ONLINE MONITORING AND ANALYSIS OF INDUCTION ......N. Hariharavarshan, Jeyaram Durga Manian and R....

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International Journal of Electrical Engineering & Technology (IJEET) Volume 7, Issue 6, Nov–Dec, 2016, pp.36–54, Article ID: IJEET_07_06_004

Available online at

http://www.iaeme.com/IJEET/issues.asp?JType=IJEET&VType=7&IType=6

ISSN Print: 0976-6545 and ISSN Online: 0976-6553

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© IAEME Publication

ONLINE MONITORING AND ANALYSIS OF

INDUCTION MOTOR USING CURRENT SIGNATURE

ANALYSIS IMPLEMENTING WAVELET ANALYSIS

AND FFT ANALYSIS

N. Hariharavarshan, Jeyaram Durga Manian and R. MelvinaMinny

Electrical and Electronic Engineering, Panimalar Engineering College, Chennai, India

ABSTRACT

Three phase squirrel cage induction motors are widely used in industrial applications.Thus

fault occurring in these motors will adversely affect the Industrial products and increase the shut

down time. As time moves, the machine develops major faults and disturbs the production line

leading to major financial losses and production instability. In order to overcome these losses at

critical circumstances, the fault could be found out prior to it rather than becoming into a major

fault. Condition Monitoring System helps in predicting and identifying the pre fault condition in

faster and effective way, which prevents the unwanted breakdown timein working hours. Among the

various Condition Monitoring Systems such as thermal monitoring, vibration monitoring and so on

,Motor Current Signature Analysis (MCSA) is the best possible solution.As among all the available

solutions are intrusive in operation (need shutdown period for testing), maintenance of the

measuring equipment, bulk initial capital required, very high sensitivity and unreliable source with

even small unsuitable environmental factors, slower in fault detection .All the undesirable factors

of the other methods made MCSA an undisputable, viable, feasible ,reliable and best solution for

fault detection mechanism to be followed in even small scale industries. They are being followed in

many places often due to it’snon-intrusive approach to detect the fault, in addition a fast identifying

method using FFT analysis .Normally MCSA are performed with FFT analysis which is effective in

cases of constant load but as of today it is being an emergent problem to find fault in changing and

fluctuating load conditions, where FFT fails its viability and this condition made the solution to

choose wavelet analysis. In this paper, we have designed a complete3 phase induction motor with

matlab toolsand induced faults into the motor externally (by varying the input frequency) and

analysed the problem by using wavelet analysis. By analyzing the frequency spectrums of the stator

current, electrical torque and angular speed, we were able to determine the total harmonic

distortion for different faults and it will be produces clear explanation about the fault that is being

introduced into the machine which will finally pollute the power system by injecting harmful

harmonics into it.This method helps in envisioning the fault and thus helps in reducing the

occurrence of severe faults (Brinelling, Arcing, Rotor Failure etc.) in the motor. Thus it acts as an

important tool to protect machines from falling into unrepairable form. The MCSA model takes the

initial step in improving the efficiency of the system and the technique utilizes the result of spectral

analysis of stator current, torque and speed characteristics of the motor model.

Online Monitoring and Analysis of Induction Motor Using Current Signature Analysis Implementing Wavelet

Analysis and FFT Analysis


Key words: MCSA (Motor Current Signature Analysis), FFT, Induction motor, wavelet.

Cite this Article: N. Hariharavarshan, Jeyaram Durga Manian, and R. MelvinaMinny. Online

Monitoring and Analysis of Induction Motor Using Current Signature Analysis Implementing

Wavelet Analysis and FFT Analysis. International Journal of Electrical Engineering &

Technology, 7(6), 2016, pp. 36–54.

http://www.iaeme.com/IJEET/issues.asp?JType=IJEET&VType=7&IType=6

1. INTRODUCTION

As the backbone of modern industry, induction motors are virtually used in every industry. Online fault

identification of induction motors are very important to ensure safe operation, timely maintenance,

increased operation reliability, and preventive rescue especially in high power applications. The induction

motor faults are generally classified as either mechanical or insulation system faults. An induction motor is

an AC electric motor in which the electric current in the rotor is needed to produce torque which is

obtained by electromagnetic induction produced from the magnetic field of the stator winding. An induction

motor therefore does not require mechanical commutation.

