Electric Motor Basics

Electrical MachinesIntroductionDr. Tahir IzharProfessor Electrical Engineering, UET, Lahore11IntroductionElectric energy can be used to generate light and heat.Sometimes energy is required in mechanical form such as fans and rolling mills.Electrical machines are energy converters.These are used to continuously translate electrical energy into mechanical energy or vice versa.22Electromechanical Conversion The process of energy conversion used in electric machines is known as electromechanically energy conversion

33Electromechanical ConversionIn electrical system the primary quantities involved are Voltage and CurrentIn mechanical system, the analogous quantities are Torque and SpeedElectric power is the product of Voltage and Current-WattsMechanical power is the product of Torque and Speed-HP44Electromechanical Conversion The coupling medium between electrical and mechanical systems is the magnetic field.

55Type of Electric Motors6Electric MotorsAlternating Current (AC) MotorsDirect Current (DC) MotorsSynchronousInductionThree-PhaseSingle-PhaseSelf ExcitedSeparately ExcitedSeriesShuntCompound6Electric Motor Basic PrinciplesInteraction between magnetic field and current carrying wire produces a forceOpposite of a generator

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7A Simple AC GeneratorWe noted earlier that Faradays law dictates that if a coil of N turns experiences a change in magnetic flux, then the induced voltage V is given by

If a coil of area A rotates with respect to a field B, and if at a particular time it is at an angle to the field, then the flux linking the coil is BAcos, and the rate of change of flux is given by

88Thus for the arrangement shown below

99Therefore this arrangement produces a sinusoidal output as shown below

1010Wires connected tothe rotating coilwould get twistedTherefore we usecircular slip ringswith slidingcontacts calledbrushes

1111A Simple DC GeneratorThe alternating signal from the earlier AC generator could be converted to DC using a rectifierA more efficient approach is to replace the two slip rings with a single split slip ring called a commutatorThis is arranged so that connections to the coil are reversed as the voltage from the coil changes polarity.Hence the voltage across the brushes is of a single polarity.Adding additional coils produces a more constant output.1212Use of a commutator

1313A simple generator with two coils

1414Use of Iron CoreThe ripple can be further reduced by the use of a cylindrical iron core and by shaping the pole piecesThis produces anapproximatelyuniform field in thenarrow air gapThe arrangementof coils and coreis known as thearmature

151516Field poleNorth pole and south poleReceive electricity to formmagnetic fieldArmatureCylinder between the polesElectromagnet when current goes throughLinked to drive shaft to drive the loadCommutatorOverturns current direction in armatureDC Motors Components

16Direct-Current motors, as the name implies, use a direct-unidirectional current. A DC motor is shown in the figure and has three main components:Field pole. Simply put, the interaction of two magnetic fields causes the rotation in a DC motor. The DC motor has field poles that are stationary and an armature that turns on bearings in the space between the field poles. A simple DC motor has two field poles: a north pole and a south pole. The magnetic lines of force extend across the opening between the poles from north to south. For larger or more complex motors there are one or more electromagnets. These electromagnets receive electricity from an outside power source and serve as the field structure.Armature. When current goes through the armature, it becomes an electromagnet. The armature, cylindrical in shape, is linked to a drive shaft in order to drive the load. For the case of a small DC motor, the armature rotates in the magnetic field established by the poles, until the north and south poles of the magnets change location with respect to the armature. Once this happens, the current is reversed to switch the south and north poles of the armature.Commutator. This component is found mainly in DC motors. Its purpose is to overturn the direction of the electric current in the armature. The commutator also aids in the transmission of current between the armature and the power source.Commutator of Actual Machine

Commutator Cross-Section1718DC motor wiring topologies

19Separately excited DC motor: field current supplied from a separate forceSelf-excited DC motor: shunt motor

