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Circuit breakers

When selecting a circuit breaker for a particular application the principal factors toconsider are; system voltage, rated load current, and fault level at the point ofinstallation

Voltage rating

At medium voltages the phase to neutral voltage may be 250v but the potentialdifference between two phases with the neutral insulated would be 440v At thesevoltages no difficulties should arise in selecting the circuit breaker equipment.However, on a 3.3kV insulated neutral system the phase to neutral voltage is3.3kV/ж 3 = 1.9kV If an earth fault develops on one phase the potential of the othertwo phases to earth is 3.3kV To ensure the insulation is not subject to excessivestress a circuit breaker designed for a normal system voltage of 6.6kV may be fitted.Also on insulated neutral systems high over voltages may be caused by arcing faults.Medium voltage systems switch gear insulation should be able to withstand suchvoltages, but 3.3kV and above, the margin of safety is reduced When a high voltagesystem is installed both the voltage rating of the circuit breaker and the method ofearthing must be considered

Current rating

Consider three factors;

a Maximum permissible temperature of circuit breaker copperwork and contacts

b temperature due to LOAD CURRENT

c Ambient temperature

In industrial use the ambient temperature considered is usually 35oC Ifuses in a marine environment temperature of 40oC (Restricted areas) and 45oC(unrestricted areas) are used, therefore the circuit breaker rating may be 'free air'value and this does not consider the degree of ventillation, the number and position

of the circuit breakers or the layout of the bus bars The final switchboardarrangement could be only 80 to 90% of the free air rating

Fault rating

Breakers should be rated to accept a breaking current of about 10 times the full loadcurrent The breaker should also be able to make against a fault condition where themaking current may be 25 times the full load current when the contact first make.Circuit breakers must remain closed for a short time when a fault occurs in order toallow other devices which are nearer to the fault to trip first The breaker should becapable of carrying its breaking current for a specified time of usually about onesecond

Arc suppression

Blow force at right angles to arc and field.The blow out coils, which are connected in series with the circuit breaker contacts,form an electro-magnetic field which reacts with the arc to give a deflecting forcewhich tends to bloe the arc outwards The increase in effective length of the arccauses it to extinguish more quickly The blow out coils are protected form the arc byarc resistant material which may be in the form of an air shute

Hot ionised gases around the arc and contacts are displaced by cold airforming eddy current air flow This helps to increase resistance between contacts

Contacts

Attention should be paid to all contacts likely to deteriorate due to wear, burning,inadequate pressure, the formation of a high resistance film or becoming weldedtogether Faulty contacts are often indicated by overheating when loaded Differentcontact materials may need different treatment

Copper is widely used but is liable to develop a high resistance film, andcopper contacts may become welded together if the contact pressure is low and thecontents have to carry a high current Copper is commonly used for contacts whichhave a wiping action when closing and opening., this action removing the film.Copper contacts are used on knife switches, laminated (brush) contacts of regulatorsand other controllers, drum contacts, etc

Carbon and metallized carbon contacts are unsuitable for carrying highcurrents for long periods but, as they do not weld together, they are used for arcingcontacts on some control gear Pure silver and silver ally contacts tends to blacken inservice but the oxide film has a low resistance Copper- tungsten (sinteredcompound), grey I colour, is used in contact facing This material has a high surfaceresistance which resists heavy arcing and does not weld Silver tungsten (sintered)has similar properties to copper tungsten but has a lower contact resistance and isless liable to overheat on continuous load

Servicing contacts

Copper contacts should be filed up if necessary to restore the profile required toensure correct wiping action Copper contacts which have become burnt or pitted orotherwise damaged, may be carefully dressed with a file Emery cloth should not beused Some contacts are provided with pressure adjustment, so if the contactpressure is reduced by dressing it should be readjusted Using a spring balancepulled in a direction normal to the contact surface a reading should be taken when apiece of paper placed between the contacts is released Inadequate spring pressuremay also be due to the pressure springs becoming weak due to fatigue oroverheating