An induction motor's rotor can be either wound type or squirrel-cage; they both are the big

classifications between them in the view of rotor winding. Three phase induction motor or asynchronous

motors are widely used in industrial drives because they are rugged, reliable and have an economical

driving force. Common mechanical faults include rotor bar breakage, rotor end ring cracking, static and/or

dynamic air-gap irregularities, stator winding faults, bent shaft, misalignment, and bearing gearbox

failures. Statistical data indicates that the mechanical faults are responsible for more than 95% of all

failures. Single-phase induction motors are used extensively for smaller loads, such as household

appliances like fans, mixers and other house hold items. This makes induction motor an indisputable

competitor and indispensable position in the industries. The advantage of self-starting makes it more

efficient in the felid of electrical; this made the motor put into an immense research for improving its

efficiency and reducing its work spot failures.

The parameters that are considered in the construction of a induction motor is given in Table 1.These

parameters are made standard all over the construction process.

Table 1 Parameters of SCIM considered

Parameter Value

Power Rating of SCIM 40 KW

Frictional coefficient (B) 0.0100N-m/(rad/sec)

Moment of inertia (J) 1.662 kgm2

Mutual inductance (Lm) 0.040H

Rotor inductance (Lr) 0.043H

Poles (P) 2

Rotor resistance (Rr) 0.187Ω

Stator resistance (Rs) 0.087Ω

3 phase voltage(Vph) 375.588V

Peak voltage (Vm) 258.588V



2. CONSTRUCTION OF INDUCTION MOTOR SIMULINK MODEL

Figure 1 Generalized block representation of simulink system

The figure 1 is the overall block diagram that we followed to get the complete current spectrum that is

required to obtain the MCSA. The motor environment which is mentioned in the diagram is completely

implemented in the simulink using matlab and the formulae of designing induction motor .The formulae

have been implanted for each part of its construction. The total induction motor is distinguished into: • Electrical Subsystem model

• Torque Subsystem Model

• Mechanical subsystem model

• Stator Current Output Subsystem model

2.1. Electrical Subsystem Model

= 10−1/2√3 2 −1/2−√3 2 (1)

Equation (1):(Park’s Transformation, here we don’t need zero axis component and thus the equation is

simplified) is used to covert three phase voltageto two phase voltage

Where ,, and are the three-phase stator voltages, while and are the two-axis

components of the stator voltage vector . In the two-axis stator reference frame, the current equation of

an induction motor can be written as:

iiii = ! L0L#0

0L0L#L#0L0

0L#0L$%&

'()

× +,-

+-VVVV/0

−+,- R 0 0 000−P2 ω)L#

RP2 ω)0 L#0R−P2 ω)L

0P2Rω)L

/40 iiii

/440567

68 dτ

(2)

In the electrical model, the three-phase voltage [, ,] is the input and the current vector

[i, i, i,i] is the output vector. The rotor voltage vectors normally zero because of the short-circuited

cage rotor winding in the SCIM, i.e. Vdr=0 and Vqr=0.




Figure 2 Park’s transform usage subsystem

In the figure 2 the inputs 1,2 represent Vdq and Vqs respectively. In electrical engineering, direct

quadrature zero(ordq0ordqo) transformation or zero–direct-quadrature (0dq) transformation is a

mathematical transformation that rotates the reference frame of three-phase systems in an effort to simplify

the analysis of three-phase circuits. This transform is referred to as Park’s transformation (Equation (1)).

2.2. Torque Subsystem Model

The electrical transient model in terms of voltages and currents can be given in matrix form as:

<==>??@AA

B = <==> C + EF GHF EFI GHFI−GHFEFI−(GH − G?)FI

C + EF(GH − G?)EFI FI −GHFI EFIC? + EF? (GH − G?)F?−(GH − G?)F? C? + EF?@AAB

<==>LLL?L?@AA

B

(3)

Where

S is the Laplace operator .The speed G?is considered constant for an infinite inertia load.

The electrical dynamics of the machine are given by a fourth-order linear system.

The speed can be related to torque as

MH = MN + O PQR (4)

GI = ST G? (5)

By substituting equation (4) in (5)

MH = MN + ST O PUR (6)

Where TLis the load torque.