Field winding parallel with armature windingCurrent = field current + armature currentSpeed constant independent of load up to certain torqueSpeed control: insert resistance in armature or field currentDC motors19If field is supplied from a separate source it is called as separately excited DC motor. (Click once) In a shunt motor, the field winding (shunt field) is connected in parallel with the armature winding (A) as shown in the. The total line current is therefore the sum of field current and armature current The following can be said about the speed of shunt motors:(Click once) The speed is practically constant independent of the load (up to a certain torque after which speed decreases) and therefore suitable for commercial applications with a low starting load, such as machine tools(Click once) Speed can be controlled by either inserting a resistance in series with the armature (decreasing speed) or by inserting resistance in the field current (increasing speed)20Self-excited DC motor: series motorDC motors

Field winding in series with armature windingField current = armature currentSpeed restricted to 5000 RPMAvoid running with no load: speed uncontrolledSuited for high starting torque: cranes, hoists20In a series motor, the field winding (shunt field) is connected in series with the armature winding (A) as shown in the figure. The field current is therefore equal to the armature current. (Click once) The following can be said about the speed of a series motorSpeed is restricted to 5000 RPMIt must be avoided to run a series motor with no load because the motor will accelerate uncontrollably(Click once) Series motors are suited for applications requiring a high starting torque, such as cranes and hoists 21DC compound motor

DC motorsField winding in series and parallel with armature windingGood torque and stable speedHigher % compound in series = high starting torqueSuited for high starting torque if high % compounding: cranes, hoists21A DC compound motor is a combination of shunt and series motor. In a compound motor, the field winding (shunt field) is connected in parallel and in series with the armature winding (A) as shown in the figure.(Click once) For this reason this motor has a good starting torque and a stable speed. (Click once) The higher the percentage of compounding (i.e. percentage of field winding connected in series), the higher the starting torque this motor can handle. (Click once) For example, compounding of 40-50% makes the motor suitable for hoists and cranes, but standard compound motors (12%) are not. AC Generators or AlternatorsAlternators do not require commutationThis allows a simpler construction.The field coils are made to rotate while the armature windings are stationary.Note: the armature windings are those that produce the output.Thus the large heavy armature windings are in the stator.The lighter field coils are mounted on the rotor and direct current is fed to these by a set of slip rings.2222AC GeneratorA four-pole alternator

2323Synchronous Machines

24Most hydraulic turbines have to turn at low speeds (between 50 and 300 r/min)

A large number of poles are required on the rotorHydrogeneratorTurbineHydro (water)D 10 mNon-uniform air-gapNSSNd-axisq-axisSalient-Pole Synchronous Generator

25Salient-Pole Synchronous Generator

StatorSalient-pole rotor26 L 10 mD 1 mTurbineSteamStatorUniform air-gapStator windingRotorRotor windingNSHigh speed 3600 r/min 2-pole 1800 r/min 4-pole Direct-conductor cooling (using hydrogen or water as coolant) Rating up to 2000 MVATurbogeneratord-axisq-axisCylindrical-Rotor Synchronous Generator27Cylindrical-Rotor Synchronous Generator

Stator

Cylindrical rotor28Construction of synchronous machines

Salient-pole rotor: four and more poles.Rotors are made laminated to reduce eddy current losses.Construction of synchronous machines

Salient pole with field windingsSalient pole without field windings observe laminationsA synchronous rotor with 8 salient polesConstruction of synchronous machines

Construction of synchronous machines

Slip ringsBrush

Motors OperationWhen current flows in a conductor it produces a magnetic field about it - as shown in (a) belowWhen the current-carrying conductor is within an externally generated magnetic field, the fields interact and a force is exerted on the conductor - as in (b)3333AC MotorsAC motors can be divided into two main forms:Synchronous motorsAsynchronous (Induction) motorsHigh-power versions of either type invariably operate from a three-phase supply.Single-phase versions of each are also widely used particularly in a domestic setup.3434Synchronous motorsJust as a DC generator can be used as a DC motor, so AC generators (or alternators) can be used as synchronous AC motors.Three phase motors use three sets of stator coils.The rotating magnetic field drags the rotor around with itSingle phase motors require some starting mechanism.Torque is only produced when the rotor is in sync with the rotating magnetic field.Not self-starting may be configured as an induction motor until its gets up to speed, then becomes a synchronous motor3535Induction motorsThese are perhaps the most important form of AC motor.Rather than use slip rings to pass current to the field coils in the rotor, current is induced in the rotor by transformer action.The stator is similar to that in a synchronous motor.The rotor is simply a set of parallel conductors shorted together at either end by two conducting rings.3636Induction motorsA squirrel-cage induction motor

