Dccc Moter Question

8/12/2019 Dccc Moter Question

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lectrical Question (6 Wh armature teeth of !" machine is ske#ed ?

5he air gap length changes bet#een the slot and teeth of the armature #hen the armature is

rotating under constant magnetic field. 5his results in the production of small variation in the

torue hence this results can be observed as small vibrations in the frame of the machine. 7o inorder to avoid this vibration effect armature teeth is ske#ed.

lectrical 8ntervie# Questions With Ans#ers7ite contains electrical intervie# uestions #ith ans#ers related to dc machines, electrical

transformers, po#er sstem, po#er plant, intervie# uestions and intervie# tips...

6 What is meant b regenerative braking?

Ans#er

6 When the suppl is cut off for a running motor, it still continue running due to inertia. 8n order

to stop it uickl #e place a load resistor4 across thearmature #inding and the motor should have

maintained continuous field suppl.so that back e.m.f voltage is made to appl across the resistor

and due to loadthe motor stops uickl. 5his tpe of breaking is called as

9:egenerativeBreaking9.

lectrical intervie# uestion

6 Wh is the starting current high in a !" motor?

Ans#er

6 8n !" motors, ;oltage euation is ;otor,8a

< Armature current,:a < Armature resistance4.At starting,b is ero. 5herefore, ;


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lectrical Question -6 xplain Armature reaction in !" machine ?

Ans#er6Armature reaction is the effect of the magnetic field set up b the armature current on the

distribution of the flux under the main poles of the !" machine.mf #ill be induced in the armature of the dc machine b the 0leming1s electromagnetic

induction principle "onductor and magnetic field reuired to produce emf4. As the conductors of

the armature are shorted current starts flo#ing and this current #ill produce a counter magnetic

filed #hich opposes the main field. 5his effect of armature flux on the main flux so as to distort

or #eaken the main field flux is called armature reaction.

lectrical Question 26 What is the effects of the armature reaction in a dc machine?

Ans#er6

!ue the armature reaction the armature flux #ill have t#o impacts on the main field flux

8t partl #eakens or distorts the main field flux

8t cross magnetises or distorts the main field flux

lectrical 8ntervie# Questions With Ans#ers7ite contains electrical intervie# uestions and ans#ers. @bDective Questions and Ans#ers.

5opics includes !" machines, 8nduction machines, Eo#er sstems, control sstems, po#er

electronics,po#er plant, substation,microprocessors,electrical drives...

lectrical Question 36 xplain the methods to reduce the armature reaction in !" machine ?Ans#er6

Armature reaction causes the distortion in the main field flux. 5his can be reduced if the

reluctance of the path of cross magnetiing filed is increased.5he armature teeth and air gap at the

pole tips offer high reluctance to armature flux. 5hereb increasing the air gap armature reaction

can be reduced.

B special arrangements such that leading and trailing pole tips of the poles are alternativel

omitted.

B providing compensating #inding. "ompensating #inding #ill be in series #ith the armature#inding

B providing 8nter poles #hich are placed at the geometrical neutral axis at the mid #a bet#een

the main poles.

lectrical Question %6 Wh brushes of a !" machine is slightl shifted from the neutral axis b

small angle?

Ans#er6!uring no load Feometrical neutral pole F*E4 coincides #ith the magnetic neutral pole>*E4.

5his poles are reuired to place the brushes in neutral poles to collect the current from the

armature #ith out the sparking effect. When the machine is loaded the >agnetic neutral pole

shifts slightl b small angle due to armature reaction. 7o brushes of the dc machine is placed bslightl shifting from the neutral axis b an angle.

lectrical Question &6 xplain "ommutation in !" machine ?Ans#er6

"ommutator and the brushes in a dc machine are plas a ver important role6

8t receives electrical current from the rotating armature and deliver it to the static external load in

dc generator and vice verse in case of dc motor the collect the current from the terminals and

deliver to the rotating armature.

5he converts generated alternative current in the armature to dc current Bi directional variable

in to a uni directional variable4 to deliver to the load and vise verse in the case of motor

operation.
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5he first t#o methods involve the adDustments to be done on the armature side of the dc machine

#hereas the third method reuires adDustment in the field side of the dc machine. 5herefore speed

control of the dc machine are classified as armature control method and field control method. 8n

armature control method the speed of the dc machine decreases as the appling voltage #ill be

reduced on the other hand in field control method #eakens the flux per pole of the dc machineand speeds above the rated speeds are attained

Armature "ontrol method of 7peed "ontrol68n armature control method voltage applied to the armature terminals is varied #ith out changing

the field current.

7peed of the dc shunt motor b armature control can be obtained b6

B connecting an variable external resistance in series to the armature circuit so that voltage

applied to the dc machine can be varied

B connecting an variable resistance in shunt to the armature circuit and other variable resistance

in series to the armature shunt circuit.

B providing the varing voltage to the armature terminals. 5he voltage provided should be

separate from the voltage that provide to the field circuit

i4 Armature :esistance "ontrol >ethod6

8n armature resistance control method a variable resistance is connected in series to the armature

circuit. When the resistance of the rheostat is increased, current flo#ing through the circuitreduces and the voltage drop at the armature is less compared to line voltage. 5hus speed of the

machine reduces in proportional to the applied voltage.

5his method is emploed onl in applications #here speed variation is reuired for shorter

duration and not continuous. 5his is because of the larger resistance reuired and high po#er lossduring the operation at the rheostat. x6 printing machines, cranes, hoists

ii4 7hunt :esistance "ontrol >ethod6

Armature resistance method is simple and eas but have disadvantages as the speed not onl

depends on the resistance value but also on the load current variation in the load during the drive

operation results in the variation in the speed of the machine4. 5herefore a divertor shuntresistance4 is emploed along #ith the series resistance.

8n this method the change in the armature current because of the load torue4 does not affect the

change in the voltage across the terminals hence the speed.

iii4. Armature ;oltage "ontrol6

8n this method of speed control a variable source of voltage is provided to suppl the po#er to the

armature circuit. ;oltage to the field circuit should be different from the variable voltage sourceprovided to the armature.

B this method disadvantages such as poor voltage regulation and lo# efficienc in case of

voltage resistance method is avoided, ho#ever this method is costlier compared to the other. 5he

adDustable voltage source can be from a adDustable voltage generator or from a adDustable rectifier0ield "ontrol >ethod of speed control6

8n !" machines #eakening of the field of the dc machine increases the speed of the dc machineand strengthening of the field #ill cause to reduction in the speed of the machine. /ence b

#eakening the field of the machine speed above the rated speed can be attained.

7peeds above the rated speed can be attained b6

Eroviding a variable resistance in series to the field circuit so that variation in the resistance value

results in increase or decrease in the field of the machine resulting in the speed control.

B variation in the reluctance of the magnetic circuit of the motor.

B varing the applied voltage to the field circuit of the dc motor b keeping the voltage supplied

to the armature circuit constant


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i4 0ield :esistance "ontrol6

8n this method variable resistance is emploed in series to the field circuit. As in this method

#hen resistance value increases onl field #eakens. B #eakening the field speed above the

normal speed can be attained

ii4 :eluctance "ontrol >ethod65his method reuires motor to be constructed #ith special mechanical features such that

reluctance of the magnetic circuit can be changed. 5his is more expensive and rarel emploediii4 0iled ;oltage "ontrol6

5his method reuires variable voltage source #hich is different from the voltage applied to the

armature circuit.

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Dccc Moter Question