DC Machines MCQs 2026

Three-point starter protects DC shunt motor from

1. Alow field (high speed runaway) and overload✓
2. Bonly reverse rotation always✓
3. Conly low voltage starting✓
4. DAC supply only✓

Sparkless commutation requires

1. Acurrent in coil near zero at brush short-circuit✓
2. Bmaximum Ia always✓
3. Czero field flux✓
4. Dopen circuited armature✓

Commutator pitch relates to

1. Acoil span and number of commutator segments✓
2. Bonly field poles count✓
3. Conly supply frequency✓
4. Donly AC phase✓

Equalizer connection in lap winding

1. Aincreases field flux✓
2. Bbalances parallel path currents✓
3. Creduces Eb to zero✓
4. Deliminates armature✓

Swamping resistance in generator

1. Astarts motor only✓
2. Bmeasures speed✓
3. Cmakes parallel field diverter for voltage regulation✓
4. Dreplaces commutator✓

DC motor mechanical output is

1. AV times Ia always✓
2. BEb times Ia (approx developed power)✓
3. CIa squared R only✓
4. Dfield copper loss✓

Torque constant relates T to

1. Aonly speed✓
2. Bonly Eb✓
3. Conly field resistance✓
4. Dflux and armature current✓

Back EMF constant relates Eb to

1. Aonly armature resistance✓
2. Bonly brush drop✓
3. Cspeed and flux✓
4. Donly load torque✓

Permanent magnet DC motor has

1. Avariable shunt field always✓
2. Bfixed flux from magnets (no field winding)✓
3. Cseries field only✓
4. Dno armature✓

Field diverter in DC series motor

1. Aincreases starting current only✓
2. Bshorts armature✓
3. Cmeasures Eb only✓
4. Dreduces series field current for speed control✓

Demagnetizing component of armature reaction

1. Astrengthens field always✓
2. Bweakens main field under one pole pair✓
3. Caffects speed only✓
4. Deliminates torque✓

Stray load losses in DC machine include

1. Aonly I²R copper✓
2. Bonly friction✓
3. Conly hysteresis at zero load✓
4. Dbrush contact and eddy effects under load✓

Plugging (reverse voltage braking)

1. Aonly reduces field✓
2. Bonly adds resistance✓
3. Cdisconnects armature✓
4. Dreverses supply or field to oppose rotation✓

Regenerative braking occurs when

1. Amotor draws more current always✓
2. Bmotor operates as generator returning power to supply✓
3. Cfield is removed✓
4. Dspeed is zero only✓

Starting resistor for DC motor is used because

1. Aincreases starting torque only by raising flux✓
2. Beliminates commutation✓
3. Climits starting current while Eb is low✓
4. Dincreases field current always✓

No-load speed of DC motor is high when

1. AEb is zero✓
2. Bback EMF nearly equals supply (light load, low Ia drop)✓
3. Cload is maximum✓
4. Dfield is shorted✓

Iron (core) losses consist of

1. AI squared R armature only✓
2. Bhysteresis and eddy current losses✓
3. Cbrush friction only✓
4. Dfield copper only✓

Copper losses in DC machine are

1. AI squared R in windings✓
2. Bcore hysteresis only✓
3. Cfriction and windage only✓
4. Dstray load only✓

Efficiency of DC machine is

1. Aoutput power divided by input power times 100 percent✓
2. Binput over output✓
3. Closses over output✓
4. DEb over V only✓

Critical resistance in DC generator is

1. Aarmature resistance only✓
2. Bload resistance minimum✓
3. Cbrush contact always zero✓
4. Dmaximum field circuit resistance for self-excitation✓

Self-excited generator builds voltage when

1. Aresidual flux and field connection provide positive feedback✓
2. Barmature is open only✓
3. Cspeed is zero✓
4. Dload is infinite✓

Separately excited generator has

1. Afield supplied from external source✓
2. Bfield from armature only always✓
3. Cno field✓
4. Dpermanent magnet only always✓

DC generator converts

1. Aelectrical to mechanical always✓
2. Bheat to light only✓
3. Cchemical to magnetic only✓
4. Dmechanical energy to electrical via electromagnetic induction✓

