DC Machines MCQs 2026

Back EMF in a DC motor is

1. A supply voltage always ✓
2. B brush voltage drop only ✓
3. C capacitor charging voltage ✓
4. D voltage induced by armature conductors cutting flux ✓

Motor speed N is proportional to

1. A armature current only ✓
2. B phi squared only ✓
3. C back EMF Eb divided by flux phi ✓
4. D independent of Eb ✓

Motor torque is proportional to

1. A Eb only ✓
2. B speed only ✓
3. C 1/phi always ✓
4. D flux phi times armature current Ia ✓

In a DC shunt motor, field winding is

1. A in series with armature only ✓
2. B absent always ✓
3. C in parallel with armature across supply ✓
4. D connected to rotor only ✓

In a DC series motor, field winding carries

1. A only field current separate ✓
2. B zero current ✓
3. C capacitor current only ✓
4. D armature current (series connection) ✓

DC compound motor has

1. A only permanent magnets always ✓
2. B no field winding ✓
3. C only AC field ✓
4. D both shunt and series field windings ✓

Commutation in DC machine is

1. A reversal of current in coil as it passes brush axis ✓
2. B speed control method ✓
3. C field excitation only ✓
4. D starting resistor only ✓

Armature reaction in DC machine

1. A increases speed only ✓
2. B eliminates back EMF ✓
3. C distorts and shifts main field axis ✓
4. D removes need for brushes ✓

Interpoles are used to

1. A improve commutation and reduce sparking ✓
2. B increase flux only ✓
3. C measure speed ✓
4. D replace armature ✓

Compensating windings oppose

1. A back EMF entirely ✓
2. B supply voltage ✓
3. C field resistance ✓
4. D armature reaction MMF under poles ✓

DC generator converts

1. A electrical to mechanical always ✓
2. B heat to light only ✓
3. C chemical to magnetic only ✓
4. D mechanical energy to electrical via electromagnetic induction ✓

Separately excited generator has

1. A field supplied from external source ✓
2. B field from armature only always ✓
3. C no field ✓
4. D permanent magnet only always ✓

Self-excited generator builds voltage when

1. A residual flux and field connection provide positive feedback ✓
2. B armature is open only ✓
3. C speed is zero ✓
4. D load is infinite ✓

Critical resistance in DC generator is

1. A armature resistance only ✓
2. B load resistance minimum ✓
3. C brush contact always zero ✓
4. D maximum field circuit resistance for self-excitation ✓

Efficiency of DC machine is

1. A output power divided by input power times 100 percent ✓
2. B input over output ✓
3. C losses over output ✓
4. D Eb over V only ✓

Copper losses in DC machine are

1. A I squared R in windings ✓
2. B core hysteresis only ✓
3. C friction and windage only ✓
4. D stray load only ✓

Iron (core) losses consist of

1. A I squared R armature only ✓
2. B hysteresis and eddy current losses ✓
3. C brush friction only ✓
4. D field copper only ✓

Speed control of DC shunt motor by field flux weakening

1. A decreases speed always ✓
2. B stops motor instantly ✓
3. C increases speed above rated ✓
4. D reverses rotation always ✓

Armature voltage control of DC motor

1. A varies speed below rated by changing V ✓
2. B increases flux only ✓
3. C controls only field ✓
4. D eliminates back EMF ✓

Ward-Leonard system provides

1. A only AC starting ✓
2. B smooth wide-range DC motor speed control ✓
3. C only field reversal without speed change ✓
4. D only generator protection ✓

