EE082-Fault Finding in Electrical Control Systems-Th-Inst.pdf

7/25/2019 EE082-Fault Finding in Electrical Control Systems-Th-Inst.pdf

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SRI LANKA INSTITUTE of ADVANCED TECHNOLOGICAL EDUCATION

Training Unit

Fault Finding in ElectricalControl Systems

Theory

No: EE 082

INDUSTRIETECHNIKINDUSTRIETECHNIK

ELECTRICAL and ELECTRONIC

ENGINEERING

Instructor Manual


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Training Unit

Fault Finding in Electrical Control Systems

Theoretical Part

No.: EE 082

Edition: 2008All Rights Reserved

Editor: MCE Industrietechnik Linz GmbH & CoEducation and Training Systems, DM-1Lunzerstrasse 64 P.O.Box 36, A 4031 Linz / AustriaTel. (+ 43 / 732) 6987 3475Fax (+ 43 / 732) 6980 4271Website: www.mcelinz.com


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FAULTFINDING IN ELECTRICAL CONTROL SYSTEMS

CONTENTS Page

LEARNING OBJECTIVES...................................................................................................4

1 INTRODUCTION ..........................................................................................................5

1.1 Safety rules .......................................................................................................... 5

1.2 Accident prevention .............................................................................................6

1.2.1 Rescue procedure in the event of electrical accidents.....................................7

2 TYPES OF FAULT ..................................................................................................... 11

2.1

Short circuits to frame ........................................................................................11

2.2 Short-circuits ......................................................................................................11

2.3 Earth faults.........................................................................................................11

2.4

Winding short circuits.........................................................................................12

2.5 Turn-to-turn faults ..............................................................................................12

3 TEST AND MEASURING EQUIPMENT FOR FAULTFINDING.................................13

3.1

Basic equipment ................................................................................................13

4

FAULTHNDING METHODS.......................................................................................14

5 FAULTFINDING IN LIGHT CIRCUITS .......................................................................16

5.1 Faults on filament lamps ....................................................................................16

5.2 Faults on fluorescent lamps Lamp does not come on: ......................................19

6 FAULTFINDING ON ELECTRIC HAND TOOLS........................................................25


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7 FAULTFINDING ON ELECTRICAL CONTROL SYSTEMS USING CIRCUIT

DIAGRAMS........................................................................................................................27

8 FAULTFINDING ON ELECTRICAL MACHINES........................................................30

8.1 Cage rotor machines..........................................................................................30

8.2 Slipring rotor machines ......................................................................................35

8.3 D.C. motors........................................................................................................40

8.4

Single-phase transformers.................................................................................45

9 FAULTFIND1NG ON EXCESS VOLTAGE PROTECTION EQUIPMENT .................49

9.1 Residual current devices....................................................................................49

9.1.1

Faults on residual current devices .................................................................50

9.1.2

Testing circuits ...............................................................................................53

9.2 Protective earthing conductors .......................................................................... 54

9.3

Protective earth and neutral conductors (PEN) .................................................55

9.4 Protective multiple earth systems (IT systems)..................................................57

10 FAULTFINDING ON RECTIFIER CIRCUITS ........................................................ 60

10.1

Testing power semiconductors with an ohmmeter.............................................60

10.2 Testing power semiconductors with an oscilloscope .........................................61

10.2.1 Half-wave rectification ................................................................................62

10.2.2

Full-wave rectification (centre tapped transformer)....................................63

10.2.3 Bridge circuit ..............................................................................................64

10.2.4

Threephase Bridge circuit ..........................................................................65


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LEARNING OBJECTIVES

The trainee should...

name the types of faults which occur in electrical circuits and equipment

name and use test and measuring equipment used for faultfinding in electrical

engineering

list methods of assisting fault-finding

detect and repair faults in lighting circuits and in lighting units

describe methods for faultfinding on electrical hand tools, such as hand drilling and

hand grinding machines

read electrical circuit diagrams find and remove open-circuits and short-circuits in electrical control systems using

circuit diagrams

test cage motors for operation and condition

locate and repair faults in rotor and stator circuits

test windings of d.c. shunt wound machines

test single-phase transformers

locate and repair faults on excess earth voltage protection equipment (residual current

devices)

test the protective earthing circuits for perfect operational condition

locate and repair faults on protective earth and neutral circuits

locate and repair faults on protective conductor circuits

detect and repair faults in semiconductor rectifier circuits


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FAULT FINDING IN ELECTRICAL CONTROL SYSTEMS

1 INTRODUCTION

When faultfinding it is assumed that the system was operating properly before the fault

occurred. However, it is advisable to check again the connections to the mains, load and,

if provided, any remote control system. If there is no fault until the load is connected, the

drive motors must first be examined for possible earth and turn-to-turn faults.

The measuring Instruments to be used must be double insulated, or at least connected

through an isolating transformer (without protective conductor), because there is a high

potential to earth for some measurements.

WARNING

Capacitors may remain charged long after the mains has been switched off. To avoid

damage to the measuring Instrument (e.g. when measuring with an ohmmeter), capacitors

must be discharged beforehand by short circuiting them for a short time.

1.1 Safety rules

For carrying out work on electrical installations:

- Isolate all poles of the supply, on all sides of the installation.

- Protect against reconnection.

- Test for no voltage.

- Earth and short-circuit (remembering correct sequence).

- Cover adjacent live parts and enclose any danger points.

Connection is carried out in the reverse order.


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1.2 Accident prevention

Causes of accidents:

- tools in poor condition

- carelessness and thoughtlessness

- haste

- inexperience

Electrical accident:

If an electrical circuit is completed through the human body, current flows through the

body and the person is at risk. The current flowing through the human body depends on

the voltage and resistance, according to Ohm's law.


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The resistance of the human body depends on:

- contact pressure: strong pressure - low resistance

- contact surface: large surface area - low resistance

- humidity: high humidity or wet conditions - low resistance

- voltage: the higher the voltage the higher the current

- location: only relevant when the circuit is closed above ground Level and depends on

the footwear to earth resistance and the resistance of the ground near the point of

contact

Body resistance at voltages of over 100 V is approx. 3000. The effect of the electrical

current on the human body is solely dependent on the current magnitude, the voltage

causing the flow through the body.

1 mA Effect becomes perceptible

15 mA The person may be unable to free himself

from 5041A Highly dangerous!

Current over 50 mA and voltages from 50 V upwards can be fatal.

The 50 V mark provides a safety factor since body resistance and the current magnitude

dangerous to the human body are approximate values.

