100 Most Important MCQ on Electric Drive | Industrial Drive MCQ

100 Most Important MCQ on Electric Drive | Industrial Drive | Objective Type Question Electric Drive with Explanation

Ques 1. The consideration involved in the selection of the type of electric drive for a particular application depends upon

  1. Speed control range and its nature
  2. Starting Nature
  3. Environmental condition
  4. All of the above

Answer.4. All of the above

Explanation:-

Motor control is required in the large number of industrial and domestic applications such as transportation systems, rolling mills, paper machines, textile mills, machine tools, fans, pumps, robots, and washing machines. Systems employed for motion control are called drives and may employ any of the prime movers.

Drives employing electric motors are known as electric drives.

or

The system which is used for controlling the motion of an electrical machine, such type of system is called an electrical drive.

Factors Affecting the Selection of Electric Drive

The selection of electric drive basically means the selection of drive motor. Following are the various factors which influence the selection of motor to drive the load:

  1. The limits of speed range: The range over which the speed control is necessary, for the load, similarly how hard is it to control the speed and the speed regulation also affects the choice of the motor.
  2. Motor efficiency: The motor efficiency varies as load varies so the efficiency consideration under variable speed operation affects the choice of the motor.
  3. Braking: The braking requirements from the load point of view. Easy and effective braking are the requirements of a good drive.
  4. Starting requirements: The starting torque necessary for the load, the corresponding starting current is drawn by the motor also affects the selection of drive.
  5. Power factor: It is well known that running of motors with low power factor values is not at all economical. While the power factor vanes with the load conditions in some motors. Hence the type of the load and the running power factor of the motor are the essential considerations while selecting a drive.
  6. Load factor: There are the variety of types of load conditions possible Idle continuous, intermittent, impact etc. Such load variations are discussed later in the chapter. Such load variation factor and duty cycle of the motor influences the selection of drive.
  7. Reliability of operation: It is important to study the conditions of stable operation of an electric drive. This includes the investigation of the reliability of operation of an electric drive.
  8. Availability of supply: The motors available are a.c. or d.c. But the availability of supply decides the type of motor to be selected for the drive.
  9. Effects of supply variations: There is a possibility of frequency supply variations. The motor selected must be able to withstand such supply variations.
  10. Economical aspects: The size and rating of motor decide its initial cost while the various losses, temperature rise decides its Truing cost. These economical aspects must be considered while selecting a drive.
  11. Environmental effects: It Is possible that the atmosphere where an electric drive is to be used may contain some chemical gases, fumes, humidity etc. Such a contaminated atmosphere or a humid atmosphere also affects the choice of motor for a drive.

 

Ques 2. Which of the following motor is preferred for automatic drives?

  1. Ward Leonard controlled dc motors
  2. Squirrel cage induction motor
  3. Synchronous motors
  4. Shunt Motor

Answer 1. Ward Leonard controlled dc motors

Explanation:

Ward Leonard Speed Control

The Ward-Leonard system consists of a fixed speed 3-phase AC induction motor driving by separately excited DC generator that, in turn, feeds a variable voltage to a shunt wound DC motor. So this is essentially a DC variable speed drive.

The basic principle of the DC variable speed drive is that the speed of a separately excited DC motor is directly proportional to the voltage applied to the armature of the DC motor. The main changes over the years have been concerned with the different methods of generating the variable DC voltage from the 3-phase AC supply.

Ward Leonard control of dc machines is considered as one of the most elegant and efficient methods of speed control over a wide range. The schematic diagram of the control technique is shown in Figure. The system consists of a separately excited generator feeding power to the dc motor to be controlled. The generator, “G” is driven at a constant speed by an ac motor connected to the ac mains. The driving motor may be an induction motor or a synchronous motor. Where ac mains are not available, a diesel engine or a gas turbine acts as a prime mover to run the generator. This generator drives a motor ‘ M”. The motors “G” and ‘ M” must be of equal rating.

Adjusting the field current of the generator, the motor terminal voltage can be controlled. The field winding voltage of the generator can be smoothly varied in either direction. Therefore, the motor terminal voltage and the speed of the motor can be varied smoothly from full negative to full positive. Modern installations use thyristor converter circuit for variable voltage dc supply.

One of the advantages of Ward Leonard drive system is its ability for regenerative braking, thereby delivering power to the grid, corresponding to low motor speeds. For regenerative braking, the output voltage of generator “G” is reduced below the induced voltage of the motor “M” by decreasing the generator field excitation. This reverses the current flow in the armature of the machines “G” and “M”.

