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Chapter 1

Introduction

1.1: Introduction

Motors are critical components for electrical utilities and process industries. A

motor failure can result in the shutdown of a generating unit or production line.

The operators of motors or electrical drive systems are under continual pressure to

reduce maintenance costs and prevent unscheduled downtimes, which result in lost

production and financial income Early detection of fault within a motor prior to

complete failure provides an opportunity for maintenance to be performed on

scheduled routine without loss of production. This has arises the need for

condition based maintenance strategies i.e. monitoring the condition of motors and

planning the maintenance based on an indication that is a problem about to occur.

Condition monitoring implies monitoring various parameters of a machine in order

to assess the health of the machine. Condition monitoring is equivalent to

cardiogram analysis of the human heart. A cardiogram assesses the state and health

of the human heart. Similarly the various parameters measured during the

condition monitoring of the electrical equipment assess the health of the machine.

Many operators now use online condition based maintenance strategies in parallel

with conventional planned maintenance schemes. This has reduced unexpected

failures, increased the time between planned shutdowns and reduced operational

cost. During past fifteen years there has been substantial amount of research into

the creation of new condition monitoring techniques. New methods have been

developed which are now being used by the operators and research is continuing

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with the development of new and alternative online diagnostic techniques. This

basic objective of this work is to diagnose the different types of motors faults

online through processing and analysis of motor current. This technique is often

called "current signature analysis"

1.2: CURRENT SIGNATURE ANALYSIS

Signature analysis is the procedure of acquiring the motor current and voltage

signals, performing signal conditioning and analyzing the derived signals to

identify the various faults. Motor current acts as an excellent transducer for

detecting fault in the motor. Spectrum analysis of the motor's current and voltage

signals can hence detect various faults without disturbing its operation. Current

signature analysis involves the measurement of electric current around any one

phase either through clamp on meters or through CT's. This current is then

transformed into its frequency spectra and analyzed for detection of fault.

Information on the application of specific condition monitoring techniques in

industry is not available and evidence of diagnosing problem via reference to

actual on site case histories are also not considered. There is no clear distinction

made between monitoring techniques, which are at the R & D stage in comparison

to those which are being successfully applied in industry. It is a fact that the

operator of induction motor requires evidence of the successful application of

monitoring systems to assist him in their selection of appropriate systems. An

operator must treat each induction motor drive as a unique entity and the potential

failure modes, fundamental causes, mechanical load characteristics and operational

conditions have all to be taken into consideration when a condition monitoring

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system is being selected. The focus has been on the use of condition monitoring

using Electrical signature analysis technique.

Figure 1.1: Current Signature with No bar Failure

Figure 1.2: Current Signature with bar Failure

1.3: METHOD OF MEASUREMENT

The method of measurement involves measurement of electric current around any

one phase or three phases. Current can be measured with the help of current

transformers. The current signal is conditioned. DAQ (Data acquisition device

USB 6009) digitizes the signal to permit the time and frequency spectra for

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analysis. The time varying current signal is then transformed into frequency

domain through FFT algorithm.

1.4: FAULTS IN INDUCTION MOTOR

The most prevalent faults in Induction Motor are briefly categorized as:

Rotor Faults (10%)

Bearing Faults (41%)

Eccentricity Faults (12%)

Stator Faults (37%)

The surveys indicate that in general, failures in electrical machines are dominated

by bearing and stator faults with rotor winding problems being less frequent.

1.4.1: ROTOR FAULT

Each individual rotor bar can be considered to form a short pitched single turn,

single-phase winding. The air gap field produced by a slip frequency current

flowing in a rotor bar will have a fundamental component rotating at a slip speed in

the forward direction with respect to rotor and one of equal magnitude that rotates

at the same speed in the backward direction. With symmetrical rotor, the backward

component sums to zero. For a broken bar rotor, however the resultant is non-zero.

The field, which rotates at slip frequency backward with respect to the rotor, will

induce EMF on stator side that modulates the main frequency component at twice

slip frequency.

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The measurements on totally enclosed IHP motor with cast rotor was undertaken to

verify the presence of broken bar signals predicted by the above analysis. The

motor was fed with 3ph-balanced supply through measurement panel. The line

current was recorded through the non-inductive shunt. The signal was then given to

Oscilloscope for the processing. Initially the no load current and no load losses

were also calculated and then the motor was operated at rated output. The slip and

efficiency of the motor were calculated. The same experiments were repeated first

for motor with one broken bar, two broken bar and three broken bars. With one

broken bar, there is increase in the magnitude of LSB 1.The ratio also drops down

to 31 indicating the fault within rotor. The constant losses and no load current of

the motor also increase. There is also decrease in the efficiency of the motor. With

increase in the bar failure there is increase in magnitude of LSB1 and ratio and

efficiency goes on decreasing. The slip, constant losses and no load current go on

increasing.

1.4.2: BEARING FAULTS

The relationship of the bearing vibration to the stator current spectrum can be

determined by remembering that any air gap eccentricity produces anomalies in the

air gap flux density. As with the air gap eccentricities, these variations generate

stator current at predictable frequencies. Where m = 1, 2, 3 ... and f is one of the

characteristics vibration frequencies calculated based on bearing dimensions. The

characteristic frequencies for ball bearings are based upon dimensions.

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where n(b) is the number of balls, f(r) the mechanical rotor speed in Hz, PD the

bearing pitch diameter, BD the ball diameter. The Bearing fault was simulated by

replacing the driving end bearing with the degraded bearing of same size and

number. The fault frequencies were predicted as these frequencies were visible in

the frequency spectrum.

1.4.3: ECCENTRICITY FAULTS

The use of current monitoring for detection of air gap eccentricity to identify the

frequency component in the current spectrum .The eccentricity fault was simulated

by inserting the 0.1mm copper strip in the end shield of driving end. This will

create the static eccentricity in the motor. The motor will be allowed to run at rated

speed to create the dynamic eccentricity. These frequencies were visible in the

frequency spectrum.