1.2.1 Rescue procedure in the event of electrical accidents

- Switch off the circuit.

- Release the victim from circuit.

- Apply first aid.

- Inform doctor, rescue service.

- Report accident to police.


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1.2.1.1 Rescue from the circuit

Up to 1000 V:

- switch off safety switch and remove all fuses.

- in the case of portable equipment, remove plug from socket.

- in the case of fixed equipment, disconnect electrical apparatus and cables (N.B. -

there may perhaps be two power supplies).

- pull away victim and insulate (intermediate layer of rubber, plastic,...).

Over 1000 V:

The circuit must first be switched off because it is very dangerous to approach the victim.

1.2.1.2 First aid

First aid cannot replace the doctor, but it may preserve the life of the injured person.

Burns - cover wound with sterile material, do not apply any ointments or cotton wool.

Unconsciousness, breathing discernible - loosen tight clothing, lay the injured person

down correctly.


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Unconsciousness, no breathing - commence mouth-to-mouth resuscitation or use a

respirator.

Unconsciousness, no breathing, no pulse (ventricular fibrillation) - apply artificial

respiration immediately, also heart massage! A human being can only live for three

minutes without oxygen!

Artificial respiration:

Bend the head right back. This ensures that the victim's tongue does not drop back into

his/her throat. (Risk of choking).

Hold nose and blow air in slowly; if this proves too difficult, bend the head back further.

Helper breathes in again, and observes the victim's chest for signs of breathing activity.

About 15 to 20 breaths/minute.

If a respirator is available, use it.


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Heart massage and artificial respiration:

Heart massage should only be done by helpers who are well acquainted with this therapy

and it always has to be combined with artificial respiration (mouth to mouth, if a respirator

is available, use it.)

Application

If there is

- no breathing

- a dilation of the pupil and no reaction at incidence of fight

- no pulse to feel at the carotid artery

Performance

- Place victim on a solid surface (lying on the Back)

- Press the lower half of the breastbone with the eminence of the hands (put the second

hand exactly on the first - the fingers don't touch the chest) about 3 to 4 cm downward.

- Keep your arms stretched!

- As soon as the heart massage is starting also artificial respiration is necessary

because the lungs need oxygen.

- The heart massage ensues 60 to 80 times/min.

- The procedure always has to start with artificial respiration (5 times).

Heart massage and artificial respiration have to ensue in an equable rhythm.

1 : 5 (1 time artificial respiration -

5 times heart massage etc.)

if there are two helpers

2 : 15 (2 time artificial respiration -

15 times heart massage etc.)

if there is only one helper


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2 TYPES OF FAULT

An insulation fault occurs when the insulation of electrical conductors, in equipment with a

metal housing or in cables, is damaged. These insulation faults may lead to short circuits

to frame or earth.

2.1 Short circuits to frame

These are electrical connections between active and inactive conductive parts.

Inactive conductive parts:These are conducting parts which do not belong to the operating circuit (e.g. housings).

They are separated from active conductors by insulation.

Active conductors:

These are conductive parts which are live during normal operation and form the operating

circuit (e.g. cable conductors, plug pins, terminals, neutral conductors, but not PE or PEN

conductors).

2.2 Short-circuits

These are conducting connections between operationally live parts brought about by an

insulation fault.

2.3 Earth faults

These are conducting connections between conductors insulated from earth and earth or

earthed parts, brought about by insulation faults or by bridging of the insulation.


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2.4 Winding short circuits

These are conducting connections between two conductors in the winding of an electric

motor brought about by insulation faults.

2.5 Turn-to-turn faults

These are conducting connections between two or more turns on the same coil or winding

of an electric motor brought about by insulation faults.

1 = Fault to frame

2 = Short circuit

3 = Earth fault

4 = Phase to phase short circuit

5 = Turn-to-turn fault


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3 TEST AND MEASURING EQUIPMENT FOR FAULTFINDING

3.1 Basic equipment

Continuity tester:

- with lamp or buzzer.

Test lamp:

- for voltages ranging from 100 to 500 V

Multimeter:

- this is a multiple range measuring Instrument suitable for voltages up to 1000 V,currents up to approx. 30 A and for resistance measurement

Insulation tester:

- for determining the insulation resistance of installations, cables and equipment.

Measuring voltages: 100, 250, 500, 1000 V

Single knob Wheatstone bridge:

- for accurate measurement of ohmic resistances within a measurement range of 40

mto 50 ka

Phase sequence indicator:

- for establishing the phase sequence in three-phase networks

Oscilloscope:

- for measurements in rectifier circuits

Special measuring instruments are required for testing protective equipment.

They include instruments and equipment for testing contact voltage, earth resistance, loop

resistance, local insulation resistance, total current and residual current devices.


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4 FAULTHNDING METHODS

The consuming component is tested for mechanical and electrical faults. There are two

types of mechanical fault or damage - internal and external. Intern & faults include bearing

faults, and external faults include damage to the housing.

The electrical faults are mainly insulation faults (short circuits to frame, short-circuits, earth

faults).

Safety devices which have been connected in the circuit are switched off or disconnected

in order to locate these faults (e.g. motor safety switch, automatic cut-out). Where the

protective conductor is separate, the neutral conductor is also disconnected.

The fault is then located by means of testing and measuring equipment (e.g. insulation

meter). The winding is tested for short circuits to frame or phase-to-frame contact and

cables are tested for insulation from earth and from each other. Each winding isindividually measured, which means that they must be disconnected.

If the fault is not at the consuming component, but in the supply cable, a test is carried out

with the voltmeter or a test lamp to determine whether the circuit is live (switch,

consuming component). This will indicate if the supply cable is open circuited, if neither

the fuse, automatic cut-out, residual current device nor other protective devices have

tripped. The switches are switched on. An open circuit may be at a switch or at the

terminals, and it is located accurately with the voltmeter or test lamp.

If one of the protective devices trips, an insulation fault has occurred. Measurement is

again carried out to earth or to conductors with an insulation tester, remembering that the

high voltage generated by the insulation tester may damage the equipment.

In the case of contactor circuits it must be remembered that there is a main circuit and a

control circuit. Both must be tested. In the control circuit one terminal after the other is

tested as far as the contactor coil to determine whether voltage is present. Depending on

the circuit, there are voltages of 24 V to 220 V.


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A rectifier circuit may also be present and this must also be tested. The diodes or

thyristors are measured with the ohmmeter in the forward and reverse direction. In the

case of the thyristor it must be remembered that it can be triggered in both directions!

