Marine engineering vol 2 part 16

FIG.16 - Variation of speed with load for various resistances in series with shunt motor armature at fixed field strength.. 43 -Armature reversing thyristor motor control scheme.. 44 - F

MARINE ENGINEERING PRACTICE

THE INSTITUTE OF MARINE ENGINEERS

Published by The Institute of Marine Engineers

80 Coleman Street

London

EC2R 5BJ

Copyright © 1978 The Institute of Marine Engineers

A Charity Registered in England and Wales

All rights reserved No part of this publication may be reproduced, stored

in a retrieval system, or transmitted in any form of by any means, electronic,mechanical, photocopying, recording or otherwise, without the priorpermission of the publisher Enquiries should be addressed to The Institute

of Marine Engineers

ISBN: 0 900976 78 0

Printed by Hobbs the Printers in the UK

LIST OF ILLUSTRATIONS

FIG 1 - Typical windlass arrangement.

FIG 2 - Typical winch/windlass arrangement.

FIG 3 - Typical anchor capstan arrangement.

FIG 4 - Union purchase rig.

FIG 5 - Patented swinging derrick system.

FIG 6 - Heavy derrick rig.

FIG 7 - Patented heavy lift derrick.

FIG 8 - A S-ton deck crane.

FIG 9 - Two 1S-ton twin deck cranes.

FIG 10 - Heavy lift deck crane combined with S-ton deck crane.

FIG 11 - Typical electro-hydraulic grab.

FIG 12 - Typical 4-rope grab.

FIG 13 - Steam valves for one double acting cylinder of a two-cylinder steam engine FIG 14 - Back pressure valve for automatically regulating differential pressure across

mooring winch engine.

FIG.15 - Relationship between speed and load for shunt, series and compound d.c motors

at rated supply voltage.

FIG.16 - Variation of speed with load for various resistances in series with shunt motor

armature at fixed field strength.

FIG 17 - Contactor switched series resistance control of D.C winch.

FIG 18 - Single-speed A C motor reversing drive.

FIG 19 - Typical torque/speed curves for single and double-cage rotor induction motors FIG 20 - Two-speed reversing power circuit for A.C motor with independent windings.

FIG 21 - Two-speed reversing power circuit for A.C motor with tapped winding.

FIG 22 - Schematic diagram for three-speed pole-changing cargo winch induction motor FIG 23 - Typical load/speed characteristics for a slipring induction motor with rotor

resistance control.

FIG 24 - A C slipring motor control system for windlass drive.

FIG 25 - Basic Ward Leonard control system.

FIG 26 - Speed/load characteristic of basic Ward Leonard system.

FIG 27 - Modified Ward Leonard control system.

FIG 28 - Speed/load characteristic of modified Ward Leonard system.

FIG 29 - Control schematic for hoist, luff and slew motions of Ward Leonard driven

deck crane.

FIG 30 - Differential crane limit switch arrangement.

FIG 31 - Slack rope switch.

FIG 32 - A selection of thyristors, illustrating range and encapsulation.

FIG 33 - Typical thyristor characteristics.

FIG 34 - The control of voltage and power by delaying the thyristor gate signal.

FIG 35 Schematic power circuit for the thyristor control of a Ward Leonard drive.

FIG 36· Thyristor and diode reversing drive for squirrel cage induction motor.

FIG 37 - Back-to-back thyristor reversing drive for slipring induction motor.

FIG 38 - Torque/speed curves for thyristor-controlled induction motor drive.

FIG 39 - Full bridge thyristor power circuit.

FIG 40 - Phase and d.c voltages relating (0 full thyristor bridge at firing pulse delays of

0° 30° 60° 90° and 120°.

FIG 41 - Transformer-fed half-bridge circuit.

FIG.42 - Voltage-wave form from controlled transformer-fed half bridge.

FIG 43 -Armature reversing thyristor motor control scheme.

FIG 44 - Field reversing thyristor motor control scheme.

FIG.45 - 'Back-to-Back' bridge or 'static' thyristor motor control scheme.