1.4.4: STATOR FAULTS

Stator winding failures are also a major problem in low and medium voltage

induction motors. It should be noted that volume of low voltage motors is much

greater than high voltage machines. In motors it is normally the case that insulation

degradation cannot be initially diagnosed via on line measurements and the first

indication of a problem will be that a fault actual develops. It is important to

appreciate that there is clear distinction between insulation degradation prior to a

fault and an actual fault. Stator winding faults can be classified as follows:

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1. Turn to turn short within coil

2. Short Circuit between coils of the single phase motor

3. Phase to phase short circuit

4. Phase to earth short

5. Open circuit in a single phase (Single Phasing)

Pre warning of serious problem (3 and 4 above) can only be achieved if shorted

turns within coil (one or two shorted turns) can be initially diagnosed via online

diagnostic techniques. This requires continuous online monitoring to diagnose the

faults state in 1 and 2 above. There is also the question of how long does it take for

shorted turns within coil to develop into phase to phase or phase to earth fault and

motor failure? This question has not been resolved and will be function of many

variables and will be unique to each motor. Some operators and manufacturers

have previously considered that it is not worth diagnosing shorted turns or coils in

stator windings since the lead time to failure is too short to merit a continuous

online diagnostic system. The concept that the motor has already developed a fault

and will need to be repaired has prevailed. This philosophy is generally now

considered to be somewhat out dated and defeatist. In modern production process

any lead time can be extremely advantageous since unexpected failure of a drive

can be very costly and in some industries it can also be a serious safety hazard. If

shorted turns in a stator coil can be diagnosed a preplanned shut down can be

arranged for the motor to be replaced by healthy one and the faulty one sent for

repair With respect to the problem of single phasing it is relatively easy to

diagnose the problem provided the correct protection equipment is used to cater for

all load conditions. It is also possible to monitor and analyze signals such as

current to diagnose single phasing under any load operating condition. The stator

winding itself is used as the sensor for the detection of abnormalities in the

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windings. The harmonics which are expected to vary and which have their origin

in the stator currents.

The stator fault i.e. inter-turn short circuit in the winding was simulated by the

rewinding the 1 (h. p), 4 pole motor. The winding of the motor was re-winded and

the tapings at the end of the coils and the tapings at 1, 3, 5, 20 and 25turns were

taken out on the terminal box. The inter-turn short circuit was created by shorting

the turns through rheostat to limit the short circuit current to rated current of the

winding. Fig. 7 shows the arrangement for simulating the stator fault. The fault

frequencies related to stator fault were predicted and obtained in the spectrum It

was found that for the 2 turn inter-turn short circuit, frequency component 398 was

most sensitive to fault. From no fault to maximum fault current variations in the

range of 49% was recorded. Components 498 Hz were found to be less sensitive.

The component 198 and 298Hz was decreasing in all the phases. But decrease is

more pronounced in the faulty phase. For 4 turn inter-turn short circuit, frequency

component398 Hz also increased from no fault conditions. The variations of more

than 100% were observed from no fault to maximum fault current. For 20 turn

inter-turn short circuit, frequency component398Hz also increased. The variation

of more than 400%was observed from no fault to maximum fault current.

From the experiments it was apparent that some of the specified frequency

components were redundant, but whether or not this would be the case for other

machines.

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Chapter 2

Induction Motor

2.1: Introduction

An electric motor is an electrical machine that converts electrical energy into

mechanical energy. The reverse of this would be the conversion of mechanical

energy into electrical energy and is done by an electric generator.

In normal motoring mode, most electric motors operate through the interaction

between an electric motor's magnetic field and winding currents to generate force

within the motor. In certain applications, such as in the transportation industry with

traction motors, electric motors can operate in both motoring and generating or

braking modes to also produce electrical energy from mechanical energy.

Figure 2.1: Energy Conversion

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AC induction motors are commonly used in industrial applications. This type

of motor has three main parts rotor, stator and enclosure. The stator and rotor do

the work and the enclosure protects the stator and rotor.

Figure 2.2: Induction Motor

2.2: Stator Core

The stator is the stationary part of the motor’s electromagnetic Circuit. The stator

core is made up of many thin metal sheets called laminations. Laminations are

used to reduce energy loses that would result if a solid core were used.

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2.3: Stator Windings

Stator laminations are stacked together forming a hollow cylinder. Coils of

insulated wire are inserted into slots of the stator core.

Figure 2.3: Stator Core and Winding

When the assembled motor is in operation the stator windings are connected

directly to the power source. Each grouping of Coils together with the steel core it

surrounds becomes an electromagnet when current is applied. Electromagnetism is

the basic principle behind motor operation.

2.4: Rotor Construction

The rotor is the rotating part of the motor’s electromagnetic circuit. The most

common type of rotor used in an induction motor is a squirrel cage rotor. Another

type is wound rotor.

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Figure 2.4: Rotor

2.4.1: Wound Rotor Motor

A major difference between the wound rotor motor and the squirrel cage rotor is

that the conductors of the wound rotor consist of wound coils instead of bars.

These coils are connected through slip rings and brushes to external variable

resistors. The rotating magnetic field induces a voltage in the rotor winding.

Increasing the resistance of the rotor windings causes less current to flow in the

rotor windings decreasing rotor speed. Decreasing the resistance causes more

current to flow increasing rotor speed.

2.5: Synchronous Speed

The speed of the rotating magnetic field is referred to as the synchronous speed of

the motor. Synchronous speed is equal to 120 times the frequency (F) divided by

the number of motor poles (P).

The synchronous speed for a two-pole motor operated at 60Hz is 3600 RPM.

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Synchronous speed decreases as the number of poles increases. The following

table shows the synchronous speed at 60 Hz for several different pole numbers.

Table 1: Synchronous Speeds

2.6: Slip

For an induction motor the rotating magnetic field of stator must rotate faster than

the rotor to induce current in the rotor. When power is first applied to the motor

with the rotor stopped this difference in speed is at its maximum and a large

amount of current is induced in the rotor.

After the motor has been running long enough to get up to operating speed the

difference between the synchronous speed of the rotating magnetic field and the

rotor speed is much smaller. This speed difference is called slip. Slip is necessary

to produce torque. Slip is also dependent on load. An. Increase in load causes the

rotor to slow down increasing slip. A decrease in load causes the rotor to speed up

decreasing slip. Slip is expressed as a percentage and can be calculated using the

following formula.

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For example a four-pole motor operated at 60 Hz has a synchronous speed of 1800 RPM. If its

rotor speed at full load is 1775 RPM then its full load slip is 1.4%

2.7: Types of Torque

There are different types of torque associated with induction motor. They are

briefly described below.

2.7.1: Starting Torque

Starting torque also referred to as locked rotor torque is the torque that the motor

develops each time it is started at rated voltage and frequency. When voltage is

initially applied to the motor’s stator there is an instant before the rotor turns.

2.7.2: Pull-out Torque

As the motor picks up speed, torque decreases slightly until a point is reached. The

torque available at this point is called pull-up torque.

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2.7.3: Breakdown Torque

As torque increases up to a maximum value at approximately 200% of full-load

torque. This maximum value of torque is referred to as breakdown torque.

2.7.4: Full-Load Torque

Torque decreases rapidly as speed increases beyond breakdown torque until it

reaches full-load torque at a speed slightly less than 100% of synchronous speed.

Full- load torque is developed with the motor operating at rated voltage, frequency,

and load.

Figure 2.5: Torque Speed Characteristics of Induction Motor

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Speed-torque curves are useful for understanding motor performance under load.

The following speed-torque curve shows four load examples. This motor is

appropriately sized for constant torque load 1 and variable torque load 1. In each

case, the motor will accelerate to its rated speed. With constant torque load 2, the

motor does not have sufficient starting torque to turn the rotor. With variable

torque load 2, the motor cannot reach rated speed. In this example, the motor will

most likely overheat until its overload relay trips.

