DC Motor

Electrician Trade Theory, Module-2

Posted May 15, 2025 Updated Dec 12, 2025

By Roni

10 min read

DC Motor

⭕️ Show all Answers

Q1: Which instrument is used to measure armature winding resistance?

Megger
Multimeter
Series type Ohm meter
Kelvin bridge

Q2: Which instrument is used to test armature winding for short and open circuit?

Tong Tester
Internal Growler
External Growler
Digital multimeter

Q3: What is the name of the speed control method of DC motor?

Field diverter method
Field tapping method
Voltage control method
Armature diverter method

Q4: Which winding wire is used for DC field coil?

Super enameled copper wire
Single silk covered copper wire
Double silk covered copper wire
PVC covered copper winding wire

Q5: Which formula is used to calculate the speed of DC motor?

$N = \frac{E_b}{\phi}$
$N = \frac{\phi}{E_b}$
$N = \frac{E_b \cdot \phi}{120}$
$N = \frac{E_b \cdot \phi}{60}$

Q6: How many parallel paths in duplex lap winding in the armature of 4 pole D.C Motor?

Q7: Which rule determines the direction of rotation of armature in D.C motor?

Right hand grip rule
Right hand palm rule
Fleming's left hand rule
Fleming's right hand rule

Q8: What is the name of D.C motor?

D.C shunt motor
D.C series motor
D.C differential compound motor
D.C cumulative compound motor

Q9: Which rule determines the direction of current in D.C motor?

Right hand grip rule
Right hand palm rule
Fleming's left hand rule
Fleming's right hand rule

Q10: What is the formula to calculate the current taken by D.C shunt motor armature?

$I_a = \frac{V}{R_a}$
$I_a = \frac{E_b}{R_a}$
$I_a = \frac{V-E_b}{R_a}$
$I_a = \frac{V+E_b}{R_a}$

Q11: Which rule is applied to identify the direction of flux in DC motor?

Cork's screw rule
Right hand grip rule
Fleming's left hand rule
Fleming's right hand rule

Q12: Name the type of DC motor.

Shunt motor
Series motor
Long shunt compound motor
Short shunt compound motor

Q13: What is the formula to calculate back EMF in a DC motor?

$E_b = \frac{ZNP\phi}{60A}$
$E_b = \frac{NP}{Z\phi 60A}$
$E_b = \frac{ZNP}{60A\phi}$
$E_b = \frac{60A\phi}{ZNP}$

Q14: What is the name of the equipment?

Megger
Earth resistance tester
Internal growler
External growler

Q15: What is the name of winding, if coil pitch is less than pole pitch?

Full pitch winding
Half pitch winding
Long chorded winding
Short chorded winding

Q16: What is the purpose of series resistor connected with holding coil in a D.C four point starter?

Limit the current in holding coil
Increase the current in holding coil
Increase the voltage in holding coil
Decrease the voltage in holding coil

Q17: Which speed control method of D.C series motor is used for electric train?

Field diverter method
Field tapping method
Armature diverter method
Supply voltage control method

Q18: Why shunt field coil is connected in series with holding coil in D.C three point starter?

Increase the holding coil current
Decrease the holding coil current
Protect the shunt field from over current
Protect the motor in case of open in shunt field

Q19: Why the direction of rotation is changed only by changing the armature current direction in a D.C compound motor?

Maintain rated speed
Maintain motor characteristics
Avoid armature reaction effect
Prevent motor from over loading

Q20: Which speed control methods offers below normal speed in DC shunt motor?

Field control method
Voltage control method
Armature control method
Ward Leonard system of speed control

Q21: Why starters are required to start D.C motors in industries?

Regulate the field voltage
Reduce the armature current
Control the armature reaction
Smooth operation of motors

Q22: Which insulating material belongs to class 'B' insulation?

Cotton
Bamboo
Fiber glass
Leatheroid paper

Q23: What is the temperature value of class F insulation?

90°C
105°C
120°C
155°C

Q24: Which type of D.C motor is used for constant speed drives?

DC series motor
DC shunt motor
Differential long shunt compound motor
Differential short shunt compound motor

Q25: Which type of DC motor is used in elevators?

DC series motor
DC shunt motor
DC differential compound motor
DC cumulative compound motor

Q26: Which method of speed control gives below the rated speed in DC series motor?

Field diverter method
Tapped field method
Voltage control method
Armature diverter method

Q27: What is the effect, if a four point starter resistance is cutoff during running?

Motor stopped
Runs at slow speed
Runs at very high speed
Runs at reverse direction

Q28: Why carbon composition brush requires in the armature circuit to operate the D.C motor?

Increases the starting torque
Protects from armature reaction
Protects armature from over loading
Reduces the spark in the commutator segment

Q29: Why series motor produce high torque and speed initially without load?

Absence of back emf
Load current flows through field winding
Armature current and field current are same
Series field winding wound with thick wire

Q30: Why the series field is short circuited at the time of starting in differential compound motor?

To reduce the starting current
To increase the speed of motor
To decrease the speed of motor
To maintain proper direction of rotation

Q31: Which is the most effective method of balancing armature?

Static balancing
Dynamic balancing
Attached with counter balancing
Plugged with lead weight balancing

Q32: Which material is used for starting resistance of DC starters?

Eureka
Nichrome
Manganin
Constantine

Q33: Which DC compound motor is operated at constant speed under varying load?

Differential long shunt
Cumulative long shunt
Differential short shunt
Cumulative short shunt

Q34: How No volt coil is connected in a three point starter with DC shunt motor?

Directly connected to supply
Connected in series with armature
Connected in parallel with armature
Connected in series with shunt field

Q35: Which type of armature winding is illustrated?

Duplex lap winding
Triplex lap winding
Simplex lap winding
Quadruplex lap winding

Q36: Which growler test for armature is illustrated?

