# DC Machines Questions and Answers – Operating Characteristics of DC Series Motor – 2

This set of DC Machines MCQs focuses on “Operating Characteristics of DC Series Motor – 2”.

1. Which DC motor is generally preferred for cranes and hoists?
a) Series motor
b) Shunt motor
c) Cumulatively compounded motor
d) Differentially compounded motor

Explanation: DC series motor is used where high starting torque is required, thus in cranes and hoists DC series motor is used widely. Torque-speed characteristics of DC series motor suits perfectly to these applications.

2. Which of the following motor has the poorest speed regulation?
a) Shunt motor
b) Series motor
c) Differential compound motor
d) Cumulative compound motor

Explanation: If the load is applied to the any DC motor or machine, the speed of the motor decreases automatically. Thus, in order to maintain the constant speed, the difference between no load and full load speed called as speed regulation should be maintained very less. In series motor this difference is almost equal to infinity.

3. Buses, trains, trolleys, hoists, cranes make use of DC series motor because __________
a) It provides low starting torque
b) Cost efficient
c) It provides high starting torque
d) It requires minimum power compare to other motors

Explanation: DC series motor provides very high starting torque, and at higher torques speed of the DC series motor is also maintained to constant. Both of these properties are used in applications of DC series motor.

4. DC motor is to drive a load which is having certain minimum value for most of the time and some peak value for short duration. We will select the _______
a) Series motor
b) Shunt motor
c) Compound motor
d) Cannot be determined

Explanation: When load is minimum, we will get very high speed at shaft in DC series motor, but it is advisable that DC series motor should not be used on no-load application. For the application stated in question DC series motor is perfect option.

5. Which DC motor has got maximum self-relieving property?
a) Series motor
b) Shunt motor
c) Cumulatively compounded motor
d) Differentially compounded motor

Explanation: DC series motor develops large starting torque initially and slow down with increase in load and so get automatically relived from heavy excessive load. Thus, DC series motor is called as self-reliving machine.
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6. Which motor should not be started on no-load?
a) DC Series motor
b) DC Shunt motor
c) Synchronous motor
d) Induction motor

Explanation: No-load speed of the DC motor is very high, it’ll create a dangerous situation and the centrifugal forces will destroy the armature and may harm the personnel. As, flux is inversely proportional to speed, speed will be infinity by calculations.

7. Which of the following is the correct equation for DC series motor ________
a) [ Vt / kφ]- [ Rse +Ra / kφ] Ia
b) [ Vt / kφ]- [ Ra/ kφ2] Ia
c) [ Vt / kφ2]- [Rse +Ra/ kφ2] Ia
d) [ Vt / kφ2] + [ Ra/ kφ2] Ia

Explanation: Speed-torque characteristic equation is obtained by back emf = (Vt -IaRa-IaRse) and back emf= kφn. Thus, by combining these equations and by assuming all constants equal to k we get, the final equation shown above.

8. Actual speed obtained at any value of armature current comparing to speed obtained by assuming linear magnetic assumption in DC series motor is _______
a) Same
b) Higher
c) Lower
d) Cannot say

Explanation: From speed current characteristics of a DC series motor is easily seen that, initial value of speeds in both of the cases is same or rather infinite, but as a armature current or load current increases speed starts dropping down relatively more in the case of linear magnetization.

9. Torque of e DC series motor is __________
a) Directly proportional to armature current
b) Inversely proportional to armature current
c) Directly proportional to square of armature current
d) Inversely proportional to square of armature current

Explanation: Torque in a DC series motor, in the case of linear magnetization varies directly with armature current square. Practically at higher values of armature current it tends to settle at constant value.

10. Practically, at higher values of armature current torque does not follow its relationship with armature current.
a) True
b) False

Explanation: At higher values of armature current or load current, saturation and demagnetization due to armature reaction becomes prominent, and DC machine shows its effect on torque. Thus, torque tends to remain at constant value.

11. What will happen to a torque, if a DC series motor is accidentally connected to single phase ac supply voltage?
a) Pulsating and unidirectional
c) Oscillating
d) Cannot be determined

Explanation: In DC series motor, the AC currents through the field and armature winding will be made unidirectional by the function of commutator and brushes. So, torque will be unidirectional but it will be pulsating also due to AC.

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