BCIT ELECTRICAL DC MACHINES LEVEL 3 EXAM QUESTIONS AND
CORRECT ANSWERS
Question 1
To avoid the problem of voltage buildup during the startup of a generator, the operator should:
Correct Answer
C. Set the field rheostat to minimum resistance
Question 2
What is the difference between long-shunt and short-shunt connections?
Correct Answer
C. Long-shunt connects the shunt field across the line; short-shunt connects it across the armature only
Question 3
The condition where the magnetic flux of the series and shunt fields oppose each other is called:
Correct Answer
B. Differential compounding
Question 4
What is the difference between separately excited and self-excited fields?
Correct Answer
B. Separately excited fields receive field current from an external source
Question 5
The rotating part of a DC generator is called the
Correct Answer
C. Armature
Question 6
Which factors cause terminal voltage to decrease as load increases in a generator?
Correct Answer
A. Armature IR drop and armature reaction
Question 7
A locked rotor usually means that the motor will:
Correct Answer
D. Both A and B
Question 8
A compound motor has both __________ and __________ field coils.
Correct Answer
A. Series and shunt
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,Question 9
How can the voltage characteristic of a series field coil be useful?
Correct Answer
A. It can compensate for voltage drop in a compound generator
Question 10
Interpoles are connected in series with the armature because:
Correct Answer
A. Their strength changes with armature strength
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,Question 11
Will changing the polarity of the line leads change the direction of rotation of a series motor?
Correct Answer
c. No, because both armature and field current reverse together
Question 1: In a DC machine, the primary function of the commutator is to:
A. Increase the armature resistance
B. Convert alternating current induced in the armature windings into direct current output
C. Provide mechanical support to the shaft
D. Reduce eddy current losses in the core
CORRECT ANSWER: B. Convert alternating current induced in the armature windings into direct current output
Rationale: The commutator acts as a mechanical rectifier, reversing the connections of the armature coils to the external circuit at
the appropriate instants so that the generated alternating emf appears as unidirectional voltage at the brushes.
Question 2: The residual magnetism in the field poles of a DC generator is essential for:
A. Reducing armature reaction
B. Initiating the voltage build-up process during self-excitation
C. Improving commutation
D. Increasing the efficiency of the machine
CORRECT ANSWER: B. Initiating the voltage build-up process during self-excitation
Rationale: Residual magnetism provides the initial small flux that induces a low emf when the armature begins to rotate, allowing the
self-excitation process to start and the voltage to build up to the rated value.
Question 3: Armature reaction in a DC machine primarily causes:
A. An increase in the main field flux under all load conditions
B. Distortion and weakening of the main field flux
C. Improvement in the commutation process
D. A reduction in the armature current
CORRECT ANSWER: B. Distortion and weakening of the main field flux
Rationale: The magnetic field produced by the armature current interacts with the main field, resulting in a demagnetizing and
cross-magnetizing effect that distorts the flux distribution and generally reduces the total useful flux.
Question 4: The purpose of interpoles (commutating poles) in a DC machine is to:
A. Increase the main field strength
B. Neutralize the armature reaction in the interpolar zone and improve commutation
C. Reduce the speed of the machine
D. Provide mechanical balance to the rotor
CORRECT ANSWER: B. Neutralize the armature reaction in the interpolar zone and improve commutation
Rationale: Interpoles are series-connected auxiliary poles placed in the geometric neutral plane. Their flux opposes the armature
reaction flux in the commutation zone, thereby assisting spark-free commutation.
Question 5: In a separately excited DC generator, the field winding is:
A. Connected in series with the armature
B. Connected in parallel with the armature
C. Supplied from an independent external DC source
D. Short-circuited
CORRECT ANSWER: C. Supplied from an independent external DC source
Rationale: In a separately excited generator the field winding receives its excitation current from a separate DC supply, allowing
independent control of the field current and generated voltage.
Question 6: The critical field resistance of a DC shunt generator is the:
A. Maximum field circuit resistance above which the generator fails to build up voltage
B. Minimum field circuit resistance required for rated voltage
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, C. Resistance of the armature winding
D. Resistance of the interpoles
CORRECT ANSWER: A. Maximum field circuit resistance above which the generator fails to build up voltage
Rationale: The critical field resistance is the highest value of field-circuit resistance for which the field resistance line is tangent to the
open-circuit characteristic; any higher resistance prevents voltage build-up.
Question 7: The torque developed by a DC motor is proportional to:
A. The square of the armature current only
B. The product of flux per pole and armature current
C. The speed of the motor only
D. The supply voltage only
CORRECT ANSWER: B. The product of flux per pole and armature current
𝑃𝑍
Rationale: The electromagnetic torque in a DC machine is given by 𝑇 = Φ𝐼𝑎 T = 2πAP ZΦIa, showing direct proportionality to the
2𝜋𝐴
flux per pole and the armature current.
