BCIT ELECTRICAL DC MACHINES LEVEL 3 EXAM –
UPDATED ACTUAL QUESTIONS AND CORRECT ANSWERS
(2026 EDITION)
FQuestion Bank With Correct ANSWER & Rationale For
Every Option
Question 1:
What is the main difference in the construction of a DC generator and a DC motor?
A. Type of winding – Wrong: both machines use identical lap/wave armature windings.
B. Number of poles – Wrong: pole count depends on rating, not on machine type.
C. Armature material – Wrong: both use laminated silicon-steel cores with copper windings.
D. Type of enclosure – Correct: generators and motors are built with different enclosures (drip-
proof/screened vs. open/pipe-ventilated) to suit their service environments.
ANSWER: D. Type of enclosure
Question 2:
The rotating part of a DC generator is called the
A. Yoke – Wrong: the yoke is the stationary outer frame.
B. Field pole – Wrong: poles are bolted to the frame and stay still.
C. Armature – Correct: the armature rotates in the field and carries the induced EMF.
D. Brush assembly – Wrong: brushes are stationary and ride on the commutator.
ANSWER: C. Armature
Question 3:
,What are the three basic types of armature windings?
A. Series, shunt, compound – Wrong: those are field connection types, not armature windings.
B. Lap, wave, frog-leg – Correct: these are the three fundamental closed-drum armature windings.
C. Progressive, retrogressive, duplex – Wrong: those describe winding progression/multiplexity only.
D. Single-layer, double-layer, triple-layer – Wrong: layer arrangement is not a winding type.
ANSWER: B. Lap, wave, frog-leg
Question 4:
Which armature winding is used in high-current, low-voltage applications?
A. Wave – Wrong: wave has only 2 parallel paths, suited to high voltage/low current.
B. Frog-leg – Wrong: used where self-equalizing is wanted, not the standard high-current choice.
C. Lap winding – Correct: lap provides as many parallel paths as poles, dividing current for high-current
service.
D. Duplex wave – Wrong: still few paths compared with lap.
ANSWER: C. Lap winding
Question 5:
What is the electrical function of the commutator?
A. To increase the air-gap flux – Wrong: flux is set by the field poles, not the commutator.
B. Reverse armature connections to maintain unidirectional current in the external circuit – Correct: it
rectifies the AC generated in the armature into DC externally.
C. To eliminate armature reaction – Wrong: that is done by interpoles/compensating windings.
D. To store electrical energy – Wrong: it is a mechanical switch, not a storage device.
ANSWER: B. Reverse armature connections to maintain unidirectional current in the external circuit
Question 6:
Which of the following is a common brush material used in DC machines?
A. Carbon – Correct but incomplete alone.
B. Graphite – Correct but incomplete alone.
C. Metal–graphite (copper–carbon) – Correct but incomplete alone.
D. All of the above – Correct: carbon, graphite and metal–graphite brushes are all standard DC machine
brush materials.
ANSWER: D. All of the above
,Question 7:
Suitable brush pressure is usually between:
A. 0.1 and 0.5 psi – Wrong: too light; brushes chatter and spark.
B. 1 and 2 psi – Correct: 1–2 psi gives good contact without excessive wear.
C. 5 and 8 psi – Wrong: excessive pressure causes rapid brush wear and heating.
D. 10 and 15 psi – Wrong: far too high; would grind brushes and commutator.
ANSWER: B. 1 and 2 psi
Question 8:
What is the function of field poles in a DC machine?
A. To support the brush rigging – Wrong: brushes are mounted separately on the brush yoke.
B. Produce the magnetic field required for operation – Correct: poles carry the field winding that creates
the working flux.
C. To carry the load current – Wrong: load current flows in the armature and brushes.
D. To damp machine vibration – Wrong: that is the job of the frame and foundations.
ANSWER: B. Produce the magnetic field required for operation
Question 9:
The yoke of a DC machine serves to
A. Provide the return path for magnetic flux and give mechanical support – Correct: the yoke completes
the magnetic circuit and carries the core and end shields.