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37Induction MotorsPoly-phase AC supply is used to create a rotating magnetic field on the stator.This induces a magnetic field on the rotor, which tries to follow stator - slipping required to produce torque Workhorses of the industry - high powered applications

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38Single Phase Induction motorsThe single phase distributed winding of a single phase induction motor does not produce a rotating magnetic field, but a pulsating field reaching maximum intensity at 0o and 180o electrical. The operation of the single phase induction motor can be explained by the double revolving field theory.A pulsating field is equivalent to two rotating fields of half the magnitudes but rotating at the same synchronous speed in opposite directions.3939Double Revolving FieldTwo counter rotating magnetic field can be represented by phasors, coinciding twice per revolution at 0o (Figure below-a) and 180o (figure e). When the phasors rotate to 90o and -90o they cancel in figure c. 40

Single phase stator produces a non-rotating, pulsating magnetic field.40Double Revolving FieldAt 45o and -45o (figure b) they are partially additive along the +x axis and cancel along the y axis. An analogous situation exists in figure d. The sum of these two phasors is a phasor stationary in space, but alternating polarity in time. Thus, no starting torque is developed. 41

41Torque production Rotor at Standstill:If the rotor is at standstill, the pulsating stator flux will induced current in the rotor circuit.The rotor current will produce rotor pulsating flux acting in the same axis.The two fluxes tend to oppose each other (Lenzs Law).As the angle between these fluxes is zero no starting torque is developed and the rotor will remain at standstill.4242Torque production Rotor at Running:If the rotor is running, the torque can be developed.This can be explain using double revolving field theory.Both rotating mmf s will produce component torques in opposite directions.At standstill, these component torques are equal and opposite resulting zero starting torque.At any other speed the two torques are unequal and the resultant torque will keep the motor rotating in the direction of rotation.The torque-speed relation is shown in Figure-below.

4343Torque-Speed CharacteristicsTorque-Speed characteristics of a single phase induction motor based on constant forward and backward flux waves.44

44Single Phase Motors StartingInherently, 1-phase motor can not start.Some arrangement is required to produce starting torque.The simplest method is to provide an auxiliary starting winding on the stator in addition to main winding.The two windings are displaced by 90o in space. The motor is started as two phase motor.The two windings can be designed to make the motor as a balanced two phase motor.However, the windings must be energized by two sources with phase displacement of 90o.451/4/201345Capacitor Run MotorA capacitor can be used to split the single phase into two phases as shown below.

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46Capacitor Run MotorThis motor configuration works well up to 1/4 horsepower (200watt), though, usually applied to smaller motors. The direction of the motor is easily reversed by switching the capacitor in series with the other winding. This type of motor can be adapted for use as a servo motor.The capacitor value is of the order of 2-50uF.The capacitor is a compromise between the best starting and running values.4747Torque-SpeedCapacitor run motorTypical torque-speed characteristics of capacitor-run, split phase motor.48

48Capacitor Start motorA capacitor start motor is shown below.The switch in on at standstill and turned off after starting.49

49Capacitor Start motorA larger capacitor may be used to start a single phase induction motor via the auxiliary winding.It is switched out by a centrifugal switch once the motor is up to speed. Moreover, the auxiliary winding may be many more turns of heavier wire than used in a resistance split-phase motor to mitigate excessive temperature rise. The result is that more starting torque is available for heavy loads like air conditioning compressors. This motor configuration works so well that it is available in multi-horsepower sizes. 501/4/201350Torque-Speed Capacitor Start motor51