Open coil test
Grounded coil test
Shorted coil test
Shorted commutator test

Q37: Which speed control method is applied to obtain both below normal and above normal speed in DC motor?

Field control method
Armature control method
Tapped field speed control
Ward Leonard speed control

Q38: Why commutators are sparking heavily?

Incorrect brush position
Incorrect field connection
Incorrect direction of rotation
Incorrect armature connection

Q39: What is the action of the induced emf in a running D.C motor?

Assists the applied voltage
Opposes the applied voltage
Increases the armature current
Decreases the armature current

Q40: Which motor has this characteristics curve?

Series motor
Shunt motor
Cumulative compound motor
Differential compound motor

Q41: What is the purpose of resistor connected with holding coil in 4 point starter?

Limit current in NVC
Protect the coil from short circuit
Protect the motor from overload
Protect the armature from short circuit

Q42: Why the D.C series motor field winding is wound with thick wire?

To regulate field voltage
To carry the load current
To keep maximum inductance
To reduce the armature reaction

Q43: Which type of speed control of D.C series motor?

Field parallel method
Field diverter method
Field tapping method
Armature diverter method

Q44: Which type of D.C motor is suitable for shearing machines?

Shunt motor
Series motor
Cumulative compound motor
Differential compound motor

Q45: Where D.C compound motors are preferred?

Constant load requirements
Constant speed requirements
High starting torque requirements
Constant speed under varying load requirements

Q46: What is the necessity of starter for D.C motor?

Limit the field current
Limit the field voltage
Control the motor speed
Limit the armature current

Q47: Which type of instrument is used to test the armature winding?

Megger
Growler
Multimeter
Ohmmeter

Q48: Why the holding coil of a 3 point starter is connected in series with shunt field?

To limit the load current
To run motor at low voltage
To hold the handle plunger firmly
To protect the motor from high speed

Q49: What is the best method to change the DOR of a compound motor without change of its characteristics?

Change armature current direction
Change shunt field current direction
Change series field current direction
Change the current in armature and shunt field together

Q50: What is the purpose of NVC connected in series with the field in 3 point starter?

To improve the torque
Reduce the field current
To decrease the back emf
To prevent increase in speed

Q51: Which type of DC motor is used for sudden application of heavy loads?

Shunt motor
Series motor
Differential compound motor
Cumulative compound motor

Q52: Which speed control method is used in food mixture motors?

Voltage control method
Field diverter control method
Armature diverter method
Series field tapping method

Q53: Which speed control system provides a smooth variation of speed from zero to above normal?

Field control
Armature control
Field diverter control
Ward-Leonard system control

Q54: What is the purpose of tapes in winding?

Insulate slots
Bind the coils
Wrap the conductor
Insulate exposed conductors

Q55: Which type of DC armature winding the front pitch ($Y_f$) is greater than back pitch ($Y_b$)?

Lap winding
Wave winding
Progressive winding
Retrogressive winding

Q56: What reduces the cross sectional area of core material for VA rating?

Dynamo sheet
Low alloy sheet
High alloy sheet
Normal steel sheet

Q57: How to obtain opposite polarity in adjacent poles of a 4 pole DC motor?

Varying the number of turns in coil
Making series connection of coils
Making parallel connection of coils
Making current flow in different direction

Q58: What is the operation in the rewinding process?

Cleaning of slots
Removing of winding
Removing of wedges
Cutting of winding wire

Q59: Which insulating material used in winding is a highly non -hygroscopic and possess good electrical strength?

Empire cloth
Triplex paper
Millinex paper
Leatheroid paper

Q60: Which type of armature winding is illustrated?

Triplex wave winding
Duplex wave winding
Progressive lap winding
Retrogressive lap winding

Q61: Calculate the average pitch ($Y_A$) for retrogressive wave winding, if No. of armature conductor =14, No. of slots=7, No. of poles 2

Show Calculation

$$Y_A = \frac{Z \pm 2}{P}$$ Where: * $Z$ = Number of armature conductors * $P$ = Number of poles * For **Progressive** winding, use $+2$ * For **Retrogressive** winding, use $-2$ **Given:** * Number of conductors ($Z$) = $14$ * Number of poles ($P$) = $2$ * Winding type = Retrogressive **Calculation:** Since it is a retrogressive winding, we use the minus sign: $$Y_A = \frac{Z - 2}{P}$$ $$Y_A = \frac{14 - 2}{2}$$ $$Y_A = \frac{12}{2}$$ $$Y_A = 6$$

Q62: Which type of test is illustrated for the armature after rewound?

Open coil test
Shorted coil test
Voltage drop test
Grounded coil test

Q63: Why the newly rewound armature must be preheated before varnishing?

Drive out the moisture from it
Help for quick drying of varnish
Make easy to penetrate varnish inside
Maintain uniform spreading of varnishing

Q64: How the direction of rotation of a DC compound motor is changed?

By changing the direction of armature current
By interchanging the supply terminals
By changing the direction of both field and armature current
By changing the direction of series field current

Q65: What is the effect in a D.C shunt motor, if its supply terminals are interchanged?

Runs in slow speed
Runs in high speed
Runs in the same direction
Runs in the reverse direction

Q66: What is the speed, if field winding of a DC shunt motor is in open circuit?

Stop running
Motor runs normally
Runs at slow speed
Runs in very high speed

Q67: What is the reason for reduction in speed of a D.C shunt motor from no load to full load?

Shunt field current increases
Shunt field current decreases
Armature voltage drop increases
Armature voltage drop decreases

Q68: Which winding fault is determined by the test?

Open coil fault
Short coil fault
Grounded coil fault
Grounded core fault

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2nd year, Trade Theory

Theory ITI

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