21. Which DC motor is generally preferred for cranes and hoists?
Series motor
Shunt motor
Cumulatively compounded motor
Differentially compounded motor
Answer: 1. Series motor
Explanation:
The outstanding characteristic of a series motor is that it has high torque at low speeds and vice-versa.
The power of a series motor tends to be constant because high torque is accompanied by low speed and vice-versa.
DC Series motors are also used in electric cranes and hoists; light loads are lifted quickly and heavy loads more slowly.
22. DC series motors are used _____
Where load is constant
Where load changes frequently
Where constant operating speed is needed
High starting torque
Answer: 4. High starting torque
Explanation:
In DC series Motor, the torque is directly proportional to the square of the armature current.
T = Ia2
If the current is increased two-fold, the torque will increase four-fold. Consequently, the starting torque of a d.c. the series motor is very high. For this reason, it is used to start heavy loads.
23. In conductively compensated DC series motor compensating winding is provided in _______
As separately wound unit
In series with armature winding
In parallel with armature winding
In parallel with field winding
Answer: 2. In series with the armature winding
Explanation:
In a conductively compensated DC series motor, the compensating winding is connected in series with the armature circuit. The winding is put in the stator slots. The axis of the compensating winding is 90° (electrical) with the main field axis.
24. Buses, trains, trolleys, hoists, cranes make use of DC series motor because ______
It provides low starting torque
Cost-efficient
It provides high starting torque
It requires minimum power compared to other motors
Answer: 3. It provides high starting torque
Explanation:
The outstanding characteristic of a series motor is that it has high torque at low speeds and vice-versa.
It is well-suited for traction purposes such as in electric trains. Acceleration is rapid because the torque is high at low speeds.
Furthermore, the series motor automatically slows down as the train goes up an inclined surface, yet turns at top speed on the flat ground.
The power of a series motor tends to be constant because high torque is accompanied by low speed and vice-versa.
Series motors are also used in electric cranes and hoists; light loads are lifted quickly and heavy loads more slowly.
25. No-load speed of which of the following DC motor will be highest?
Shunt motor
Series motor
Cumulative compound motor
Differentiate compound motor
Answer: 2. Series motor
Explanation:
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 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.
26. DC motor is to drive a load which is having a certain minimum value for most of the time and some peak value for a short duration. We will select the _______
Series motor
Shunt motor
Compound motor
Cannot be determined
Answer: 1. Series motor
Explanation:
The outstanding characteristic of a series motor is that it has high torque at low speeds and vice-versa. If the load has a certain minimum value for most of the time and some peak value for a short duration the best-suited motor is the DC series motor.
27. The direction of rotation of a DC series motor shaft can be changed by _______
Interchanging supply terminals
Interchanging field terminals
Not possible
Cannot be determined
Answer:2. Interchanging field terminals
Explanation:
In the case of the DC series motor, the field and armature winding are connected in series. If the polarity of the input voltage or supply terminals are interchanged, then it changed the direction of the field and armature current both get changed so the direction of speed will remain the same.
The direction of rotation of the DC series motor may be reversed by changing the polarity of either field or the armature winding.
28. Which DC motor has got maximum self-relieving property?
Series motor
Shunt motor
Cumulatively compounded motor
Differentially compounded motor
Answer:1. Series motor
Explanation:
Self-relieving property – Generally dc series motors are used for traction purposes. In a dc series motor, the speed is inversely proportional to torque: or in other words the product of speed and torque is constant. Therefore, the power developed (i.e. the motor h.p.) which is proportional to the product of torque and speed. will remain constant. This gives the motor a self-protective property against excessive overloading.
29. Which of the following motor has the poorest speed regulation?
Shunt motor
Series motor
Differential compound motor
Cumulative compound motor
Answer:2. Series motor
Explanation:
The speed regulation of a DC motor is defined as the change in speed from no load to full load. It is expressed as a fraction or a percentage of the full load speed.
The no-load speed of a DC series motor is very high. So, it has the poorest speed regulation.
If the load is applied to 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 speed regulation should be maintained very less. In the series motor, this difference is almost equal to infinity.
30. Which of the following DC motor can be used at conveyors?
Series motor
Shunt motor
Differentially compound motor
Cumulative compound motor
Answer: 1. Series motor
Explanation:
The outstanding characteristic of a series motor is that it has high torque at low speeds and vice-versa.
The power of a series motor tends to be constant because high torque is accompanied by low speed and vice-versa.
DC Series motors are also used in electric cranes and hoists; light loads are lifted quickly and heavy loads more slowly.