51Capacitor-Start Capacitor-Run MotorThe circuit of this motor is shown below.52

1/4/201352Capacitor-Start Capacitor-Run MotorA variation of the capacitor-start motor is to start the motor with a relatively large capacitor for high starting torque, but leave a smaller value capacitor in place after starting to improve running characteristics while not drawing excessive current.The additional complexity of the capacitor-run motor is justified for larger size motors. A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be two + to + (or - to -) series connected polarized electrolytic capacitors. Such AC rated electrolytic capacitors have such high losses that they can only be used for intermittent duty (1 second on, 60 seconds off) like motor starting. A capacitor for motor running must not be of electrolytic construction, but a lower loss polymer type. 5353Torque-SpeedCapacitor-Start Capacitor-Run54

54Resistance split-phase motorThe circuit of this motor with phasor is shown below55

55Resistance split-phase motorIf an auxiliary winding of much fewer turns of smaller wire is placed at 90o electrical to the main winding, it can start a single phase induction motor.With lower inductance and higher resistance, the current will experience less phase shift than the main winding. About 30o of phase difference may be obtained.This coil produces a moderate starting torque, which is disconnected by a centrifugal switch at 3/4 of synchronous speed. This simple (no capacitor) arrangement serves well for motors up to 1/3 horsepower (250 watts) driving easily started loads. 5656Shaded pole induction motor 57

Shaded pole induction motor, (a) dual coil design, (b) smaller single coil version.57Shaded pole induction motorAn easy way to provide starting torque to a single phase motor is to embed a shorted turn in each pole at 30o to 60o to the main winding. Typically 1/3 of the pole is enclosed by a bare copper strap.These shading coils produce a time lagging damped flux spaced 30o to 60o from the main field. This lagging flux with the undamped main component, produces a rotating field with a small torque to start the rotor. 5858Shaded pole induction motorStarting torque is so low that shaded pole motors are only manufactured in smaller sizes, below 50 watts. Low cost and simplicity suit this motor to small fans, air circulators, and other low torque applications. Motor speed can be lowered by switching reactance in series to limit current and torque, or by switching motor coil taps as in Figure below.

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59Electronically Operated MotorsThere is an emerging growth of Electronically Operated motors in numerous applications.This is due to the recent development in permanent magnet materials, efficient high power semiconductor devices, and high power single chip control devices.These motors offer, long life, high reliability, lower cost, and higher efficiency.Elimination of brushes, commutator and slip rings makes the design compact and robust. Simple, light weight and lossless rotor construction lead to low inertia, high efficiency and low cost. This presentation will introduce permanent magnet Brushless, and Switched Reluctance motors. The operation and characteristics of these motors will be introduced.

60Due to the recent development in permanent magnet materials, efficient high power semiconductor devices, and high power micro-controllers, there has been an emerging growth of Electronically Operated Special motors in numerous applications. The use of electronically operated special motors is increasing due to safety, comfort, and better performance. Drive-by-wire and intelligent cruise control systems with Electric Power Assisted Steering (EPAS) could be realized in future. These motors offer improved response, long life, and high reliability, lower cost and higher efficiency which is required in many servo-drive applications.These machines are highly suitable for servo-drive applications due to their characteristics. Elimination of brushes, commutator and slip rings makes the design compact and robust. Simple, light weight and lossless rotor construction lead to low inertia and high efficiency. Low rotor inertia leads to quick dynamic response.The lecture will include permanent magnet Brushless DC, Switched Reluctance, and stepper motors. The physical design, operation and characteristics of these motors will be introduced. The applications, pros and cons will be discussed. The issues related to the problems, future trends and research areas will also be identified.