Compensating windings oppose

1. Aback EMF entirely✓
2. Bsupply voltage✓
3. Cfield resistance✓
4. Darmature reaction MMF under poles✓

Interpoles are used to

1. Aimprove commutation and reduce sparking✓
2. Bincrease flux only✓
3. Cmeasure speed✓
4. Dreplace armature✓

Armature reaction in DC machine

1. Aincreases speed only✓
2. Beliminates back EMF✓
3. Cdistorts and shifts main field axis✓
4. Dremoves need for brushes✓

Commutation in DC machine is

1. Areversal of current in coil as it passes brush axis✓
2. Bspeed control method✓
3. Cfield excitation only✓
4. Dstarting resistor only✓

DC compound motor has

1. Aonly permanent magnets always✓
2. Bno field winding✓
3. Conly AC field✓
4. Dboth shunt and series field windings✓

In a DC series motor, field winding carries

1. Aonly field current separate✓
2. Bzero current✓
3. Ccapacitor current only✓
4. Darmature current (series connection)✓

In a DC shunt motor, field winding is

1. Ain series with armature only✓
2. Babsent always✓
3. Cin parallel with armature across supply✓
4. Dconnected to rotor only✓

Motor torque is proportional to

1. AEb only✓
2. Bspeed only✓
3. C1/phi always✓
4. Dflux phi times armature current Ia✓

Motor speed N is proportional to

1. Aarmature current only✓
2. Bphi squared only✓
3. Cback EMF Eb divided by flux phi✓
4. Dindependent of Eb✓

Back EMF in a DC motor is

1. Asupply voltage always✓
2. Bbrush voltage drop only✓
3. Ccapacitor charging voltage✓
4. Dvoltage induced by armature conductors cutting flux✓

Back EMF in a DC motor is

1. Asupply voltage always✓
2. Bbrush voltage drop only✓
3. Ccapacitor charging voltage✓
4. Dvoltage induced by armature conductors cutting flux✓

Motor speed N is proportional to

1. Aarmature current only✓
2. Bphi squared only✓
3. Cback EMF Eb divided by flux phi✓
4. Dindependent of Eb✓

Motor torque is proportional to

1. AEb only✓
2. Bspeed only✓
3. C1/phi always✓
4. Dflux phi times armature current Ia✓

In a DC shunt motor, field winding is

1. Ain series with armature only✓
2. Babsent always✓
3. Cin parallel with armature across supply✓
4. Dconnected to rotor only✓

In a DC series motor, field winding carries

1. Aonly field current separate✓
2. Bzero current✓
3. Ccapacitor current only✓
4. Darmature current (series connection)✓

DC compound motor has

1. Aonly permanent magnets always✓
2. Bno field winding✓
3. Conly AC field✓
4. Dboth shunt and series field windings✓

Commutation in DC machine is

1. Areversal of current in coil as it passes brush axis✓
2. Bspeed control method✓
3. Cfield excitation only✓
4. Dstarting resistor only✓

Armature reaction in DC machine

1. Aincreases speed only✓
2. Beliminates back EMF✓
3. Cdistorts and shifts main field axis✓
4. Dremoves need for brushes✓

Interpoles are used to

1. Aimprove commutation and reduce sparking✓
2. Bincrease flux only✓
3. Cmeasure speed✓
4. Dreplace armature✓

Compensating windings oppose

1. Aback EMF entirely✓
2. Bsupply voltage✓
3. Cfield resistance✓
4. Darmature reaction MMF under poles✓

Iron (core) losses consist of

1. AI squared R armature only✓
2. Bhysteresis and eddy current losses✓
3. Cbrush friction only✓
4. Dfield copper only✓

Copper losses in DC machine are

1. AI squared R in windings✓
2. Bcore hysteresis only✓
3. Cfriction and windage only✓
4. Dstray load only✓

Permanent magnet DC motor has

1. Avariable shunt field always✓
2. Bfixed flux from magnets (no field winding)✓
3. Cseries field only✓
4. Dno armature✓

Back EMF constant relates Eb to

1. Aonly armature resistance✓
2. Bonly brush drop✓
3. Cspeed and flux✓
4. Donly load torque✓

Torque constant relates T to

1. Aonly speed✓
2. Bonly Eb✓
3. Conly field resistance✓
4. Dflux and armature current✓

DC motor mechanical output is

1. AV times Ia always✓
2. BEb times Ia (approx developed power)✓
3. CIa squared R only✓
4. Dfield copper loss✓

Swamping resistance in generator

1. Astarts motor only✓
2. Bmeasures speed✓
3. Cmakes parallel field diverter for voltage regulation✓
4. Dreplaces commutator✓