The steady state equations can always be derived by substituting the time derivative components to

zero.

The steady state equations can be given as = CV + WGHX (7)



0 = YUZ V?+ WGHX? (8)

Where the complex vectors have been substituted by the rmsphasors. These equations satisfy the steady state

equivalent circuit if the parameter CI is neglected. The torque can be generally expressed in the vector form as:

MH = [S (TS)XI\\\\\] × V?\\\] (9)

Resolving the variables into ^H − _H components

MH = [S (TS)(XIL? − XIL?) (10)

The other forms of torque equations can be given as:

MH = [S (TS)(XIL − XIL) (11)

MH = [S (TS)(XL − XL) (12)

MH = [S (TS)FI(LL? − L?L) (13)

MH = [S (TS)(X?L? − X?L?) (14)

In the two-axis stator reference frame, the electromagnetic T is given by

M = TNQ[ (L?L − L?L) (15)

Figure 3 Torque and speed subsystem




Figure 4 overall construction of induction motor

The figure 4 gives the overall construction of 3phaseinduction. The separate subsystems which are

explained in the earlier blocks, that constitute the induction motor. The variation of frequency is being

provided in the repeating frequency, this adds up as a external disturbance. The frequency that are

produced by certain faults are determined by experiments and formulae. The output is being obtained in

the workspace that is being further processed using wavelets (splitting the wave into smaller parts) and

FFT analysis.

This processing is very sensitive that, it will be able to find a smallest disturbance that is trying to

ingress into the system. This saves a lot of man, cost, and time waste and makes power system a proactive

area than reactive which normally it is. Further the faults that are being commonly found and not taken

timely reaction are jot down.

3. FAULT ANALYSIS

3.1. Introduction to Faults

The major faults in induction motor are due to stator, rotor and bearings. These faults can be classified as:

• Stator faults like shaft speed oscillation, Stator inter turn faults and grounded faults. The stator related faults

contribute upto 38% of total fault conditions.

• Rotor related faults like rotor asymmetry, rotor end ring faults and broken rotor bars. The rotor related

problems constitute to about 10 % of the faults occurring in the system.

• Bearing fault mainly includes the bearing ball failure. Here the racing of balls may lead to major damage of

induction motor. The bearing failure corresponds to 40% of the total failure.

3.2. Type of Faults

The induction motor is modeled using matlab and the frequency value for each fault is varied depending on

the formula. The frequency for healthy condition is 50 Hz. The frequency changes for faulty conditions.

The faulty conditions are broken rotor bar, bearing failure, shaft speed oscillation and rotor asymmetry.

Here the major faults are broken rotor bar and bearing failure. They are given as

3.3. Broken Rotor Bar

The broken rotor bar usually occurs mainly if there is a large load stress or subsequent heating on the rotor

bars. The uneven heating may lead to cracking and as the cracking develops it increases the resistance of

the rotor bar and hence it will cause increased current flow in the neighboring rotor bar and reduced current

flow in the broken rotor.



When the rotor is broken, the induction motor will not act at the fundamental frequency of the rotor but

it will operate at a new frequency which is given by H. The broken rotor bar can be distinguished by

presence of side bands. The new frequency is given by

H = ` a(bc? ± bH) e%$f g ± bh (16)

Where,

H=frequency for the broken rotor bar

`=supply frequency (50Hz)

Nr=rotor bars number=30

bH=eccentricity order number =0

s=slip per unit=0.4

Ns=supply frequency harmonic rank

P=number of pole pair

By using the values we have the values of H = 180.00kl

3.4. Bearing Failure

Bearing failure mainly occurs when a large load is applied on small area. This causes brinelling which may

lead to a permanent dent in the induction motor. The bearing failure can be found from the stator current

spectra. Since, the ball bearing will produce motions along with the stator. It produces a frequency greater

than the original frequency. The frequency variation depends on the bearing parameters and hence it varies

for each bearing. The frequency is given by

mn = ` ± b o,) (17)

o,p = mS ? a1 ± qrTr cos vh (18)

Where,

nb=number of balls

n=1,2,3,….