B. Generate the main EMF – Wrong: EMF is generated in the armature conductors.
C. Improve commutation – Wrong: commutation is handled by interpoles and brushes.
D. Cool the bearings – Wrong: bearings are cooled by their own housing/air flow.
ANSWER: A. Provide the return path for magnetic flux and give mechanical support
Question 10:
The yoke of a modern DC machine is usually made of
A. Cast iron – Wrong: poor magnetic permeability; used only on very old machines.
B. Rolled or cast steel – Correct: steel has high permeability and good mechanical strength.
C. Aluminum – Wrong: non-magnetic, cannot carry flux.
D. Laminated silicon steel – Wrong: laminations are needed only where flux is alternating (cores).
, ANSWER: B. Rolled or cast steel
Question 11:
The armature core is laminated in order to
A. Reduce hysteresis loss – Wrong: hysteresis is reduced by using silicon steel, not lamination.
B. Reduce eddy-current loss – Correct: thin insulated laminations break up eddy-current paths.
C. Reduce copper loss – Wrong: copper loss depends on winding resistance and current.
D. Reduce weight only – Wrong: weight is a side benefit, not the purpose.
ANSWER: B. Reduce eddy-current loss
Question 12:
Pole cores are laminated mainly to
A. Reduce eddy currents caused by flux pulsation – Correct: pulsating flux in the pole face would
otherwise heat solid poles.
B. Reduce weight – Wrong: not the primary reason.
C. Reduce hysteresis – Wrong: material grade handles hysteresis.
D. Make assembly easier – Wrong: laminations make assembly harder, not easier.
ANSWER: A. Reduce eddy currents caused by flux pulsation
Question 13:
The purpose of the pole shoe is to
A. Spread the flux uniformly over the armature and support the field coil – Correct: the shoe widens the
pole face and retains the winding.
B. Carry armature current – Wrong: armature current never flows in the pole.
C. Act as a bearing surface – Wrong: bearings are in the end bells.
D. Prevent brush sparking – Wrong: sparking is controlled by interpoles and brush setting.
ANSWER: A. Spread the flux uniformly over the armature and support the field coil
Question 14:
Commutator segments are made of
A. Soft iron – Wrong: iron is magnetic and resistive, unsuitable.
B. Hard-drawn copper – Correct: hard-drawn copper resists wear while staying highly conductive.
C. Aluminum – Wrong: too soft and high-resistance for commutation duty.
UPDATED ACTUAL QUESTIONS AND CORRECT ANSWERS
(2026 EDITION)
FQuestion Bank With Correct ANSWER & Rationale For
Every Option
Question 1:
What is the main difference in the construction of a DC generator and a DC motor?
A. Type of winding – Wrong: both machines use identical lap/wave armature windings.
B. Number of poles – Wrong: pole count depends on rating, not on machine type.
C. Armature material – Wrong: both use laminated silicon-steel cores with copper windings.
D. Type of enclosure – Correct: generators and motors are built with different enclosures (drip-
proof/screened vs. open/pipe-ventilated) to suit their service environments.
ANSWER: D. Type of enclosure
Question 2:
The rotating part of a DC generator is called the
A. Yoke – Wrong: the yoke is the stationary outer frame.
B. Field pole – Wrong: poles are bolted to the frame and stay still.
C. Armature – Correct: the armature rotates in the field and carries the induced EMF.
D. Brush assembly – Wrong: brushes are stationary and ride on the commutator.
ANSWER: C. Armature
Question 3:
,What are the three basic types of armature windings?
A. Series, shunt, compound – Wrong: those are field connection types, not armature windings.
B. Lap, wave, frog-leg – Correct: these are the three fundamental closed-drum armature windings.
C. Progressive, retrogressive, duplex – Wrong: those describe winding progression/multiplexity only.
D. Single-layer, double-layer, triple-layer – Wrong: layer arrangement is not a winding type.
ANSWER: B. Lap, wave, frog-leg
Question 4:
Which armature winding is used in high-current, low-voltage applications?
A. Wave – Wrong: wave has only 2 parallel paths, suited to high voltage/low current.
B. Frog-leg – Wrong: used where self-equalizing is wanted, not the standard high-current choice.
C. Lap winding – Correct: lap provides as many parallel paths as poles, dividing current for high-current
service.
D. Duplex wave – Wrong: still few paths compared with lap.
ANSWER: C. Lap winding
Question 5:
What is the electrical function of the commutator?