Carbon contacts should receive the same attention as copper contactsexcept that they should not need lubrication Silver, Silver alloy and copper tungstencontacts do not require cleaning As there is no need to remove surface film frompure silver contacts they may be used for light butt contacts

Where some contacts have the appearance of pitting on both faces this issometimes referred to as being 'burnt in' Some manufacturers recommend that thecontacts, unless there is loss of material, are not dressed as this may destroy thecontact area

Certain instruments and controls require a feed direct from the bus bars.Any connection between the bus bars and protecting fuses must be capable ofwithstanding maximum fault level Standard practice is to provide a three phase set

of fuses, known as 'Back Up' fuses, as near to the bus bars as possible Connectionsare then led to the racks of the many instruments fuses fitted

Circuit breakers

Must be capable of making and breaking under normal conditions and also abnormalconditions such as a short circuit As the circuit breaker must be able to withstandclosing onto a fault conditions without sustaining damage, it is of heavy construction.Fitted with an over current release and overloads with time lags, a circuit breakercan be used as follows;

a To control the output of a generator

b As a direct on line starter

c Control outgoing feeder circuits

On modern switchboards 'draw out' circuit breakers may be fitted In the openposition the whole circuit breaker can be wound clear of the bus bars, thus fullinspection and maintenance can be achieved without the necessity of de-energisingthe bus bars so providing a separate isolating switch

The 'plug in' contacts joining the circuit breaker to the bus bars are notcapable of taking the breaking load and it is essential that the circuit breaker is in theopen position before any attempt is made to withdraw it A mechanical interlock isfitted arranged to trip the circuit breaker before the winding handle can be inserted,

The breaker also has a mid position, in this position the control circuitsare still connected with the bus bar connection isolated The electrical operation ofthe breaker can then be tested

Circuit breakers are normally fitted with under voltage protection andtripping is accomplished by shorting or open circuiting the no-volt coil which releases

the latching in mechanism The no-volt coil may also be open circuited by a reversepower relay and an overload trip fitted with a time delay

Instruments

The following instruments are the minimum that must be fitted;

 Bus bar voltmeter and frequency meter

 Volt meter and frequency meter, with selector switch to measure incoming machine conditions

 Ammeter with phase selector switch for each alternator

 Watt meter for each alternator

 Synchroscope and if check synchroscope not fitted lamps

 Earth leakage indicator

Additional instruments that may be fitted

 Watt hour meter

 Power factor meter

 Alternator excitation ammeter

 Alternator excitation volt meter

 kVAr meter

 Share connection supply meter

 Emergency batteries on discharge meter

When a check synchroniser is fitted it is there to prevent connecting an incomingmachine to the bus bars whilst out of phase, it is not there as aid to synchronising

In an emergency the 'in synch' light may be used to indicate when the breaker may

be closed

When an incoming machine is selected, its no-volt coil and circuitbreaker contactor relay coil are connected in series with contacts on the checksynchroniser These contacts must be closed, that is the machine in phase with thebus bars, before the breaker contactor relay may be energised If starting from adead ship the check synchroniser must be switched to off before the first generator isput on the board

a Overload protection-fitted to circuit breakers

b Reverse power-When motive power is removed an alternator will try to become asynchronous motor and draw current from the circuit A reverse power relay willoperate after about 2 seconds and about 2-3% reverse power for turbines, 10-12%reverse power for diesels The time delay prevents tripping during paralleling andtaking the alternator off the board

c Preference trip-automatically , and sometimes sequentially, sheds load from board tomaintain supply to essential services during periods of overload

d Fuses-Usually of the HRC type

e Discrimination-The protective device closest to the fault should operate and protectother services

f Group starter board-Large demand sections may be separated from the mainswitchboard by fuses and circuit breakers