60Brushless DC Motors6161

Brushless DC Motors (BLDCM)Essential difference - commutation is performed electronically with controller rather than mechanically with brushes

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BLDCM Commutation

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Commutation is performed electronically using a controller.Needs rotor positional closed loop feedback: hall effect sensors, back EMF, photo transistors63Applications, Pros and ConsApplicationsCPU cooling fansCD/DVD PlayersElectric automobilesPros (compared to brushed DC)Higher efficiencyLonger lifespan, low maintenanceClean, fast, no sparking/issues with brushed contactsConsHigher costMore complex circuitry and requires a controller,6464Definition of BLDCA synchronous motor with permanent magnets in the rotor and operated in self controlled mode using rotor position sensors and solid state electronic commutator-is generally known as a brushless DC motor.Due to the recent development in permanent magnet materials and power electronic devices, there has been an emerging growth of permanent magnet brushless motors in numerous applications. 6565Brushless DC Motors:CharacteristicsThese machines are highly suitable for servodrive applications due to following characteristics:Elimination of brushes, commutator and slip rings makes the design compact and robust.Simple, light weight and lossless rotor construction lead to low inertia and high efficiency.Low rotor inertia leads to quick dynamic response.High efficiency allows a reduction in frame size of the machine.6666Brushless Motors: major classesThere are two major classes of permanent magnet brushless motors drives.These are characterised by the shapes of their respective back EMF waveforms-sinusoidal and trapezoidal . The sinusoidal back EMF motor is usually excited with perfectly sinusoidal current The trapezoidal back EMF motor is fed with quasi-square current waveform with two intervals of zero-current excitation per cycle. 6767Brushless Motors: Major ClassesThe trapezoidal back EMF motor, driven with DC current which switches polarity in synchronism with the passage of alternate magnet poles, is known as brushless DC motor. The nature of the rectangular current with only six commutation instants of a three phase brushless DC motor, requires very simple position sensing system as compared to sinusoidal back EMF motor.The characteristics of brushless DC machines, in its ideal form, are exactly the same as the DC machine6868axial-field and radial-field machinesPermanent magnet brushless DC motors can be constructed with disc type rotors or cylindrical rotors. The former type are referred to as axial-field machines and the later as radial-field machines. Each type has its own advantages and drawbacks. The axial-field machine can yield high dynamic performance as compared to radial-field machine. However, its efficiency and maximum torque can be lower. Moreover, the manufacturing cost of axial-field machine is higher than that for radial-field machine.6969Magnet Mounting: SurfacePermanent magnets can be mounted with a variety of ways. The magnets can be mounted on the surface of the steel rotor using high-strength adhesive material.In surface mount magnet machines, the effective airgap is large because the recoil permeability of the magnet material at 1.02 to 1.06 is close to that of air.

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70Magnet Mounting: SurfaceLarge airgap yields negligible armature reaction and small magnetising inductance.The structure is magnetically non-salient.Ld =Lq Due to a large airgap, the electrical time constant of the stator winding is small which is important as far as the dynamic response of the machine is concerned.71

71Magnet Mounting: InsetThe arrangement shown in the figure is known as inset magnets.The permanent magnets are inserted in the steel structure.This construction is more secure compared to surface magnets.The structure is salient.72

72Magnet Mounting: InsetThe reluctance along q-axis and d-axis are different.Ld < LqSame magnet size, the peak torque is large, due to additional reluctance torque.The electrical time constant of the stator winding is large.73

73Magnet Mounting: InteriorLarger air-gap flux due to flux-focusing effect.Ld > LqThe structure is magnetically salient.Ferrite magnets can be employed and large air-gap flux can be achieved.This type is used for motors with sinusoidal back emf designs.