When selecting test equipment ensure that the equipment is also operating correctly and

accurately. Always set measuring Instruments to highest range, then reduce to the correct

range later (the Pointer should be located in the upper third of the scale).


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5 FAULTFINDING IN LIGHT CIRCUITS

If faults occur, the following points must be observed:

a) In the event of a fault, either the line fuse or the automatic cut-out or residual current

device trips. If the residual current device has tripped, the search for the defective line

may give problems (see section 9.1 - Faultfinding on current-operated earth leakage

circuit breakers).

b) If point a) does not apply, test the lamp.

c) If points a) and b) do not apply, test the defective line for open circuits (switch does not

operate, terminals open or poor contacts in the lamp socket)

d) If point a) applies - i.e. the line fuse, automatic cut-off or residual current device has

tripped, a fault to frame may have occurred. Locate the fault by means of an insulation

tester. Fault to frame can occur in the socket of the lamp!

5.1 Faults on filament lamps

Lamp does not come on:

Possible causes:

- Filament burnt out

- No contact or poor contact, open circuit (switches, terminals)

- Fault in the Installation of the circuit (e.g. confused L-PE with N-PE, line-earth with

neutral-earth) - Short circuit to frame in the socket

Remedy:

- Bend up the contacts in the lamp socket.

- Replace lamp.


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System type: Single-phase 220 V a.c.


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E.g. Two-way lamp circuit:

a) Test power supply and lamp.

Even if the fuses or the lamp have no visible defects they should also be checked with

a continuity tester.

b) With the test lamp (or voltmeter) on the socket contacts, determine if voltage is

present. Actuate a switch and measure again.

c) If voltage is present, switch off circuit and bend up contacts slightly.

If no voltage is present, there must be an open circuit or one of the switches is

defective (e.g. contacts broken off or burnt off). In order to test a switch, switch off the

circuit, disconnect the wires, then check with a continuity tester.


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5.2 Faults on fluorescent lamps Lamp does not come on:

- Mains voltage too low

- Lamp defective

- Poor contact, open circuit, choke, capacitor

- Starter damaged.

Remedy:

- Mains voltage must be greater than 190 V;

- Replace lamp

- Test connections, contacts and ballast

- Replace starter

Lamp does not come on properly:


- Temperature too low or too high

- Poor contacts Remedy:

- At mains voltage below 190V = ignition uncertain

- Use incandescent starter if possible

- Reverse polarity of the starter in the case of series connection

- Test connections and contacts

Lamp flickering:

- Lamp voltage too high; end of lamp life

Remedy:

- Replace lamp


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Faultfinding on fluorescent lamps:

E.g. fluorescent lamps switched with current impulse switch.

First establish whether it is only a single tube, a lamp with two tubes, or a group of lamps

which is not working.

If several lamps are not working, test power supply, press buttons, and observe impulse

switch to see whether it is also switching. Measure voltage at the impulse switch terminals

and at the terminals. If voltage is not present there is an open circuit in the wiring.

If a tube does not come on, test the starter and tube, replace if necessary, check wiring.

Possibly poor contacts at terminals; choke or capacitor may have an open circuit.


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Fluorescent lamp circuits:

In industrial lighting systems, lamps are divided between the three phases (in duo circuit).

Hence load division and reduction in stroboscopic effect.

Lamp does not come on; starter switch operates continuously

- Starter is located in supply cable between lamp and ballast

- Lamp burning voltage too high; end of lamp life

Remedy:

- Change starter

- Replace lamp

Lamp does not come on, starter does not switch, lamp electrodes glow:

- Starter is jammed, short-circuit in starter capacitor

Remedy:

- Replace starter


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Uneven illumination in lamp

- Occurs in new lamps due to contamination

Remedy:

- Allow the lamp to burn in properly

Camp comes on brightly then fails to come on again:

- Turn-to-turn fault on ballast

- Incorrectly chosen ballast

- Connection to D. C. voltage - choke ineffective

- Voltage too high (e.g. connection to two line conductors)

Remedy:

- Replace ballast

- Fit correct ballast for lamp

- For d.c. voltage, fit a resistor instead of the choke

- Change circuit

Blackening near electrodes:

- End of lamp life

- Current intensity too high

- Considerable voltage variations or supply voltage too low

Remedy:

- Replace lamp

- Test ballast

- Test mains voltage


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Dark spots on lamp:

- Mercury precipitation due to cooling

- Regular precipitations on louvered lamps

Remedy:

- Make the lamp temperature distribution even

- Increase distance between louvre and lamp

Ballast giving disturbing buzzing sound:

- Components loose

- Not properly fitted

- Lamp parts or cases too small (vibration phenomena due to magnetic stray field)

Remedy:

- Tighten screws

- Use elastic supports

- Use correct cover for ballast

Radio interference:

- Suppressor condenser not connected or defective

- Interference due to radiation

Remedy:

- Fit or replace suppressor condenser

- Place radio further away; use screened antenna feeder cable if possible.


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Lamp flickers:

- Ignition without adequate preheating of the electrodes

- New lamp not properly burnt in

Remedy:

- Replace starter

- Flickering may stop after a short burning time.

Lamp flickers, even though switched off:

Switch in neutral conductor, lamp remains live.


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6 FAULTFINDING ON ELECTRIC HAND TOOLS

Electrical hand tools should belong to protection class II, (double insulated) and have

contour or flat plugs. The plugs are permanently connected to the supply cables.

Drilling machines, which have plastic reduction gears, are fully insulated; there can be no

short-circuit to frame.

The following must be tested:

- Connecting cables and plugs for open circuit.

- Switches and speed controls for open circuit and serviceability.

- Brushes and brush holders; if the commutator is pitted, this is due to the brushsparking. Brush sparking occurs, for example, when a commutator segment is

disconnected, when the commutator is not completely circular, when the armature

winding is open circuited, if the brushes are in the wrong position. When the motor

stops at the disconnected segment, it no longer starts up again. If it is rotated a little

further, it will run again and skips this disconnected segment.

- Check motor windings for turn-to-turn fault and open circuits.

Worn brushes, connector, plugs, switches and speed controls can be replaced. When

replacing brushes make sure that they are of the same type as the ones being replaced.

The pitted commutator must be skimmed and the insulation between segments cleaned

(e.g. scraped out).

If a winding is open circuit (burnt), it must be replaced, even if there is only a turn-to-turn

fault.

A mechanical fault may also have occurred, e.g. on the reduction gearing, chuck, etc.


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E.g. hand drilling machine:

1. First carry out a visual test (cables), turn the armature by hand; in most cases carbon

brushes can easily be tested from the outside (length free movement, sufficient

contact pressure?)