Figure 2.6: Torque Speed Characteristics w.r.t Loads

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2.8: Types of Induction Motors

There are two types of induction motor with respect to supply voltages.

Single phase induction motor

Three Phase induction motor

2.8.1: Single Phase Induction Motor Types

There are four types of single phase induction motor.

1. Split Phase induction motor.

2. Capacitor Starts induction motor.

3. Capacitor Start-Capacitor Run induction motor.

4. Shaded Poles induction motor.

2.8.1.1: Split Phase Induction Motor

In addition to the main winding or running winding, the stator of single phase

induction motor carries another winding called auxiliary winding or starting

winding. A centrifugal switch is connected in series with auxiliary winding. The

purpose of this switch is to disconnect the auxiliary winding from the main circuit

when the motor attains a speed up to 75 to 80% of the synchronous speed. We

know that the running winding is inductive in nature. Our aim is to create the phase

difference between the two winding and this is possible if the starting winding

carries high resistance. Let us say Irun is the current flowing through the main or

running winding, Istart is the current flowing in starting winding, and VT is the

supply voltage.

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Figure2.7: Circuit Diagram of Split Phase Induction Motor

We know that for highly resistive winding the current is almost in phase with the

voltage and for highly inductive winding the current lag behind the voltage by

large angle. The starting winding is highly resistive so, the current flowing in the

starting winding lags behind the applied voltage by very small angle and the

running winding is highly inductive in nature so, the current flowing in running

winding lags behind applied voltage by large angle. The resultant of these two

current is IT. The resultant of these two current produce rotating magnetic field

which rotates in one direction. In split phase induction motor the starting and main

current get spitted from each other by some angle so this motor got its name as

split phase induction motor.

2.8.1.1.1: Applications of Split Phase Induction Motor

Split phase induction motors have low starting current and moderate starting

torque. So these motors are used in fans, blowers, centrifugal pumps, washing

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machine, grinder, lathes, air conditioning fans, etc. These motors are available in

the size ranging from 1 / 20 to 1 / 2 KW.

2.8.1.2: Capacitor Start Induction Motor

The working principle and construction of Capacitor start inductor motors and

capacitor start capacitor run induction motors are almost the same. We already

know that single phase induction motor is not self-starting because the magnetic

field produced is not rotating type. In order to produce rotating magnetic field there

must be some phase difference. In case of split phase induction motor we use

resistance for creating phase difference but here we use capacitor for this purpose.

We are familiar with this fact that the current flowing through the capacitor leads

the voltage. So, in capacitor start inductor motor and capacitor start capacitor run

induction motor we are using two winding, the main winding and the starting

winding. With starting winding we connect a capacitor so the current flowing in

the capacitor i.e. Ist leads the applied voltage by some angle.

Figure 2.8: Capacitor Start Induction Motor

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The working principle and construction of Capacitor start inductor motors and

capacitor start capacitor run induction motors are almost the same. We already

know that single phase induction motor is not self-starting because the magnetic

field produced is not rotating type. In order to produce rotating magnetic field there

must be some phase difference. In case of split phase induction motor we use

resistance for creating phase difference but here we use capacitor for this purpose.

We are familiar with this fact that the current flowing through the capacitor leads

the voltage. So, in capacitor start inductor motor and capacitor start capacitor run

induction motor we are using two winding, the main winding and the starting

winding. With starting winding we connect a capacitor so the current flowing in

the capacitor i.e. Ist leads the applied voltage by some angle.

The running winding is inductive in nature so, the current flowing in running

winding lags behind applied voltage by an angle, φm. Now there occur large phase

angle differences between these two currents which produces a resultant current, I

and this will produce a rotating magnetic field. Since the torque produced by these

motors depends upon the phase angle difference, which is almost 90°. So, these

motors produce very high starting torque. In case of capacitor start induction

motor, the centrifugal switch is provided so as to disconnect the starting winding

when the motor attains a speed up to 75 to 80% of the synchronous speed but in

case of capacitor start capacitors run induction motor there is no centrifugal switch

so, the capacitor remains in the circuit and helps to improve the power factor and

the running conditions of single phase induction motor.

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2.8.1.3: Capacitor Start-Capacitor Run Induction Motor (Two Value

Capacitor Method)

The working principle and construction of Capacitor start inductor motors and

capacitor start capacitor run induction motors are almost the same. We already

know that single phase induction motor is not self-starting because the magnetic

field produced is not rotating type. In order to produce rotating magnetic field there

must be some phase difference. In case of split phase induction motor we use

resistance for creating phase difference but here we use capacitor for this purpose.

We are familiar with this fact that the current flowing through the capacitor leads

the voltage. So, in capacitor start inductor motor and capacitor start capacitor run

induction motor we are using two winding, the main winding and the starting

winding. With starting winding we connect a capacitor so the current flowing in

the capacitor i.e. Ist leads the applied voltage by some angle. In capacitor start-

capacitor run induction motor capacitor is no excluded with the use of centrifugal

switch. Capacitor remains in the circuit during motor running.

2.8.1.3.1: Application of Capacitor Start IM and Capacitor Start

Capacitor Run IM

These motors have high starting torque hence they are used in conveyors, grinder

and air Conditioners, compressor, etc. They are available up to 6 KW.

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2.8.1.4: Shaded Pole Single Phase Induction Motors

The stator of the shaded pole single phase induction motors has salient or projected

poles. These poles are shaded by copper band or ring which is inductive in nature.

The poles are divided into two unequal halves. The smaller portion carries the

copper band and is called as shaded portion of the pole.

Figure 2.9: Shaded Pole Induction Motor

When a single phase supply is given to the stator of shaded pole induction motor

an alternating flux is produced. This change of flux induces emf in the shaded coil.

Since this shaded portion is short circuited, the current is produced in it in such a

direction to oppose the main flux. The flux in shaded pole lags behind the flux in

the un shaded pole. The phase difference between these two fluxes produces

resultant rotating flux.

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We know that the stator winding current is alternating in nature and so is the flux

produced by the stator current. In order to clearly understand the working of

shaded pole induction motor consider three regions.

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Chapter 3

Faults In Induction Motor

3.1: Introduction

The SQUIRREL-GAGE induction motors are most widely used electrical

machines for industrial commercial and domestic applications. They are more

widespread than any other electric machine in industry due to their intrinsic

ruggedness and reduced cost. Surveys have found that these machines demand

around 40- 50% of the total energy generated in a developed country. These

machines have created a revolution in world economy as most of the production

processes in a developed country is carried out by utilizing an induction machine.

Recently, the use of adjustable speed drives has also spread in many applications.

Hence, sudden failures in these machines can be catastrophic for the processes in

which they are involved. These machines are therefore seeking more attention

from researchers to diagnose the various faults occurring in these machines and to

develop various monitoring and signal processing techniques that can be applied

for prognosis.

Electrical machines and drive systems are subject to many different types of faults.

