# DC Machines Questions and Answers – DC Motor Basics -2

This set of DC Machines online test focuses on “DC Motor Basics – 2”.

1. Which of the following motor is used where high starting torque and wide speed range control is required?
a) All motors
b) Induction motor
c) Synchronous motor
d) DC motor

Explanation: DC series motor gives the maximum starting torque at starting. With the help of Ward Leonard speed control method various speeds are also available. It provides one mode to othe transmission also.

2. The armature voltage control of DC motor will provide ___________
a) Constant power drive
b) Constant voltage drive
c) Constant current drive
d) Constant torque drive

Explanation: By changing the armature voltage we can run motor to give various speeds at constant torque. While changing the field is also another idea to change speed but it’ll be variable torque constant power operation.

3. As there is no back emf at the instant of starting a DC motor, in order to prevent a heavy armature current from flowing though the armature circuit ___________
a) Series resistance is connected with armature
b) Parallel resistance is connected to the armature
c) armature is temporarily open circuited
d) a high value resistor is connected across the field winding

Explanation: To prevent large armature current from damaging the armature circuit we connect series resistance with armature and in order to avoid losses we reconnect the same resistance with field circuit once machine is started, i.e. the exact principle of starters.

4. What will happen to torque if back emf and speed of the DC motor is doubled?
a) Remain unchanged
b) Reduce to one-fourth value
c) Increase four folds
d) Be doubled

Explanation: Torque depends on armature current, so as long as armature current remains constant torque will not change. Speed depends directly with back emf thus, it will definitely become double when back emf is doubled.

5. At the instant of starting, when a DC motor is put on supply, it will behave like ______________
a) Highly resistive circuit
b) Low resistance circuit
c) Capacitive circuit
d) Inductive circuit

Explanation: At the starting when back emf is very low, machine will fetch large amount of armature current from supply acting as a low resistive circuit. In order to avoid such case, we use starters in DC machine which add some resistance to armature at the starting.
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6. All others are advantages of DC motor over AC motor except _______________
a) Low cost
b) Wide speed range
c) Stability
d) High starting torque

Explanation: Dc machine is very useful over AC when operation is concerned but its superior speed control system itself requires 2 more machines. DC machine requires high maintenance due to commutator and brushes so it is an expensive machine.

7. If a DC motor designed for 45°C ambient temperature is to be used for 55°C ambient temperature, then the motor ___________
a) Of lower HP should be selected
b) Of higher HP should be selected
c) Can be used for 50°C ambient temperature also
d) Is to be de-rated by a factor recommended by manufacturer and select the next higher H.P. motor

Explanation: The ambient temperature is one of a factor in derating. If the ambient temperature given is high, the insulation may reach its maximum temperature limit quickly. Hence, the motor may have to be de-rated, and for application next motor must be used.

8. Torque developed by a DC motor depends upon _____________
a) magnetic field
b) active length of the conductor
c) current flow through the conductors
d) Current, active length, no. of conductors, magnetic field all

Explanation: Torque produced by a DC motor is directly proportional to an armature current flowing through the machine. While it also depends on some constant values like active length, no. od armature conductors and magnetic field which are constant for a given machine.

9. For very sensitive and wide speed control, which of the following method is preferred?
a) Armature control
b) Ward-Leonard control
c) Multiple voltage control
d) Field control

Explanation: In ward-Leonard method, by using big circuitry, very fine speed control over the whole range from zero to normal speed and that too in both directions can be obtained. The motor-generator set can provide speed both below and above the rated speed and in both direction, by this method.

10. Field weakening control method is used for the speed _________
a) Above rated speed
b) Below rated speed
c) Can be used anywhere
d) Can’t tell

Explanation: By reducing of field current, flux will also reduce, because flux is directly proportional to the field current passing through the winding. As speed is inversely proportional to flux, so this method can be used for only above rated speed.

11. Armature resistance control method is used for the speed _____________
a) Above rated speed
b) Below rated speed
c) Can be used anywhere
d) Can’t tell

Explanation: In armature circuit resistance method armature resistance can be only increased, by adding another resistance in series. As a result, the back emf will decrease. Speed is directly proportional to back emf so, it will also decrease. Therefore, this method is used for below rated speed.

12. Which function is performed by counter emf of a DC motor?
a) Exceeds supply voltage
b) Aids applied voltage
c) Helps in energy conversion
d) Regulates its armature voltage

Explanation: Back emf or counter emf makes the DC a self-regulating machine i.e. it makes the DC motor to draw as much armature current as it is just enough to develop the required load torque. Thus, it helps in energy conversion and it is always less than applied voltage.

13. The output power of any electrical (AC or DC) motor is taken from the ______________
a) Field
b) Coupling mounted on the shaft
c) Armature
d) Motor frame

Explanation: The output power of any electrical motor is always taken from the coupling mounted on the shaft because the motor converts input electrical power into output mechanical power. So, we get mechanical power at shaft coupling.

14. Why field winding of a DC series motor is provided with thick wire?
a) As it carries large load current
b) To provide large flux
c) In order to reduce eddy current to provide large flux
d) To reduce the of insulting materials

Explanation: In DC series motor, the cross-sectional area of wire used for field coils has to be large because the in this case field coils carry the load or armature current. As the cross-sectional area is large, the resistance is minimum and series voltage drop is also minimum.

Sanfoundry Global Education & Learning Series – DC Machines.

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