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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.

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

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.

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.

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.

Orthogonality ofMagneticFieldsinDCMotors

Torqueproducedbythearmatureandmmf producedbythefieldwindingareorthogonal.

Torqueindcmotors

The average force acting on the rotor conductor , isexpressed as

Torqueequation

Torqueequation

Majortypesofdcmotors

Series dc motor Shunt dc motor Compound dc motor Separately excited dc motor Permanent magnet dc motor

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.

Shuntmotor

TvsIa characteristicsofashuntmotorwithARandwithoutAR

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)

TorqueandPowerDevelopedasafunctionofSpeedofaTypicalShuntMotor

Example

A240Vshuntmotortakesacurrentof3.5Aonnoload.Thearmaturecircuitresistanceis0.5andshuntfieldwindingresistanceis160.Whenthemotoroperatesatratedloadat2400rpm,ittakes24A.Determine

(a)Itsefficiencyatfullload(b)Torquedevelopedandtheusefultorque(c)Thenoloadspeed(d)Percentspeedregulation

Solution

Solution.

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.

Seriesmotor.

Torquedeveloped,powerdevelopedandSpeedCharacteristicsofaSeriesMotorasaFunctionofArmatureCurrent

TorqueandPowerCharacteristicsofaSeriesMotorasaFunctionofSpeed

Example2A220V,7hp seriesmotorismechanicallycoupledtoafananddraws25A

andrunsat300rpmwhenconnectedtoa220Vsupplywithnoexternal

resistanceconnectedtothearmaturecircuit.Thetorquerequiredbythefan

isproportionaltothesquareofthespeedandRa=0.6andRse=0.4.

Neglectarmaturereactionandrotationalloss.1. Determinethepowerdeliveredtothefanandthetorquedevelopedby

themachine2. Thespeedistobereducedto200rpmbyinsertingaresistanceinthe

armaturecircuit.Determinethevalueofthisresistanceandthe powerdeliveredtothefan.

Solution

Compoundmotors

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.

SpeedCurrentCharacteristicsofShunt,CumulativeCompoundandDifferentialCompoundMotor