Basic Principles And Functions Of Electrical Machines

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Basic Principles and Functions of Electrical MachinesO.I. Okoro, Ph.D.1*, M.U. Agu, Ph.D.1, and E. Chinkuni, Ph.D.21Department of Electrical Engineering, University of Nigeria, Nsukka, Enugu State, Nigeria* E-mail: oiokoro@hotmail.com2Polytechnic University of Namibia, Windhoek, NamibiaE-mail: chikuni@yahoo.comABSTRACTRecent advances in power electronics and highspeed microprocessors have led to considerableattention in electrical machines with regard totheir applications in industrial drives. This paperbrings to the fore, various types of electricalmachines, their operations, and applications, aswell as the method of determining theirparameters. Various ways of protecting electricmachines against overloads and mechanicalfaults are also highlighted. It is anticipated thatthe work presented in this paper will be ofimmense benefit to practicing engineersespecially in areas of machine design,maintenance, and protection.(Keywords: electrical machines, operation design,maintenance, protection, stator)INTRODUCTIONThe Direct Current (D.C.) machine, thesynchronous machine, and the inductionmachine are the major electromechanicalconversion devices in industry [1]. The merits ofthe squirrel cage induction machine are:lightness, simplicity, ruggedness, robustness,less initial cost, higher torque-inertia ratio,capability of much higher speeds, ease ofmaintenance, etc [2, 3]. The most importantfeature which declares it as a tough competitorto D.C. machines in the drives field is that itscost per KVA is approximately one fifty of itscounter-part and it possesses higher suitability inhostile environment.Unfortunately, induction machines suffer fromthe drawback that, in contrast to D.C. machines,their speed cannot be easily and effectivelyadjusted continuously over a wide range ofoperating conditions [4]. On the other hand, thesynchronous machine has the merit of beingoperated under a wide range of power factors,both lagging and leading, and are much bettersuited for bulk power generation.In theinduction motor, alternating current is applied toThe Pacific Journal of Science and he stator and alternating currents are induced inthe rotor by transformer action.In the synchronous machine, direct current issupplied to the rotor and Alternating Current(A.C.) flows in the stator. On the other hand, aD.C. machine is a machine that is excited fromD.C. sources only or that itself acts as a sourceof D.C. [5]. It is a common practice in industry toemploy A.C. motors whenever they areinherently suitable or can be given appropriatecharacteristics by means of power electronicsdevices. Yet, the increasing complexity ofindustrial processes demands greater flexibilityfrom electrical machines in terms of specialcharacteristics and speed control. It is in thisfield that the D.C. machines, fed from the A.C.supply through rectifiers, are making their mark.In this paper, we shall discuss the various typesof electric machines, thereafter, we shall look atthe basic features and principles of operation ofelectric machines. Determination of machineparameters, basic protections, maintenance, andelectric machine applications are also discussed.CLASSIFICATION OF ELECTRIC MACHINESThere are several methods of classifying electricmachines [6]: Electric power supply - Electric machinesare classified as D.C. and A.C. machines aswell as according to their stator and rotorconstructions as shown in Figure 1. NationalElectricManufacturers’Association (NEMA) Standards - NEMAstandards are voluntary standards of theNational Electric Manufacturers Associationand represent general practice in industry.They define a product, process, orprocedure with reference to lerances,operatingcharacteristics,performance, quality, rating, and testing.NEMA classifications of Electric Machinesare summarized in Table 1.–45–Volume 7. Number 1. May 2006 (Spring)

Figure 1: Classification of Electric Machines.Table 1: NEMA Classifications.TypesFeaturesA. Open:i.Drip-proofOperate with dripping liquids up to 150 from verticalii.GuardedGuarded by limited size openings (less than 0.75 inch)iii. Externally ventilatedVentilated with separate motor driven blower, can have other typesof protectionB. Totally enclosed:i.Non-ventilated (TENV)Not equipped for external coolingii.Fan-cooled (TEFC)Cooled by external integral faniii. Water-cooledThe Pacific Journal of Science and ooled by circulating water–46–Volume 7. Number 1. May 2006 (Spring)