These faults include:

1. Stator faults which are defined by stator winding open or short-circuited.

2. Rotor faults which include rotor winding open or short circuited and broken

bar(s) or cracked end-ring for squirrel cage machines.

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3. Mechanical faults such as bearing damage, eccentricity, bent shaft, and

misalignment.

4. Failure of one or more power electronic components of the drive system.

Induction machines are highly symmetrical machines, so any kind of fault modifies

their symmetrical properties.

Characteristics fault frequencies therefore appear in the measured sensor signals,

depending on the type of fault. The factors responsible for failure of three-phase

induction motor are highlighted in Fig.3.1. It is evident from the below given chart

that the highest contributor for failure in a three-phase induction motor is the

bearing fault. This fault is categorized as a mechanical fault.

Figure 3.1: Distribution of Faults In Induction Motor

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3.2: Classification of Induction Motor Faults

The induction motor faults can be classified in given manner:

Figure 3.2: Faults Classification

3.3: MECHANICAL FAULTS

About 40-50% of induction motor faults are related to mechanical defects.

Classification of these faults includes the following:

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1. Damage in rolling element bearing.

2. Eccentricity fault.

3.3.1: Bearing Faults

Most electrical machines use either ball or rolling element bearings which consists

of outer and inner rings. Balls or rolling elements rotate in tracks inside the rings.

Bearing faults may be reflected in defects of outer race, inner race, ball or track.

Vibrations, internal stresses, inherent eccentricity, and bearing currents have

effective influence on the development of such faults.

Taking a step back and looking at the big picture, it is found that motors which

were controlled using variable frequency drives tend to show more premature

failures. Variable frequency drives (VFDs, ADSs, or inverters) regulate the speed

of motor by converting sinusoidal line AC voltage to DC voltage, and then back to

pulse width modulated (PWM) AC voltage of variable frequency. The switching

frequency of these pulses ranges from 1 kHz up to 20 kHz and is referred as the

“Carrier frequency”. The ratio of change of the ΔV/ΔT creates a parasitic

capacitance between the motor stator and the rotor, which induces a voltage on the

rotor shaft. If this voltage referred as “Shaft voltage”, builds up to a sufficient

level, it can discharge to ground through the bearings. This current is called as

“bearing current”.

The bearing current results from voltage pulse overshoot created by the fast-

switching IGBT in the VFD. Other reasons of shaft voltage include non-symmetry

of motor’s magnetic circuit, supply unbalances, transient conditions and others.

Any of these conditions can create bearing currents. Shaft voltage accumulates on

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the rotor until it exceeds the dielectric capacity of the motor bearing lubricant, then

the voltage discharges in a short pulse to ground through the bearing. After

discharge, the voltage again accumulates on the shaft and the cycle repeats itself.

This random and frequent discharging has an electric discharge machining (EDM)

effect, causing pitting of bearings rolling elements and raceways. The first effect of

bearing current damage is the audible noise created by rolling elements riding over

these pits in the bearing race. This deterioration causes a groove pattern in the

bearing race, which indicates that the bearing has sustained severe damage. This

can lead to complete bearing failure.

Figure 3.3 Time Vs Amplitude

The bearing current results from voltage pulse overshoot created by the fast-

switching IGBT in the VFD. Other reasons of shaft voltage include non-symmetry

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of motor’s magnetic circuit, supply unbalances, transient conditions and others.

Any of these conditions can create bearing currents. Shaft voltage accumulates on

the rotor until it exceeds the dielectric capacity of the motor bearing lubricant, then

the voltage discharges in a short pulse to ground through the bearing. After

discharge, the voltage again accumulates on the shaft and the cycle repeats itself.

This random and frequent discharging has an electric discharge machining (EDM)

effect, causing pitting of bearings rolling elements and raceways. The first effect of

bearing current damage is the audible noise created by rolling elements riding over

these pits in the bearing race. This deterioration causes a groove pattern in the

bearing race, which indicates that the bearing has sustained severe damage. This

can lead to complete bearing failure.

Bearing faults may be reflected in defects of outer race, inner race, ball or track.

Fault in the load part of the drive system, load imbalance, shaft misalignment,

gearbox faults, or bearing faults, gives rise to a periodic variation of the induction

machine load torque. Torque oscillations already exist in a healthy motor owing to

space and harmonics of the air-gap field but fault-related torque oscillations are

present at particular frequencies often related to the shaft speed.

Shaft vibration frequencies associated with different ball-bearing faults were given

in. Different fault gives rise to different harmonic frequencies which are listed

below:

FC = 1/2 FR (1-Dbcosβ/Dc)

FO = NB/2 FR (1-Dbcosβ/Dc)

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FI = NB/2 FR (1+Dbcosβ/Dc)

FB = DC/DB FR [1-(Dbcosβ/Dc)]

They define cage fault frequency, outer raceway fault frequency, inner raceway

fault frequency, ball fault frequency.

Typically bearing faults are detected through vibration signals. Internal vibrations

are caused by asymmetries and construction details. Vibration and current have

different natures. Vibration is acceleration, and is bound to the square of the

frequency, while current is a displacement. Hence current is mainly sensitive to

low-frequency phenomena. Link between vibration and current component was

presented using two different approaches and vibration was seen as a torque

component that generates two frequency components Fbe in the stator current.

Fbe=|f±kfcar|

Industrial systems are however, still based on vibration signals as they are the only

reliable media. However, use of electrical signals is, preferable in many

applications. Extensive research activity focuses on bearing fault detection based

on current Signals. Current signals can be used for bearing fault detection only in

the case of large failures where it is desirable to detect incipient faults that quickly

degenerate into other defects.

It was shown that mechanically induced speed oscillations give rise to sidebands

components of the fundamental stator current frequency. It was also demonstrated

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that shaft misalignment causes modulation of current by the shaft rotational

frequency.

The use of dedicated signal processing techniques is essential in order to extract

the fault signature from current efficiently.

3.3.2: Eccentricity Faults

The eccentricity of a cylinder rotating around an air gap can be classified as static,

dynamic, or mixed eccentricity (Fig.3.). Air gap eccentricity is one of the common

failure conditions in an induction motor. For static eccentricity the center of

rotation is displaced from the original center, for dynamic eccentricity, the center

of rotation is at origin while the cylinder is displaced. Finally, for mixed

eccentricity, both the cylinder and center of rotation are displaced from their

respective origin.

An eccentricity may be caused by many problems such as bad bearing positioning

during the motor assembly, worn bearings, bent rotor shaft or operation under a

critical speed creating rotor whirl. The eccentricity causes extensive stressing on

the machine and greatly increases the bearing wear. Also, the radial magnetic field

owing to the eccentricity can act on the stator core exposing the stator windings to

potentially harmful vibrations. More recently, the rotor eccentricity was evaluated

through different signal analysis such as vibration, flux and current.

