In a DC series motor, When the load is removed the armature current as well as the field flux becomes very small, and at the same time, the back emf increases. According to the speed relation, the motor overruns and attains a dangerously high speed.
32. .Which statement is correct about DC series motor?
a) Has its field winding consisting of thick wire and more turns
b) Has a poor torque
c) Can be started easily without load
d) Has its field winding consisting of thick wire and less turns
Answer: 4. Has its field winding consisting of thick wire and less turns
Explanation:
For a dc series, motor and generator respectively the field winding is connected in series with the armature. In the case of a dc series machine, therefore, the field winding is made up of thick winding wires such that the armature current can flow through wires of the field winding without overheating.
33. In which of the following applications DC series motor is invariably tried?
a) Starter for a car
b) Drive for a water pump
c) Fan motor
d) Home appliances
Answer: 1. Starter for a car
Explanation:
DC Series motors are known for their high starting torque and ability to respond effectively to heavy loads, like turning the crankshaft of a cold engine, pushing a number of pistons, and operating all the accessories that are connected to that crankshaft. The starter of the car simply implies the start of operation at maximum load. Thus, a series motor can be used in this application. For higher torques, since speed remains constant car starter is one of the best applications of DC series motor.
34. Torque of a 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
Answer: 3. Directly proportional to square of armature current
Explanation:
The torque of a DC series motor is Directly proportional to the square of armature current.
In a DC series machine, both field and armature winding is connected in series.
Therefore, field current will be equal to armature current.
Torque produced ∝ If × Ia
i.e., T ∝ ϕ . Ia
∵ ϕ ∝ Ia (in DC series motor)
∴ T ∝ I2a
35. The speed of a DC series motor is ___________
a) Proportional to field current
b) Proportional to the square of the armature current
c) Proportional to the armature current
d) Inversely proportional to the armature current
Answer: 3. Directly proportional to square of armature current
Explanation:
In a DC series machine, both field and armature winding is connected in series.
Therefore, field current will be equal to armature current.
Torque produced ∝ If × Ia
i.e., T ∝ ϕ . Ia
∵ ϕ ∝ Ia (in DC series motor)
∴ T ∝ I2a
or, √T ∝ Ia
we also know that,
N ∝ 1/φ ∝ 1/Ia ∝ 1/√T
N ∝ 1/√T
Speed of DC series motor is inversely proportional to the armature current because, as armature current increases the flux produced also increases due to the series combination. Similarly, if armature current is reduced flux is reduced which will increase speed.
36. The actual operating efficiency of a series motor is _______ than that of a shunt motor.
Equal
More
Less
Twice
Answer: 3. Less
Explanation:
The actual operating efficiency of a series motor less than that of a shunt motor. Although the efficiency of a series motor varies about the same as that of a shunt motor, the actual operating efficiency is usually considerably lower. This is due to the fact that a series motor seldom runs at its rated load, but is continuously accelerating and decelerating.
37. In a DC series motor, if the armature current is halved, the torque of the motor will be equal to
a) 100% of the previous value
b) 50% of the previous value
c) 25% of the previous value
d) 10% of the previous value
Answer: 2. 25% of the previous value
Explanation:
In DC series Motor is directly proportional to the square of the armature current.
T ∝ I2a
So if the armature current is Half i.e Ia = 1/2 then torque will become 1/4 which is 25% of the initial value.
38. What will happen to a torque, if a DC series motor is accidentally connected to single-phase ac supply voltage?
a) Pulsating and unidirectional
b) Steady and unidirectional
c) Oscillating
d) Cannot be determined
Answer: 1. Pulsating and unidirectional
Explanation:
If a DC series motor is accidentally connected to single-phase ac supply voltage then the current direction through a series motor is reversed, the polarities of the field poles and the armature conductors are both reversed, resulting in a pushing effort between them in the same direction.
Because of this, a series motor will operate on a.c. supply, its direction of rotation is the same for both half cycles. The lower the frequency of a.c., the greater the torque that can be produced. If the frequency is high, the inductive reactance of the coil limits the current that can flow and the torque of the motor.
39. Which of the following are characteristics of DC series Motor?
Large Starting torque
Infinite Speed
Low speed at high load
All of the above
Answer: 1. Pulsating and unidirectional
Explanation:
Large starting torque, the infinite speed at no-load, Low speed at high load, low running torque for steady speed operation are called Series characteristics. This are shown by DC series motor, so applications like cranes, traction, etc. use DC series motor.
40. The DC series motor is useful in the application when
Traction
Crane
Hoist
All of the above
Answer: 4. All of the above
Explanation:
DC series motor is a variable speed motor, and hence best suited for traction work. (if) The motor develops high starting torque. Therefore it is used in cranes, elevators, hoists, air compressors.
At no load, the DC series motor rotates at dangerously high speed. Hence DC series motor should always be started on load.
When the motor is connected across the supply mains without load, it draws a small current from the supply mains.
In the DC series motor, the speed is inversely proportional to the flux. And flux is directly proportional to the armature current in the series motor. In no-load, ideally armature current is zero that’s why flux is also zero. Hence Speed of the motor is infinite
As a result of that series motor never start at no-load condition