2. Test cables and plugs for open circuits; connecting cables are very often broken close

to the plug or the machine connection. When checking windings, make sure that only

the winding concerned is being checked, open the switch connections if necessary.


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7 FAULTFINDING ON ELECTRICAL CONTROL SYSTEMS USING CIRCUIT

DIAGRAMS

Distinguish between faults in the main circuit and faults in the control circuit. Tripping of

the thermal overload switch indicates a fault in the main circuit!

Thermal relay or main circuit fuse trips: Fault on machine (motor), power supply (phase

failure), major voltage fluctuations, mechanical overloading: Observe the motor starting

current, relay contacts defective (burnt off or welded together).

Control circuit does not work:

- Thermal overload trip also opens control circuit.

- Check control circuit fuse and control voltage, if control circuit fuse is defective, test

coils for short-circuits.

- Measure voltage on coils with voltmeter, note any voltage drop. (particularly at low

control voltages, voltage must be 85 % of the rated voltage).

- If possible carry out an operational test on the circuit (remove main circuit fuses).

- Contactor remains actuated, although no voltage on coil:

- Clean magnet body.

- Voltage present on coil, contactor is not actuated.

- Coil interrupted, seized voltage too low.


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E.g. Reversing contactor circuit

Fault - contactors are actuated, motor is not running.

a) Main circuit diagram:

Testing the voltage.

Testing on the main circuit:

1. Test power supply (L1 - L2; L1 - L3; L2 - L3)

2. Test operation of relay contacts

3. Measure motor terminals, test thermal overload trip and cable as far as motor

terminals for wire breakage, poor terminal connection.


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Control circuit of a reversing contactor circuit:

E.g. fault - K2 not actuated.

Measure control voltage with voltmeter on N conductor and phase L, then check for open

circuit, step by step (a, b, c, d, e, f). Remember that the measurements at points e and f

can only be carried out with push button S 3 pressed (contact 3-4 closed). If there is

voltage at a-b on the coil, the coil is defective or the contactor has seized (also note

voltage level).


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8 FAULTFINDING ON ELECTRICAL MACHINES

Electrical and mechanical faults may arise.

The machine is tested for mechanical damage:

- turn the rotor shaft by hand to check whether the rotor is jammed or is otherwise

seized (bearing).

- broken feet, defective terminal boxes, burst end housings, external damage

- test supply: supply cables, transformers, fuses, motor safety switch, rectifier

- test electrical machine components

8.1 Cage rotor machines

Jerky running:

- Bearing damage, bearing support or balls damaged

- Out of balance or loose pulley or coupling

- Failure of one phase

Remedy:

- Replace bearing

- Balance rotor

- Test phases with test lamp

Motor does not start:

- Open circuit in supply cable, fuse or motor safety switch has tripped

- Open circuit in stator circuit

- Open circuit in rotor circuit

- Bearing seized.


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Remedy:

Test power supply, fuses, thermal overload trips, measure voltage at motor terminals.

Cage motors have a very low starting torque, and must not therefore start under heavy

load1 lf there is voltage at the motor terminals, check the motor. Switch off the voltage,

disconnect at the terminal board and check windings.

Motor terminal board for star and delta connections:

1. Measure winding resistance with measuring bridge and compare:

U1 - U2

V1 - V2

W1 - W2

2. Insulation resistance of windings relative to each other:

U - V

U - W

V - W

3. Insulation resistance of windings to earth:

At least 1000 ohms per volt of operating voltage


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The resistance of the individual windings should not differ substantially (5 % max.).

The insulation resistance between the windings, and between the windings and earth is

measured with an insulation tester or insulation resistance bridge.

A cage motor does not start in the star position of the star-delta switch, only on the delta

stage:

- Contact broken in the star-delta switch

- Motor torque in star connection is so low that it cannot operate the driven machine; too

high a load

Remedy:

- Test star-delta switch and remedy open circuit, if necessary

- Reduce starting lad, use a starting V-belt or use a motor of higher rating - slipring

motor.

Motor starts in the operating position of the star-delta switch, speed greatly reduced under

load:

- Rotor bars have become unsoldered

- Overloading

Remedy:

- Replace rotor bars

- Measure input with an ammeter; reduce load or use motor with higher rating.

Motor has difficulty starting; excessive speed reduction on load:


- Voltage drop in the supply cables too high

- Stator wrongly connected in star-delta

- Phase and neutral conductor interchanged


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Remedy:

- Measure mains voltage; replace transformer

- Check the cable current ratings

- Test circuit Test connections

Motor has difficulty in starting, loud humming noise when switched on, motor heats up

quickly:

- Worn bearing bushes or fracture in the ball bearing

- Rotor rubbing against stator

Remedy:

- Check bushes, ball bearings and shaft clearance

- Replace bearing bushes or ball bearings if necessary

Three-phase motor runs sluggishly in star connection, or not at all:

- Load too high


- Star-delta switch contact seized

Remedy:

- Reduce load, or install larger motor

- Test mains and cables

- Overhaul switch

Motor humming very loudly on load; overheating:

- Winding of stator winding has turn-to-turn or phase-to-phase fault

Remedy:

- Measure phase currents; if they differ seriously from specification, the motor must be

rewound.


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Motor becomes very hot; motor safety switch does not trip:

- Overloading

- Too frequent starting up.

- Voltage too high or too row; one phase disconnected on terminal board

- Incorrect corrections

- Turn-to-turn or phase-to-phase fault

- Fan not working

Incorrect mode of operating Remedy:

- Reduce load or use larger motor

- Use motor with slipring rotor

- Measure voltages with voltmeter

- Compare circuit with wiring diagram

- Replace winding

- Test fan

- Use motor according to operating mode (according to rating plate)

When motor is switched an, fuses blow or the motor safety switch trips:

- Fuse or motor safety switch too small

- Short circuit in cable from star-delta switch to stator

- Short circuit between two lines or phases of the stator winding or to frame.

Remedy:

- Replace with correct fuses; adjust motor safety switch correctly

- Disconnect cables from terminal board, and test insulation; remedy insulation faults

- Disconnect cables from terminal board, and remove links from terminal board.

Measure insulation between phases and to case; replace winding.


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8.2 Slipring rotor machines

Jerky running:

- Bearing damage, bearing arrangement or balls damaged

- Out of balance or Loose flange or pulley

- Phase failure

Remedy:

- Replace bearing

- Balance rotor and pulley

- Test phases with test lamp

Motor does not start:- Interruption in the supply cable; fuse or motor circuit breaker has tripped

1. Open circuit in stator

2. Open circuit in rotor

3. Brushes do not bed on slipring

4. Bearing seized

1. Test stator circuit:

Test supply, fuses, switches, measure voltage at the stator terminals, check the

windings as for the cage motor.