FIG 46 -A Typical Arrangement of one type of watertight motor brake.

The authors would like to thank the undernoted companies for their kindpermission to reproduce illustrations.

FIG I Clarke Chapman MarineFIG 2 Clarke Chapman MarineFIG 3 Clarke Chapman MarineFIG 4 Clarke Chapman MarineFIG 5 Cargospeed Equipment Ltd

FIG 6 British Standards InstitutionFIG 7 Blohm &Voss A G

FIG 8 Clarke Chapman MarineFIG 9 Clarke Chapman MarineFIG.10 Clarke Chapman MarineFIG 11 Clyde Booth - RodleyFIG 12 Butters Cranes Ltd

INTRODUCTION This booklet describes electrical and steam-powered machinery including its control system Hydraulic power transmission is covered

in Marine Engineering Practice Volume I Part 7 - Hydraulic Power Transmission in Marine Machinery by C.M Joy C.Eng and electric power is covered in Electricity Applied to Marine Engineering by

W Laws.

The range of deck machinery currently in use is extensive and varied tosuit the owner's particular requirements However, the equipment generally usedcan be classified into three categories:

1) Anchor handling equipment;

2) Mooring equipment;

3) Cargo handling equipment

Although other types of deck machinery are in use e.g towing winches,trawl winches, etc, it is not the intention to discuss these highly specializedmachines, but merely to describe, in general terms, the type of equipment to

be found on the decks of most modern vessels

1 ANCHOR HANDLING EQUIPMENT1.1 WINDLASSES (see Fig 1)

The efficient working of the anchor windlass is essential to the safety ofthe ship and therefore its design and performance is subject to the approval ofthe appropriate classification society Classification society rules governingwindlass performance vary Basically they require that:

a) the windlass cable lifter brakes are able to control the running anchorand cable when the cable lifter is disconnected from the gearing during

"letting go"; average cable speeds vary between 5-7.5 m/s (1000-1500)during this operation;

b) the windlass can heave a certain weight of cable at a specified speed;this "full load" varies but is generally between 4 and 6 times the weight

of one anchor and the speed of haul at full load is usually between0.12-0.2 m/sec (25-40 ft/min)

The normal windlass arrangement utilizes one prime mover to drive twodeclutchable cablelifters and also two warping ends The warping ends are notdeclutchable and rotate continuously when the windlass is in use When mooring,light line speeds of 0.75-1.0 m/sec (150-200 ft/min) are required

Due to the low speed of rotation of the cablelifter whilst heaving anchor(2-7 rev/min), a high gear reduction is required when the windlass is driven by

a high-speed electric or hydraulic motor This reduction is generally obtained

by the use of a high-ratio worm gear, followed by one or two steps of spurgearing between the warping end shaft and the cable lifter Alternatively,multi-steps of spur gearing are used

As windlasses are required for intermittent duty only, the gearing isdesigned with an adequate margin on strength rather than wear Slipping clutchesare commonly fitted on electrically driven windlasses, either between the motorand the gearbox or incorporated in the gearbox This avoids the inertia of thedriving motor being transmitted through the gear system in the event of shockloading on the cable Such shocks can occur, for example, when the anchor ispulled hard into the hawse pipe when being housed

Windlasses are normally controlled from a local position, the operatormanually applying the cablelifter brake, as required, to control the speed of therunning cable Whilst heaving anchor, the operator is positioned at the windlass

or at the ships side if he needs to watch the anchor being housed RemotelycOntrolled systems are available which permit all normal windlass functions to

to be carried out from the bridge, thus obviating the need for crew members to

be on standby duty on the forecastle for long hours while the ship is negotiating

an estuary or other restricted waterway

The windlass is in the most vulnerable position as far as exposure to theelements is concerned and should be designed and constructed so that mainten-ance is reduced to the absolute minimum Normally primary gearing is enclosedand splash lubricated, maintenance being limited to pressure grease points forgunmetal sleeve bearings However, due to the large size of the final set of bevel

or spur reduction gears, and the clutching arrangements required, these gearsare often of the open type and are lubricated with open gear compound

