INDUSTRIAL TRAINING AT:

VISAKHAPATNAM STEEL PLANT

VISAKHAPATNAM

PRESENTED BY:

BATTULA PURNACHAND

11115021

E-2

TYPES OF MACHINE FAILURES

Machine failures are of two types:

1. Mechanical failures

2. Electrical failures

Major mechanical failures:

Bearing failures.

Alignment failures.

Brush failures

REASONS FOR BEARING FAILURES

The main reasons for bearing problems are: Mechanical

Electrical

Lubrication

Fatigue

•

MECHANICAL: Poor Bearing Matching: All the bearings used in the couplings must be of same size otherwise

bearing mismatching will result in vibrations. Tight Bearing Clearance. External Axial Thrust must not be too high because this will lead to heating of machine due to

high frictional losses. Abnormal Vibration.

ELETRICAL: Shaft & external current: When energized, electric motor, develops a voltage differential between shaft ends. In many cases, voltage compels current to flow from one end of the shaft, then through the machine housing and on to the opposite bearing and shaft end. Motor shaft current can create arcing that damages both bearings and couplings. To prevent the arc-induced damage is by applying insulated couplings.

LUBRICATION : Oil Starvation: The machines will be affected by lack of lubricating oils due to continuous running. This results in wearing and heating of bearings and couplings. Contaminated Oil with Impurities: The dirt particles present in the oil will increase the friction at the bearings and leads to heating of machine parts.

FATIGUE FAILURE: Aging. Improper Bonding of Babbitt with Shell.

Effects of Bearing Failures: Overheating of machine parts due to friction losses. Uneven vibrations and noise will be produced. Due to wearing of bearings uneven air gaps will be produced which result in sparking. Wear and tear of couplings and shaft

Remedies for Bearing Failures: Lubricating the bearings regularly. Bearings should be perfectly matched i.e., the size of bearings used must be equal. Avoiding bearing currents

:

ALIGNMENT Instruments used for checking alignment Dial indicator:(lever arm test indicator or finger indicator) A dial test indicator has a smaller measuring range than a standard dial indicator. A test indicator measures the deflection of the arm, the probe does not retract but swings in an arc around its hinge point.. The model shown is bidirectional; some types may have to be switched via a side lever to be able to measure in the opposite direction.

These indicators actually measure angular displacement and not linear displacement. If a force is perpendicular to the finger, the linear displacement error is acceptably small within the display range of the dial. However, this error starts to become noticeable when the force is as much as 10 degrees off the ideal 90.

Other instruments used: Outside micrometer Feeler gauge

MisalignmentShafts are said to be misaligned if their rotational centerlines are not co-linear, when the machines are operating. Radial Misalignment Radial is the distance between the two shaft axis and is quantified by measuring the radial distance between the centerline of one shaft if it were to be extended to overlap the other.

Axial Misalignment Axial misalignment is the variation in axial distance between the shafts of the driving and driven machinery. Radial & axial misalignment (Angular misalignment): Angular misalignment is the effective angle between the two shaft centerlines and is usually quantified by measuring the angle between the shaft centerlines as if they were extended till they intersect.

EFFECTS OF MISALIGNMENT Burn marks will be produced in commutator. Excessive vibration Excessive noise during operation Wear & tear of machine parts Overheating of bearings, gears and lubricating oils. In some cases shearing of coupling bolts, shaft, collapse of bearings may take place. In some extreme cases vibration of complete structural building also may occur .

CAUSES FOR BRUSH FAILURES: As brushes are placed in brush holders improper bedding of brushes will result in brush failure. Excess oil from lubricants and presence of dirt on commutator will result in unequal wearing of brushes. The brushes wear due to continuous running of machines.

Exact spring pressure of 180g/cm.sqr must be applied constantly on the brushes any excess or less pressure leads to sparking and damage of brushes.

EFFECTS OF BRUSH FAILURES: Spankings at the entering and leaving edge of brushes. Commutator and slip rings gets too hot. Brushes and brush holders gets too hot. Generation of insufficient voltage on self-exciting machine.

TESTING METHODS OF MACHINES IN CME

Insulation resistance test: For LT motors (<415Volts) the winding insulation resistance to be measured with 500 Volts Megger. The winding insulation resistance should be more than 5MΩ with respect to body. And the same is measured between the each phase. If the megger reads below the mentioned value, the winding should be thoroughly cleaned and dried. The stator and slip ring rotor of the induction motor has a three-phase winding wound on core. Each phase has a starting and ending. Each phase of the winding is insulated from the other and form the core .To check the perfect ness of the insulation the insulation resistance is measured .The megger is a piece of instrument used to measure the insulation resistance .It works on the principle of dynamo. A liver connected rotor is rotated rotor is rotated in the magnetic field and current is passed to the testing terminals

Insulation resistance between core and winding

The testing terminals of the megger is connected to A1and B1

Initially the megger is under the open circuit condition indicating infinite resistance between

open terminals. After connecting liver is rotated and observed weather the needle moves to zero then we

can assume that there is insulation failure. Similarly it is tested for windings of different phases.

Insulation Resistance between phases

Testing terminals of megger are connected

For HT motors (>415Volts i.e. 3.3kv, 6.6kvand 11kv)

winding insulation resistance to be measured with >500Volts megger i.e. 1000kv, 2.5kv and

5kv.The winding insulation resistance should be >100MΩ

.