Automatic voltage regulators

Shall be supplied separately from all other instrument circuits Protection should be

by fuses mounted as close to the supply connections as possible

Shore supply connections

a Where arrangements are made for the supply of electricity from a source on shore orother location a suitable connection box has to be installed in a position in the shipsuitable for the convenient reception of flexible cables, it should contain a circuitbreaker or isolating switch, fuses, and terminals of adequate size to receive the cableends

b For three phase shore supplies with earthed neutral terminals are to be provided forconnecting hull to shore earth

c An indicator for shore side connection energised is to be provided

d A means for checking polarity or phase rotation is to be provided

e At the connection box a notice indicating ships requirements with respect to supply aswell as connection procedure

f Alternative arrangements may be submitted for consideration

Operating coil-Along with carbon pile form the controlling elements CCT and PT-Are the detecting elements, the CCT acts as a feed forward device

indicating future voltage changes by detecting variation in current flow

Stabilising element-Is the capacitor across the Exciter (may be replaced by a

resistor)

The A.C voltage is applied to the operating coil through a full waverectifier This A.C voltage supply induced in the potential transformer and thecirculating current transformer may vary under varying load conditions such as direct

on line starting of relatively large motors The capacitor connected across the coilsmoothes the D.C output from the rectifier

If the A.C applied voltage falls, the field of the solenoid weakens, andthe resistance of the carbon pile decreases With less exciter circuit resistance thecurrent in the exciter field increases thus increasing the output voltage of the A.C.generator

The automatic voltage regulator voltage output may be adjusted with thehand regulator R1 in the exciter field Before synchronising the alternator the opencircuit voltage is adjusted with the hand regulator R1

After synchronising, and after the kW loading has been adjusted on theprime mover governor, the field excitation under steady load conditions may beadjusted using the Trimming resistor R2 Using the trimming resistor the powerfactor of the incoming machine will be equalised with the machines already in use

If the load power factor now changes then the terminal voltage willregulate badly, e.g a rise from 0.8 to Unity Power factor will cause a rise in terminalvoltage of about 20 % So a small Voltage Trimmer R3 is provided across eachcurrent transformer to adjust terminal voltage when there is a change in overallpower factor

Modern A.V.R (Zener Bridge)

Voltage across the Zener diodes remains almost constant independent ofcurrent variations Smoothed D.C output is applied to the voltage reference bridge.This bridge is balanced at the correct generator voltage output with no potentialdifference between 'A' and 'B'

If the generator voltage fails, current through the bridge arms falls andcurrent flows from 'A' to 'B' through the amplifier

If the generator voltage falls, current through the bridge arms falls andcurrent flows from 'B' to 'A' through the amplifier

If the generator voltage rises, Current through the bridge arms rises withcurrent flow from 'A' to 'B' through the amplifier

The signal from the amplifier will automatically vary the field excitationcurrent, usually through a silicon controlled rectifier ( Thyristor) control element

The Silicon Controlled Rectifier (Thyristor) is a four layer, three terminal,solid state device with the ability to block the flow of current, even when forwardbiased, until the gate signal is applied This gate signal could come from a Zenerdiode Voltage reference bridge The gate signal will switch on the forward biasedS.C.R and current flows through the exciter field When reverse biased the S.C.R.will again block current flow Due to inductance of the field winding the S.C.R wouldcontinue to pass current for a part of the negative cycle By fitting a 'free wheeling'diode the current though the Thyristor falls quickly at the end of the positive cycle

In some circuits the excitation current is designed to be excess of requirements, sothat the gate signal reduces flow

Limiting voltage dip and response time

under impact loading

The effect of a large load suddenly switched on to a small power installation such as

a ships plant will be an instantaneous dip in the generator voltage

This effect, due to the transient reactance on starting, cannot beobviated either in a self regulated machine, or in a conventional generator withA.V.R

The sluggish response of the excitation systems limits the speed ofvoltage recovery

