Estimation of induction motor operating power factor

Estimation of Induction Motor

Operating Power Factor From

Measured Current and

Manufacturer Data

Sl

NoTittle 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

index

Estimation of IM Operating PF From Measured Current and Manufacturer Data

1


introduction

“ THREE-PHASE induction motors (IM) are industrial work-

horses, responsible for consumption of 40–50% of generated

electrical power. ”

2


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

BACKGROUND INFORMATION

IM equivalent circuit.

v1

r1 X1

I1

rC Xm

I1’

X2’

r2’s

I1 = Im +I1 ’ Load

componentExciting

component

Im

core-loss

resistance magnetizing

reactance

Leakage reactance sta

tor

Roto

r


4

𝜽𝟎 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

Power Factor and Motor Load


5


CURRENT –ONLY, PF ESTIMATION


Total input electrical apparent power , 𝑷 = 𝟑𝑽𝑰

Active power, for supplying the load ,𝑷 = 𝟑𝑽𝑰 cos ∅

PF =𝑷

𝑷= cos ∅ Eqn 1

I= 𝑰𝒂𝒄𝒕𝒊𝒗𝒆𝟐 + 𝑰𝒓𝒆𝒂𝒄𝒕𝒊𝒗𝒆

𝟐

𝑰𝒂𝒄𝒕𝒊𝒗𝒆 = I cos ∅ , 𝑰𝒓𝒆𝒂𝒄𝒕𝒊𝒗𝒆= I sin ∅ 𝑰𝒓𝒆𝒂𝒄𝒕𝒊𝒗𝒆=I sin cos−𝟏(𝑷𝑭) Eqn 2

PF = 𝑐𝑜𝑠 ∅ = 1 − 𝑠𝑖𝑛∅2 = 1 − (𝐼𝑟𝑒𝑎𝑐𝑡𝑖𝑣𝑒

𝐼)2 Eqn 3

6


𝑰𝒓𝒆𝒂𝒄𝒕𝒊𝒗𝒆 remains constant, it can be estimated from

nameplate data using 𝑰𝒓𝒆𝒂𝒄𝒕𝒊𝒗𝒆= I sin cos−𝟏(𝑷𝑭)

At no-load condition, there is no active current flow.

So, at no-load, I= 𝑰𝒓𝒆𝒂𝒄𝒕𝒊𝒗𝒆 PF = 0 (

PF= 1 − (𝐼𝑟𝑒𝑎𝑐𝑡𝑖𝑣𝑒

𝐼)2 )

As motor load increases the PF will increase toward

unity. Motor

load

increases

Total

motor I

increase

𝑰𝒓𝒆𝒂𝒄𝒕𝒊𝒗𝒆remains

constant

(𝑰𝒓𝒆𝒂𝒄𝒕𝒊𝒗𝒆

𝑰)𝟐

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

Estimate the 𝑰𝒓𝒆𝒂𝒄𝒕𝒊𝒗𝒆 from the nominal PF out of

the nameplate data. ( 𝑰𝒓𝒆𝒂𝒄𝒕𝒊𝒗𝒆=I sin cos−𝟏(𝑷𝑭)) Estimate the operating PF from measured motor

current & constant 𝑰𝒓𝒆𝒂𝒄𝒕𝒊𝒗𝒆

“ It would not require synchronized voltage and current measurement like in the displacement PF measurement principle.”

Basic Algorithm


8

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

NOMINAL VOLTAGE 400V

NOMINAL CURRENT 4.9A

RATED POWER 2.2KW

RATED PF 0.81

RATED SPEED 1430 RPM

NAME PLATE RATING OF THE

TEST MOTOR TYPE ABB M2AALA4.


𝑰𝒓𝒆𝒂𝒄𝒕𝒊𝒗𝒆=I sin cos−𝟏(𝑷𝑭)= 4.9* Sin (cos−1 0.81)

=2.87

Measured

CURRENT

I(A)

I/𝑰𝒏𝒐𝒎𝒊𝒏𝒂𝒍 (%) 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

MEASURED I,CALCULATED PF

FROM

CURRENT-ONLY METHOD AT

DIFFERENT LOAD

PF= 1 − (𝐼𝑟𝑒𝑎𝑐𝑡𝑖𝑣𝑒

𝐼)2=

1 − (2.87

3.01)2 = 0.3

10


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,

𝒇𝟎 ∶ 𝐒upply frequency.

PF=𝒄𝒐𝒔(𝒙

(𝟏

𝒇𝟎)

X 𝟑𝟔𝟎𝟎)

The current ZC deviation is positive or negative w.r.t.

The voltage would decide whether the PF is lagging

or leading.

11

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

Tektronix Voltage probe

Current hall-sensor probe

Main supply (50 Hz)

oscilloscope

Agilent

IM 3 ph

12

±x ms : 3.4 ms ,𝒇𝟎 =𝟓𝟎

Pf =𝒄𝒐𝒔(𝒙

(𝟏

𝒇𝟎)

X 𝟑𝟔𝟎𝟎)

=cos(3.4

20𝑿 𝟑𝟔𝟎𝟎) =

0.48

PF MEASUREMENT USING

DISPLACEMENT IN THE ZC OF THE

SYNCHRONIZED SUPPLY VOLTAGE

AND MOTOR CURRENT AT 66%

CURRENT 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

RMS CURRENT,TIME DEVIATION

BETWEEN ZC OF VOLTAGE

&CURRENT, PF FROM

DISPLACEMENT METHOD AT

DIFFERENT LOAD

13

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 ( 𝑃 ) per phase from the

measurement in the scope.

Estimate the PF using the equation : PF = 𝑃

𝑉𝐼

14

Power factor measurement using

instantaneous power from the measured

synchronized supply voltage and the

motor current at 66% current load.

𝑃=328.525 V=230 I=3

PF = 𝑃

𝑉𝐼=

328.525

230 𝑋 3= 0.476

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

RMS CURRENT,RMS VOLTAGE ,

INSTANTANEOUS POWER ,POWER

FACTOR AT DIFFERENT LOAD


15


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

Power

3.00 0.48 4.24 0.79

DISCUSSION OF RESULTS

“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


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


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.

Under no-load condition, the referred reactance of the rotorX2’ 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.

18


Measure

d c

urr

ent

(A)

Measured current (A)

𝑁𝑜𝑚𝑖𝑛𝑎𝑙 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 (𝐴)

Pow

er

facto

r

Measured current (A)

𝑁𝑜𝑚𝑖𝑛𝑎𝑙 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 (𝐴)

Underload protection using motor current & power factor

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


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

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

http://www.abb.com/motors

http://www.abb.com/drives

Thank you!

22

ANY

QUESTION

S?


23

Engineering

Estimation of induction motor operating power factor