Under mixed eccentricity conditions, the stator currents contain the following

frequencies:

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𝐹𝑒𝑐𝑐 =f ± k 1 − s

𝑝 𝑓

Where, “s” is the machine slip. Since the frequencies related to the eccentricity and

to the load torque overlap on the current sidebands, the frequencies provided by

above equation are no longer enough for the diagnosis. The model of eccentricity

using both analytical and finite element (FE) approach is still investigated so that it

can be improved.

Figure 3.4: Eccentricity Faults(a) Without eccentricity(b) Static eccentricity(c) Dynamic

eccentricity(d) Mixed eccentricity

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3.4: ELECTRICAL FAULTS

In electrical faults there are two types of faults.

1. Rotor Faults

2. Stator Faults

3.4.1: Rotor Faults:

In rotor, there can be some faults which are given below.

3.4.1.1: Unbalanced Currents

Unsymmetrical faults may produce more severe heating in machines than

symmetrical faults or balanced three-phase operation. The negative sequence

currents which flow during these unbalanced faults induce 120 Hz rotor currents

which tend to flow on the surface of the rotor forging and in the nonmagnetic rotor

wedges and retaining rings. The resulting I 2R loss quickly raises the temperature.

If the fault persists, the metal will melt, damaging the rotor structure.

A basic question concerns the cause of the system unbalance. For generators, such

operation is very often the failure of the protection or equipment external to the

machine. For large motors, the unbalance can be caused by the supply equipment,

34

e.g. fused disconnects. Typical conditions that can give rise to the unbalanced

generator currents are:

1. Accidental single-phasing of the generator due to open leads or bushing.

2. Unbalanced generator step-up transformers.

3. Unbalanced system fault conditions and a failure of the relays or breakers.

4. Planned single-phase tripping without rapid reclosing.

3.4.1.2: Rotor Mass Unbalance

From the knowledge of construction of motor it is known that rotor is placed inside

the stator bore and it rotates coaxially with the stator. In a healthy motor, rotor is

centrally aligned with the stator and the axis of rotation of the rotor is the same as

the geometrical axis of the stator. This results in identical air gap between the outer

surface of the rotor and the inner surface of the stator. However, if the rotor is not

centrally aligned or its axis of rotation is not the same as the geometrical axis of

the stator, then the air gap will not be identical and the situation is referred as air-

gap eccentricity. In fact air-gap eccentricity is common to rotor fault in an

induction motor. Air-gap eccentricity may occur due to any of the rotor faults like

rotor mass unbalance fault, bowed rotor fault, etc. Due to this air-gap eccentricity

fault, in an induction motor electromagnetic pull will be unbalanced. The rotor side

where the air gap is minimum that will experience greater pull and the opposite

side will experience lower pull and as a result rotor will tend to move in the greater

pull direction across that gap. The chance of rotor pullover is normally greatest

during the starting period when motor current is also the greatest. In severe case

35

rotor may rub the stator which may result in damage to the rotor and/or stator. Air-

gap eccentricity can also cause noise and/or vibration.

This rotor mass unbalance occur mainly due to manufacturing defect, if not may

occur even after an extended period of operation, for nonsymmetrical addition or

subtraction of mass around the center of rotation of rotor or due to internal

misalignment or shaft bending due to which the center of gravity of the rotor does

not coincide with the center of rotation. In severe case of rotor eccentricity, due to

unbalanced electromagnetic pull if rotor rubs the stator then a small part of

material of rotor body may wear out which is being described here as subtraction

of mass, resulting in rotor mass unbalance fault.

3.4.1.3: Classification of Mass Unbalance

There are three types of mass unbalanced rotor:

1. Static mass unbalanced rotor.

2. Couple unbalance rotor.

3. Dynamic unbalance rotor.

36

3.4.1.3.1: Static Mass Unbalanced Rotor

For this fault shaft rotational axis and weight distribution axis of rotor are parallel

but offset, as shown in below Figure. Without special equipment this type of

eccentricity is difficult to detect.

Figure 3.5: Static Mass Unbalance Rotor

3.4.1.3.2: Couple Unbalance Rotor:

It is shown in below given Figure that if this fault occurs then the shaft rotational

axis and weight distribution axis of rotor intersect at the center of the rotor.

Figure 3.6: Couple Unbalanced Rotor

37

3.4.1.3.3: Dynamic Unbalance Rotor

It is shown in below given figure that if this fault occurs then shaft rotational axis

and weight distribution axis of rotor do not coincide. It is the combination of

coupling unbalance and static unbalance.

Figure 3.7: Dynamic Unbalanced Rotor

3.4.1.4: Causes of Rotor Unbalance

The main causes of rotor mass unbalance in an induction motor can be mentioned,

point wise, as follows:

1. Manufacturing defect.

2. Internal misalignment or shaft bending.

3. It may occur after an extended period of operation, for nonsymmetrical

addition or subtraction of mass around the center of rotation of rotor.

38

3.4.2: Stator Faults

This fault is due to failure of insulation of the stator winding. It is mainly termed as

inter-turn short-circuit fault. Different types of stator winding faults are (i) short

circuit between two turns of same phase—called turn-to-turn fault, (ii) short circuit

between two coils of same phase—called coil to coil fault, (iii) short circuit

between turns of two phases—called phase to phase fault, (iv) short circuit

between turns of all three phases, (v) short circuit between winding conductors and

the stator core— called coil to ground fault, and (vi) open-circuit fault when

winding gets break.

Different types of stator winding faults are shown in below figure. Short-circuit

winding fault shows up when total or a partial of the stator windings get shorted.

Open-circuit fault shows up when total or a partial of the stator windings get

disconnected and no current flows in that phase/line.

Figure 3.8: Star-Connected Stator Showing Different Types of Stator winding Fault

39

Figure 3.9: Photograph of Damage Stator Winding

40

Chapter 4

Electrical Signature Analysis (ESA)

4.1: Introduction

Electric Signature Analysis (ESA) is a technique used for condition, monitoring

and incipient fault detection in motors, generators and transformers. By using ESA

we can identify electrical and mechanical faults in electrical equipment when are in

operation. As a preventive-maintenance tool, Electrical Signature Analysis can be

used for analyzing the performance of many different types of equipment while in

operation.

We can also identify several mechanical and electrical problems; rotor-stator

eccentricity, bearing failures, stator winding short circuits and misalignments. It is

a more preventive and predictive approach towards the incipient fault detection and

gives us a level of redundancy.

As already mentioned, the Electric Signature Analysis (ESA) and condition

monitoring techniques both are used for studying faults in electrical equipment’s so

in electrical motor’s the ESA provides the ability to identify connection problems

and rotor-stator eccentricity (air gap) characteristics. If the rotor-stator eccentricity

misaligns then it will cause the vibrations in motor. As these vibrations will

possess a certain pattern which will repeat itself due to the periodic nature of motor

operation. These vibrations will affect the electrical signature of INDUCTION

motor and will cause the periodic noise in the signal. As mentioned earlier,

41

frequency domain possesses strong immunity to time-domain signal processing.