As the current is reversed, the machine “G” will act as a motor and the machine “M” will act as a generator. The mechanical energy supplied to the machine “M” is converted into electrical energy; which is now supplied to the machine “G’ [presently working as a motor]. The machine “G” will act as a prime mover for the ac machine which will now work as generator feeding power to the grid. The nature of the speed-torque characteristic is similar to that shown in Figure. Drop in motor speed due to change in load torque is caused by Iara drop in both the machines. When the generator output voltage is high, the motor speed is automatically high. The Iara drop in both machines will be negligible compared to their outputs. Therefore, the % speed regulation of the motor will be excellent. The Ward Leonard drive is used in steel rolling mills, paper plants, elevators, machine tools.

The main advantages of Ward Leonard scheme are:

  1. Capable of producing regenerative braking thus delivering electrical energy to the grid. Therefore, the scheme adopts conservation of energy. Besides, it is suitable for four-quadrant operation.
  2. This system is commonly employed for elevators, hoists, and main drive in steel mills, as this method can give unlimited speed control in either direction. Since the generator voltage can be varied gradually from zero, no extra starting equipment is required to start up the main motor smoothly.
  3. Speed reversal is smooth and does not require any external resistance or any special starting gear;
  4. The arrangement is neat and clean and relatively free from maintenance problem.
  5. The lagging reactive volt-amperes of a plant can be neutralized, by using an over-excited synchronous motor. The overall power factor of the plant also improves.
  6. When the load is intermittent as in rolling mills, the drive motor used is an induction motor with a flywheel mounted on its shaft to smooth out the intermittent loading to a low value. The M-G set provided with a flywheel is known as Ilgner set. The synchronous motor is not suitable for intermittent loading because of its constant speed characteristic.

The main disadvantages of Ward Leonard scheme are:

  1. The initial cost of the drive system is costly since to drive 1 dc motor, additional 1 dc generator of the same rating and a 3-phase ac motor are needed;
  2. The scheme needs more floor space and produces more noise compared to a single machine undergoing speed-control mechanism.
  3. The efficiency of the scheme becomes poor due to the inclusion of two addition machines viz. the machine G and the prime mover which is generally a 3-phase motor.
  4. The drive produces noise and requires frequent maintenance.

Nowadays, static Ward-Leonard drives, using two thyristor bridge rectifiers (converters) connected back to back, are used to eliminate the above drawbacks.

More precise and smooth speed control over a wide range such as 200:1, and even higher, is accomplished in which there are no sources of noise and vibration. In such an adjustable voltage system, higher torque-speed characteristics can be obtained by providing id compensation. Soft starting can also be incorporated.

Non-electrical prime movers can be used to drive the dc generator of the Ward-Leonard drive. For example, in diesel-electric locomotives and ship propulsion drives, the generator is driven by a diesel engine or a gas turbine.

Examples of applications are rolling mills, paper mills, mine hoists, etc.

 

Ques 3. The consideration involved in the selection of the type of electric drive for the Load Variation application depends upon

  1. Constant Load
  2. Continuous Variable Load
  3. Pulsating Load
  4. All of the above

Answer.4. All of the above

Explanation:-

Selection of Motor based on Load Variation

While selecting a motor it is necessary to consider the variation of load torque with speed and time. This is related to the torque rating of the motor i.e. how much and what type of torque motor can produce safely. The variation of load torque with speed basically decides the type of motor to be selected. While the variation of load torque with time decides the rating of the motor to be selected. Such a factor which influences the selection of rating of the motor based on the load variation with time is called load variation factor. One cycle of variation of the load is called a duty cycle. The different types of load variations with time and corresponding examples of load are given below:

  1. Continuous or constant loads: In this type load occurs for a long time under the same conditions.
    e.g. fan type loads, paper making machines etc.
  2. Continuous variable loads: The load is variable over a period of time but occurs repetitively for a longer duration.
    e.g metal cutting lathes, conveyors etc
  3. Pulsating loads: A torque which exhibits a combination of constant load torque superimposed by pulsations. 
    e.g. reciprocating pumps, compressors, all loads having Bank shaft.
  4. Impact loads: These are peak loads occur at regular intervals of time. Motors for such loads are provided with heavy flywheels.
    e.g rolling vanilla, presses, sharing machines, forging hammers
  5. Short time internment loads: The load appears periodically on identical duty cycles, each consisting of a period of applications of load and one or rest.
    e.g. all forms of cranes, hoist, elevators.
  6. Short time loads: A constant load appears on the drive and Me system rests for the remaining period of the cycle.
    e.g. motor-generator sets for charging batteries, household equipment.

 

Ques 4. __________ drive is also called as Line shaft drive

  1. Individual drive
  2. Multimotor drive
  3. Group Drive
  4. None of the above

Answer 4. Group Drive

Explanation:-

CLASSIFICATION OF ELECTRICAL DRIVES

Electrical drives may be grouped into the following three cate^lpriee:

  1. Group drive.
  2. Individual drive.
  3. Multimotor drive.

Group drive:- A drive in which a single electric motor drives a line shaft by means of which an entire group of working machines may be operated is called group drive. It is also sometimes called as the line shaft drive. The line shaft is fitted with multi-stepped pulleys and belts that connect these pulleys and the shafts of the driven machines serve to vary their speed.