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74Brushless DC motor: constructionA brushless DC motor, is a motor without brushes, slip-rings or mechanical commutator The airgap flux is produced by the permanent magnets. The characteristics of the brushless DC motor are identical to the conventional DC motor as seen from the DC supply. The brushless DC motor is constructed with permanent magnets on the rotor and current carrying conductors are placed in the stator. The function of the commutator and brushes is implemented by maintenance-free solid-state switches. Therefore, in this respect the brushless DC motor is equivalent to an inverted DC commutator motor.7575Brushless DC motor: ConstructionThe basic components of a typical brushless DC motor. 76

76Brushless DC motor: ConstructionThe permanent magnet brushless DC motor, essentially requires a rotor position sensor to detect the pole position in order to switch the current in the stationary winding so that the current pattern in the stationary winding rotates in self-synchronism with the rotating field, thus producing continuous torque.An inverter is needed to switch the currents into the windings. Note that the inverter takes the place of the commutator and a three phase inverter is equivalent to a three segment commutator of a conventional DC motor. 7777Brushless DC motor: ConstructionThe motor uses a simple optical position sensor. It comprises of a shutter disc and three slotted-optical sensors. The shutter disc has one tooth and one slot because this motor has two permanent magnet poles. The slotted sensor has an integrated light source, photo transistor and necessary circuit to generate standard logic signals. When the shutter tooth covers the sensor slot, the sensor generates logic '0' and when uncovered, it generates a logic '1' signal. 7878Brushless DC motor: ConstructionThe shutter disc is mounted on the rotor shaft and aligned with the poles. The three sensors are mounted on a stationary frame and aligned with their respective phases. This typical brushless DC motor uses two-phase feeding scheme with magnet arc of 180o.A conventional 3-phase inverter is used to energised the motor winding. A DC voltage source is connected across the top and bottom power rail while three motor terminals are connected to the middle points of each leg of the inverter.

7979Brushless DC motor: ConstructionClamping diodes are connected across each power transistor to free-wheel the current through the inductive winding which is turned off. Rotor position feedback signals are required to turn 'on' and turn 'off' the respective power transistors of the inverter. The three position sensors generate the commutation logic signals. However, these logic signals must be encoded using combinational logic circuit to switch the six inverter transistors. The three phase winding of the motor can either be connected in star configuration or in delta configuration.8080Star Configuration 81

81Star Configuration Figure shows the drive circuit of a 3-phase, star-connected brushless DC motor.The motor windings are represented with its equivalent circuit of induced voltage source in series with winding inductance and its copper resistance.The switching sequence of the inverter transistors with respect to the rotor position is shown in the following Table.8282Star Configuration: Switching Sequence 83

83Induced Voltage If the permanent magnet rotor of the brushless DC motor, shown, is rotated at a constant speed, with the stator windings open circuited then the induced EMFs observed are shown in Figure below. For a 60o distributed, full-pitch winding with ideal rectangular airgap flux, the ideal induced EMF in each phase is trapezoidal in shape. The important feature of the waveform is constant amplitude for 120o during the positive and negative half cycles. 8484Induced Voltage 85

85Delta ConfigurationIt is also possible to connect the windings in delta configuration. However, the switching pattern of the inverter devices must be modified. The operation of a delta connected motor can be understood from the circuit diagrams of Figure below.

8686Delta Configuration

8787Delta Configuration: Switching Sequence88

88Delta ConfigurationIt can be seen that at any instant in time, three inverter transistors are 'on' to energise two motor phases while the third remains idle. Each phase conducts for 120o rotation. Therefore, the ideal phase induced EMF and feed current waveforms shown above, also apply to delta configuration. 8989Rotor Position Sensors In permanent magnet brushless motors, some means must be incorporated for the detection of rotor position to run the motor in self synchronism. A very simple low bandwidth position sensor is required for brushless DC motor to produce only six commutation pulses per cycle. However, high resolution position decoder is required for permanent magnet brushless AC motor to feed sinusoidal current. 9090Hall-Effect SensorsThe most commonly used position sensor in brushless DC motors is the Hall-effect element. The Hall-effect element is a DC-coupled device which detects the magnetic field. It is constructed using a semiconductor pellet with four terminals attached to each side of the pellet.A DC bias is applied across two terminals of the element while an EMF (HALL-voltage) is produced across the other two terminals if it is placed in a magnetic field due to Hall effect. 9191Hall-Effect SensorsHall IC chips are also available which have Hall element, amplifier and logic converter circuit in a single package.The output of the Hall IC can be used directly to drive the power transistors. The Hall elements or ICs can be easily mounted inside the stator of the motor to detect the polarity of the rotor magnets. Therefore, the cost of the motor can be reduce considerably by using build-in Hall-effect sensors. 9292Sensorless Operation The sensorless operation of brushless DC motor has been investigated by many researchers.In all the cases the rotor position is estimated through indirect means such as induced EMF.Since the rotor has permanent magnets, the induced EMF generated in the phase windings or in separate search coils can be used to detect the rotor position.However, the induced EMF is zero when the motor is not moving. Therefore, some sort of star-up mechanism is required before using the induced EMF signals to lock the machine in self synchronism. 9393Control of Brushless DC DriveFrom the control point of view the brushless DC motor is similar to the DC commutator motor.To control the DC motor drive, a separate chopper circuit is required if the motor is to be driven from a DC voltage source. However, in a brushless DC drive, the inverter is an integral part of the motor which can be used to implement the speed and current controls.9494Control of Brushless DC Drive