Equalizer connection in lap winding

1. Aincreases field flux✓
2. Bbalances parallel path currents✓
3. Creduces Eb to zero✓
4. Deliminates armature✓

Commutator pitch relates to

1. Acoil span and number of commutator segments✓
2. Bonly field poles count✓
3. Conly supply frequency✓
4. Donly AC phase✓

Sparkless commutation requires

1. Acurrent in coil near zero at brush short-circuit✓
2. Bmaximum Ia always✓
3. Czero field flux✓
4. Dopen circuited armature✓

Three-point starter protects DC shunt motor from

1. Alow field (high speed runaway) and overload✓
2. Bonly reverse rotation always✓
3. Conly low voltage starting✓
4. DAC supply only✓

Four-point starter separates

1. Aarmature from supply✓
2. Bcommutator from brushes✓
3. CEb from speed✓
4. Dfield circuit from OLR so field is not affected by OLR drop✓

Motor characteristic curve plots

1. Aflux vs voltage only✓
2. BPF vs frequency✓
3. CB vs H only✓
4. Dspeed vs torque or speed vs current✓

Generator external characteristic plots

1. Aspeed vs torque only✓
2. BB vs H only✓
3. CXL vs f only✓
4. Dterminal voltage vs load current✓

Voltage regulation of generator is

1. AIa times Ra only✓
2. BEb only✓
3. C(V no-load minus V full-load)/V rated times 100%✓
4. Dspeed times flux only✓

Reason for armature resistance drop is

1. AEb increases with load always✓
2. Bfield flux zero✓
3. CIa times Ra reduces terminal voltage in generator✓
4. Dspeed zero✓

Motor armature equation V equals

1. AEb minus Ia Ra only always✓
2. BEb plus Ia Ra plus brush drop✓
3. CIa Ra only✓
4. DEb only✓

If load on DC shunt motor increases, armature current

1. Adecreases always✓
2. Bincreases and speed drops slightly✓
3. Cstays exactly constant always✓
4. Dbecomes zero✓

DC motor V=440 V, Eb=400 V: armature voltage equation net driving (V-Eb) is

1. A40 V drop✓
2. B840 V✓
3. C40 V✓
4. D-40 V✓

Input 1000 W, output 800 W: efficiency is

1. A80%✓
2. B20%✓
3. C160%✓
4. D40%✓

Input 500 W, output 400 W: efficiency is

1. A20%✓
2. B160%✓
3. C40%✓
4. D80%✓

Input 2000 W, output 1700 W: efficiency is

1. A15%✓
2. B170%✓
3. C85%✓
4. D42.5%✓

Input 750 W, output 600 W: efficiency is

1. A20%✓
2. B80%✓
3. C160%✓
4. D40%✓

Input 100 W, output 70 W: efficiency is

1. A30%✓
2. B70%✓
3. C140%✓
4. D35%✓

Input 1000 W, total losses 150 W: output power is

1. A850 W W✓
2. B1150 W✓
3. C150 W✓
4. DNaN W✓

Input 500 W, total losses 75 W: output power is

1. A575 W✓
2. B75 W✓
3. CNaN W✓
4. D425 W W✓

Input 2000 W, total losses 300 W: output power is

1. A2300 W✓
2. B300 W✓
3. C1700 W W✓
4. DNaN W✓

Input 800 W, total losses 120 W: output power is

1. A680 W W✓
2. B920 W✓
3. C120 W✓
4. DNaN W✓

Input 400 W, total losses 60 W: output power is

1. A460 W✓
2. B60 W✓
3. C340 W W✓
4. DNaN W✓

Brush and commutator function is to

1. Aincrease frequency✓
2. Bconvert internal AC in armature to external DC✓
3. Cmeasure flux only✓
4. Deliminate back EMF✓