2. Test rotor circuit:

Disconnect rotor terminals and measure rotor stalled voltage with the stator switched

on. Compare measured voltage with the voltage specified on the rating plate.

If no voltage is measurable - open circuit in the rotor.

If rotor voltage corresponds to value specified - switch off stator and test sliprings,

carbon brushes and resistances for open circuit.


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Starting circuit of a slipring rotor with contactor control:

Motor terminal board of a slipring motor:

E.g.: Stator Rotor

When inspecting the rotor winding, two windings are measured (in series).

K L

K M in mor

L M


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Insulation measurement: test windings to earth.

Jerky starting:

- Starter too small

- Starter has burnt contacts or contacts do not make

- Starter incorrectly connected

Remedy:

- Replace starter

- Overhaul starter; locate the open circuit with test lamp and bridge or replace starter

- Compare starter connections with wiring diagram

Motor has difficulty in starting, speed drops under load

- Motor is connected in star instead of delta


- Voltage drop in cables excessive

- Phase and neutral conductors cross connected.

Remedy:

- After checking rated data on rating plate: reconnect the motor in delta at the terminal

board.

- Measure mains voltage, replace transformer if necessary

- Check cable cross-sections

- Check connections


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Motor has difficulty starting, loud humming when switched on, motor warms up quickly:

- Worn plain bearings or fracture in ball bearing; rotor rubs against stator

Remedy:

- Test plain bearings, ball bearings, shaft clearance; if necessary replace bearing

bushes or ball bearings

When motor is switched on, fuses blow or motor circuit breaker trips

- Short circuit in cables between switch and stator

- Short circuit in cables between starter and motor

- Short circuit between turns or phases or short circuit to frame

- Short circuit between slip rings

Remedy:

- Disconnect cables from the terminal board and test insulation, remove any faults found

- Disconnect supply cables from the starter, insulate brushes from slip rings with piece

of paper or wood, and test starter cable insulation; remove faults

- Detach supply cables from terminal board, and remove links on terminal board; test

insulation between phases and to earth; replace winding

- Lift brushes from the sliprings; remove belt from pulley, motor will then idle; repair as

required.

Motor humming loudly at high input; overheating:

- Stator winding has short circuit between turns or phases

- Open circuit in rotor

Remedy:

- Establish, by touching, whether the winding is heating irregularly, considerable heating

at location of fault indicates rewinding


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Spasmodic humming and stator current fluctuations

- Open circuit in rotor

Remedy:

- Test brushes (brush holders and brush springs)

- Measure with voltmeter or test lamp to check that full voltage is present

- Test cables between motor and starter

Motor is becoming very hat, motor overhead switch cuts out

- Overload

- Voltage too high or too low

- One phase open circuit at terminal board

- Circuit incorrect

- Short circuit between turns or phases

- Fan not working

- Incorrect motor rating

Remedy:

- Reduce load (test with ammeter) or change motor

- Test phase with test lamp


- New winding

- Test fan

- Use motor according to rating plate


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8.3 D.C. motors

Jerky running:

- Bearing damage - Bearing ring or ball bearings

- Unbalanced or loose coupling or pulley

Remedy:

- Replace bearing

- Balance armature or replace bush

Motor does not start:

- Open circuit in the supply cable

- Brushes do not bed on commutator- Field winding open circuit

- Terminal voltage too low

- Bearing seized

- Fault in starter

- Armature open circuit

Remedy:

- Test terminals and cables

- Remove terminals in brush holder, replace worn brushes

- Test field winding

- Measure terminal voltage

- Replace bearing

- Test contacts and connections in starter Check connections on commutator

- Test voltage supply, inspect starter and windings (open or short-circuit or earth fault).

Check type of motor, because different types have different characteristics.


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The different motor types:

Shunt wound motor Series motor Compound motor

Faultfinding on separately excited D.C. motor:

(Most commonly used)

First establish whether armature and field voltage are present. Many motor control

systems are provided with field monitoring devices to prevent the armature from taking too

high a current or becoming damaged in the event of field failure. This point must also be

given attention when faultfinding.


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a) Testing the field circuit:

Voltmeter at L2+ and L2-, then at F1 and F2, if no voltage is present at F1 and F2, test the

field circuit step by step.

Voltmeter at L2 and terminal t of field control element, then at terminal s, noting the

position of the field control element.

If voltage is present at F1 - F2, test armature circuit and winding FT - F2 for open circuit,

turn-to-turn fault or earth fault.

b) Testing the armature circuit:

Voltmeter at L1+ and L1-, then at A1 and A2, if no voltage at A1 - A2, inspect armature

circuit step by step.

Voltmeter at L1- and contact L of starter, then L1 - contact R (note position of the starter),

then at L1- and A1, A1 - B2, A1 - B1, A1 - A2.

If voltage is present at F1 - F2 and A1 - A2, and if motor is still not running, check load

and armature.


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Jerky starting:

- Starter too small

- Starter contacts burnt or not making correctly

- Starter incorrectly connected

- Short-circuit between laminations

- Turn-to-turn fault in armature

Remedy:

- Replace starter

- Overhaul starter

- Compare starter connection with wiring diagram

- Overhaul laminations

- Rewind armature

D.C. motor has excessive sparking at brushes:

- Motor overloaded

- Brushes displaced

- Projecting mica an commutator

- Out-of-round commutator, dirty

- Jerky running, bearing damage

- Turn-to-turn fault in the field or inter-pole winding

Remedy:

- Measure current, reduce load

- Test marking of brush position

- Mica must be scraped to approx. 0.8 mm below segments with special saw

- Rotate commutator, clean and remove oil and dirt

- Replace bearing

- Replace winding


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D.c. series motor running too quickly

- Load too low

- Brush gear incorrectly set

- Terminal voltage too high

- Turn-to-turn fault in field winding

Remedy:

- Test and increase load

- Set brush gear to marked setting

- Measure terminal voltage; test mains voltage

- Replace winding

Blackening of commutator in places:

- Lamination short-circuiting

Remedy:

- Overhaul laminations

Burning out of lamination insulation at one point:

- Armature coil has become unsoldered

Remedy:

- Overhaul armature coil.