In a self excited generator the dip is less and the recovery time greatlyimproved (say 0.3s against 0.7s)

In order to maintain constant voltage, under varying conditions,excitation must be varied

Variation of voltage at constant excitation

Variation of excitation at constant voltage

Air Gap

If the air gap around a rotor is not uniform the motor may not start in certainpositions Because the rotor is not centred, probably due to worn bearings, there is

an out of balance magnetic pull

Radial play in between the shaft and the housing should be detected byhand and bearing wear detected by feeler gauge between the rotor and the stator, orarmature and field poles may be measured at three or four fairly equidistant pointsaround the machine If possible one measurement should be made at the bottom ofthe machine and another in line with the drive Compare with previous records tocheck wear At minimum air gap Clearance of the bearings should be renewed toavoid the possibility of the rotor rubbing on the stator

On small machines two feelers on opposite sides of the rotor should beused to avoid error caused by rotor movement from normal position when only onefeeler gauge is used In synchronous motors and D.C motors sparking may occur ifthe radial air gaps between the armature and the field poles are unequal Ifnecessary renew bearings or add or remove soft iron shims from under the poleshoes Unequal field strength has a similar effect of sparking at the brushes Thismight be due to short circuit or earth fault on the field coils, or a short circuit on theshunt and field coils An increase of air gap gives an increase in 'reluctance'

In a salient pole A.C generator this fact may be used to produce a sinusoidal fluxdensity curve by gradually increasing the length if the air gap towards the pole tips

In the induction motor the airgap should be as small as possible if the motor is to act with a high power factor Anincrease in air gap increases the reactance of the motor and lowers its power factor.Small motors are accurately machined and centring of the rotor is very important soball or roller bearings are fitted

Air gap Motor size

Parallel operation of generators

D.C generators

For compound wound D.C generators it is usually sufficient to ensure that thevoltages of the incoming generator is the same as the bus bar voltage Theequalising connection joining the junctions between the armatures and their seriesfields is incorporated in the circuit breaker in such a way that the equalisingconnection is automatically closed before and opens after, the main contacts Byadjustment of the shunt field regulator the load sharing may be controlled

A.C alternators

To parallel alternators the following conditions are required;

1 Same voltage-checked with the voltmeter

2 Same frequency-checked with the frequency meter and synchroscope

3 Same phase angle-checked with synchroscope

4 Same phase rotation-checked with rotation meter Only important when connecting shore supply, or after maintenance on switchgear or alternator

Load Sharing Of Alternators In Parallel

Alternators in parallel must always run at the same speed After a machine has beenparalleled and is required to take up its share of the load, this will not be achieved byadjusting the field excitation current Although the increase in e.m.f will cause acurrent to flow in the busbars, and this will show on the machines ammeters, this is

a reactive current that lags the e.m.f by 90o and produces a reactive (kVAr) but not

kW Its only effect is to alter the operating power factor of the alternator

More power may be obtained at the bus bars from the incomingalternator only by supplying more power to its prime mover This increase of steam

or fuel supply is achieved by altering the governor setting either electrically ormanually

After adjusting the governor the incoming machine takes up its desiredamount of the kW loading and this is recorded on the machines watt meter However,

if the kW loading is shared equally between two machines it may be found that theLoad Current of the incoming machine is more or less than the other machine This isfue to the incoming machine having a different power factor This may be corrected

by adjusting the excitation of the incoming alternator

Thus after paralleling an alternator;

i Adjust prime mover governor until kW loading is correct

ii Adjust field excitation current until current sharing is correct

If the alternators have similar load characteristics, once adjusted, the load willcontinue to be shared If the load characteristics of alternators vary, the kW loadingand load current sharing may require readjusting under different load conditions

Load sharing of alternators

No1 on load

No1 on load, No2 synchronised and taking 100kW

No1 and No2 sharing load after adjusting governor settings, excitation adjusted to prevent excessive volt drop in No2

