McGraw-Hill CreateTM Review Copyfor Instructor Nicolescu.Not fordistributionIntroduction to Mechatronics and Measurement Systems,Fourth Edition9343710.4Electric Motorsis a small air gap between the rotor and the stator where the magneticfields interact.In many DC motors,the rotor also includes a commutator that delivers and con-trols the direction of current through the armature windings.For motors with a com-mutator,“brushes"provide stationary electrical contact to the moving commutatorconductingsegments.Brushes inearlymotors consisted of bristles of copper wireflexedagainst thecommutator,hencethetermbrush;butnowtheyare usuallymadeof graphite,which provides a larger contact area and is self-lubricating.Thebrushesare usually spring-loaded to ensure continual contact with the commutator.VideoDemo10.11 showsa small,brushed,permanent-magnetDCmotordisassembled soyou can see the various components and how theyfunction.Abrushless DC motor has permanent magnets on the rotor and a rotatingfieldin the stator.The permanent magnets on the rotor eliminate the need for a commuta-tor.Instead, the DC currents in the stator coils are switched in response to proximitysensors that are triggered as the shaft rotates.Video Demos 10.12 and 10.13 showtwoexamplesof brushlessDC motors.One advantageofa brushlessmotoristhatit does notrequiremaintenancetoreplaceworn brushes.Also,becausethere arenorotor windings or iron core, the rotor inertia is much smaller, sometimesmakingcontrol easier.There arealsonorotor heat dissipationproblems,becausethere are noVrotor windings and henceno FRheating.Anotheradvantage of not havingbrushesVideo Demois that there is no arcing associated withmechanical commutation.Therefore,brush-lessmotorscreatelessEMI andaremoresuitableinenvironmentswhereexplosive10.11DC motorgases mightbepresent.Onedisadvantage ofbrushlessmotors is that they can costcomponentsmore due to the sensors and control circuitry required.10.12BrushlessFigure 10.7 shows examples of commercially available assembled motors.InDCmotorfromathe top figure, the motor on the left is an AC induction motor with a gearhead speedcomputerfanreduction unit attached.The motor on the right is a two-phase stepper motor.Motors10.13Brushlesscome in standard sizes with standard mounting brackets,and theyusuallyincludeDC motorgearnameplates listing someof themotor's specifications.Thebottomfigure showsthepumpinternalconstructionofapermanent-magnet-rotorsteppermotor.VideoDemo1o.1410.14DCandsteppermotorshowsotherexamplesof commerciallyavailableregularDC motors and stepperexamplesmotors.CLASS DISCUSSIONITEM1O.2Eddy CurrentsDescribe the causes of eddy currents that are induced in a conducting material expe-riencing a changing magnetic field.The iron core in a motor rotor is usuallylami-nated.Explain why.What is the best orientation for the laminations?Torque is produced by an electric motor through the interaction of either statorfields and armaturecurrents or statorfields and armaturefields.We illustrate bothprinciples starting with the first.Figure 10.8 illustrates a DCmotor with six armaturewindings.Thedirection of current flow in the windings is illustrated in the figure
Confirming Pages is a small air gap between the rotor and the stator where the magnetic fields interact. In many DC motors, the rotor also includes a commutator that delivers and controls the direction of current through the armature windings. For motors with a commutator, “brushes” provide stationary electrical contact to the moving commutator conducting segments. Brushes in early motors consisted of bristles of copper wire flexed against the commutator, hence the term brush; but now they are usually made of graphite, which provides a larger contact area and is self-lubricating. The brushes are usually spring-loaded to ensure continual contact with the commutator. Video Demo 10.11 shows a small, brushed, permanent-magnet DC motor disassembled so you can see the various components and how they function. A brushless DC motor has permanent magnets on the rotor and a rotating field in the stator. The permanent magnets on the rotor eliminate the need for a commutator. Instead, the DC currents in the stator coils are switched in response to proximity sensors that are triggered as the shaft rotates. Video Demos 