1.2. WINCH WINDLASS UNIT

This arrangement (Fig 2) uses a forward mooring wi!1ch to drive a windlassunit thus reducing the number of prime movers required The port and starboardunit can, if required, be interconnected mechanically by means of clutches toprovide the following facility:

a) a standby drive should one prime mover fail;

b) the power of both prime movers to one windlass should this berequired

1.3 ANCHOR CAPSTAN

With this type of equipment the driving machinery is situated below deck andthe cablelifters are mounted on ve"rtical shafts The capstan barrel may bemounted on top of the cablelifter but on larger equipment above 76 rom (3 in.)diameter cable - it is usual to have the capstan barrel mounted on a separateshaft as shown in Fig 3

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2 MOORING EQUIPMENT

These are, at the present time, no accepted Classification Society rules forthe selection of mooring equipment, the size and type of equipment adoptedfor a vessel generally being based on past experience However, certain indepen-dent authorities and major oil companies have instituted their own investigationsinto the question of mooring techniques and equipment and a basis for selection

of tanker mooring equipment is available Hauling load duties of warping capstansand mooring winches vary between 30-300 kN (3-30 tons) at 0.3-0.6 m/sec(60-120 ft/min) and twice full load is normally provided for recovering lightlines

The size of wire rope used on mooring winch barrels is limited by the weight

of wire manageable by the crew; this is currently accepted as 48 mm (2 in )diameter maximum The basic problems associated with the use of wire ropes

is that they are difficult to handle, they do not float and when used in layers, due to inadequate spooling, the top tensioned layer cuts down into theunderlying layers causing damage In order to counteract this latter problem, adivided barrel can be used so that the wire may be stored in one portion and asingle layer of wire transferred to the second portion when tensioned The lowdensity, high breaking strength synthetic rope (polypropylene, nylon, terylene,etc.) offers certain advantages over the wire rope, its one main disadvantagebeing a tendency to fuse if scrubbed against itself or the barrel These ropes arecurrently in use with double drum warping equipment and storage reels ontankers and bulk carriers

multi-The fitting of synthetic ropes direct to mooring barrels is also currently beingtested in practice, apparently with some success

Mooring winches are currently manufactured with steam, electric or hydraulicdrives The two basic types are described below

2.1 NON-AUTOMATIC MOORING WINCHES

These winches provide the facility for tensioning the hauling wire up to thestalling load of the winch, usually 1.5 time the hauling load; thereafter theload is held by the prime mover brake or preferably by the barrel brake withthe barrel de-clutched The winch cannot payout wire unless the brake isoverhauled (when the rope tension overcomes the braking torque) or recoverwire unless manually operated Thus wire will become slack during service unlessconstantly attended, because of tidal variations, loading/unloading, etc

2.2 AUTOMATIC MOORING WINCHES

These winches provide the maRual control facilities of the non-automaticwinches However, in addition they incorporate a control feature such that, inthe "automatic" setting, the winch may be overhauled and wire is paid off thebarrel at a predetermined maximum tension In addition, wire is recovered at

a lower tension should it tend to become slack Thus there is a certain range of

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tension associated with each step of automatic control when the wire is stationary;

it is not desirable to reduce this ran~e to the minimum possible as this results inhunting of the controls

It should be noted that the principal reason for incorporating automatic controls

as described is to limit the tension at which the winch automatically rendersthe wire so avoiding broken wires and also preventing mooring wires becomingslack

The load sensing devices used with automatic mooring winches are varied andcannot be described here in detail

Spring loaded gearwheels and torsion bars are widely used for sensing variations

in rope tension with steam and electric winches With this "dead motor" system,the prime mover is normally un~nergized and only becomes active in response

to predetermined variations in rope tension

Conversely, the "live motor" system makes use of a prime mover which iscontinuously energized in the hauling mode With steam and hydraulic mooringwinches, pressure sensing devices can be used and the motor current can bemonitored with electric winches