Motor is becoming too hot:

- Load too high

- Voltage at the terminals too low

- Incorrect circuit

- Turn-to-turn fault

- Fan not operating

- Incorrect operating mode


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Remedy:

- Reduce load, use larger motor

- Measure mains voltage; check cross-section of cables


- New winding

- Test fan Use motor according to rating plate

Motor has difficulty in starting, speed variation under load:

- Voltage too low at terminals

Excessive voltage drop

- Turn-to-turn fault

- Field incorrectly connected

- Earth fault

Remedy:

- Measure mains voltage

- Test supply cable cross-section

- New winding in case of turn and earth fault

- Test terminal board

8.4 Single-phase transformers

Transformer oil too hot

- Overloading, note ambient temperature, cooling surface contaminated;

- Primary voltage too high

Remedy:

- Reduce load, install ventilation, clean surface;- Compare voltage with type plate and change transformer


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High primary current:

- Short circuit between turns or to earth

Remedy:

- Switch off load, measure current and voltage on high and low voltage side

- Replace winding

No output voltage:

- Break in secondary or primary winding

- No input voltage - open circuit of mains - disconnection at transformer terminals

Remedy:

- Replace windings

- Measure input voltage; replace supply cable

Reduced output voltage:

- Short circuit between turns or to earth

Remedy:

- Replace winding

Fuse trips:

- Short-circuiting

Remedy:

- Locate and remove fault


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In parallel operation the following must be remembered:

- Primary and secondary must have the same voltage

- Short-circuit voltages must be the same

- Phase angle between high and low voltages is either 0 or 180. If the phase angle is

not the same, only the low voltage connections need be replaced.

If measurement is carried out with a voltmeter, there must be no voltage between two

winding terminals which are to be connected to a conductor on the output side.

Checking the phase position with a voltmeter:

In the case of parallel connection of three-phase transformers, the following must be

remembered:

- Primary and secondary power ratings must be the same.- Short-circuit voltages must be the same

- They must belong to the same switching group, with the possibility of equalising

different characteristics by making suitable terminal connection.

A voltmeter is installed between terminals (phases) of the same name. If this voltmeter

does not indicate a voltage, the circuit is in order.


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Checking the phase position with a voltmeter:

_ _ _ _ _ Make connections only when voltmeter reads zero


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9 FAULTFIND1NG ON EXCESS VOLTAGE PROTECTION EQUIPMENT

9.1 Residual current devices

The supply cable for the equipment to be protected is monitored by a total current

converter for residual current. The equipment casings are earthed. If fault to frame occurs

in equipment thus protected, a fault current flows via earth; the sum of the currents in the

supply cables is no longer 0, and the residual current device operates. The connected

equipment is now protected against contact voltage (UB = 50 V). The switch also protects

against contact of live-conductors to earth (earth fault).

Example of a fault current protective circuit:


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9.1.1 Faults on residual current devices

Safety switch does not trip

1. Testing the residual current device

When the test knob is pressed, only the switch is tested, not the protective circuit.

Fault - Connection between neutral (N) and protective conductor (PE) after the residual

current device

RB = Operating earth

RS = Auxiliary earth electrode

PE = Protective conductor

N = Neutral conductor

L = Outer conductor

If the residual current device falls to trip despite exceeding the rated fault current, this

leads to the conclusion that there is a fault between the neutral (N) and the protective

conductor (PE), after the residual current device. Most of the fault current flows back via

the neutral conductor.

In the event of fault to frame after rectifier circuits, the unidirectional fault current flow may

lead to magnetisation of the core of the residual current device, so preventing tripping.


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Residual current devices (FI safety switches) bearing the symbol must cut out reliably

even in the event of fault to frame after rectifier circuits (pulsating direct fault currents).

2. Testing the residual current

Fault - Defective insulation before the residual current device

RB = Supply earth

RS = Auxiliary earth

PE = Protective conductor

L = Live conductor

N = Neutral conductor

An insulation fault between the outer and protective conductor occurs before the residual

current device. An earth leakage current flows via the earth electrode and an earth

electrode voltage is generated. The residual current device does not trip.

To avoid such faults in the area of the residual current device, the protective conductor

before the residual current device should not installed together with the supply cable in

one conduit.

Safety switch trips:

One reason for this might be a connection between neutral conductors in different residual

current circuits behind the safety switch. In this case possible fault currents are distributedin any neutral conductors, and this may lead to undesirable tripping of a safety switch.


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Safety switch trips too early:

This means that it trips before the permissible tripping range is reached. Either the

protective conductor is defective, or the system already has a certain basic fault,

producing a current below the release current. This occurs in measuring circuits where the

basic leakage current is added to the test current supplied by the measuring instrument

therefore the safety switch trips earlier.

If the residual current device has tripped, all poles of the supply are disconnected. This

means that the circuit in which the fault has occurred cannot be identified immediately. All

the circuit breakers are now switched off or all the fuses are unscrewed, and the residual

current device is reset. One of the circuit breakers is now switched in, or a fuse is screwed

in until the residual current device trips again. The defective circuit is now identified.

The main circuit breaker is now switched off or the fuse unscrewed, and the residual

current device as well as the remaining circuits which have no faults are connected.

In the defective circuit switch off all current using equipment, if plug-in pieces of

equipment are being used, unplug them and reconnect the circuit. If the residual current

device now holds, one of the pieces of equipment is defective. If the residual current

device fails again, the circuit must be examined with insulation testers and the circuits

sub-divided at the terminals to isolate the fault. The insulation of the N conductor must

also be tested.

Damp and wet areas are particular sources of faults. Sometimes faults occur where the

residual current device trips irregularly. In this case install a device with a lower tripping

current, these trips more easily and faultfinding is therefore simpler to carry out.

If the residual current device repeatedly cuts out and all the circuit breakers are switched

off; this indicates a fault between neutral and earth!


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WARNING

The fault current release value of the residual current device may only be changed if

permitted by the earthing condition of the system, i.e. a higher fault current release value

requires a lower system earthing resistance.

9.1.2 Testing circuits

Testing the residual current device

- on single-phase alternating current:

- on three-wire three-phase current:


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- on four-wire three-phase current:

9.2 Protective earthing conductors

In protective earthing, the casings of the equipment to be protected are connected via a

protective conductor to a suitable earth electrode.

1. Fault current flows back through the ground

It must be remembered that the specific earth resistance varies due to seasonal climatic

fluctuations (dry weather, wet weather).


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2. Fault current flows back via a metal water pipe network to the electricity generator or

transmitter (e.g. in shipbuilding).

If fault to frame takes place in protectively earthed equipment, a high current flows via the

fault location and the protective earth electrode to the supply earth of the network, and the

series-connected fuse cuts out. If several Current using units are protected by a single

fuse, the fault must be located by measuring the insulation resistance to determine which

of the current using units is defective.

