EED
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Transcript of EED
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Slideslastclass(632014)
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EqualizerConnections
The heavy current machine exaggerates a problem that exists in anymachine.
problem is due to same voltage will not be generated in each parallelpath.
Results in heavy circulating currents around the various parallel paths. These circulating currents move out and in in-the brush commutator
connections, causing heating, losses, and short commutator and brushlife.
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Dummycoilsinwavewinding In a simplex wave winding, the average pitch YA (or commutator pitch YC)should be a whole number. Sometimes the standard armature punchingsavailable in the market have slots that do not satisfy the aboverequirement so that more coils (usually only one more) are provided thancan be utilized. These extra coils are called dummy or dead coils. Thedummy coil is inserted into the slots in the same way as the others to makethe armature dynamically balanced but it is not a part of the armaturewinding.
Suppose the number of slots is 22 and each slot contains 2 conductors. Thenumber of poles is 4. For simplex wave wound armature
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Dummycoils.. Ya=(Z 2)/P Ya=(44 2)/4=11.5or10.5 Sotomakewholenumbernoofcoilstobereduced. Ya=(42 2)/4=11or10 This means that armature can be wound only if we use 21 coils and21 segments. The extra coil or dummy coil is put in the slot. One endof this coil is taped and the other end connected to the unusedcommutator segment (segment 22) for the sake of appearance. Sinceonly 21 segments are required, the two (21 and 22segments) areconnected together and considered as one.
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DCMotors The greatest advantage of DC motors may be speed control. Since speed is
directly proportional to armature voltage and inversely proportional to the
magnetic flux produced by the poles, adjusting the armature voltage and/or
the field current will change the rotor speed.
Today, adjustable frequency drives can provide precise speed control for AC
motors, but they do so at the expense of power quality, as the solid-state
switching devices in the drives produce a rich harmonic spectrum. The DC
motor has no adverse effects on power quality.
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Disadvantages Power supply, initial cost, and maintenance requirements are the negatives
associated with DC motors
Rectification must be provided for any DC motors supplied from the grid. It can
also cause power quality problems.
The construction of a DC motor is considerably more complicated and expensive
than that of an AC motor, primarily due to the commutator, brushes, and armature
windings.
An induction motor requires no commutator or brushes, and most use cast
squirrel-cage rotor bars instead of true windings --------two huge simplifications.
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Orthogonality ofMagneticFieldsinDCMotors
Torqueproducedbythearmatureandmmf producedbythefieldwindingareorthogonal.
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Torqueindcmotors
The average force acting on the rotor conductor , isexpressed as
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Torqueequation
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Torqueequation
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Majortypesofdcmotors
Series dc motor Shunt dc motor Compound dc motor Separately excited dc motor Permanent magnet dc motor
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Shuntmotors Shunt motors use highresistance field windings connected in
parallel with the armature.
Varying the field resistance changes the motor speed.
Shunt motors are prone to armature reaction, a distortion and
weakening of the flux generated by the poles that results in
commutation problems evidenced by sparking at the brushes.
Installing additional poles, called interpoles, on the stator
between the main poles wired in series with the armature
reduces armature reaction.
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Shuntmotor
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TvsIa characteristicsofashuntmotorwithARandwithoutAR
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ShuntmotorWhen the load on the motor increased,, the following
sequence of events takes place
Ia increases to satisfy the demand of load Hence the IaRa drop increases For fixed Va, Eb
decreases
Since flux is constant, if AR is neglected, the decreasein Eb accompanied by a decrease in speed as shown in
Fig. (curve d)
considering the effect of armature reaction results indecrease in flux in the motor, which in turn causes
increase in speed (curve c)
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TorqueandPowerDevelopedasafunctionofSpeedofaTypicalShuntMotor
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Example
A240Vshuntmotortakesacurrentof3.5Aonnoload.Thearmaturecircuitresistanceis0.5andshuntfieldwindingresistanceis160.Whenthemotoroperatesatratedloadat2400rpm,ittakes24A.Determine
(a)Itsefficiencyatfullload(b)Torquedevelopedandtheusefultorque(c)Thenoloadspeed(d)Percentspeedregulation
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Solution
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Solution.
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Seriesmotors
Series motors connect the field windings in
series with the armature.
Series motors lack of good speed
regulation, but are well-suited for high
torque loads like power tools and
automobile starters because of their high
torque production and compact size.
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Seriesmotor.
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Torquedeveloped,powerdevelopedandSpeedCharacteristicsofaSeriesMotorasaFunctionofArmatureCurrent
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TorqueandPowerCharacteristicsofaSeriesMotorasaFunctionofSpeed
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Example2A220V,7hp seriesmotorismechanicallycoupledtoafananddraws25A
andrunsat300rpmwhenconnectedtoa220Vsupplywithnoexternal
resistanceconnectedtothearmaturecircuit.Thetorquerequiredbythefan
isproportionaltothesquareofthespeedandRa=0.6andRse=0.4.
Neglectarmaturereactionandrotationalloss.1. Determinethepowerdeliveredtothefanandthetorquedevelopedby
themachine2. Thespeedistobereducedto200rpmbyinsertingaresistanceinthe
armaturecircuit.Determinethevalueofthisresistanceandthe powerdeliveredtothefan.
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Solution
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Compoundmotors
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Compoundmotors
The (increase/decrease) in theflux with an increase in thecurrent causes the torque to(increase/decrease) at a fasterrate in a (cumulative/differential )compound motor than in a shuntmotor.
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SpeedCurrentCharacteristicsofShunt,CumulativeCompoundandDifferentialCompoundMotor