The control system for an electric mooring winch is described in part 2 of thispart

Mooring winches are usually controlled from the local position i.e at the winch.However, on vessels of large beam, such as tankers or where docking operationsare a regular occurrence (for example, with vessels regularly navigating the St.Lawrence Seaway), remote and shipside controllers are of great advantage Asmooring techniques vary so widely the position and type of control must beengineered to suit the application It is considered, especially on vessels wheremooring lines may be long and ship position critical, that the greatest asset

to the operator is knowledge of the wire tensions existing during the mooringoperation, coupled with an indication of the amount of wire paid off the barrel:

It is quite feasible to record these at a central position and mooring lines wouldthen only have to be adjusted periodically as indicated by the recording instru-ments At present, this practice is used extensively on oil drilling rigs but could

be developed for mooring ships

The majority of automatic mooring winches are of spur geared design in order

to improve the backward efficiency of the gear train for reversing purposes,the gearing and bearings being totally enclosed and splash lubricated from theoil sump On larger mooring winches for VLCC it is now common practice todesign the brake to withstand the breaking load of the mooring wire, althoughBritish Standard Specification BS.MA 32 only calls for a holding load of not less

than 80 per cent of the breaking strength pf the rope Worm geared automatic

mooring winches are uncommon as the low forward, backward and standstillefficiencies associated with this type of gearing create design and operationalproblems

3 CARGO HANDLING EQUIPMENTEverything on board a modern cargo ship is geared to achieve the shortestpossible port turnround time and this requirement is especially applicable to itscargo handling equipment.

Broadly speaking cargo handling can be divided into the following categories.3.1 DERRICK RIG SYSTEMS

Although gradually being ousted by deck cranes, this system has been themost popular method of handling cargo since the conception of the moderncargo vessel and even before that

Derricks rigs are usually based on the fixed outreach or swinging derrick system.3.1.1 Fixed Outreach Systems

The union purchase system (Fig 4) is the most common fixed outreachsystem The system utilizes two derricks and two winches with the derricks in

fixed positions One derrick is arranged over the quayside and the other overthe hold By a combination of hoisting on both winches and hoisting on onewinch and paying out on the other winch it is possible to transfer from thequayside to the hold or vice versa Although it may appear crude, this system hasbeen a tried favourite for years but it has several important disadvantages:I) It can only be used between a fixed point on the quayside and afixed point in the hold; this creates a serious problem of manhandlingthe cargo into its stowed position;

2) Due to the sharing of the load between two derricks, overloading ofone derrick can occur if the operation is not properly managed andarising from this, many harbour authorities have expressed theirconcern about its use

3.1.2 Swinging Derrick Rigs

Many patent swinging derrick rigs have appeared over recent years (see Fig 5).All these rigs have been developed to enable accurate "spotting" of the load.These designs usually (but not always) include a hoisting, topping and slewingwinch and many ingenious ideas have been devised to overcome or alleviatethe stability problem which can arise when the derrick is in certain attitudes(usually the outboard position)

Whilst these rigs can perform all the functions of a deck crane they have thebasic disadvantage of being confined to one hatch, i.e they cannot slew through

3600• The rewards will be great for the inventor who can overcome this defectand no doubt many minds are already on the problem

3.2 HEAVY LIFTING SYSTEMS

The conventional method of lifting heavy loads is shown in Fig 6 andconsists of one or two hoisting winches lifting on a multi-fall system In addition,the topping and slewing motions operate thrQugh a multi-part rope system toensure control of the load ilt all time

The introduction of patented heavy lift systems (and the introduction of heavydeck cranes) has created a decline in the heavy lift derrick A typical patentedheavy derrick is shown in Fig 7

With this system loads up to 300 tons can be lifted and it has the uniqueadvantage of be~ng able to operate in two holds as the derrick (in the unloadedcondition) is capable of being traversed between the derrick posts

The load is raised on two winches through multi-part tackle on an endless ropeprinciple