9.3 Protective earth and neutral conductors (PEN)

The cases of the equipment to be protected are connected to the PEN conductor. Shortcircuits to frame on equipment employing PEN conductors causes the series-connected

fuse to trip, for a large current flows through the fault and the PEN conductor.

PEN System

In the event of fault to frame the fault current flows

through the PEN conductor. In cables having a cross

section of less than 10mm2, the PEN conductor must

be divided into an N and a PE conductor (on the right

in the diagram).


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The risk of conductor breakage is avoided by division in the case of small conductor

cross-sections. The PE and N conductors must not be reconnected after the division.

Example of PEN conductors in a three-phase four-wire network (standard system):

Example of PEN conductors in a three-phase four-wire network. The neutral and

protective conductors are laid separately. (No current normally in the PEN conductor).


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Like protective earthing, PEN systems require a low earth resistance

"PEN system" by connecting the system components to be protected to the neutral

conductor (normal system)

PEN Protective earth and neutral conductor

Ra supply earth

Li .. L3 Line conductors

HA supply connection

During installation the fuse in the defective circuit blows. It must then be checked whether

the current using unit is short-circuiting or has an earth fault. If several current using units

are protected by a

single fuse, the fault must also be isolated between the units, i.e. disconnect the units,

check the insulation resistances of each unit with suitable measuring instruments until the

fault is found.

9.4 Protective multiple earth systems (IT systems)

A high degree of operational safety is achieved with the protective cable system (e.g. in

operating systems).


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Layout example of a protective cable system in a three-phase three-wire network:

If fault to frame occurs, a fault current flows due to the insulation and capacitance

condition of the network. No contact voltage occurs due to the connections of the

protective conductor to the numerous earth electrodes. For immediate location of the fault

to frame the mains - earth insulation condition is monitored (signal is received in the

control room). The fault must then be removed as quickly as possible.

The rise in voltage in fault-free conductors increases the possibility of a second fault to

frame. This causes the equipment with the least protection to be disconnected.

The double fault to frame (earth short-circuiting) then becomes a single fault again.

Protective cable system with a 2-voltmeter circuit for insulation monitoring:


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Fault V1 V2

None approx. half

phase voltage

approx. half

phase voltage

Earth fault

of N 0 Phase voltage

Earth fault

of L1 Phase voltage 0

Earth fault

of L2

or L3

Phase voltage Phase-to-phase

voltage

The first fault is indicated by the monitoring system, but there is no indication of where the

fault is located.

The faultfinding is carried out by measuring the total current of the individual circuits from

the supply transformer. Since an earth fault gives rise to a fault current (albeit small), the

sum of the currents in the outer conductors and the neutral conductor is no longer zero

(1stlaw of Kirchhoff).

The individual circuits are measured until the damaged circuit is found.

A second possibility would be to switch off the individual circuits and observe the

monitoring device until it no longer indicates a fault, but this procedure is not often

possible because of operational breakdown.


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10 FAULTFINDING ON RECTIFIER CIRCUITS

Testing of voltages U1, U2, U- with voltmeter and oscilloscope for open circuits.

Compare the measured voltages U1, U2with the data on the type plate.

If the transformer voltages conform to the specifications, the fault lies in the diodes. Each

diode must then be checked individually (oscilloscope).

If a pulsating d.c. voltage is present, but no smoothing is visible, the capacitor is defective.

Diodes and capacitors can only be replaced.

In most cases the r.m.s. value is indicated on the transformer, and the value measured

with an oscilloscope is the peak to peak value Uss.

10.1 Testing power semiconductors with an ohmmeter

In most cases a defective power semiconductor can be checked with an ohmmeter (1.5 V

battery in ohmmeter).

Diodes:

Disconnect to measure values. Apply a threshold voltage of approx. 0.7 V in the forwarddirection. If the measurement is carried out so that the positive pole of the ohmmeter is

located at the anode of the diode to be tested, the resistance value must be lower than

that in the reverse direction.


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If the positive pole is located at the cathode, the instrument indicates a very high

resistance value. If the diode is destroyed, a low resistance value is indicated in both

directions.

If the diode has a high resistance in both directions, there is an open circuit and the diode

must be replaced.

Thyristors:

These can have a high resistance in both directions. If the thyristors are intact, the

Instrument does not show any deflection in either direction, but if the thyristors are

destroyed, 0 ohm can be indicated.

If a power semiconductor is defective, it can only be replaced by the same type.

The following data must be observed:

- Type designation

- Inverse voltage

- Release time

- Permissible voltage and current rise time

10.2 Testing power semiconductors with an oscilloscope

Among other things, the following is of vital importance to a rectifier circuit:

- the output voltage supplied by the circuit

- the voltage drop under load

- the amplitude of the superimposed a.c. voltage (hum voltage)

- the testing of diodes, if possible at rated voltage.

The following knowledge is required for designing a rectifier circuit:

- Average forward current- Peak current

- Max. inverse voltage at the diodes

- Internal transformer resistance


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In addition to the arithmetical mean value and r.m.s. value, the peak factor S, the form

factor F and the ripple W are also frequently used to determine the raw D.C. voltages and

direct currents.

10.2.1 Half-wave rectification

Distribution of A.C. voltage between load and rectifier:

Voltage and current waveforms for load:

This circuit is used for rectifying low currents.


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10.2.2 Full-wave rectification (centre tapped transformer)

Transformer voltages

Voltage and current waveforms in the load:

This circuit is used for rectifying low currents.


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10.2.3 Bridge circuit

Transformer voltage U2 (A.C):

Output voltage of the bridge circuit (voltage across load):

This circuit is most frequently used for powers of up to 2 kW.

1. with smoothing choke


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a) Waveform of the load voltage ua

b) Inductance opposes all variations in the current, and so smoothes it.

The load current iais continuous.

10.2.4 Three-phase Bridge circuit

This circuit is used most frequently for powers of over 2 kW.


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EE082 -Fault Finding in Electrical ControlSystems

Theoretical Test


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TEST 1

1. What are the safety rules when carrying out work on electrical systems?

2. What is the rescue procedure in the event of electrical accidents particularly for

release from the circuit?

3. What types of faults are there?

4. In the sketch below, draw circuits to illustrate the following: earth fault, short-circuiting

and fault to frame.

5. With reference to insulation testing in an installation, name the individual insulation

resistances which should be measured.