A. To increase the air-gap flux – Wrong: flux is set by the field poles, not the commutator.
B. Reverse armature connections to maintain unidirectional current in the external circuit – Correct: it
rectifies the AC generated in the armature into DC externally.
C. To eliminate armature reaction – Wrong: that is done by interpoles/compensating windings.
D. To store electrical energy – Wrong: it is a mechanical switch, not a storage device.
ANSWER: B. Reverse armature connections to maintain unidirectional current in the external circuit
Question 6:
Which of the following is a common brush material used in DC machines?
A. Carbon – Correct but incomplete alone.
B. Graphite – Correct but incomplete alone.
C. Metal–graphite (copper–carbon) – Correct but incomplete alone.
D. All of the above – Correct: carbon, graphite and metal–graphite brushes are all standard DC machine
brush materials.
ANSWER: D. All of the above
,Question 7:
Suitable brush pressure is usually between:
A. 0.1 and 0.5 psi – Wrong: too light; brushes chatter and spark.
B. 1 and 2 psi – Correct: 1–2 psi gives good contact without excessive wear.
C. 5 and 8 psi – Wrong: excessive pressure causes rapid brush wear and heating.
D. 10 and 15 psi – Wrong: far too high; would grind brushes and commutator.
ANSWER: B. 1 and 2 psi
Question 8:
What is the function of field poles in a DC machine?
A. To support the brush rigging – Wrong: brushes are mounted separately on the brush yoke.
B. Produce the magnetic field required for operation – Correct: poles carry the field winding that creates
the working flux.
C. To carry the load current – Wrong: load current flows in the armature and brushes.
D. To damp machine vibration – Wrong: that is the job of the frame and foundations.
ANSWER: B. Produce the magnetic field required for operation
Question 9:
The yoke of a DC machine serves to
A. Provide the return path for magnetic flux and give mechanical support – Correct: the yoke completes
the magnetic circuit and carries the core and end shields.
B. Generate the main EMF – Wrong: EMF is generated in the armature conductors.
C. Improve commutation – Wrong: commutation is handled by interpoles and brushes.
D. Cool the bearings – Wrong: bearings are cooled by their own housing/air flow.
ANSWER: A. Provide the return path for magnetic flux and give mechanical support
Question 10:
The yoke of a modern DC machine is usually made of
A. Cast iron – Wrong: poor magnetic permeability; used only on very old machines.
B. Rolled or cast steel – Correct: steel has high permeability and good mechanical strength.
C. Aluminum – Wrong: non-magnetic, cannot carry flux.
D. Laminated silicon steel – Wrong: laminations are needed only where flux is alternating (cores).
, ANSWER: B. Rolled or cast steel
Question 11:
The armature core is laminated in order to
A. Reduce hysteresis loss – Wrong: hysteresis is reduced by using silicon steel, not lamination.
B. Reduce eddy-current loss – Correct: thin insulated laminations break up eddy-current paths.
C. Reduce copper loss – Wrong: copper loss depends on winding resistance and current.
D. Reduce weight only – Wrong: weight is a side benefit, not the purpose.
ANSWER: B. Reduce eddy-current loss
Question 12:
Pole cores are laminated mainly to
A. Reduce eddy currents caused by flux pulsation – Correct: pulsating flux in the pole face would
otherwise heat solid poles.
B. Reduce weight – Wrong: not the primary reason.
C. Reduce hysteresis – Wrong: material grade handles hysteresis.
D. Make assembly easier – Wrong: laminations make assembly harder, not easier.
ANSWER: A. Reduce eddy currents caused by flux pulsation
Question 13:
The purpose of the pole shoe is to
A. Spread the flux uniformly over the armature and support the field coil – Correct: the shoe widens the
pole face and retains the winding.
B. Carry armature current – Wrong: armature current never flows in the pole.
C. Act as a bearing surface – Wrong: bearings are in the end bells.
D. Prevent brush sparking – Wrong: sparking is controlled by interpoles and brush setting.
ANSWER: A. Spread the flux uniformly over the armature and support the field coil
Question 14:
Commutator segments are made of
A. Soft iron – Wrong: iron is magnetic and resistive, unsuitable.
B. Hard-drawn copper – Correct: hard-drawn copper resists wear while staying highly conductive.
C. Aluminum – Wrong: too soft and high-resistance for commutation duty.