A similar system of two winches is used for topping and slewing such that whenboth winches heave the derrick will rise -and when one winch is heaving and theother paying out the derrick will slew Recent new buildings have been fittedwith two of these patented heavy lifting derricks and with the use of a liftingbeam connected to the two hooks it is possible to lift loads of up to 600 tons.Special precautions have to be taken when lifting loads of this magnitude toensure adequate stability of the vessel and correct functioning of ships equipment

as angles of inclination of up to 17~0are often encountered

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3.3 DECK CRANES

Historically this is the most recent method of handling cargo onboard ship

In spite of the fact the deck cranes have been known for some time (in the

literature advertising the maiden voyage of Titanic, mention is made of the fast

electric cranes for handling passenger's baggage) nevertheless they were slow to

be widely adopted Thanks mainly to Scandinavian ship owners, this resistancehas been broken and even the most conservative shipping companies are turning

to deck cranes

3.3.1 Cargo Cranes (Hook)

A few years ago cargo cranes rarely exceeded 20 tonnes lifting capacity Today,with increased volume of heavy machinery being tansported, particularly to theemerging nations where port facilities are often totally inadequate or non-existent,the advantages of the deck crane have proved to be very useful With its facilityfor accurately "spotting" cargo, readiness for action and requiring only oneoperator, the deck crane has now firmly established a place for itself and it isquite common to see a ship's outfit of six cranes which can range in size from 5

to 40 tonnes capacity

As can be seen in Fig 8, a typical crane consists of a jib which can be lowered tothe horizontal for stowage purposes, and a hoisting unit which can be on a singlefall of rope or multi falls dependin§ on the lifting capacity Facility is providedfor slewing the crane through 360 , but limit switches are usually provided torestrict rotation if there are deck obstructions

Cranes are mostly of the "self contained" type utilizing the available ship'selectrical supply to operate a suitable motor for driving a Ward LeonardGenerator or a hydraulic power pack Power is then supplied to individualmotors for hoisting, luffing and slewing Only occasionally does one see cranesusing a.c squirrel cage motors, although they are becoming popular in Japan.3.3.2 Twin Cranes

The introduction of the twin crane is without doubt, the greatest innovation

in the deck crane and heavy lift market

The basic idea, like all great ideas, is extremely simple and consists of twoindependent cranes of equal capacity mounted on a common platform whichcan be rotated independently (see Fig 9)

Each crane can be used individually for normal cargo working, but when a heavylift is required the jibs are slewed parallel to each other and a lifting beam isconnected between the cargo hooks Special arrangements are usually made tosynchronize the hoisting and luffmg motions of the two cranes and with thisarrangement it is pOSSible (ignoring for the moment the weight of the liftingbeam) to lift a load of twice the safe working load of one crane

When operating in the twin mode, the individual slewing motions are renderedinoperative and only the platform slewing motion can be used Although withtwin crane arrangements the cranes involved are of equal capacity and identicalspeeds, a variation of the idea has been used with cranes of unequal capacity

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The circumstances arise when a heavy lift crane (say 50 tonnes) is required tooperate in two holds and a smaller crane (say 5 tonnes) is required to operate

in the hatch not being used by the hea~ lift crane

The heavy lift crane is mounted concentric with the crane foundation (seeFig 10) to reduce the loading and the light crane is offset With this system thecranes do not work in tandem, but nevertheless the facility for using the heavylift crane in either hold outweighs the cost involved

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3.3.3 Grabt>ing Cranes

With the increasing numbers of dry bulk carriers, it was inevitable that cranes(and derrick rig systems) with the facility for using grabs would be required.Originally, these cranes were normal ~argo cranes adapted for grabbing duties.These adaptations consisted in the beginning, of fitting dumping or handreleased grabs operating on a single fall of rope Due to the slowness of operation,limitation of size and inability to handle cargoes such as iron ore, rock phosphate.etc these grabs have now almost disappeared

13

The electro-hydraulic grab (see Fig 11), which has been steadily replacing thedumping and hand released type of grabs, has several good features:

1) high payload/weight ratio;

2) no shock loading during closing or opening;

3) easily fitted to existing cranes

FIG 11 - Typical electro-hydraulic grab.