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

1. A 40 V drop ✓
2. B 440 V ✓
3. C -40 V ✓
4. D 40 V ✓

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

1. A 40 V ✓
2. B 40 V drop ✓
3. C 400 V ✓
4. D -40 V ✓

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

1. A 20 V ✓
2. B 20 V drop ✓
3. C 440 V ✓
4. D -20 V ✓

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

1. A 20 V drop ✓
2. B 200 V ✓
3. C 20 V ✓
4. D -20 V ✓

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

1. A 40 V drop ✓
2. B 840 V ✓
3. C 40 V ✓
4. D -40 V ✓

Input 1000 W, output 800 W: efficiency is

1. A 80% ✓
2. B 20% ✓
3. C 160% ✓
4. D 40% ✓

Input 500 W, output 400 W: efficiency is

1. A 20% ✓
2. B 160% ✓
3. C 40% ✓
4. D 80% ✓

Input 2000 W, output 1700 W: efficiency is

1. A 15% ✓
2. B 170% ✓
3. C 85% ✓
4. D 42.5% ✓

Input 750 W, output 600 W: efficiency is

1. A 20% ✓
2. B 80% ✓
3. C 160% ✓
4. D 40% ✓

Input 100 W, output 70 W: efficiency is

1. A 30% ✓
2. B 70% ✓
3. C 140% ✓
4. D 35% ✓

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

1. A 850 W W ✓
2. B 1150 W ✓
3. C 150 W ✓
4. D NaN W ✓

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

1. A 575 W ✓
2. B 75 W ✓
3. C NaN W ✓
4. D 425 W W ✓

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

1. A 2300 W ✓
2. B 300 W ✓
3. C 1700 W W ✓
4. D NaN W ✓

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

1. A 680 W W ✓
2. B 920 W ✓
3. C 120 W ✓
4. D NaN W ✓

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

1. A 460 W ✓
2. B 60 W ✓
3. C 340 W W ✓
4. D NaN W ✓

Brush and commutator function is to

1. A increase frequency ✓
2. B convert internal AC in armature to external DC ✓
3. C measure flux only ✓
4. D eliminate back EMF ✓

Lap winding in DC machine has

1. A always one path ✓
2. B no commutator ✓
3. C parallel paths equal to number of poles (typical) ✓
4. D only AC output ✓

Wave winding in DC machine has

1. A paths equal to poles always ✓
2. B zero paths ✓
3. C only shunt field ✓
4. D two parallel paths typically ✓

Cumulative compound motor has

1. A series flux opposing shunt ✓
2. B series flux aiding shunt flux ✓
3. C no shunt field ✓
4. D only AC excitation ✓

Differential compound motor has

1. A series aiding shunt ✓
2. B only series field ✓
3. C only permanent magnet ✓
4. D series flux opposing shunt flux ✓

No-load speed of DC motor is high when

1. A Eb is zero ✓
2. B back EMF nearly equals supply (light load, low Ia drop) ✓
3. C load is maximum ✓
4. D field is shorted ✓

Starting resistor for DC motor is used because

1. A increases starting torque only by raising flux ✓
2. B eliminates commutation ✓
3. C limits starting current while Eb is low ✓
4. D increases field current always ✓

Regenerative braking occurs when

1. A motor draws more current always ✓
2. B motor operates as generator returning power to supply ✓
3. C field is removed ✓
4. D speed is zero only ✓

Plugging (reverse voltage braking)

1. A only reduces field ✓
2. B only adds resistance ✓
3. C disconnects armature ✓
4. D reverses supply or field to oppose rotation ✓

Demagnetizing component of armature reaction

1. A strengthens field always ✓
2. B weakens main field under one pole pair ✓
3. C affects speed only ✓
4. D eliminates torque ✓

Stray load losses in DC machine include

1. A only I²R copper ✓
2. B only friction ✓
3. C only hysteresis at zero load ✓
4. D brush contact and eddy effects under load ✓

Field diverter in DC series motor

1. A increases starting current only ✓
2. B shorts armature ✓
3. C measures Eb only ✓
4. D reduces series field current for speed control ✓

Permanent magnet DC motor has

1. A variable shunt field always ✓
2. B fixed flux from magnets (no field winding) ✓
3. C series field only ✓
4. D no armature ✓

Back EMF constant relates Eb to

1. A only armature resistance ✓
2. B only brush drop ✓
3. C speed and flux ✓
4. D only load torque ✓

Torque constant relates T to

1. A only speed ✓
2. B only Eb ✓
3. C only field resistance ✓
4. D flux and armature current ✓

DC motor mechanical output is

1. A V times Ia always ✓
2. B Eb times Ia (approx developed power) ✓
3. C Ia squared R only ✓
4. D field copper loss ✓

Swamping resistance in generator

1. A starts motor only ✓
2. B measures speed ✓
3. C makes parallel field diverter for voltage regulation ✓
4. D replaces commutator ✓

Equalizer connection in lap winding

1. A increases field flux ✓
2. B balances parallel path currents ✓
3. C reduces Eb to zero ✓
4. D eliminates armature ✓

Commutator pitch relates to

1. A coil span and number of commutator segments ✓
2. B only field poles count ✓
3. C only supply frequency ✓
4. D only AC phase ✓

Sparkless commutation requires

1. A current in coil near zero at brush short-circuit ✓
2. B maximum Ia always ✓
3. C zero field flux ✓
4. D open circuited armature ✓