BASIC FEATURES OF ELECTRIC MACHINESThe basic structural features of a D.C. machineare: induced voltage and the current in thearmature both of which are A.C. Stator - The stator carries the field winding.The stator together with the rotor constitutesthe magnetic circuit or core of the machine. Itis a hollow cylinder.Brushes - These are conducting carbongraphite spring loaded to ride on thecommutator and act as interface between theexternal circuit and the armature winding. Rotor - It carries the armature winding. Thearmature is the load carrying member. Therotor is cylindrical in shape.Poles - The field winding is placed in poles, thenumber of which is determined by the voltageand current ratings of the machine. Slot/Teeth - For mechanical support,protection from abrasion, and further electricalinsulation, non-conducting slot liners are oftenwedged between the coils and the slot walls.The magnetic material between the slots iscalled teeth. Figure 3 shows a cross-sectionalviews of slot/Teeth geometryArmature Winding - This winding rotates in themagnetic field set up at the stationary winding.It is the load carrying member mounted on therotor. An armature winding is a continuouswinding; that is, it has no beginning or end. It iscomposed of a number of coils in series as isshown in Figure 2.Depending on the manner in which the coil endsare connected to the commutator bars, armaturewindings can be grouped into two: lap windingsand wave windings. Wave winding gives greatervoltage and smaller current ratings while the lapwinding supplies greater current and smallervoltage ratings [7].Figure 3: Typical Armature Slot Geometry.On the other hand, the basic constructionalfeatures of an A.C. machine (e.g inductionmachine) are: Rigid Frame - The whole construction ensurescompact and adaptable design at low weightand low vibration level in all operatingconditions and throughout the whole speedrange [8]. Figure 4 shows a basic ABB RigidFrame Design.Figure 2: An Armature Coil. Field Winding - This is an exciting systemwhich may be an electrical winding or apermanent magnet and which is located on thestator.Commutator - The coils on the armature areterminated and interconnected through thecommutator which comprised of a number ofbars or commutator segments which areinsulated from each other. The commutatorrotates with the rotor and serves to rectify theThe Pacific Journal of Science and igidFrameFigure 4: ABB Rigid Frame–47–Volume 7. Number 1. May 2006 (Spring)

Stator Package - The stator core is a stack ofthin electrical sheet steel laminations insulatedby a heat resistant inorganic resin. The radialcooling ducts ensure uniform and efficientcooling. The stator package, shown in Figure 5forms a solid block which retains its rigiditythroughout the long lifetime of the machine.RuggedBearingAssembliesFigure 7: ABB Rugged Bearing AssembliesStatorPackageBASIC PRINCIPALS OF OPERATIONFigure 5: ABB Stator Package Rotor Construction - The rotor of A.C.machines can be of wound type or squirrelcage type. A typical squirrel cage rotor isshown in Figure 6. Depending on the numberof poles and whether the shaft is of the spideror cylindrical type, the rotor core is shrunk ontothe shaft and the conductor bars tightlycaulked into the slots to prevent bar vibration.Rotor ConstructionElectric motors and generators are a group ofdevices used to convert mechanical energy intoelectrical energy, or electrical energy intomechanical energy, by electromagnetic means. Amachine that converts mechanical energy intoelectrical energy is called a generator, alternator,or dynamo, and a machine that converts electricalenergy into mechanical energy is called a motor.Two related physical principles underlie theoperation of generators and motors. The first is theprinciple of electromagnetic induction discoveredby Michael Faraday in 1831 [9]. If a conductor ismoved through a magnetic field, or if the strengthof a stationary conducting loop is made to vary, acurrent is set up or induced in the conductor. Theconverse of this principle is that of electromagneticreaction, first discovered by Andre’ Ampere in1820 [10]. If a current is passed through aconductor located in a magnetic field, the fieldexerts a mechanical force on it.Both motors and generators consist of two basicunits, the field, which is the electromagnet with itscoils, and the armature (the structure that supportsthe conductors which cut the magnetic field andcarry the induced current in a generator or theexciting current in a motor). The armature isusually a laminated soft-iron core around whichconducting wires are wound in coils.Figure 6: ABB Rotor ConstructionDETERMINATION OF MACHINE PARAMETERS Rugged Bearing Assemblies - The bearingsare designed for reliable continuous operationand ease of maintenance. Depending on therated power, either spherical seated selfaligning sleeve bearings or anti-frictionbearings with a life time of over 100,000 hoursare available [8]. See Figure 7 for ABB ruggedbearing assemblies.The Pacific Journal of Science and he nameplate gives sufficient information on therated current, power, frequency, voltage, windingtemperature, and stator winding connection.However, it may be necessary to determine thewinding resistances and reactances as well as themechanical properties of the machine in order toevaluate the machine performance under bothsteady and dynamic conditions. For instance, the–48–Volume 7. Number 1. May 2006 (Spring)