Lap winding in DC machine has

1. Aalways one path✓
2. Bno commutator✓
3. Cparallel paths equal to number of poles (typical)✓
4. Donly AC output✓

Wave winding in DC machine has

1. Apaths equal to poles always✓
2. Bzero paths✓
3. Conly shunt field✓
4. Dtwo parallel paths typically✓

Cumulative compound motor has

1. Aseries flux opposing shunt✓
2. Bseries flux aiding shunt flux✓
3. Cno shunt field✓
4. Donly AC excitation✓

Differential compound motor has

1. Aseries aiding shunt✓
2. Bonly series field✓
3. Conly permanent magnet✓
4. Dseries flux opposing shunt flux✓

Brush contact drop is typically assumed

1. Aproportional to square of speed only✓
2. Bzero always✓
3. Cnearly constant (~2 V total for carbon brushes pair)✓
4. Dequal to Eb✓

Reversal of rotation of DC motor can be by

1. Areversing armature OR field (not both together for same direction change twice)✓
2. Bshorting armature only✓
3. Copening field only✓
4. Dincreasing Ra only✓

Maximum efficiency of DC machine often occurs when

1. Acopper loss zero✓
2. Biron loss zero✓
3. Cvariable loss equals constant loss approximately✓
4. Dspeed zero✓

Armature reaction shift causes

1. Abrush position adjustment (neutral axis shift)✓
2. Bno effect on commutation✓
3. Cinfinite speed✓
4. Dzero Eb always✓

Main poles produce

1. Aonly armature copper✓
2. Bprimary working flux in air gap✓
3. Conly brush friction✓
4. Donly reactive power✓

Yoke in DC machine provides

1. Amechanical support and part of magnetic flux path✓
2. Bonly electrical insulation✓
3. Conly commutation✓
4. Donly cooling air path only without structure✓

Cooling methods for DC machines include

1. Aonly vacuum✓
2. Bonly heating✓
3. Cnatural air, forced air, and liquid cooling✓
4. Dno cooling needed ever✓

Rating of DC machine specifies

1. Aonly color of paint✓
2. Boutput power, voltage, speed (and current)✓
3. Conly brush count without power✓
4. Donly field resistance✓

Eb in generator on open circuit equals

1. Azero always✓
2. BIa Ra only✓
3. Cterminal voltage (no Ia Ra drop)✓
4. Dtwice supply V✓

Load characteristic of motor shows

1. Aspeed always increases with load✓
2. Btorque independent of Ia✓
3. CEb equals zero at load✓
4. Dspeed decreasing as torque (load) increases for shunt type✓

Over-compounded generator has

1. Afalling voltage always✓
2. Brising terminal voltage with load due to series field✓
3. Czero regulation✓
4. DAC output only✓

Speed control of DC shunt motor by field flux weakening

1. Adecreases speed always✓
2. Bstops motor instantly✓
3. Cincreases speed above rated✓
4. Dreverses rotation always✓

Armature voltage control of DC motor

1. Avaries speed below rated by changing V✓
2. Bincreases flux only✓
3. Ccontrols only field✓
4. Deliminates back EMF✓

Ward-Leonard system provides

1. Aonly AC starting✓
2. Bsmooth wide-range DC motor speed control✓
3. Conly field reversal without speed change✓
4. Donly generator protection✓

DC motor V=240 V, Eb=200 V: armature voltage equation net driving (V-Eb) is

1. A40 V drop✓
2. B440 V✓
3. C-40 V✓
4. D40 V✓

DC motor V=220 V, Eb=180 V: armature voltage equation net driving (V-Eb) is

1. A40 V✓
2. B40 V drop✓
3. C400 V✓
4. D-40 V✓

DC motor V=230 V, Eb=210 V: armature voltage equation net driving (V-Eb) is

1. A20 V✓
2. B20 V drop✓
3. C440 V✓
4. D-20 V✓

DC motor V=110 V, Eb=90 V: armature voltage equation net driving (V-Eb) is

1. A20 V drop✓
2. B200 V✓
3. C20 V✓
4. D-20 V✓