ALIGNMENT TEST:Using dial gauge, outside micrometers etc., the alignment of the shaft and drives is tested.

Problems in Alignment lead to vibrations, noise and wear and tear of the machine. If the machine doesn’t have any alignment problems then it is sent for running test.

RUNNING TEST:Running test is namely No load running test. In this test rated voltage will be applied and run

the motor as per the duty cycle. The bearing condition, noise level, vibration, temperature of the motor body etc. are being observed. If it is a DC motor observe the sparking level.

If any sounds and vibration are observed then there is a problem with bearings or alignments. During this test temperature at different parts of the machine will be noted. The temperatures must be almost constant and nearly equal to the room temperature. If there are any abnormal temperature changes then it shows that there are some frictional loses due to contacts, bearings and alignments problems.

HIGH VOLTAGE TEST:A high voltage which is nearly 2times of rated voltage is given to both ARMATURE CIRCUIT and

FIELD CIRCUIT for a very short duration of time (60seconds) and seen whether the IR value it is changed or not. If the IR value is healthy and remains constant then there will be no insulation problems in the machine. If there is any change in IR value then the IR value must be improved using drying or the insulation must be changed.

OVER LOAD TEST:

A over load of nearly 120% of rated load is applied for about 60 seconds to the machine and seen whether it withstands or not.

OVER SPEED TEST:

The machines is driven with a speed of 1.2 times the rated speed for about 120s and seen whether it withstands or not

DC MOTOR TESTING :

• In this testing the dc motor is connected to the testing panel

• The connections are made such that the armature and field windings are connected to their

respective testing panels.

• The motor is then driven for some time.

• TESTING PARAMETERS:

Speed, temperature rise, voltage ,current are checked with respective to the rated readings of

the motor

The motor is driven for some time and checked for the Sparking

Check for bearing sound and vibration

• If there are any faults the then the motor is sent for repair

ARMATURE TESTING PANEL FIELD TESTING PANEL

• The speed of the motor can also be controlled by the knob • The current, voltage, speed are seen in the meters attached

DRYING: The need for drying the machines is caused by the fact that the insulation of winding of the machines which were out of Operation for a long time (particularly of those which stayed in rooms with high moisture content) dampens and its properties deteriorate. It is recommended that all newly installed machines and those which were long out of operation or in repair should be dried before reusing.

Drying of the electric machine can be carried out by external heating method. In this method a hot air blower is used to produce dry hot air. This hot air is sent through the stator windings or rotor windings which removes the moisture and also the dust particles if present in the windings.

PRE-REWINDING ACTIVITIES

• Heating of windings: For removal of windings we have to soften and loosen the insulating materials.

Windings are placed in a charging Oven and heated up to 4 to 5 hours which make the windings to expand and loosen. The maximum temperature for standard motor is 700°F and 600°F for T-frame motors before placing windings in the oven. Frequently observe temperature of the windings during the heating process.

FAULTS & THEIR REMIDIES The following list gives an aid to the rectification of various possible faults: FAULTS PROBABLE CAUSE REMEDY

Motor does not start Open circuit. Reconnect connections.

In correct connections. Correct connections.

Too low voltage. Set correct voltage.

Undue overloading Check the driven

Equipment

In correct brush rocker

position

Reset the rocker in correct

position

In correct motor speed. In correct connections. Correct connections as pre

drawings.

In correct field current. Set the field current as

mentioned in name plate.

In correct armature voltage. Set the correct voltage of

the armature.

In correct brush rocker

position.

Reset the rocker in correct

position.

Bad commutation or sparks

at the brushes.

In correct brush rocker position.

Reset the rocker in correct

position.

Un suitable brushes. Fit the brushes of the

specified quality.

Brush badly bedded. Bed the brush properly. Brushes do not slide freely

in the brush holder.

Make the brush to slide

freely.

Brush pressure too low. Replace the brush holder or

spring.

Over loading. Set the correct load.

Field current too low. Set the field current as

mentioned in name plate.

FAULTS

PROBABLE CAUSE

REMEDY

FAULTS

PROBABLE CAUSE

REMEDY

Low insulation resistance, earth

fault, insulation damage.

Damage insulation, dirty

windings.

Re insulate, clean windings, dry

& re varnish

Noisy bearings. Bearing damaged. Change bearings.

Too low grease. Re grease.

Generator produces

incorrect voltages or no

voltages.

In correct connection open

circuit.

Correct connection or re do

connections.

Machine too hot. Air blocked. Open the machine and clean it.

Filter chocked. Clean the filter.

High excitation current. Correct excitation current.

Short circuit of field windings. Repair or replace the coils.

CONCLUSION• In addition to the theoretical knowledge, practical knowledge is also needed to know the

operating procedure, characteristics of the different machines to deal and use the equipment in

optimum level in any industry or organization.

• In this regard, we physically observed the different parts of the machine (in

dismantled condition of the various AC/DC motors) repair procedures, trouble shooting,

maintenance, testing of various electrical equipment's in CME & ERS department of VSP.

• By putting utmost observation of testing of electrical machines to know the different

types of tests performed in the every event of certain repair.

THANK YOU