No1 and No2 sharing load with balanced power factors by adjusting excitation

The effects of altering Torque and Excitation on single

phase alternator plant-and by extrapolation a

3-phase circuit

Before paralleling, by varying Rb, adjust the excitation current in therotor field of 'B' until Va=Vb When in phase and at the same frequencysynchronising may take place

If there was no external load on the bus bars the torque on the primemovers of A and B is only that required by its own alternator and Ra and Rb areadjusted so that Ea and Eb are equal

Relative to the bus bars Ea and Eb are acting in the same direction witheach other making the top bar positive with respect to the bottom bar.

Varying the driving torque

If the driving torque of 'B' is reduced (less fuel supplied) the rotor fallsback by an angle say p.f.(b) giving a resultant e.m.f of Ez in the closed circuit

The e.m.f Ez circulates a current I which lags behind Ez by angle p.f.(a).This circulating current Iis more or less in phase with Ea and in opposition to Eb This means that A is generating power to motor B and this will compensate for any

Once the power increase in A equals the power loss of B balance is restored and Aand B continue to run in synchronism

Therefore the power is shared by adjusting the torque ( fuel input.)

Slight loss of power in B-is taken up by an increase in power from A.

The terminal voltage will not vary and the speed and frequency will stay the same ordrop only very slightly

Large loss of power in B-with a large circulating current from A to B

the alternator A will try to drive B as a synchronous motor The amount of full loadpower required to drive an alternator as a motor is only 2 to 3% for a turbine and 10

As the circulating current flows from A to B the reverse power trip on B will operate

All the load now falls on A which will probably cause the overload trip to operate and'black out'

V1 = Eb - Н Ez = Ea + Н Ez

Therefore increasing the excitation current will increase the terminal voltage

As p.f.(a) is almost 90o the Power circulating from B to A is very small

Ez I Cos [ p.f.(a)] approx equals Zero (Cos 90o = Zero)

Effect of reducing Excitation

By increasing Rb the reduction of the field excitation current of B will reduce theterminal voltage

Ea>Eb terminal Voltage V = Ea - Н Ez = Eb + Н Ez

The circulating current I from A to B will have a large 'Wattless'component Machine A now has more of the lagging reactive current and its powerfactor is reduced Too large a reduction in excitation current in B with subsequentincrease in load current in A could cause the current overload trip of A to operate.This could be followed by the low voltage or the overload trip of B operating causing

a black out

Voltage regulation

The graph demonstrates that excitation must be increased (generally)with increasing load to maintain terminal voltage

The worse the power factor the worse the terminal voltage changeduring load change

Voltage regulation = DV when load removed/ Full load terminal voltage

At 1.0 p.f = AC/ OA

At 0.8 p.f = AD/ OA

Therefore lower p.f = greater voltage regualtion

If the two fields are not at the same frequency then the armature willrotate at a speed equal to the difference

In the modern rotary synchroscope there are no slip rings The rotor hastwo soft iron pole pieces and with its shaft carrying the pointer it is magnetised bycoil R from the bus bars With this coil is fixed adjacent to the shaft, therefore, thereare no moving coils, contacts or control springs

Single PhaseSingle phase synchronising with lamps Lamps Dark

Lamps bright

If using single phase synchronising it is considered better to use thelamp bright method as it is easier to judge the middle of the bright sequence ratherthan the middle of the dark sequence

Three phase synchronisingSynchroscope with two lamps (lamps dark)

The secondary windings of transformer T1 supplies field coil F of thesynchroscope The secondary windings of T2 supplies the rotating coils R of thesynchroscope

If the incoming machine is in antiphase with the bus bar the voltage differencebetween the output of the secondary of T1 and T2 is double the normal voltagegiving normal volt drop across each lamp When in phase there is no voltagedifference between the outputs of T1 and T2 and therefore lamps are dark whensynchronised

Synchroscope with two lamps (Lamps bright)