Three-point starter protects DC shunt motor from

1. A low field (high speed runaway) and overload ✓
2. B only reverse rotation always ✓
3. C only low voltage starting ✓
4. D AC supply only ✓

Four-point starter separates

1. A armature from supply ✓
2. B commutator from brushes ✓
3. C Eb from speed ✓
4. D field circuit from OLR so field is not affected by OLR drop ✓

Motor characteristic curve plots

1. A flux vs voltage only ✓
2. B PF vs frequency ✓
3. C B vs H only ✓
4. D speed vs torque or speed vs current ✓

Generator external characteristic plots

1. A speed vs torque only ✓
2. B B vs H only ✓
3. C XL vs f only ✓
4. D terminal voltage vs load current ✓

Voltage regulation of generator is

1. A Ia times Ra only ✓
2. B Eb only ✓
3. C (V no-load minus V full-load)/V rated times 100% ✓
4. D speed times flux only ✓

Reason for armature resistance drop is

1. A Eb increases with load always ✓
2. B field flux zero ✓
3. C Ia times Ra reduces terminal voltage in generator ✓
4. D speed zero ✓

Motor armature equation V equals

1. A Eb minus Ia Ra only always ✓
2. B Eb plus Ia Ra plus brush drop ✓
3. C Ia Ra only ✓
4. D Eb only ✓

If load on DC shunt motor increases, armature current

1. A decreases always ✓
2. B increases and speed drops slightly ✓
3. C stays exactly constant always ✓
4. D becomes zero ✓

If field flux of shunt motor decreases, speed

1. A decreases always ✓
2. B increases ✓
3. C unchanged always ✓
4. D becomes zero instantly ✓

Series motor should not be run on no-load because

1. A Eb becomes infinite always ✓
2. B field is too strong ✓
3. C torque is maximum at no load ✓
4. D speed may dangerously increase (flux proportional to Ia) ✓

DC machine armature is made of

1. A solid copper block ✓
2. B laminated silicon steel to reduce eddy losses ✓
3. C plastic only ✓
4. D air gap only ✓

Number of parallel paths A affects

1. A armature current per path and conductor sizing ✓
2. B only field turns ✓
3. C only brush material ✓
4. D only supply frequency ✓

Hysteresis loss in core depends on

1. A armature I only ✓
2. B brush voltage only ✓
3. C material, flux density, and frequency of reversal ✓
4. D external load resistance only ✓

Windage and friction losses depend on

1. A field current only ✓
2. B speed primarily ✓
3. C load current only at standstill ✓
4. D supply PF only ✓

Brush contact drop is typically assumed

1. A proportional to square of speed only ✓
2. B zero always ✓
3. C nearly constant (~2 V total for carbon brushes pair) ✓
4. D equal to Eb ✓

Reversal of rotation of DC motor can be by

1. A reversing armature OR field (not both together for same direction change twice) ✓
2. B shorting armature only ✓
3. C opening field only ✓
4. D increasing Ra only ✓

Maximum efficiency of DC machine often occurs when

1. A copper loss zero ✓
2. B iron loss zero ✓
3. C variable loss equals constant loss approximately ✓
4. D speed zero ✓

Armature reaction shift causes

1. A brush position adjustment (neutral axis shift) ✓
2. B no effect on commutation ✓
3. C infinite speed ✓
4. D zero Eb always ✓

Main poles produce

1. A only armature copper ✓
2. B primary working flux in air gap ✓
3. C only brush friction ✓
4. D only reactive power ✓

Yoke in DC machine provides

1. A mechanical support and part of magnetic flux path ✓
2. B only electrical insulation ✓
3. C only commutation ✓
4. D only cooling air path only without structure ✓

Cooling methods for DC machines include

1. A only vacuum ✓
2. B only heating ✓
3. C natural air, forced air, and liquid cooling ✓
4. D no cooling needed ever ✓

Rating of DC machine specifies

1. A only color of paint ✓
2. B output power, voltage, speed (and current) ✓
3. C only brush count without power ✓
4. D only field resistance ✓

Eb in generator on open circuit equals

1. A zero always ✓
2. B Ia Ra only ✓
3. C terminal voltage (no Ia Ra drop) ✓
4. D twice supply V ✓

Load characteristic of motor shows

1. A speed always increases with load ✓
2. B torque independent of Ia ✓
3. C Eb equals zero at load ✓
4. D speed decreasing as torque (load) increases for shunt type ✓

Over-compounded generator has

1. A falling voltage always ✓
2. B rising terminal voltage with load due to series field ✓
3. C zero regulation ✓
4. D AC output only ✓