9595Equivalent Circuit96

96Switched Reluctance Motors(SRM)9797Variable Reluctance MotorThe variable reluctance motor is based on the principle that an unrestrained piece of iron will move to complete a magnetic flux path with minimum reluctance, the magnetic analog of electrical resistance. 2/4/201398

98Variable Reluctance MotorThis type of motor consists of a soft iron multi-toothed rotor and a wound stator. When the stator windings are energized with DC Current, the poles become magnetized.Rotation occurs when the rotor teeth are attracted to the energized stator poles.9999Synchronous Reluctance MotorIf the rotating field of a large synchronous motor with salient poles is de-energized, it will still develop 10 or 15% of synchronous torque. This is due to variable reluctance throughout a rotor revolution. There is no practical application for a large synchronous reluctance motor. However, it is practical in small sizes. If slots are cut into the conductor-less rotor of an induction motor, corresponding to the stator slots, a synchronous reluctance motor results.100100Synchronous Reluctance MotorIt starts like an induction motor but runs with a small amount of synchronous torque. The synchronous torque is due to changes in reluctance of the magnetic path from the stator through the rotor as the slots align. This motor is an inexpensive means of developing a moderate synchronous torque. Low power factor, low pull-out torque, and low efficiency are characteristics of the direct power line driven variable reluctance motor. Such was the status of the variable reluctance motor for a century before the development of semiconductor power control. 101101Switched reluctance MotorSequential switching of the stator phases moves the rotor from one position to the next. The magnetic flux seeks the path of least reluctance. This is an over simplified rotor and waveforms to illustrate operation. 2/4/2013102

102Four Rotor PolesDoubling the number of stator poles decreases the rotating speed and increases torque. This might eliminate a gear reduction drive. If smooth rotation is the goal, there is an electronic driven version of the switched reluctance motor. 2/4/2013103

1036-Rotor and 8-Stator Poles104

104Electronic drive for SRMWhile the variable reluctance motor is simple, even more so than an induction motor, it is difficult to control.Electronic control solves this problem and makes it practical to drive the motor well above and below the power line frequency. A variable reluctance motor driven by a servo, an electronic feedback system, controls torque and speed, minimizing ripple torque.2/4/2013105