Without doubt, however, the real work-horse in the marine grabbing field isthe rope operated grab, which is based on the well proven principle used on landbased cranes

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Two rope drums are used One rope drum holds the grab stationary during theopening and closing action The other drum controls the opening and closing

of the grab When the grab is being hoisted or lowered both drums operatesimultaneously

Refinements in the control system ensure that the grab remains closed whenbeing hoisted in "closed" position and open when being lowered in the "open"position

The most common arrangement utilizes four ropes, Le two holding ropes andtwo closing ropes (see Fig 12)

Clam shell jaws

FIG.12 - Typical 4-rope grab.

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Pre-programmed grabbing cycles have been introduced to reduce operatorfatigue and increase output but to date they have not been used extensively.REFERENCES

British Standard: Marine Series:

"Glossary of Terms and Graphical Symbols for Ship's Deck Machinery"B.S MA30;

"Ships' Deck Machinery - Cargo Winch" B.S MA31;

"Ships' Deck Machinery - Mooring Winches" B.S MA32;

"Ships' Deck Machinery - Warping Winches" B.S MA33;

"Ships Deck Machinery - Capstans" B.S MA34

For background on the design methods used by the manufacturers of theequipment, interested readers are referred to the standard works on the theory

of machines, mechanics, structural design, etc

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4 THE CONTROL OF DECK EQUIPMENT

Since all traditional deck equipment has been evolved to assist manual effort

in the handling of ship and cargo it is reasonable that, with the introduction ofpower to drive the machinery, the aim has been for the control systems tosimulate the continuous speed control and load limiting characteristics of humanmuscle

4.1 STEAM

The first introduction of power was in the form of the steam engine withsingle or multiple double acting cylinders In the simplest form reversing wasobtained by the classic Stephensons link mechanism, and control obtained bythrottling the steam through an adjustable supply valve The natural characteristic

of these steam engines was self regulating as the greater expansion of the steam

on light load gave higher speeds Conversely, as the load increased the speeddecreased and the winch would stall safely if overloaded

It was also relatively easy to requce the steam flow to a winch so as to hold

a suspended load by maintaining pressure against piston and valve leakage.Foot-operated brakes were added to give better control in operation and, whererequired, manual screw-applied brakes were fitted to provide holding effort

As ship sizes increased the maintenance of steam lines on deck became more of

a problem, and the introduction and increasing use of motor ships led to adecline in the use of steam in favour of electrical drives, except in certainsituations where the risk of fire or explosion precluded the use of electricity forexample, on the main deck of tankers

The modem steam engine controls are readily recognisable as variants of thebasic form, engineered for greater reliability

Steam admission to the cylinder is usually by a double piston valve moved by aneccentric fixed on the crankshaft This type of valve is pressure balanced and theengine can be reversed by a similar valve, controlled by a hand lever, which re-verses the steam and exhaust connections to the cylinders A typical arrange-ment of reversing and engine valves is shown in Figure 13

When steam engines are used on mooring winches the same controls are provided,but an additional valve is required to control the tension in the mooring linewhen stalled The engine is set to heave and the inlet valve is opened fully.The pressure on the exhaust side of the engine is controlled by a "back pressurevalve" to maintain a pressure differential across the piston which corresponds tothe required mooring line tension

In its simplest form the back pressure valve may be held shut by a spring ofadjustable force; hence it is able to open when the exhaust pressure on the top

of the valve is sufficient to overcome the spring PressUI"Cin the axhaust portt jl~ , ';',

of a stalled steam engine is provided by the small, but ever present, leakage ofsteam from the high pressure line past pistons and valves

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FIG 13 - Steam valves for one double acting cylinder of a two-cylinder steam engine.