?=mechanical rotor speed in Hz(1380rpm to 23Hz)

BD=Ball diameter (0.6mm)

PD =bearing pitch diameter (0.31mm) v=contact angle=15.1degree

o,p=33.7Hz

By using the values we have the values of mn =252.23Hz


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3.5. Simulation Results for Various Fault C

The simulation results are further classified:

3.5.1. Healthy Motor Characteristics

The induction motor is modeled for 40KW, 2 Pole pair, and 460V Squirrel Cage Induction motor. The

healthy motor condition for 50Hz fundamental frequency is given by figure1

Figure

Figure

Figure



EET/index.asp 43

Simulation Results for Various Fault Conditions

results are further classified:

haracteristics


healthy motor condition for 50Hz fundamental frequency is given by figure1

Figure 5.a Speed characteristic of a healthy motor

ure 5.b.Torque Characteristics of healthy motor

ure 5.c. Stator current spectrum healthy motor


[email protected]




Figure 5.d.Quadrature Stator Current value healthy motor

All the above figures are related to the steady state 50Hz frequency. It is very clear from figure (a)

because the speed becomes constant after transient period of the induction motor.

From figure (c) the analyzer output rises peak only at its fundamental frequency (50Hz).From figure

(b) the torque characteristics is same as that of an induction motor. The Iqs is having a transient period of

4s and after that the exponential term gets cancelled, it can be seen from the fig (d).

3.5.2. The Simulation Results for Broken Rotor

When broken rotor bar occurs, the characteristics changes significantly and the value for the sideband

frequency level of this fault if found using Equation (16)

Figure 6.a Speed characteristics for broken rotor bars

Figure 6.b.Torque time charachteristics of a broken rotor




Figure 6.c.Stator current spectrum of broken rotor

Figure 6.d. Quadrature stator current value of broken rotor

Broken rotor will drastically reduce the speed and the torque generated will be zero for ms depending

upon the no of rotor broken which is quite a serious problem that cannot be identified by other methods.

3.5.3. Simulation for Bearing failure

When bearing failure occurs, the frequency change of the spectrum depends on the dimensions of the

bearing and number of bearing which is considered. The frequency is given by (17) and (18).

Figure 7.a Speed characteristics for bearing failure



Figure 7.b.Torque time charachteristics of a bearing failure

Figure 7.c. Stator current spectrum of bearing failure

Figure 7.d. Quadrature stator current value of bearing failure

The difference in all the spectrum are very prominently seen, this is because a failure of bearing will

heavy effect in the production of torque due to high friction opposing. And the spectrum peak varies quit

prominently

4. FFT AND WAVELET ANALYSIS

A Fast Fourier transform will identify the peak point and will give specific information about the output

waveform. It is very essential to identify the waveform distortion from the ideal waveform. This analysis

gives the peak location of distorted waveform and the harmonic order they create in the system. A FFT

analysis is just not enough to clearly identify the magnitude of the waveform ,so the mother wavelet is




further divided into 6 child wavelets from mother wavelet, these child wavelets gives the possible small

peak achieved in each of the separated wavelet.

An FFT analysis allows to find the component of the frequency that disturbs output,but by doing this

we lose all control over the temporal spread.Whereas the original signal, when measured at a fixed time,

gives only absolute precision on the amplitude at that fixed time, but zero information about the frequency

spectrum of the signal. This makes wavelet analysis very pressing in this part so that we get the

characteristic of the parameter ω (which is the analogue of the frequency parameter k for the Fourier

transform), we can derive a characteristic frequency k(ω)and in the characteristic time t(ω) which makes it

a extraordinary perfection to the end result.

4.1. FFT Analysis

The spectrum analysis of the stator current, speed and torque will give the harmonic order. For healthy

condition the values are found with THD as 0.00 % as given in fig8.a, fig8.b and fig8.c respectively.

Figure 8.a. Healthy FFT analysis of Stator current

Figure.8.b.Healthy FFT analysis of speed

Figure 8.c.Healthy FFT analysis of Torque



For the broken rotor by performing FFT analysis it could be found that the THD of order 8 occurs.