105OperationA variable reluctance motor is optimized for continuous high speed rotation with minimum ripple torque. It is necessary to measure the rotor position with a rotary position sensor like an optical or magnetic encoder, or derive this from monitoring the stator back EMF.A microprocessor performs complex calculations for switching the windings at the proper time with solid state devices.This must be done precisely to minimize audible noise and ripple torque. For lowest ripple torque, winding current must be monitored and controlled. 106106OperationThe strict drive requirements make this motor only practical for high volume applications like energy efficient vacuum cleaner motors, fan motors, or pump motors. One such vacuum cleaner uses a compact high efficiency electronic driven 100,000 rpm fan motor. The simplicity of the motor compensates for the drive electronics cost. No brushes, no commutator, no rotor windings, no permanent magnets, simplifies motor manufacture. The efficiency of this electronic driven motor can be high. But, it requires considerable optimization, using specialized design techniques, which is only justified for large manufacturing volumes. 107107Advantages Simple construction- no brushes, commutator, or permanent magnets, no Cu or Al in the rotor. High efficiency and reliability compared to conventional AC or DC motors. High starting torque. Cost effective compared to bushless DC motor in high volumes. Adaptable to very high ambient temperature. Low cost accurate speed control possible if volume is high enough. 108108DisadvantagesCurrent versus torque is highly nonlinear Phase switching must be precise to minimize ripple torque Phase current must be controlled to minimize ripple torque Acoustic and electrical noise Not applicable to low volumes due to complex control issues 109109Linear MotorAt present Linear Motors are not widely used. However Linear Motors has great potential in the field of high speed transportation.

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The wound stator and the squirrel cage rotor of an induction motor may be cut at the circumference and unrolled into a linear induction motor. The direction of linear travel is controlled by the sequence of the drive to the stator phases. The linear induction motor has been proposed as a drive for high speed passenger trains. Up to this time, the linear induction motor with the accompanying magnetic repulsion levitation system required for a smooth ride has been too costly for all but experimental installations. However, the linear induction motor is scheduled to replace steam driven catapult aircraft launch systems on the next generation of naval aircraft carrier, CVNX-1, in 2013. This will increase efficiency and reduce maintenance.110International Standards111111IEC Standards The International Electrotechnical Commission, in French is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies collectively known as "electrotechnology".

112112IEC Standards IEC standards cover a vast range of technologies from power generation, transmission and distribution to home appliances and office equipment, semiconductors, fibre optics, batteries, solar energy, nanotechnology and marine energy as well as many others. The IEC also manages three global conformity assessment systems that certify whether equipment, system or components conform to its International Standards.

113113IEC Standards The IEC charter includes:Energy production and distribution. Electronics.Magnetics and Electro-magnetics. Electro-acoustics.Multimedia and Telecommunication.Terminology and Symbols.Electromagnetic compatibility. Measurement and Performance. Dependability.Design and Development. Safety and Environment.114114Membership and participationThe IEC is made up of members, called national committees, and each NC represents its nation's electrotechnical interests in the IEC. This includes manufacturers, providers, distributors and vendors, consumers and users, all levels of governmental agencies, professional societies and trade associations as well as standards developers from national standards bodies.National committees are constituted in different ways. Some NCs are public sector only, some are a combination of public and private sector, and some are private sector only. About 90% of those who prepare IEC standards work in industry.

115115IEC Full Members

Full MembersAssociate MembersAffiliates116116Standard Supply Voltages

117117Standard Supply Voltage in PakistanOfficial standard is 230 V / 50Hz. Voltage tolerance is 230 V 5% (218 V to 242 V). Frequency tolerance 50Hz 2% (49Hz to 51Hz) But Karachi Electric Supply Corporation (KESC) is 240 V / 50Hz.1/4/2013118118Power and EnergyHorsepower:In 1700s, Scottish inventor James Watt need to explain why his steam engines were better than the horsesHe calculated the amount of work one horse could do in a minute. Thus the term horsepower was coined. Not only did Mr. Watt sell his steam engines, he introduced a system of measurement that is still in use today.

119119Power and EnergyHorse-power:Mr. Watt wanted to tell how powerful his engines were. After some tests (not with engines but with horses) he established that on the average, a horse could haul coal at the rate of 22,000 lb-ft per min. For some reason, he decided the raise this number by 50% to arrive at 33,000 lb-ft per minutes. So, if an engine can push 33,000 Lb of something one foot in one minute, we say that is a one-horsepower engine. 120120Electrical Machines Introduction THANK YOUFOR YOUR ATTENTION121121Y-AxisX-Axis10Percent of rated SpeedPercent of Rated Torque120SeriesCompoundShunt1008060402004003002001000ShuntSeriesCompoundShunt FieldShunt FieldSeries FieldSeries Field