Other demands on the steam supply can cause fluctuations in the main linepressure and, since the exhaust pressure is fIxed by the spring setting, it followsthat differential pressure, and hence tension, will vary To counteract thistendency the back pressure valve may be made to accept line pressure steam on adriving piston which supplements or replaces a weaker spring The drivingpiston of the back pressure valve is provided with a small controlled leak so thatthe closing force may be regulated by a flow control valve in the feed from thesteam line With this type of valve, illustrated in Figure 14, the exhaust pressure

is reduced or increased in sympathy with the supply pressure, and thus thedifferential pressure on the pistons and tension are maintained more constant.4.2 ELECTRICAL CONTROL SYSTEMS:

4.2.1 Systems with direct current supplies

The fIrst ships to use electrical power were equipped with direct currentgenerators supplying fIxed voltage busbars that fed all services on board.Widespread use of direct current was made prior to 1940

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Direct current motors are made with two distinctive characteristics determined

by the choice of field winding used for excitation These are, respectively, shuntand series see Figure 15 The shunt-excited motor is essentially a constant speedmachine but speed can be adjusted by varying the field strength Weakening thefield strength increases speed, and vice versa

FIG IS - Relationship between speed and load for shunt, series and compound d.c motors

at rated supply voltage.

When overloaded the shunt motor draws very large armature currents and thesetend to reduce the field strength by armature reaction, reducing the motor'sability to handle very large overloads The series motor field strength isapproximately proportional to the armature current, so that this type of motor

is well able to handle large overloads due to the enhanced torque and reducedspeed during starting This feature accounts for the common adoption of seriesmotors for traction duties

The disadvantage of the series motor is that, with load reduction, the fieldstrength also reduces and the speed increases These motors can assume veryhigh and dangerous speeds if unloaded and consequently they have no inherentbraking capability

The drive requirements of most deck machinery are usually obtained by usingmotors with a combination of the best features of these two basic types, obtained

by a compound field system embodying both series and shunt windings invarying proportions, according to the duty The motors then have improvedtorque for acceleration and can run up in a controlled manner on light load tospeed up the duty cycle

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It is normal practice to arrange that the shunt winding is energised first toensure that some of the field strength is established before the armature supply

is switched on

With motors of more than one horsepower the armature resistance is low, andthe instantaneous current which would be drawn on energisation could be verylarge so that, if the cables and supply system were adequate, a sudden andviolent start would occur To prevent this, and limit the supply current to valueswhere normal cable sizes and protective equipment may be safely employed, it

is usual to introduce a multi-section resistance in series with the armature whenfirst energised For windlass and warping winch drives this first step may bechosen at a fraction of the rated load, but for hoisting machinery it is usual toset the first step at a value close to rated load current to ensure that suspendedload will not fall back when hoisting begins

Since the inserted resistance absorbs a porportion of the busbar voltage thebalancing voltage (back emf) developed in the armature is reduced; hence thespeed is reduced at a given load As the added series resistance is cut out, asection at a time by advancing the controller, the available torque is increased.For a given load a lower resistance voltage drop means an increase in speed todevelop a balancing voltage

To provide effective control over the whole range of operating speeds it is alsoquite common to provide a footbrake on winches with this form of control.

A number of ingenious arrangements have been developed, all evolved fromrelatively simple control circuits such as shown in Figure 17 This circuit includesthe following common safety features

A It is not possible for the motor to start should the supply be energisedwhen the controller is not in neutral, as the no-volt relay would fail toenergise

B If the supply is lost when the winch is running the no-volt relay drops outand it is necessary to return the cOl\troller to neutral before re-starting

C The motor shunt field is energised when the supply is switched on andremains on while the winch is in use There are no protective fuses in theshunt field circuit as blowing of a fuse could be hazardous, particularly

if the brake coil remained energised

D The third and subsequent steps are controlled by a contactor with aspecial lock-out coil which prevents it closing on demand, unless thecurrent has fallen to a safe level

E A voltage-sensitive relay is connected across the armature to preventreversal until the machine speed has falIen to a safe level

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FIG 17 - Contactor switched series resistance control of D.C winch

