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BASIC NEC 2023

DC Series Motor Practice Test Questions

Free online DC series motor practice test. 25 basic electrical questions on speed-torque characteristics, field winding design, speed control methods, and typical applications such as cranes, hoists, and traction. Good for electrician exam prep.

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A DC series motor has its field winding connected in SERIES with the armature, so the same current flows through both. Because torque is roughly proportional to the SQUARE of the armature current (in the unsaturated region), a series motor gives very high starting torque — this is why it is used in cranes, hoists, and electric trains. Its biggest limitation is poor speed regulation: speed changes a lot with load, and it can reach a dangerously high speed if run with no load at all. Never connect a series motor to its load through a belt that could break or slip off.

25

Questions

38 min

Time

70%

Pass

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DC Series Motor Practice Test Questions — Question List

Question 1: The speed of DC series motor is increased by .................

increasing the field resistance
strengthening the field flux
weakening the field flux

Question 2: A DC Series motor should not be started under no load condition because .................

at no load the speed of the DC series motor is very high
at no load DC series motor cannot develop maximum torque
it cannot start under no load
It draws more current at no load.

Question 3: The value of back EMF in a DC Motor is maximum at ..................

no load
half full load
full load

Question 4: When the speed of the DC Motor increases the armature current ..............

increases
decreases
remains constant

Question 5: Which of the following motor has best speed regulation?

series motor
shunt motor
universal motor
induction motor

Question 6: When the DC series motor is running the speed is basically determined by .............

field excitation
load
armature resistance
speed is constant

Question 7: The speed of DC series motor is practically constant.

True
False

Question 8: The series motor never connected to mechanical load through a belt drive.

True
False

Question 9: The DC series motor is used in the applications requiring low torque at low speed.

True
False

Question 10: Which of the following applications does the series motor is used?

cranes and hoists
cars
constant speed applications

Question 11: The field winding of a DC series motor is normally made of:

Thin wire with many turns and high resistance
Thick wire with few turns and low resistance
Aluminum foil with no turns
Insulated cable identical to the armature winding

Question 12: Before the magnetic core saturates, the torque developed by a DC series motor is proportional to:

The armature current directly
The square of the armature current
The square root of the armature current
The supply voltage only

Question 13: Electric trains using DC series traction motors often control speed by switching motor groups between "series" and "parallel" connection. What is the main benefit of this method?

It gives step speed control while keeping efficient use of the supply voltage
It doubles the supply frequency
It converts the motors into generators permanently
It removes the need for any speed control at all

Question 14: To reverse the direction of rotation of a DC series motor, you should:

Reverse the connections of either the armature or the field winding, but not both
Reverse both the armature and field connections together
Increase the supply voltage
It cannot be reversed once running

Question 15: A diverter resistance connected in PARALLEL with the series field winding is used to:

Weaken the field and increase motor speed above base speed
Strengthen the field and reduce motor speed
Protect the motor from over-voltage only
Convert the motor to a generator

Question 16: A universal motor, commonly found in vacuum cleaners and hand power tools, is built on the same basic principle as a:

DC series motor
Three-phase synchronous motor
DC shunt motor
Squirrel cage induction motor

Question 17: A starting rheostat connected in series with the armature of a DC series motor at start-up is used to:

Limit the starting current and is cut out as speed rises
Increase the starting current for faster start
Permanently stay in the circuit during running
Reverse the motor direction

Question 18: Compared to a DC shunt motor, the speed regulation of a DC series motor is:

Much poorer, speed changes a lot with load
Much better, speed stays almost constant
Exactly the same as a shunt motor
Not related to load at all

Question 19: A compensating winding, embedded in slots in the pole faces of a large DC series motor, is used to:

Neutralize armature reaction across the whole pole face
Increase the no-load speed of the motor
Replace the need for a commutator
Convert the motor into a generator

Question 20: A DC series motor develops a certain torque at an armature current of 15 A. If the current rises to 30 A (still in the unsaturated region), what happens to the torque?

It doubles (2 times)
It becomes 4 times as large
It stays the same
It is cut in half

Question 21: For the same starting current, a DC series motor develops a much higher starting torque than a DC shunt motor mainly because:

Its flux rises with the same current that produces torque, giving a squared relationship
It has no armature resistance at all
It runs on a higher supply voltage always
Its commutator has more segments

Question 22: Reducing the speed of a DC series motor by adding resistance in series with the armature is simple, but has which drawback?

It wastes energy as heat in the resistor, lowering efficiency
It increases motor speed instead of reducing it
It has no effect on motor speed
It reverses the direction of rotation

Question 23: Why do large DC series motors often need interpoles for good commutation?

The large armature current causes a high reactance voltage during commutation
The motor has no armature winding
The motor never draws any current
Interpoles are required only for AC motors

Question 24: If the series field circuit of a running DC series motor suddenly opens (breaks), what happens?

The motor stops because the field and armature share the same current path
The motor runs away to a dangerously high speed
The motor reverses direction automatically
Nothing changes at all

Question 25: A DC series motor is a poor choice for applications where the load must run at a constant speed regardless of load changes.

True
False

EXAM CARD BASIC

Questions25

Time limit38 min

Pass score70%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

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Source: NEC 2026 (NFPA 70)