following tests are usually carried out to determinethe parameters of an asynchronous machine. E f f i c i e ncy a ga i ns t Out put P owerNo-Load Test - The aims of the no – load testare to determine:10. 9 Stator ohmic/copper losses Stator core losses due to hysteresis andeddy current Rotational losses due to friction andwindage Magnetizing inductance.0. 80. 72 Pole4 Pole6 Pole8 Pole0. 6The test is carried out at rated frequency and withbalanced polyphase voltages applied to the statorterminals. Readings are taken at rated voltage,after the motor has been running for aconsiderable period of time necessary for thebearings to be properly lubricated. At no–load, themachine slip and the rotor current are very smallthereby resulting to a negligible no–load rotor loss. 0. 50. 40. 010. 1110100100010000Out put P ow e r [ K W ]Figure 8: Efficiency of Induction Machine.Blocked–Rotor Test - The blocked–rotor testprovides information necessary to determine:P owe r Fa c t or a ga i nst Out put P owe r The winding resistances The leakage reactances10. 9In this test, the rotor is blocked by external meansto prevent rotation. In the blocked–rotor test, theslip is unity (s 1) and the mechanical loadresistance, RM is zero; thereby resulting in a verylow input impedance of the equivalent circuit. Retardation Test - The retardation test iscarried out to determine the test motormoment of inertia. In this test, a no–load iscarried out with and without additionalstandard induction machines can be obtainedfrom manufacturer’s data as well as from theFinite–Element–Analysis (FEA) calculationresults [11] such standard curves are shown inFigure 8 and Figure 9.0. 80. 72 Pole4 Pole6 Pole8 Pole0. 60. 50. 40. 010. 1110100100010000Out put P owe r [ K W ]Figure 9: Power factor of Induction MachineELECTRIC MACHINES PROTECTION ANDMAINTENANCEElectrical and mechanical faults can imposeunacceptable conditions and protective devicesare therefore provided to quickly disconnect themachine from grid. In order to ensure thatelectrical machines receive adequate protection,extensive testing is performed to verify the highquality of assembly. After a machine of a particulartype has been type tested for electricalcharacteristics, all subsequent machines of thesame type undergo a routine test programme.The Pacific Journal of Science and outine and type test programmes can take theseforms: Routine test programme Bearing control Control of the insulation Ohmic resistance measurement Vibration measurement Short circuit test No- load test High voltage test–49–Volume 7. Number 1. May 2006 (Spring)

Type test programme Routine test No-load curve Load point Heat run testAfter the type test programme, the electricalmachine is identified with a protective symbol.Degrees of protection by enclosures for electricalmachines are quoted with the letters IP and twocharacteristic numerals. The first numeraldesignates the degree of protection for personsagainst contact with live or moving parts inside theenclosure and of machines against ingress of solidforeign bodies. The second numeral designatesthe degree of protection against harmful ingress ofwater. Some of the degrees of protection are IP23,IP54, IP55, and IP56. For instance, in IP23, thefirst numeral means operation against contact by afinger with live or moving parts inside theenclosure while the second numeral denotesprotection against water.Generally, when deciding on a particular type ofmotor protection, this should be done with actualoperating conditions in mind. Motors are protectedby current–dependent motor protection circuitbreakers and/or over current relays. These areparticularly effective in cases like locked rotor orinterrupted run-up, and are therefore indispensablein large machines with thermally critical rotor [12].1. Perform visual checks to help identify theproblem.2. If previous tests were performed from thevoltage regulator service manual, use the testresults to help identify the problem.3. Check troubleshooting knowledge-bases tohelp identify the problem.4. Perform the Generator Functional Test to helpidentify the problem.After the work has been carried out the machine isto be marked by an additional repair name platewith the following data: ELECTRIC MACHINES APPLICATIONSFigure 10, Figure 11, and Figure 12 show some ofthe practical applications of electric machines [12]. Asynchronous Machines Petroleum and chemical pumps Cooling towers Air-handling equipment Compressors Process machinery Blowers and fans Drilling machines Grinders Lathes Conveyors Crushers, etc. Synchronous Machines Power generation Wind energy turbines Power factor correction Voltage regulation improvement oftransmission lines Electric clock drives Gasoline engine drives Servo drives Compressors, etc D.C. Machines Rolling mills Elevators Conveyors Electric locomotives Rapid transit systems Cranes and hoists Lathes Machine tools Blowers and fans, etc.The temperature-dependent devices serve toprotect the motor against the effects of excessivewinding heating due to overload, increasedambient temperature, impaired cooling, intermittentoperation, high switching frequency, and phasefailure.Unscheduled downtime and resultant high repaircost reduce profits. There is a need to setobjectives to manage maintenance, schedulerepair, adjustments, and control cost. Whencarrying out servicing or repairing electricmachines, like power generation equipment, dothe following: Make sure the unit is off-lineMake sure the generator engine is stoppedMake sure all batteries are disconnectedMake sure all capacitors are discharged.The generator is a component of the generator setand should be tested with the entire system. Theservice manual for the voltage regulator providetests to determine if the generator is the cause of agenerator set malfunction.The following procedure should be used to helpidentify and define the problem [13]:The Pacific Journal of Science and ateOperative firmIf necessary, mode of repairIf necessary, signature of the expert.–50–Volume 7. Number 1. May 2006 (Spring)