.The armature connections have to be reversed when rotation is reversed Thecurrent in the series field winding remains in the same sense to assist the shuntfield winding whatever the direction of rotation "Different manufacturers have evolved their own particular schemes based on thissystem and detailed descriptions are given in their literature or are written up inmore detail elsewhere

For precise control more complex control systems have been devised, generallyinvolving an additional motor-generator set or Ward Leonard system These giveexcellent speed control because the voltage applied to the motor armature isvaried by regulating the output voltage of the generator by field control

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Such sets require a starter but this need be only short-time rated and small as theconverter is run up to speed unloaded and remains running until the work ofthe controlled motor is done This is more expensive than resistance control butmuch more economical in the use of power as there are no large dissipationlosses in a series resistor This could be a significant saving when a winch has tooperate at or near stalled conditions for long periods, such as required in livemotor mooring.

Since all the motor torque is lost when the electrical supply is interrupted, it hasbeen common practice almost from the outset to provide all electric deckequipment with a fail-safe brake, usually fitted to, or an integral part of, themotor The brake is applied by springs and released by an electro-magnet.Provision is usually made for releasing the brake in a controlled manner bymanual means to allow a load to be lowered to deck if caught suspended bypower failure A typical watertight brake arrangement is shown in Figure 46

4.2.2 A.c systems

Since its introduction some 25 years ago the use of alternating current on shipsspread rapidly and for the last 10 years it has almost totally displaced directcurrent for general distribution and the powering of deck auxiliaries:

There are a number of reasons for this of which the two most important

are:-A the reduction in maintenance due to the replacement of d ••generators withtheir commutators and brush gear by brushless alternators that requirelittle attention:

B a considerable saving in cost and simplified control of numerous auxiliaries,such as pumps, fans, etc., made possible by direct-on-line switching tostart and stop the smaller units

Furthermore distribution is generally easier and special voltages for lighting,hand tools, shavers etc., are readily obtained by static transformers, as required.Ships' systems, are typically 380 or 440 V three phase, 60 Hz, the latter beingcompatible with 380 V, 50 Hz shore supplies However, motors will run slower

on a 50 Hz supply than on the designed 60 Hz and this has a marked effect onthe output of machines such as fans and pumps which are highly speed-sensitive

4.2.2.1 Squirrel cage motor controls

The squirrel cage motor, by virtue of its simplicity, robustness and low cost,

is the most common type for general purposes The simplest form of control isthe direct-on-line contactor with built-in overload protection It can also beprovided with single-phase protection which detects any unbalance in its threesupply lines, and disconnects the motor before damage is done

Reversing requires a second contactor to transpose any two of the supply phases

It is good practice to interlock electrically and mechanically the two contactorsused for reversing as the simultaneous closing of both, for example by manualoperation of the de-energised contactor, could result in a short circuit betweenthe transposed phases, causing severe damage

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Small single-speed drives which operate through a large gear reduction to giveslow movement are generally arranged as shown in Figure 18 The brake isconnected directly across two motor terminals and may be released by anlalC.

solenoid; an additional rectifier is required if a d;c.brake solenoid is used

One disadvantage of the squirrel cage induction motor is that it is essentially aconstant-speed machine and only a small reduction in speed occurs between noload and full load

The basic load/speed characteristic of the induction motor is shown in Figure 19.The full line represents the normal torque/speed curve of the more commontype of motor and the broken curve is typical of a motor designed to give ahigher starting torque

Both types are designed to accelerate smartly to their rated speed If preventedfrom accelerating, the high slip losses will cause rapid overheating and damage.Consequently, overload devices used in conjunction with squirrel cage types areclosely matched to the motors' thermal characteristics and should not be changedfor a different rating without reference to the manufacturer

The fixed-speed disadvantage may be partly overcome by using a motor which iswound for two speeds This is accomplished either by providing two separatewindings in the motor stator or by arranging the winding connections in such away as to make it possible to change the number of poles in use by switchingthe winding externally

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