Three phase synchronising with lamps (Lamps dark)

No1 Vector is stationary, if the incoming machine is running two slowthen the No2 vector moves away from No1 vector in an anti clockwise direction Inthe position shown as the No2 vector moves progressively anti clockwise then 'a' willbrighten, 'b' will brighten shortly reaching maximum luminosity then darken, 'c' willdarken

When the machines are in phase, then 'R1' and 'R2' will be in aligntherefore 'a' will be dark, 'Y1' and 'B2' will be 120o apart and therefore 'b' will beapproaching maximum luminosity, and the same will be for 'c' with 'Y2' and 'B1' 120o

apart

AC single phase induction motors

With a single phase stator the magnetic field is purely alternating so norotating magnetic field is set up Any clockwise torque is opposed by an equal andopposite anti clockwise torque Once the rotor moves it will accelerate in the

With a split phase induction motor an auxiliary winding in series with a resistancecarries a current in which the angle of lag is less than the angle of lag in the mainwinding The two windings thus produce a rotating magnetic field which makes themotor self starting

The starting winding use fine wire of high resistance and low reactance.The current in the start winding leads the current in the main winding when themachine speed reaches 75% the start winding should be switched off This may bedone automatically using a centrifugal switch

An alternative method of starting is to use a capacitor in series with thestart winding This is more suitable for loads of higher inertia or more frequentstarting than the split phase motor

AC 3 phase induction motors

The polyphase induction motor is self starting Its speed falls to a small extent withload ( as does a D.C shunt wound motor) It has two main parts, the stator which issimilar to a stationary armature of an alternator, and the rotor, mounted on bearingswithin the stator

Stator

This is provided with three sets of windings energised separately by the individualphases of a three phase supply, giving a rotating magnetic field, constant inmagnitude and rotating at the same angular frequency as that of the supply voltage

Squirrel cage rotor

Rotor- There are no electrical or other connections made to the rotorwhich is built up of soft iron laminations fixed to the shaft and slotted to receiveconductors

The squirrel cage rotor has a single stout copper conductor bedded intoslots, these conductors being short circuited by heavy copper rings at both ends Asimilar electrical type has windings on the rotor which are short circuited but are not

in the form of a squirrel cage

Current flowing in the squirrel cage rotor conductors is an inducedcurrent and cannot be controlled When it is necessary to vary the rotor current athree phase wound rotor is used and the connections to the windings are brought out

to slip rings across which variable resistance's are connected and can reduce startingcurrent, improve starting torque and control speed

Principal of operation

If a conductor set at right angles to a magnetic fields moves across the flux from left

to right, the direction of the induced voltage will be out of the paper, by lenz's law Ifthis conductor is part of a complete circuit, them a current will flow in the direction ofthis voltage, and there will be a force on the conductor tending to urge it from right

relative speed of conductors and field, and the greater the force on each conductorwith more torque exerted on the whole

Slip = Field speed - Rotor Speed/ Field speed

The greater the slip the greater the torque exerted Light load slip isabout 2%, full load slip is about 4 to 5%

Torque and slip

Consider the frequency of the stator fields relative to each conductor When the rotor

is at reat this equals the alternation of the supply If lightly loaded the slip is small,say only one or two cycles per second

Resistance of a squirrel cage rotor as a rule will be very small and itsinductance high Its reactance will thus be large at the frequency of the supply andmuch less when it is running (induced reactance depends upon frequency)

As the frequency difference between the rotating field and the rotatingconductors reduces so the Inductive resistance component reduces and so the powerfactor increases improving efficiency and reducing current draw

Wound rotor

If the resistance of the conductors was increased, the starting power factor isincreased There are tow possible methods for attaining this The first is to have arotor squirrel cage made of a suitable high resistance material say bronze ratherthan copper, a second method is to use a wire wound rotor with the ends of thewindings brought out via slip rings and attached to high resistance's However atworking speeds more slip is needed for a given torque So what is gained in starting

is lost in steady running When a large starting torque is essential a Wound Rotormay be used, with external variable resistances which can be cut out as the rotorspeed increases A less expensive solution is a dual squirrel cage rotor