Question 8: In a DC series motor, the speed is approximately inversely proportional to:
A. The square of the armature current
B. The armature current under unsaturated conditions
C. The supply voltage
D. The field flux only when the motor is heavily loaded
CORRECT ANSWER: B. The armature current under unsaturated conditions
𝑉 − 𝐼𝑎 𝑅𝑎
Rationale: In a series motor the flux is proportional to armature current (unsaturated). Since 𝑁 ∝ N ∝ ΦV −IaRaand Φ ∝ 𝐼𝑎
Φ
Φ ∝ Ia, speed becomes inversely proportional to armature current.
Question 9: The main disadvantage of a DC series motor is that:
A. It cannot be started on no-load
B. It has very low starting torque
C. Its speed remains constant under all loads
D. It requires a separate field supply
CORRECT ANSWER: A. It cannot be started on no-load
Rationale: On no-load the armature current and therefore the flux become very small, causing the speed to rise to dangerously high
values; hence a series motor must never be started or run without load.
Question 10: Equalizer rings are used in DC machines with lap windings to:
A. Reduce the armature reaction
B. Equalize the circulating currents caused by differences in pole strengths or brush contact
C. Increase the generated emf
D. Improve the mechanical strength of the armature
CORRECT ANSWER: B. Equalize the circulating currents caused by differences in pole strengths or brush contact
Rationale: Equalizer rings interconnect points of equal potential under ideal conditions, providing low-resistance paths for
circulating currents that arise from magnetic or geometric asymmetries, thereby preventing uneven brush loading.
Question 11: The back emf of a DC motor:
A. Aids the applied voltage
B. Opposes the applied voltage and limits the armature current
C. Is equal to the applied voltage under all conditions
D. Is generated only when the motor is stalled
CORRECT ANSWER: B. Opposes the applied voltage and limits the armature current
Rationale: As the armature rotates, an emf is induced that opposes the supply voltage (Lenz’s law). The net voltage across the
armature resistance is 𝑉 − 𝐸𝑏 V − Eb, so the back emf automatically limits the steady-state armature current.
Question 12: The speed of a DC shunt motor can be controlled above base speed by:
A. Armature voltage control
B. Field flux weakening
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CORRECT ANSWERS
Question 1
To avoid the problem of voltage buildup during the startup of a generator, the operator should:
Correct Answer
C. Set the field rheostat to minimum resistance
Question 2
What is the difference between long-shunt and short-shunt connections?
Correct Answer
C. Long-shunt connects the shunt field across the line; short-shunt connects it across the armature only
Question 3
The condition where the magnetic flux of the series and shunt fields oppose each other is called:
Correct Answer
B. Differential compounding
Question 4
What is the difference between separately excited and self-excited fields?
Correct Answer
B. Separately excited fields receive field current from an external source
Question 5
The rotating part of a DC generator is called the
Correct Answer
C. Armature
Question 6
Which factors cause terminal voltage to decrease as load increases in a generator?
Correct Answer
A. Armature IR drop and armature reaction
Question 7
A locked rotor usually means that the motor will:
Correct Answer
D. Both A and B
Question 8
A compound motor has both __________ and __________ field coils.
Correct Answer
A. Series and shunt
Page 1 of 50
,Question 9
How can the voltage characteristic of a series field coil be useful?
Correct Answer
A. It can compensate for voltage drop in a compound generator
Question 10
Interpoles are connected in series with the armature because:
Correct Answer
A. Their strength changes with armature strength
Page 2 of 50
,Question 11
Will changing the polarity of the line leads change the direction of rotation of a series motor?
Correct Answer
c. No, because both armature and field current reverse together
Question 1: In a DC machine, the primary function of the commutator is to:
A. Increase the armature resistance
B. Convert alternating current induced in the armature windings into direct current output
C. Provide mechanical support to the shaft
D. Reduce eddy current losses in the core
CORRECT ANSWER: B. Convert alternating current induced in the armature windings into direct current output
Rationale: The commutator acts as a mechanical rectifier, reversing the connections of the armature coils to the external circuit at
the appropriate instants so that the generated alternating emf appears as unidirectional voltage at the brushes.
Question 2: The residual magnetism in the field poles of a DC generator is essential for:
A. Reducing armature reaction
B. Initiating the voltage build-up process during self-excitation
C. Improving commutation
D. Increasing the efficiency of the machine
CORRECT ANSWER: B. Initiating the voltage build-up process during self-excitation
Rationale: Residual magnetism provides the initial small flux that induces a low emf when the armature begins to rotate, allowing the
self-excitation process to start and the voltage to build up to the rated value.