These vibrations which seem to be a noise in time domain will possess a certain

frequency domain signature. By just eliminating the healthy signature out of this

faulty signature we can get the required results.

Figure 4.1: ESA

ESA is a portable, remote and non-invasive technique. Its whole assembly is

invisible to the monitored equipment. Data acquisition is very simple and it takes

less than three minutes to get all the required signatures of voltage or currents out

of the INDUCTION motor. The biggest advantage of this technique is that it works

while the equipment is in operation. So we don’t need to stop the operation to

perform electrical signature analysis. This compiled data can help us determine the

problems in rotor demagnetization, phase imbalance, motor load, power factor,

power harmonics and the impact of the driven equipment on the motor. ESA also

assesses rotor as well as stator health and rotor-stator eccentricity (air gap)

characteristics. In addition, the bearings condition can also be observed by the data

42

obtained. ESA is particularly helpful in accessing mechanical conditions when it is

not possible or convenient to make vibration measurements.

4.2: Importance of Electrical Signature Analysis (ESA)

ESA is an uprising and beneficial technique. The development of ESA has spanned

over fifteen years and has benefited from the testing of a wide variety of devices. It

is used in given devices.

1. Air Compressors.

2. Textile Plant Motors.

3. Navy P-3C Generator.

4. Multi-Axis Milling Machines.

5. Helicopter Tachometer Generators.

6. Air Force C-141 Fuel Pumps and Pitch Trim Actuators.

7. Large Chillers, Blowers and Fans.

8. Electric Fuel Injectors.

Figure 4.2: Air Compressor

43

Figure 4.3: Textile Plant Motor

Figure 4.4: Electric Fuel Injector

Figure 4.5: Air Force C-141

44

4.3: Condition Monitoring

On-load condition monitoring and incipient fault detection is the topic that has

recently acquired a lot of attention. As the monitored already that the equipment is

invisible to the measuring unit we don’t have to shut down the machine operation

to get the status of their health and on-load condition monitoring can use either

vibration analysis or electrical signature analysis technique. By real time analysis

we could identify the faults before they escalate into something critical. Hence,

machine down time could be minimized reflecting in less cost of the complete

operation cycle.

Trending in the field of health diagnosis of electrical machines is the introduction

of new digital signal processing techniques to extract the faulty signatures of the

machine. In case of time domain the added noise in the signatures could mislead to

wrong results. While in frequency domain, every signal possesses a particular

pattern. So we could easily identify and separate the faulty signature out of healthy

signature and we could benchmark these signatures for the future comparison.

Condition based monitoring services improves the availability and reliability of the

equipment under consideration. Dynamic forces have a direct impact on the

internal working of the electrical machines. These have to withstand many strains

over thousands of operating hours during their life time. Any breakdown would

substantially diminish the profitability of the machine unit. To prevent the

occurrence of any damage their condition monitoring is very important. The

system monitors the condition of the rotating parts inside the electrical machine. It

requires sensors to be attached to the equipment under test. Sensors measure

45

vibrations caused by the enormous forces. The information gathered by the sensors

is digitalized and recorded. After that the data get analyzed. Bearings and other

components oscillates at a certain frequency. The frequency could be calculated

based on the geometry of the bearing. If any fault occurs in the machine it would

indicate a specific signature in the analysis. Condition monitoring allows us to

reduce the down time of the electrical machines by half and gear repairs can be

quickly planned. By cutting down time by half, increases efficiency bare by

boosting the economic viability of the electrical machine. Condition monitoring

can be applied to more than just mechanical components so that no damage is

overlooked during the maintenance to reduce the down time.

By condition monitoring we minimize the risk through early fault detection. As we

are aware of the fact that rotating machine and the static electrical equipment give

us different indications that the problem is developing then if catch those signs in

enough time we can in react to them. There are lot of ways we can in react to them

e.g.

1. Vibration analysis.

2. Ultrasound.

3. Thermography.

4. Partial discharge.

5. Electrical signature analysis.

The inspection and observation both are an important part of condition monitoring.

The basic principal behind condition monitoring is that we run the test for a certain

46

period of time when the equipment is running in its normal healthy state so that we

could say it is running defect free. Then at a certain point we can say that there is

the ability to detect that the condition is degrading and the fault has developed. In

some cases ultrasound is the first technique that is employed to detect any defect in

the rotating electrical equipment. As it has the ability to locate any lubrication

problems. But generally speaking, during initial stages bearing does not make any

sound. The earlier we pick it up the lower risks associated with the failure of the

equipment and lower the cost of dealing with the problem.

The basic application of the condition monitoring is the fault detection,

maintenance, planning, and avoidance of secondary damage or collateral damage

and reduced spares inventory. After consideration of these point there are fewer

safety incidents both in terms of the equipment failing and environmental point of

view. In that case we can plan the maintenance in a better way and that would

reflect the improved safety higher likelihood of having less down time and greater

availability of spare equipment. We either let the equipment fail via getting the

advanced warning of the failure, or fix it before it fails. It avoids catastrophic

failures and reduces the likelihood that a machine will fail catastrophically. The

aim of reliability improvement is to run a plant with the lowest costs, at the highest

availability. After detection of the root cause of the fault and conditions that will

result in failure and reduced reliability, proactive measures can be taken. The

failure can be a cause of unbalance, poor installation, misalignment, soft foot,

lubrication, turbulence and cavitations. Poor lubrication increase friction in the

rotating parts, operating it incorrectly puts all kinds of load on the bearings, shields

and shafts. They all result in bearing failure, reduced efficiency, and seal failure,

coupling failure, shaft failure, leaks and contamination. From a condition

47

monitoring point of view we need to detect the occurrence of these conditions. If

we detect those issues and correct them then we are less likely to get bearing faults.

4.3.1: Advantages of Condition Monitoring

In our system, maintenance is a major problem. There are some types of

maintenance which are given.

4.3.2: Maintenance:

Maintenance, repair and operations (MRO) or maintenance, repair, and overhaul

involves fixing any sort of mechanical, plumbing or electrical device should it

become out of order or broken known as repair, unscheduled, or

casualty maintenance.

4.4: Classification of the Maintenance Activities

“Maintenance” can be understood as the action to repair or to execute

services in equipment and systems.

It can have its activities classified in five main groups:

48

4.4.1: Corrective Maintenance:

This is the most primary form of maintenance. It occurs after a failure carried out.

Usually, it becomes the unavailable equipment for use. Many disadvantages of this

type of maintenance are clear. As examples, the systematic occurrence of not

programmed stops, lesser time of useful life for the machine, bigger consumption

of energy (since with the presence of the failure the motor needs more current

keeping the constant torque) can be cited.

4.4.2: Preventive Maintenance:

This is the name that receives a set of actions developed with the intention of

preventing the occurrence of unsatisfactory conditions, and consequently, reducing

the number of corrective actions. When preventive maintenance plan is elaborated,

a set of technical measurements must be created in order to increase the machine

reliability and decrease the total cost of the maintenance.

