎ
Pf =풄풐풔(
풙
(
ퟏ
풇ퟎ
)
X ퟑퟔퟎퟎ)
=cos(
3.4
20
푿 ퟑퟔퟎퟎ) =
0.48
RMS CURRENT,TIME DEVIATION
BETWEEN ZC OF VOLTAGE
&CURRENT, PF FROM
DISPLACEMENT METHOD AT
DIFFERENT LOAD
RMS
current
I(A)
ZC time
difference
(ms)
PF
2.97 4.6 0.12
3.00 3.4 0.48
3.54 2.8 0.64
4.24 2.1 0.79
5.23 1.6 0.87
13
15. Estimation of IM Operating PF From Measured Current and Manufacturer Data
Power Factor: From Instantaneous Power
Basic Algorithm
Estimate instantaneous power per phase (i.e., point-by-
point multiplication of the two waveforms) from
the synchronized supply voltage and the motor
current
Average power (푃 ) per phase from the
measurement in the scope.
Estimate the PF using the equation : PF =
푃
푉퐼
14
16. Power factor measurement using
instantaneous power from the measured
synchronized supply voltage and the
motor current at 66% current load.
푃 =328.525 V=230 I=3
PF =
푃
푉퐼
=
328.525
230 푋 3
= 0.476
RMS CURRENT,RMS VOLTAGE ,
INSTANTANEOUS POWER ,POWER
FACTOR AT DIFFERENT LOAD
RMS
current
(A)
RMS
voltage
(v)
Instantaneou
s power(w)
PF
2.97 230 72.86 0.1
1
3.00 230 328.53 0.4
8
3.54 230 547.48 0.6
7
4.24 230 774.67 0.7
9
5.23 230 1014.32 0.8
4
Estimation of IM Operating PF From Measured Current and Manufacturer Data
15
17. Estimation of IM Operating PF From Measured Current and Manufacturer Data
DISCUSSION OF RESULTS
METHOD I(A) PF I(A) PF
From Measured Current 3.00 0.221 4.24 0.71
From Voltage, Current ZC 3.00 0.48 4.24 0.79
From Instantaneous
3.00 0.48 4.24 0.79
Power
“The PF estimation error using the current-only method is
about +0.04 , except at the no-load condition, where it is
about−0.18 . This could be because, the sensitivity of the
motor current measurement module might not be very perfect
at no-load condition.”
16
18. Estimation of IM Operating PF From Measured Current and Manufacturer Data
The proposed method relies on the fact that the
inductance of the total circuit remains constant.
This rule might be violated if the motor is supplied via a
variable speed drive.
Most modern drive systems usually measure both voltage
and current and can provide PF measurement . Hence, no
need for cheaper calculation of PF.
it is assumed that manufacturers comply with the
relevant IM manufacturing standards, providing rated
values with acceptable accuracy.
Estimation of IM operating PF could be used for PF
compensation
17
19. Estimation of IM Operating PF From Measured Current and Manufacturer Data
Under no-load condition, the referred reactance of the
rotor
X2’ would be absent.
The no-load current would comprise of the magnetizing
current only. ie the no-load current would not be a good
representative of the total reactive current
So, the reactive current estimated from the rated
condition, would not miss any inductive elements.
For bigger IMs with low I(no−load)/I(rated) ratio, the
assumption to use I reactive = constant for light loads
might lead to high errors in the PF estimation.
Higher power machines usually come with MV or LV drives
already provided with the PF computation.
18
20. Estimation of IM Operating PF From Measured Current and Manufacturer Data
Measured current (A)
Underload protection using motor current & power factor
Measured current (A)
푁표푚푖푛푎푙 푐푢푟푟푒푛푡 (퐴)
Power factor
Measured current (A)
푁표푚푖푛푎푙 푐푢푟푟푒푛푡 (퐴)
At low loads the nonlinear curve of the PF provides better
resolution
It challenging to accurately measure the low current change
from small load change at low loading.
19
21. Estimation of IM Operating PF From Measured Current and Manufacturer Data
CONCLUSION
Low cost method of determining the operating PF
of the IM using only the measured current and the
manufacturer data typically available from the
nameplate and/or datasheet.
This would provide a cheaper solution to under
load protection, e.g., in pump applications, using
the operating PF, without requiring the voltage
sensors.
Operational PF can also be used for PF
compensation to improve the power quality.
20
22. Estimation of IM Operating PF From Measured Current and Manufacturer Data
REFERENCE
P.S.Bimbhra, electrical machinery. New delhi, india:
khanna publishers,,
“Identification of induction motor equivalent circuit
parameters
using the single-phase test,” IEEE trans. Energy
convers.
Abb. (2009). Manual for low voltage motor.
Http://www.Abb.Com/motors.
Abb. (2010). LV drives, model ACS800.
Www.Abb.Com/drives
D. Sharon, “power factor definitions and power
transfer quality in no sinusoidal situations,”ieee trans.
Instrum.
“Power-factor compensation of electrical circuits,”
IEEE control syst.
21
24. Estimation of IM Operating PF From Measured Current and Manufacturer Data
ANY
QUESTION
S?
23
Hinweis der Redaktion
in Fig. 5. In Fig. 5, we have a test motor (2.2 kW) [12] supplied
from the 50-Hz mains. There is a second motor of slightly bigger
size (7.5 kW) [12] directly coupled to the test motor, acting as a
loading motor to the test motor. The loading motor is supplied
via an ABB drive [13] so that the torque of the loading motor
can be changed in order to test different loading conditions for
the test motor. In order to load the test motor, the direction of
rotation of the loading motor should be opposite to that of the
test motor. This can be done via the drive, as indicated in Fig. 5.
The test motor is an ABB three-phase, two pole pairs IM, type
M2AA LA4 [12]. Fig. 6 shows the nameplate ratings of the test
motor. Stator winding of the motor was connected in star (Y).
The effective nameplate ratings are given in Table I.
For the proposed PF measurement, the motor current of the
test motor is measured using a motor current measurement
module. To validate the proposed current-only PF estimation,
the actual PF also needs to be measured. For that purpose, a
high-resolution oscilloscope from Agilent [14] is used to make
synchronized measurement of the supply voltage and the motor
current. The oscilloscope has four input channels. So, two phases
of the supply voltage and two phases of the motor currents are
in Fig. 5. In Fig. 5, we have a test motor (2.2 kW) [12] supplied
from the 50-Hz mains. There is a second motor of slightly bigger
size (7.5 kW) [12] directly coupled to the test motor, acting as a
loading motor to the test motor. The loading motor is supplied
via an ABB drive [13] so that the torque of the loading motor
can be changed in order to test different loading conditions for
the test motor. In order to load the test motor, the direction of
rotation of the loading motor should be opposite to that of the
test motor. This can be done via the drive, as indicated in Fig. 5.
The test motor is an ABB three-phase, two pole pairs IM, type
M2AA LA4 [12]. Fig. 6 shows the nameplate ratings of the test
motor. Stator winding of the motor was connected in star (Y).
The effective nameplate ratings are given in Table I.
For the proposed PF measurement, the motor current of the
test motor is measured using a motor current measurement
module. To validate the proposed current-only PF estimation,
the actual PF also needs to be measured. For that purpose, a
high-resolution oscilloscope from Agilent [14] is used to make
synchronized measurement of the supply voltage and the motor
current. The oscilloscope has four input channels. So, two phases
of the supply voltage and two phases of the motor currents are