CONCLUSIONThis paper has presented the basic principles ofelectric machines. The basic features and theconventional method of determining the machineparameters have also been highlighted.The authors also elucidated the various ways ofprotecting electric machines against overloads andmechanical faults. Areas of application of electricmachines have been itemized in order to show theimportance of these devices in process industries.Figure 10: SCHORCH Motor 90KW, 400V,2975rpm, Compressor driveworkhascarefullyneglectedtheThismathematical analysis of electric machines. Thetreatment is beyond the scope of this endeavour.However, it must be pointed out that powerfulnumerical software such as MATLAB/SIMULINK,ANSOFT, SIMPLORER, SIMPOW, and SMT havebeen used by machine manufacturing companiesto study both the electrical and mechanicalbehaviour of electrical machines prior to design.REFERENCESFigure 11: SCHORCH Motors 160KW, 400V,1485rpm, Driving pumpsFigure12: SCHORCH Hydrogenerator 45KW,400V, 1025rpmThe Pacific Journal of Science and .MacDonald, M.L. and Sen, P.C. 1978. “ControlLoop Study of Induction Motor Drives using D.Q.Model”. Conference Record of Industry ApplicationsSociety. IEEE/IAS Annual meeting.2.Nsar, S.A. and Boldea, I. 1990. Electrical MachinesSteady-State Operation. John Wiley and Sons:London, UK.3.King, K.G. 1963. “The Application of SiliconControlled Rectifiers to the Control of ElectricalMachines”. IEE Proceedings. 110(1): 197-204.4.Slemon, G.R. and Dewan, S.B. 1974. “InductionMotor Drive with Current Source Inverter.Conference Record. IEEE/IAS Annual Meeting.411-417.5.Murphy, J.M.D. and Turnbull, F.G. 1988. PowerElectronic Control of AC Motors. Pergamon Press:New York, NY.6.Kusko, A. and Peeran, S.M. 1961. StandardHandbook for Electrical Engineers. John Wiley:London, UK.7.Theraja, B.L. 1979. A Text-book of ElectricalTechnology. S. Chand and Company Ltd.: NewDelhi, India.8.AMA. 1998. “ABB Information Brochure”. AMAModular Induction Machine: Finland.9.Giancoli, D.C. 1988. Physics for Scientists andEngineers. Prentice-Hall: Princeton, NJ.–51–Volume 7. Number 1. May 2006 (Spring)

10. Ohanian, H.C. 1989. Physics, Norton: New York,NY.11. Okoro, O.I. 2002. “Dynamic and Thermal Modellingof Induction Machine with Non-Linear Effects”.Kassel University Press: Kassel.12. SCHORCH.1999. “SCHORCH Manual onElectrical Machines, Drive Systems and SystemEngineering”. SCHORCH: Germany.Dr. Edward Chikuni holds a B.Eng. degree inElectrical Engineering from the University of SierraLeone, an M.Sc. from University of ManchesterInstitute of Science & Technology (UMIST), and aPh.D. from the University of Wales, Swansea. Heis a Chartered Electrical Engineer (MIEE) (London)and Fellow of the Zimbabwe Institution ofEngineers. At present he is a Senior Lecturer inElectrical Engineering at the Polytechnic of Namibiaon leave from the University of Zimbabwe.13. UAC. 1972. “SHIELD”. Tractor and EquipmentDivision of UAC of Nigeria Ltd: Lagos. No.3.SUGGESTED CITATIONABOUT THE AUTHORSDr.-Ing. Ogbonnaya I. Okoro received the B.Eng.and M.Eng. degrees in Electrical Engineering fromthe University of Nigeria. He holds a Ph.D. inElectrical Machines from the University of Kassel,GermanyundertheDAADscholarshipprogramme. He is a registered Electrical Engineer(COREN) and corporate member of the NigerianSociety of Engineers (MNSE) and the IEEE(MIEEE). He is currently a Senior Lecturer inElectrical Engineering at the University of Nigeria,Nsukka. (Department of Electrical Engineering,University of Nigeria, Nsukka, Enugu State,Nigeria. E-mail: oiokoro@hotmail.com ).Okoro, O.I., M.U. Agu, and E. Chinkuni. 2006.“Basic Principles and Functions of ElectricalMachines”. Pacific Journal of Science andTechnology. 7(1):45-52.Pacific Journal of Science and TechnologyDr. M. U. Agu holds a Ph.D. in Power Electronicsfrom the University of Toronto, Canada. He is anAssociate Professor in the Department of ElectricalEngineering, University of Nigeria, Nsukka. Hisresearch interests include power electronics andthe control of electric drives. He is a member of theNSE and the IEEE.The Pacific Journal of Science and ��52–Volume 7. Number 1. May 2006 (Spring)

from electrical machines in terms of special characteristics and speed control. It is in this field that the D.C. machines, fed from the A.C. supply through rectifiers, are making their mark. In this paper, we shall discuss the various types of electri

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