6. State the order in which the faultfinding procedure is carried out.

7. Name two types of faults which can occur on electrical hand tools and machines.


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8. Name three faults which could cause a motor overload to trip when a cage motor is

switched on?

9. State three possible causes and their remedies, if a slipring motor does not start.

10. State four reasons for a motor overheating.

11. What fault causes the residual current device to trip?

12. State briefly, how faults on power diodes can be checked with an

ohmmeter, giving the method of connection to the ohmmeter.


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TEST 2

1. State the range of voltages and currents which can be dangerous to man. In the

sketch below, draw in the current and voltage paths which would be highly dangerous.

2. Explain the difference between a fault to frame and short circuiting.

3. Name and describe three types of faults which may occur in windings.

4. Name five items of test and measuring equipment used for faultfinding, and state their

uses.

5. The following faults occur on fluorescent lamps:

1. Lamp does not come on

2. Lamp does not come on, starter switches continuously

3. Lamp lights brightly and fails to come on again

State the causes and their remedies.


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6. State three causes of a cage motor failing to start.

7. State two possible faults if a cage motor with star-delta starting will only start in the

delta connection.

8. State the causes and their remedy when a slipring rotor starts jerkily.

9. State five causes and their remedies, when a D.C. motor fails to start.

10. Why does excessive brush sparking occur?

11. State three types of faults which occur on the transformer.

12. State two faults which can cause the fuse or the power circuit breaker to operate.


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TEST 1

(Solution)

1. Switch off at all poles and on all sides.

Protect against accidental switching on again.

Test for freedom from voltage.

Earth and short-circuit.

Cover adjacent live parts and protect danger point.

Connection is carried out in the reverse order.

2. Switch off the supply.

Remove the victim from the circuit.

Apply first aid immediately.

Inform doctor, rescue service and police.

up to 1000 V:

4. switch off supply

5. in the case of portable equipment, remove plug from socket.

6. in the case of fixed equipment, switch off consuming units and disconnect cable.

7. remove all fuses.

8. place victim on insulated surface out of the danger area.

over 1000 V:

9. the equipment must be switched off.

3. Open circuit, fault to frame, short-circuiting, earth fault, short circuit between turns,

phases.


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4.

1 = Short circuit 2 = Fault to frame 3 = Earth fault

5. Conductors to earth, conductor to conductor and system to earth should be measured.

In the case of PEN systems the neutral to PEN conductor resistance is measured.

6. The following are tested: Connections to mains, load, remote Connection to mains,

load, remote control system or control system

Check whether the fault occurs when the current using units are disconnected

Either fault in current Fault in supply cable or

using units voltage supply

Either current using unit is Switches, terminals, transformers

tested for mechanical and etc, are- tested

electrical

Motors, Iamps,...

7. Mechanical faults (bearings, chuck in drilling machines, gears) Electrical faults

(conductor interrupted, etc...)


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8. - Rated release current set too low

- Switching on and off too frequently

- One phase missing

- Short circuit between turns or phases

9. - Break in supply cable (test fuses and motor safety switch)

- Break in stator circuit (After switching off, loosen connections on terminal board,

and examine windings, replace stator winding if necessary).

- Break in rotor circuit (inspect brushes and rotor resistances, test rotor winding,

replace if necessary).

- Bearing seized (replace bearing).

10. - Voltage too high or too low

- Incorrect circuit

- Incorrect operating mode

- Ventilation obstructed

- Motor circuit breaker set too high etc.

11. A fault to frame or earth fault

12. Diode:

Measurement in forward direction (positive pole of measuring instrument connected to

the anode) - relatively low resistance. Measurement in reverse direction (positive pole

of the measuring Instrument connected to the cathode), a high resistance is indicated.

If a low resistance is measured in both directions, the diode is destroyed.


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TEST 2

(Solution)

1. At voltages over 50 V and current over 50 mA conditions are highly dangerous

2. The fault to frame is caused by a conducting connection between a conducting part of

the operating circuit and a conducting part which does not belong to the operating

circuit.

In short-circuiting there is a conducting connection between live operating parts.

3. Conductor break - one strand is- broken

Earth fault - Contact between winding and laminations or casing

Turn-to-turn fault - Contact between two turns of a coil

Phase-to-phase fault - Contact between two winding lines

4. Ohmmeter - Cable open circuit, examine power semiconductor Resistance measuring

bridge (Wheatstone bridge) - measure winding resistance


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"Prufrex" (Growfer) - Testing armatures, and for short circuited turns

Insulation meter - Earth, phase-to-phase contact

Voltmeter, test lamp, Duspol - Voltage testing

Rotary phase sequence indicator - testing the phase sequence for three-phase current

Oscilloscope - Voltage measurement in rectifiers

5. Lamp does not come on:

- Mains voltage too low should be greater than 190 V

- Lamp defective replace

- Break in circuit test contact unit, terminals

- Starter damaged replace

Lamp does not come on, starter switches continuously

- Starter is in the supply cable

between lamp and

ballast alter circuit

- Lamp burning voltage too

high; end of lamp life replace lamp

Lamp burns brightly then does not come on again

- Turn-to-turn fault at

ballast replace

- Incorrectly chosen ballast must conform to lamp

- Connection to d.c. voltage; is only possible with

choke ineffective resistance

- Voltage too high (e.g. 380 V); alter circuit

connection to two outer

conductors

6. This fault occurs in the supply cable (motor safety switch has tripped), break in the

stator and rotor circuit, or due to seized bearing.


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7. Either the star-delta contactor has a contact break, or the torque in the star connection

is so low that the working machine cannot be started.

8. Starter too small replace

Starter has poor contact locate open circuit with test lamp;

either bridge the contact or replace starter.

Starter incorrectly connected - compare with wiring diagram.

9. Break in the supply cable - test terminals and cables

Brushes do not bed an commutator

Replace brushes if necessary.

Field winding interrupted - test

Terminal voltage too low - measure

Fault in starter - test contacts and connections

Bearing seized replace bearing

10. Brush sparking occurs where the commutator is out of round or if it is clogged with oil.

Also when the motor is overloaded, displaced brushes, turn-to-turn fault in the field or

inter-pole winding, and when the bearings are damaged.

11. Transformer oil overheating - due to overloading or too high connection voltage

high primary current - turn or earth fault

low voltage - partial turn-to-turn fault

12. The fuse or cable safety switch trips in the event of short-circuiting and overloading.


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KEY TO EVALUATION

PER CENT MARK

88 100 1

75 87 2

62 74 3

50 61 4

0 49 5

EE082-Fault Finding in Electrical Control Systems-Th-Inst.pdf

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