12- Brushless Permanent Magnet Motors


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Brushless Permanent Magnet Motors
BRUSHLESS DC MOTORS
TheweakpointinthemechanicaldesignoftheDCmotoristhebrushesrubbingagainsttherotatingcommutator(togetcurrentintothearmature).
Brusheswearout,getdirty,causedust,andareelectricallynoisy.
ThebrushlessDCmotor(BLDC)operateswithoutbrushesbytakingadvantageofmodernelectronicswitchingtechniques.
Althoughthisaddssomecomplexity,theresultisamotorthatisextremelyreliable,veryefficient,andeasilycontrolled(allverydesirablequalities).
TheBLDCisbecomingincreasinglypopular,particularlyinthosecaseswherethemotormustbeoperatedfromaDCsourcesuchasabattery.
BRUSHLESS DC MOTORS
 Figure 1(a) shows a diagram of a three-phase BLDC. The armature (called the rotor) is a
permanent magnet, and it is surrounded by three field coils.
 Each field coil can be switched on and off independently. When a coil is on, such as coil A in
Figure 1(a), the north pole of the rotor magnet is attracted to that coil. By switching the coils
on and off in sequence (A, B, C), the rotor is “dragged” around clockwise—that is, the field
has rotated electronically.
Figure 1(a) BLDCM
BLDCM,
 BLDCs have much in common with stepper motors, which are discussed in details earlier in
this semester.
 The major difference between steppers motors and BLDCM is that the BLDC is used as a
source of rotary power, like a regular electric motor, whereas the stepper motor is used when
it is necessary to step to precise positions and then stop i.e., much more suitable for servo
control applications.
 Unlike the stepper motor, the BLDC has a built-in sensor system to direct the switching from
one field coil to the next. Figure 1(b) shows the three-phase BLDC with three optical slotted
couplers and a rotating shutter (Hall-effect sensors can also be used for this application).
BLDCM, Cont.
 These position sensors control the field windings. When the shutter is open for sensor P1 as
shown, field coil A [Figure 1(a)] is energized. When the rotor actually gets to field coil A,
sensor P1 is turned off and P2 is turned on, energizing field coil B and pulling the rotor on
around to coil B, and so on. In this manner, the rotor is made to rotate with no electrical
connection between the rotor and the field housing.
 Figure 2 shows a schematic of a generalized three-phase BLDC. The three position sensors
connect to the control circuit.
 In the simplest case, these signals are passed directly on to solid-state switches that drive the
motor coils. A more sophisticated motor-control system would provide for the motor to
reverse direction (by reversing the sequencing) and would control the speed by using PWM
techniques.
BLDCM, Cont.
Fig 2 BLDCM control circuit
BLDCmotorsexhibitexcellentspeedcontrol.Infact,somemodelscomewithabuilt-intachometerthatfeedsbacktothecontrolunit,allowingaspeedregulationof0%(perfect).Whenusedinavariable-speedmotion-controlsystem,BLDCscanvarytheirspeedintherangeof100:1.However,unlikeabrushedDCmotor,theBLDChasaminimumoperatingspeed(around300rpm)belowwhichtheindividualpowerpulsescanbefelt.
Besidesbeingmorereliable,modernBLDCmotorshaveperformanceadvantagesoverbrushedDCmotorsandevenACinductionmotors.Specifically,BLDCmotorshavehigherpowerefficiency(theyuselesspowerforthesamehorsepower)andaresmallerandlighterthanothertypesofmotorswiththesamehorsepower.ItseemsreasonabletoconcludethattheBLDCmotorwillcontinuetoimproveandbeadoptedinmoreandmoreapplications.
BLDCM, Cont
Figure3showsauni-polarBLDCM.Threephoto-sensorsareusedtodetecttherotorposition.Thephotosensorsarespatiallydistributedwitha120phaseshift.
PT1,PT2andPT3,arethephoto-sensordetectors.Thesedetectors,detectlightinsequenceastherevolvingshuttermoves.
Asseeninfigure3,PT1isliabletolightwherePT2andPT3aremaskedbytherevolvingshutter.ThusprovidingafiringpulsetoTr1leadstoCCWmotionoftherotor.
Oncetherotormoves,therotorpoleaxiswillalignedwiththestatorpoleP1axis,duetotheattractingforceproducedbyrotorpoleNandstatorpoleS.uponwhich,theshutterwillmaskPT1andkeepsPT3maskedanddismasksPT2.
Uni-polar BLDCM Control
Fig 3 Uni-polar BLDCM control configuration
 Figure 4 shows a Bi-polar BLDCM control configuration. This type of motor control requires a
photo detector consisted of 6 photo transistors for rotor position detection.
 The 6-photo detectors are spatially distributed with 60 degree phase shift.
 The shutter of the position detection is arranged in such a way so that either 2-upper
transistors and one lower transistor of different legs work simultaneously or 2-lower
transistors and one upper transistor of different legs to work at one instant time.
 As seen, the shutter prevents any two transistor on the same leg to work simultaneously to
avoid (high current short circuit).
Bi-polar BLDM
Fig 4 Bi-polar BLDCM control configuration

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