A preventive maintenance program can still choose for one of the three types

of activities:

Continuous monitoring.

Periodic measurements.

Predictive techniques.

49

4.4.3: Predictive Maintenance

The predictive maintenance can be a sub-area of the preventive maintenance.

However, the predictive maintenance presents some proper characteristics as:

Support in not invasive techniques, that is, it is not necessary to stop the

operation of the machine for its application.

Elimination of corrective maintenance.

Not consideration of information as the durability of components.

On-line or off-line can be effected through techniques.

4.4.4: Systematic Maintenance

Systematic characterized for the substitution of components of the equipment or

for the substitution of the equipment as a whole.

4.4.5: Zero Hours Maintenance (Overhaul)

The set of tasks whose goal is to review the equipment at scheduled intervals

before appearing any failure either when the reliability of the equipment has

decreased considerably so it is risky to make forecasts of production capacity. This

review is based on leaving the equipment to zero hours of operation, that is, as if

the equipment were new. These reviews will replace or repair all items subject to

50

wear. The aim is to ensure, with high probability, a good working time fixed in

advance.

4.4.6: Periodic Maintenance (TBM)

The basic maintenance of equipment is the analysis of equipment with respect to

the passage of time. It consists of a series of elementary tasks i.e. data collections,

visual inspections, cleaning, lubrication, retightening screws e.g. for which no

extensive training is necessary, but perhaps only a brief training. TBM stands for

time based maintenance.

ESA is used as a special technique in Condition monitoring. So ESA is very

helpful and user friendly technique for our system maintenance.

51

Chapter 5

Procedure Observations And Calculations

5.1: Project Flow Chart

The flow diagram of our FYP is as follows:

Figure 5.1: Flow Chart

52

5.2: Hall Effect Sensor

The Hall Effect was discovered by Edwin Hall while he was a attempting to verify

the theory of electron flow proposed by Kelvin. Hall found when a magnet was

placed so that its field was perpendicular to one face of a thin rectangle of gold

through which current was flowing, a difference in potential appeared at the

opposite edges. He found that this voltage was proportional to the current flowing

through the conductor, and the flux density or magnetic induction perpendicular to

the conductor.

5.2.1: Principle

When a current-carrying conductor is placed into a magnetic field, a voltage will

be generated perpendicular to both the current and the field. This principle is

known as the Hall Effect. Below given figure illustrates the basic principle of the

Hall Effect. It shows a thin sheet of semiconducting material (Hall element)

through which a current is passed. The output connections are perpendicular to the

direction of current. When no magnetic field is present, current distribution is

uniform and no potential difference is seen across the output.

When a perpendicular magnetic field is present, a Lorentz force is exerted on the

current. This force disturbs the current distribution, resulting in a potential

difference (voltage) across the output. This voltage is the Hall voltage (VH).

53

Figure 5.2: Hall Effect Sensor

5.2.2: Advantage of Hall Effect Sensor

The reasons for using a particular technology or sensor vary according to the

application. Cost, performance and availability are always considerations. In this

project we used Hall Effect sensor as current sensing device as it was cheaper and

easily available. It helped a great deal in making our final cost small. And it also

generated the required results.

General features of Hall Effect based sensing devices are:

True solid state

Long life (30 billion operations in a continuing keyboard module test

program)

High speed operation - over 100 kHz possible

Operates with stationary input (zero speed)

No moving parts

Logic compatible input and output

54

Broad temperature range (-40 to +150°C)

Highly repeatable operation

5.2.3: Output vs. power supply characteristics

Analog output sensors are available in voltage ranges of 4.5 to 10.5, 4.5 to 12, or

6.6 to 12.6 VDC. They typically require a regulated supply voltage to operate

accurately. Their output is usually of the push-pull type and is ratio metric to the

supply voltage with respect to offset and gain. Figure on the next page illustrates a

ratio metric analog sensor that accepts a 4.5 to 10.5 V supply. This sensor has a

sensitivity (mV/Gauss) and offset (V) proportional (ratio metric) to the supply

voltage. This device has “rail-to-rail” operation. That is, its output varies from

almost zero (0.2 V typical) to almost the supply voltage (Vs - 0.2 V typical).

Figure 5.3: Input Output Circuitry of Hall Sensor

Utilization of Hall Effect sensors

There are 4 Hall Effect sensors are used in this project. Three of them are for

measurement of three phase current individually and one is for neutral current

55

measurement. On PCB, they are arranged in such a way that current flows to the

bottom copper and then by using small piece of cables ,each phase current is

allowed to pass through that cable which is wound around Hall Effect sensor

tightly. The other end of the cable is fed directly to the output terminal of data

acquisition module from which external connections to the induction motor are

provided.

5.2.4: Hall Effect Sensor Specifications

The data sheet of sensors used in this project is given in the following figure. It

shows that it has wide range of current sensing ability. The dimensions of this

sensor is just bigger than thumbnail. In comparison to CTs there is no issue of

saturation and burden resistance. So for accurate measurement of current for power

quality analysis these sensors provide the best solution. The output of the device is

fed to NI USB 6009.

Figure 5.4: Data Sheet For Hall Effect Sensor

56

5.3: Introduction to NI USB-6009

NI USB-6009 is a low cost device that provides connections to 8 analog inputs

channels, 2 analog output channels, ADC, 12 digital input output channels and 32

bit counter with full USB speed. The maximum sampling rate is 20 kS/s and USB

(2.0) bus speed is 12 Mb/s.

Figure 5.5: NI USB 6009

57

5.3.1: Block Diagram of Internal Structure

Figure 5.6: Internal Structure of NI USB 6009

58

5.3.2: Hardware Connections

To use NI USB 6009 installation of combicon screw terminal blocks are used by

inserting them into combicon jack, terminals orientation are checked and then

wires are connected to appropriate terminals. A 16 pin bus is used instead of wire

connection to keep distinction between voltage and current signals.

Figure 5.7: Connections to NI USB 6009

The Analog terminal assignments are given as follow:

59

Figure 5.8: Analog Input Terminal Assignment

The digital terminals assignments are as follow:

Figure 5.9: Digital Input Terminal Assignment

60

5.3.3: Signal Description

GND: The reference point for single ended Analog Inputs(AI),bias current return

point for differential mode measurement, AO voltages,+5V DC supply and +2.5V

DC supply.

AI (0 to 7): For single ended measurements, each signal is an analog input voltage

channel. For differential measurement positive and negative inputs are applied at

following terminals.

AI 0: AI 4, AI 1: AI 5, AI 2:AI6, AI 3: AI 7

AO (0 & 1): Supply voltage output of Analog output.

P1 (0 to 3) & P0 (0 to 7): Each signal can be individually configured as digital

input or output.

2.5V & 5V DC: Provide DC power either 2.5V or 5V according to requirements.

PFI 0: This pin is configurable as either digital trigger or event counter input.

MUX: USB 6009 has one ADC. The multiplexer route is one AI channel at a time

to PGA.