Figure 9.a.Brokenrotor FFT analysis of Statorcurrent

Figure 9.b.Brokenrotor FFT analysis of Speed

Figure 9.c. Brokenrotor FFT analysis of Torque

For bearing failure, the frequency is very high and the harmonic level is of the order 11 for stator

current

Figure 10.a. Bearing failure FFT analysis Stator current




Figure 10.b. Bearing failure FFT analysis of Speed

Figure 10.c.Bearing failure FFT analysis torque

Table 2 FFT ANALYSIS THD COMPARISONS

S.No Parameters considered Healthy Broken Bars Bearing

Failure

1. THD for speed 0.00% 108.34% 102.27%

2. Fundamental for torque 106.3 1.214 2554

3. THD for torque 0.00% 585.25% 1419.74%

4. Fundamental for Is 262.8 3.09 6064

5. THD for Is 0.00% 1912.34% 6703.86%

6. Harmonic order of THD

Mechanical speed

1st

harmonic

order

8th order

harmonics

can be

noticed

11th order

harmonics


for Is

1st

harmonic

order

8th order

harmonics

can be

noticed

11th order

harmonics


for torque

1st

harmonic

order

8th order

harmonics

can be

noticed

11th order

harmonics



4.2. Wavelet Analysis Results

The wavelet analysis is done in matlab by programming and further decomposing into smaller wavelets of

spectrum. As the wavelet transformation is having an important requirement in the field wavelet

transformation is being applies in all the comparable parameters to find the prominent differences

compared to a healthy motor characteristics:

4.2.1. Healthy motor waveforms wavelet analysis

Healthy condition waveform for the motor when wavelet analysis is done for a frequency of 50Hz.this is

the fundamental frequency of a power system.

Figure 11.a.Waveforms of stator current, speed, torque in healthy condition

Figure 11.b.Wavelet decomposition of Stator current healthy condition

Figure 11.c.Wavelet decomposition of Torque in healthy condition




Figure 11.d.Wavelet decomposition of speed in healthy condition

The above waveforms are the decomposition of mother wave form into child wavelet are clearly made

for study purpose.

4.2.2. Broken rotor bars wavelet Transform

Broken rotor is is another type of faults which is found in the induction motor very frequently the

fundamental frequency is not 50Hz but changes to 180Hz it can be calculated from Equation(16).

Figure 12.a.Waveforms of stator current, speed, torque in broken rotor bars

Figure 12.b.Wavelet decomposition of stator current waveform broken rotor bars

Figure 12.c.Wavelet decomposition of torque waveform of broken rotor bars



Figure 12.d.Wavelet decomposition of speed waveform of a broken rotor bars

Bearing failure is is another type of faults which is found in the induction motor very frequently the

fundamental frequency is not 50Hz but changes to 252Hz it can be calculated from (2),(3)

Figure 13.a.Is, Speed and Torque waveforms of bearing failure

Figure 13.b.Wavelet decomposition of stator current of bearing failure

Figure 13.c. Wavelet decomposition of torque of bearing failure




Figure 13.d.Wavelet decomposition of speed waveform of a bearing failed motor

Table 3 WAVELET ANALYSIS COMPARISONS

5. CONCLUSION

The results produced by approaching machines with these methods are very precise and the time for

identification of faults is not more than 10 s of occurring faults. In real work environment a fault finding

process is achieved with lower accuracy, though achieved the differentiation of faults and identification of

the intrusion depth of the fault in the system are not identified in a fine manner.

From table III it is tacit that the amount of energy spent recklessly, when a fault occurs. The faults

when left unidentified will slowly reduce the machine performance and will finally leads banishing of the

machine from the work environment .This makes the wastage of the life time of a machine. The table II

and table III remains console of the whole paper which precisely desiccates the fault levels identification

strategy.

REFERENCE

[1] M.E.H.Benbouzid,M.Vieira,C.Theys (January 1999): Induction motors fault detection and localization

using stator current advanced signal processing.IEEE Transactions on Power Electronics Year:

1999, Volume: 14, Issue: 1

S.No Parameters Healthy

condition

Broken

rotor bar

Bearing

Failure

1. Energy Stator

current

1.3899e+06

2.4784e+06

9.29e+05

2. Standard Deviation

Stator current

4.6970

2.4637

1.5717

3. Energy Torque

7.8176e+05

2.4238e+03

42.7357


Torque

6.6761

0.4801

0.0106

5. Energy Speed

5.1894e+07

1.8259e+06

1.8668e+07


Speed

12.3349

5.4466

7.9625



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(INDICON)Year: 2015

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