Note:It is the resistance in the closed circuit which determines thecurrent in the circuit induced by the rotating field

Comparisons of cage and slip ring rotors

Squirrel cage

 Advantages

 Cheaper and more robust

 Slightly higher efficiency and power factor

 Explosion proof, since abscence of slip rings and brushes eliminates risk of sparking

 Virtually constant speed machine

 Disadvantages

 High starting current ( 5 to 8 times F.L.)

 Low starting torque

Wound rotor with slip rings

 Advantages

 High starting torquw

 Lower starting current

 Speed can be varied if required Disadvantages

 cost

 Danger from sparking

For small squirrel cage motors direct on line starting with starter current

of about 5 x full load With larger squirrel cage where the torque increases as speed

of load increases ( fans, bow thrusters, etc.) reduced voltage starting may beobtained with star delta or auto transformer starters

In modern marine practices the wound rotor with slip rings is seldomfound To obtain high starting torque with starting currents of about 3.5 x full loadthe induction motor rotor is provided with two cages

a AN outer cage in shallow slots with high resistance ( bronze)

b An inner cage in deeper slots of low resistance (copper)

On starting the inner cage is very reactive with low torque and littlecurrent (See graph above).The outer cage has high torque with most of the rotorcurrent when starting During running most of the current is in the inner cage of lowresistance as at small slip the inductive reactance is low

These two cage induction motors may be started direct on line and arewidely used in marine practice

Skewed conductors (windings)

Magnetic hum

 Two possible sources of magnetic hum , commonly heard in transformers, are

 Attraction and repulsion alternately of laminations

 Magnetic striction i.e when the poles in a bar are aligned the bar has a tendency toexpand

Synchronous motors

 Advantages

 The ease with which the power factor can be controlled An overexcited synchronous motor with a leading power factor can be operated in parallel with induction motors having a lagging power factor to improve the overall power factor of the supply system

 The speed is constant and independent of the load This characteristic is mainly of use when the motor is required to drive another alternator to generate a supply at a frequency, as in frequency changers

Uses-A.C electric propulsion schemes but generally not for auxiliary purposes

 Disadvantages

 Cost per h.p is greater than induction motors

 D.C supply is necessary for the rotor excitation This is usually provided by a small D.C generator carried on an extension of the shaft

 Some arrangement must be provided for starting and synchronising the motor Two possible methods are by pony motor, or by incorporating a wound rotor induction windings which may be opened when up to speed and a D.C voltage applied

Graph of induction motors showing effect of increasing the ratio

of resistance to inductance

Full load occurs at around 40 % torque It can be seen that varying theresistance will change the degree of slip and hence speed For R = 2X the motor isnot self starting as it never reaches full starting torque It will also be expensive torun due to the high heat losses through the resistance's

At reduced load the power factor is much reduced Because of this it isvery inefficient to place an oversized motor on a load, or to have several motors onlypartly loaded

The effects of frequency and voltage change on an

induction motor.

Effects of voltage change

At constant voltage if frequency is increased from 50Hz to 60Hz there is an increasedInductive resistance XL As stator flux is reduced this effects the starting torqueincreasing starting current demand Higher speed increases power output If acentrifugal pump or fan the power increase is proportional to the speed cubed ( [60 /50]3 = 1.728) giving a 73% increase in power demand

At constant voltage if the frequency is decreased from 60Hz to 50Hz thestator flux is increased but the speed is reduced by a 83% Unless the load isreduced the machine will run hotter than normal Starters and contactors could beadversely affected A 440v 60Hz system supplied from a 415v 50Hz shore supplyruns at 83% speed, slightly hotter but should run without damage

Effects of frequency change