DC Machines Multiple Choice Questions on “DC Motor Basics -1”.
1. What will happen if DC shunt motor is connected across AC supply?
a) Will run at normal speed
b) Will not run
c) Will Run at lower speed
d) Burn due to heat produced in the field winding
Answer: d
Clarification: In case of parallel field connection, it won’t rotate at all and will start humming and will create vibrations, as a torque produced by positive and negative cycle will cancel out each other. DC motor will be heated up and it may burn.
2. What will happen if the back emf of a DC motor vanishes suddenly?
a) The motor will stop
b) The motor will continue to run
c) The armature may burn
d) The motor will run noisy
Answer: c
Clarification: If back emf vanishes suddenly, motor circuit will try to retain back emf by drawing more current from supply. If supplying unit didn’t trip down by this time, excess current in armature may heat up the armature.
3. What will happen, with the increase in speed of a DC motor?
a) Back emf increase but line current falls.
b) Back emf falls and line current increase.
c) Both back emf as well as line current increase.
d) Both back emf as well as line current fall.
Answer: a
Clarification: In case of DC motor, the speed is proportional to the back emf (Ea ∝ N). So, with the increase in speed, the back emf also increases. Therefore, armature current is also decreased, in case of series motor, armature current is equal to the line or load current.
4. Which part will surely tell that given motor is DC motor and not an AC type?
a) Winding
b) Shaft
c) Commutator
d) Stator
Answer: c
Clarification: All other parts except brushes and commutator are same in AC machine when outer looks are only taken in consideration. Commutator is used only in DC machine for providing mechanical rectification and not in AC machine.
5. In DC motor, which of the following part can sustain the maximum temperature rise?
a) Field winding
b) Commutator
c) Slip rings
d) Armature winding
Answer: a
Clarification: Maximum temperature rise can be sustained by field winding, as it is not involved in rotary parts. Field winding is present away from rotary parts of the machine, so temperature rise in the machine will not produce any effect in machine rotations.
6. Direction of rotation of motor is determined by ____________
a) Faraday’s law
b) Lenz’s law
c) Coulomb’s law
d) Fleming’s left-hand rule
Answer: d
Clarification: Flemings laws can be summarized as whenever, a current carrying conductor comes under a magnetic field, there will be a force acting on the conductor and on the other hand, if a conductor is forcefully brought under a magnetic field, there will be an induced current in that conductor.
7. The current drawn by the armature of DC motor is directly proportional to ________________
a) Torque
b) Speed
c) The voltage across the terminals
d) Cannot be determined
Answer: a
Clarification: From the equation of torque generated in a DC machine, we know that in both DC motor and DC generator, current drawn is directly proportional to the torque required by the machine.
8. Which power is mentioned on a name plate of a motor?
a) Gross power
b) Power drawn in kVA
c) Power drawn in kW
d) Output power available at the shaft
Answer: d
Clarification: Name plate of the motor shows rated values i.e. rated speed, rated current, rated voltage. It also shows output power available at shaft when all other quantities are set to rated values.
9. An electric motor is having constant output power. So, motor will have a torque speed characteristic _______________________
a) Circle about the origin.
b) Straight line parallel to the speed axis.
c) Straight line through the origin.
d) Rectangular hyperbola
Answer: d
Clarification: In case of DC motor for the constant output power, Ea×Ia = T×ω. As T×ω = Constant, so the torque speed characteristic is in the form of rectangular hyperbola, which represented as xy= constant.
10. Which of the following quantity will decrease if supply voltage is increased?
a) Starting torque
b) Operating speed
c) Full-load current
d) Cannot be determined
Answer: c
Clarification: When supply voltage is increased full load current will decrease in order to keep output power constant, which will decrease torque at that moment, while starting torque will remain as it is, irrespective of any change in supply voltage.
11. In which of the following case we will get maximum power?
a) Ea = 2 x supply voltage
b) Ea = supply voltage
c) Supply voltage = 2 x Ea
d) supply voltage = 4 x Ea
Answer: c
Clarification: For a motor, from power equation it is known that,
12. Sometimes motor has to be de-rated.
a) True
b) False
Answer: a
Clarification: Derating refers to the operation of equipment at reduced capacity/power or speed. Derating in motors can be caused due to the following reasons- Frequency, Voltage, Ambient temperature, Altitude.
13. The armature shaft of a DC motor must be able to withstand ______________
a) Bending moment due to weight of the armature.
b) Any unbalanced magnetic pull on the armature core.
c) Twisting stains due to transmission of torque.
d) Bending moment, unbalanced magnetic pull and twisting stains
Answer: d
Clarification: The armature shaft must be able to withstand any unbalanced magnetic pull on the armature core, bending moment due to weight of the armature and commutator, twisting stains due to transmission of torque, for a good and long run application of motor.
14. In DC machines the residual magnetism is present. The order of residual magnetism is ___________
a) 2 to 3 per cent
b) 10 to 15 per cent
c) 20 to 25 per cent
d) 50 to 75 per cent
Answer: a
Clarification: In a DC machine residual magnetism is present and it plays very important role in starting of any DC machine. It’s present because of the previous application on the same motor, it also provides some no-load voltage.
15. Sparking is discouraged in a DC motor.
a) True
b) False
Answer: b
Clarification: Sparking at brushes and commutator segments lead to damage of commutators, which is the main distinguishable component in a DC machine. So, it is advisable to fasten the commutation speed and avoid sparking.