Question 3: Armature reaction in a DC machine primarily causes:
A. An increase in the main field flux under all load conditions
B. Distortion and weakening of the main field flux
C. Improvement in the commutation process
D. A reduction in the armature current
CORRECT ANSWER: B. Distortion and weakening of the main field flux
Rationale: The magnetic field produced by the armature current interacts with the main field, resulting in a demagnetizing and
cross-magnetizing effect that distorts the flux distribution and generally reduces the total useful flux.
Question 4: The purpose of interpoles (commutating poles) in a DC machine is to:
A. Increase the main field strength
B. Neutralize the armature reaction in the interpolar zone and improve commutation
C. Reduce the speed of the machine
D. Provide mechanical balance to the rotor
CORRECT ANSWER: B. Neutralize the armature reaction in the interpolar zone and improve commutation
Rationale: Interpoles are series-connected auxiliary poles placed in the geometric neutral plane. Their flux opposes the armature
reaction flux in the commutation zone, thereby assisting spark-free commutation.
Question 5: In a separately excited DC generator, the field winding is:
A. Connected in series with the armature
B. Connected in parallel with the armature
C. Supplied from an independent external DC source
D. Short-circuited
CORRECT ANSWER: C. Supplied from an independent external DC source
Rationale: In a separately excited generator the field winding receives its excitation current from a separate DC supply, allowing
independent control of the field current and generated voltage.
Question 6: The critical field resistance of a DC shunt generator is the:
A. Maximum field circuit resistance above which the generator fails to build up voltage
B. Minimum field circuit resistance required for rated voltage
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, C. Resistance of the armature winding
D. Resistance of the interpoles
CORRECT ANSWER: A. Maximum field circuit resistance above which the generator fails to build up voltage
Rationale: The critical field resistance is the highest value of field-circuit resistance for which the field resistance line is tangent to the
open-circuit characteristic; any higher resistance prevents voltage build-up.
Question 7: The torque developed by a DC motor is proportional to:
A. The square of the armature current only
B. The product of flux per pole and armature current
C. The speed of the motor only
D. The supply voltage only
CORRECT ANSWER: B. The product of flux per pole and armature current
𝑃𝑍
Rationale: The electromagnetic torque in a DC machine is given by 𝑇 = Φ𝐼𝑎 T = 2πAP ZΦIa, showing direct proportionality to the
2𝜋𝐴
flux per pole and the armature current.
Question 8: In a DC series motor, the speed is approximately inversely proportional to:
A. The square of the armature current
B. The armature current under unsaturated conditions
C. The supply voltage
D. The field flux only when the motor is heavily loaded
CORRECT ANSWER: B. The armature current under unsaturated conditions
𝑉 − 𝐼𝑎 𝑅𝑎
Rationale: In a series motor the flux is proportional to armature current (unsaturated). Since 𝑁 ∝ N ∝ ΦV −IaRaand Φ ∝ 𝐼𝑎
Φ
Φ ∝ Ia, speed becomes inversely proportional to armature current.
Question 9: The main disadvantage of a DC series motor is that:
A. It cannot be started on no-load
B. It has very low starting torque
C. Its speed remains constant under all loads
D. It requires a separate field supply
CORRECT ANSWER: A. It cannot be started on no-load
Rationale: On no-load the armature current and therefore the flux become very small, causing the speed to rise to dangerously high
values; hence a series motor must never be started or run without load.
Question 10: Equalizer rings are used in DC machines with lap windings to:
A. Reduce the armature reaction
B. Equalize the circulating currents caused by differences in pole strengths or brush contact
C. Increase the generated emf
D. Improve the mechanical strength of the armature
CORRECT ANSWER: B. Equalize the circulating currents caused by differences in pole strengths or brush contact
Rationale: Equalizer rings interconnect points of equal potential under ideal conditions, providing low-resistance paths for
circulating currents that arise from magnetic or geometric asymmetries, thereby preventing uneven brush loading.
Question 11: The back emf of a DC motor:
A. Aids the applied voltage
B. Opposes the applied voltage and limits the armature current
C. Is equal to the applied voltage under all conditions
D. Is generated only when the motor is stalled
CORRECT ANSWER: B. Opposes the applied voltage and limits the armature current
Rationale: As the armature rotates, an emf is induced that opposes the supply voltage (Lenz’s law). The net voltage across the
armature resistance is 𝑉 − 𝐸𝑏 V − Eb, so the back emf automatically limits the steady-state armature current.
Question 12: The speed of a DC shunt motor can be controlled above base speed by:
A. Armature voltage control
B. Field flux weakening
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