10.12 and 10.13 show two examples of brushless DC motors. One advantage of a brushless motor is that it does not require maintenance to replace worn brushes. Also, because there are no rotor windings or iron core, the rotor inertia is much smaller, sometimes making control easier. There are also no rotor heat dissipation problems, because there are no rotor windings and hence no I 2 R heating. Another advantage of not having brushes is that there is no arcing associated with mechanical commutation. Therefore, brushless motors create less EMI and are more suitable in environments where explosive gases might be present. One disadvantage of brushless motors is that they can cost more due to the sensors and control circuitry required. Figure 10.7 shows examples of commercially available assembled motors. In the top figure, the motor on the left is an AC induction motor with a gearhead speed reduction unit attached. The motor on the right is a two-phase stepper motor. Motors come in standard sizes with standard mounting brackets, and they usually include nameplates listing some of the motor’s specifications. The bottom figure shows the internal construction of a permanent-magnet-rotor stepper motor. Video Demo 10.14 shows other examples of commercially available regular DC motors and stepper motors. Video Demo 10.11 DC motor components 10.12 Brushless DC motor from a computer fan 10.13 Brushless DC motor gear pump 10.14 DC and stepper motor examples ■ CLASS DISCUSSION ITEM 10.2 Eddy Currents Describe the causes of eddy currents that are induced in a conducting material experiencing a changing magnetic field. The iron core in a motor rotor is usually laminated. Explain why. What is the best orientation for the laminations? Torque is produced by an electric motor through the interaction of either stator fields and armature currents or stator fields and armature fields. We illustrate both principles starting with the first. Figure 10.8 illustrates a DC motor with six armature windings. The direction of current flow in the windings is illustrated in the figure. 10.4 Electric Motors 437 alc80237_ch10_431-477_sss.indd 437 lc80237_ch10_431-477_sss.indd 437 10/01/11 10:24 PM 0/01/11 10:24 PM Introduction to Mechatronics and Measurement Systems, Fourth Edition 93 McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution
McGraw-Hill CreateTM ReviewCopyforInstructorNicolescu.Notfordistribution94MeasurementSystems438CHAPTER1oActuatorsom电旺话0(@) AC induction and stepper motorBracketRotorStatorFlange(b) exploded view of stepper motor with apermanent magnet rotorFigure1o.7Examplesofcommercialmotors.(CourtesyofOrientalMotorTorrance,CA)stator ficldstatoXorqucrotcX?ROcurent outarmaturefieldO?currentinCarmatureoOwindingsstator ficeldFigure1o.8 Electricmotorfield-currentinteractionAs a result of Equation 10.1,the interaction of thefixed stator field and the currentsinthearmaturewindingsproduceatorqueinthecounterclockwisedirection.Youcan verify this torque direction by applying the right-hand rule to the armature cur-rent and statorfield directions.To maintain thetorque as the rotor rotates,the spatialarrangementof thearmaturecurrentsrelativetothestatorfieldmustremainfixed
Confirming Pages 438 C H A P T E R 10 Actuators As a result of Equation 10.1 , the interaction of the fixed stator field and the currents in the armature windings produce a torque in the counterclockwise direction. You can verify this torque direction by applying the right-hand rule to the armature current and stator field directions. To maintain the torque as the rotor rotates, the spatial arrangement of the armature currents relative to the stator field must remain fixed. Figure 10.7 Examples of commercial motors. (Courtesy of Oriental Motor, Torrance, CA) (a) AC induction and stepper motor (b) exploded view of stepper motor with a permanent magnet rotor Case Bracket Rotor Stator Flange Figure 10.8 Electric motor field-current interaction. 1 1 2 2 3 3 4 4 5 5 6 6 stator stator stator field stator field current out current in torque rotor armature windings armature field alc80237_ch10_431-477_sss.indd 438 lc80237_ch10_431-477_sss.indd 438 10/01/11 10:24 PM 0/01/11 10:24 PM 94 Measurement Systems McGraw-Hill Create™ Review Copy for Instructor Nicolescu. Not for distribution