PGA: Programmable gain amplifier provides input gains of 1, 2, 4, 5, 8, 10, 16 or

20 when configured for differential measurements and gain of 1 when configured

for single ended measurements. PGA gain is automatically calculated based on

voltage range selected in measurement application.

ADC: It converts AI to digital format by converting analog voltage to digital code.

AI FIFO: It holds data during AI acquisition to ensure that no data is lost.

61

5.3.4: Analog input Modes

5.3.4.1: Differential Mode

The positive terminal is connected to AI+ and negative terminal to AI-.In this

mode +20 to -20 V can be measured but voltage on one pin must not exceed 10V

(either positive or negative) with respect to GND.

5.3.4.2. Single Ended Mode

To connect single ended input, positive terminal is connected to AI terminal and

ground signal to GND.PFI 0 is configured as digital trigger input when AI task is

defined. When digital trigger is enabled, AI waits for a rising edge on PFI 0 before

starting acquisition. To use Start trigger with digital source, PFI 0 is specified as

source and rising edge is selected.

5.3.5: Specification of Analog Inputs

1. Input range….. .....…………………………….. -10 to +10 V (single ended)

.… ..……… ……….……… …. ……… ……. -20 to +20V (Differential)

2. Input impedance………………………………………………..…….144kΩ

3. Trigger source……………………………………….…..Software or external

digital source

62

4. AI FIFO ……………………………… …………………………512 bytes

5. Max sampling rate…………………… …………………………….48kS/s

6. Overvoltage protection……… … … …… …… ………………….+-35V

7. System noise………………………..…………0.73mV (rms) (Single ended)

8. System noise …………………...… ………..0.37mV (rms) (Differential)

5.3.6: Analog Output

USB 6009 has two AO channels which can generate 0-5 V. All updates of AO are

software timed. There is DAC which converts digital signal to analog voltage. To

connect load at AO, positive terminal is connected to one of the AO port and

ground to GND.

5.3.7: Digital IO

There are 12 digital lines which comprise DIO ports. GND is ground reference

point for these ports. Individually assignments of ports as DI or DO are possible.

5.3.8: Reference and Power Sources

+2.5V external reference: It creates high purity voltage supply for ADC using

multi state regulator, amplifier and filters. This reference is used as signal for self-

test.

+5 V power source: It supplies 5V, 200mA output. It can be used to power external

components.

63

5.4: MATLAB Simulation

The block diagram of MATLAB simulation is as follows:

Figure 5.10: Block Diagram

After performing analysis on different faults, we got different results for every

fault which are given in their corresponding sections.

5.5: Healthy Signature

To work on our research, we first took healthy signatures of our

healthy motor. The equipment was shown in diagram given below.

64

Figure 5.11: Hardware

After taking signatures, the results was as follows:

Figure 5.12: Healthy Signature

65

5.5.1: MATLAB Simulation for Healthy Motor

After taking simulation on MATLAB, the results of healthy motor was

as follows

Figure 5.13: Healthy Signature

In this figure, we can see that for healthy motor DC Offset is very low.

5.6: Fault of Broken Bars of Rotor

For broken bars of rotor, we opened motor and broke some bars of rotor

by grinding the rotor. The procedure is in given figures.

66

Figure 5.14 Opened motor for application of faults

Figure 5.15: Applying Rotor Fault

67

After that, we connect our faulty motor in circuit and take signatures which are as

follows:

Figure 5.16: ESA for Broken Rotor

5.7: Main Winding Fault

For this fault, we added a resistance of 7.5 ohm in main winding then took

signatures. The block diagram is as follows:

Figure 5.17: Circuit Diagram

68

After performing experiment, the results were as follows:

Figure 5.18: ESA of Main Winding Fault

5.7.1: MATLAB Simulation

In MATLAB when we add these faults, the results were as follows:

Figure 5.19: Main Winding Harmonics Waveform

69

From this figure, we can see that by adding resistance in main winding, DC offset

increases as compared to healthy winding.

5.8: Starter Winding Fault

For this fault, we added a resistance of 7.5 ohm in starter winding then took

signatures. The block diagram is as follows:

Figure 5.20: Circuit Diagram

After performing experiment, the results were as follows.

70

Figure 5.21: ESA of Starter Winding Fault

5.8.1: MATLAB Simulation

In MATLAB simulation, after adding resistance, the results was as

follows:

Figure 5.22: Starter Winding Faults

71

From this figure, we can see that by adding resistance in main winding, DC offset

increase as compared to healthy and main winding.

5.9: Supply Cable Resistance Fault

For this fault, we added a resistance of 7.5 ohm in supply cable then took

signatures. The block diagram is as follows

Figure 5.23: Resistance Added At Supply Cable

72

Figure 5.24: ESA of Supply Cable Resistance Fault

5.9.1: MATLAB Simulation

In MATLAB simulation, after adding resistance, the results was as follows:

Figure 5.25: Supply Cable Resistance Harmonics Waveform

73

Chapter 6

Conclusion

It has been theoretically verified and experimentally confirmed that Induction

motor faults can be identified at initial stages by spectrum of the stator currents.

Electrical signature analysis (ESA) technique is invisible to monitored device in

comparison to vibration analysis based technique that needs accelerometers and

other associated equipment.

It has been empirically verified that all Induction stator faults produces an

imbalance scenario which results in an increase in total harmonic distortion on the

spectrum of stator currents.

Electrical signature analysis of healthy Induction motor confirms the absence of

3rd and 9th harmonic from the spectrum of the stator current. When stator winding

is subjected to fault, triplet harmonics escalates into the signatures of stator current,

which could be used as a warning to incipient fault condition.

Moreover, an increase in total harmonic distortion is observed during varying

mechanical stresses on the Induction motor. While, vibration analysis based

monitoring technique is considered to be more reliable, but it does not give any

information on electrical characteristics of Induction motor.

74

Chapter 7

References

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as a medium for Faults Detection, IEEE Transactions on Industrial

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[2] R.R. Schoen, T.G. Habetler, F. Kamran, and R.G. Bartfield,” Motor bearing damage detection using stator current monitoring”, IEEE transaction. Ind. Appl. Vol 31, Dec 1995.

[3] Mohd El Hachemi B, “A review of induction motor signature analysis as a medium for fault detection”, IEEE transaction on Ind.Elect, vol47, Oct2000.

[4] H.A. Toliyat, M.S. Arefeen and A.G. Parlos, “A method for dynamic simulation of air-gap eccentricity in induction motors”, IEEE transaction Ind.Elect, vol32, Aug 1996.

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[10] R. M. Tallam, T. G. Habetler, and R. G. Harley, “Stator winding turn fault detection for closed-loop induction motor drives,” IEEE Trans. Ind. Appl., vol. 39, no. 3, pp. 720–724, May/Jun. 2003.

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[11] W.T.Thomson, M.Fenger, “Current signature analysis to detect induction motor faults” IEEE.Ind.Appl.Mag, Aug 2001.