 

Ques 5. The advantages of a group driver electric drive are

  1. HIgh efficiency
  2. Low Noise
  3. Constant speed
  4. All of the above

Answer 1. High Efficiency

Explanation:-

Advantages of Group driver electric drive

Saving in the initial cost: One 150 kW motor costs much less than ten 15 kW motors required to drive 10 separate machined

High Efficiency:- The efficiency and power factor of a large group drive motor will be higher, provided it is operated fairly near its rated load.

High Overload capacity:- If the machines are liable to short but sharp overloads, group drive is again advantageous, because 100 percent overload on an individual machine will cause hardly 10 percent overload when driven by group drive.

Continous start and stop:- Group drive can be used with advantage in those industrial processes where there is the sequence of continuity in operation and where it is desirable to stop these processes simultaneously as in a flour mill.

 

Ques 6. The disadvantages of group drive electric machine is/are

  1. Low efficiency
  2. Low overload capacity
  3. Can’t be used for constant operation
  4. All of the above

Answer 3. Can’t be used for constant operation

Explanation:-

Disadvantages of Group driver electric drive

  1. Group drive is seldom used these days due to the following disadvantages :
  2. Group drive does not give the good appearance (owing to the use of line shafiing pulley and
    belts) and is less safe to operate.
  3. In group drive, the speed control of the individual machine is very cumbersome using stepped
    pulleys, belts etc.
  4. Noise level at the working site is quite high.
  5. This system is unreliable since any fault in driving motor renders all the driven equipment
    idle.
  6. The considerable amount of power is lost in energy transmitting mechanism. If all the machines driven by the line shafi do not work together, the main motor runs at reduced load. Consequently, it runs with low efficiency and with poor power factor.
  7. Group drive cannot be used where constant speed is required as in paper and textile industry.
  8. The flexibility of layout of different machines is lost since they have to be so located as to suit
    the position of the line shaft.

Group drive is adopted, when existing factories are changed from engine drive to electric motor drive simply by replacing the oil or steam engine by an electric motor of corresponding output retaining all the old shafts and belts.

 

Ques 7. In __________ drive each machine is driven by its own separate motor with the help of gears and pulley

  1. Individual drive
  2. Multimotor drive
  3. Group Drive
  4. None of the above

Answer 1.  Individual Drive

Explanation:-

Individual drive. In “individual drive”, each machine is driven by its own separate motor with the help of gears, pulley etc.
Examples: Single-spindle drilling machines, various types of electrical hand tools and simple types of metal working machine tools and mechanisms.

  • For driving heavy machines such as for lifts, cranes, shapers, lathes etc. and for the purposes where constancy of speed and flexibility of control is required, such as in paper mills and textile industry, the individual drive is essential.
  • The individual drive is preferred for new factories, as it causes some saving in the cost of the superstructure which is much lighter and less expensive.

 

Ques 8. The advantages of the individual drive is/are

  1. Flexibility in operation
  2. Each machine can be run or stop as desired
  3. Maintenance of Lineshaft, bearing is eliminated
  4. All of the above

Answer 4. All of the above

Explanation:-

Advantages of Individual Drive

  1. Flexibility in the installation of different machines.
  2. Each operator has full control of the machine which can be quickly stopped if an accident occurs.
  3. Since each machine is driven by a separate motor, it can be run and stopped as desired.
  4. The motor and its control unit can be built as an integral part of the machine which results in a good appearance, cleanliness, and safety.
  5. Machines not required can be shut down and also replaced with a minimum of dislocation.
  6. In the case of the motor fault, only its connected machine will stop whereas other will continue working undisturbed.
  7. The maintenance of line shafis, bearings, pulleys, and belts etc. is eliminated. Furthermore, there is no danger of oil falling on articles being manufactured, which is very important in textile industry.

Disadvantage.

The only disadvantage of the individual drive is its high cost.

 

Ques 9. The drive which is used for metal-cutting machines tools, rolling mills etc. are

  1. Individual drive
  2. Multimotor drive
  3. Group Drive
  4. None of the above

Answer 2. Multimotor drive

Explanation:-

Multimotor drive:- In “multi-motor drives” separate motors are provided for actuating different parts of the driven mechanism.
Example: In traveling cranes, three motors are used: one for hoisting, another for long travel motion and third for cross travel motion.

Such a drive is essential in complicated metal-cutting machine tools, paper making machines, rolling mills etc.

The use of individual drives and multi-motor drives has led to the introduction of automation in production processes which, apart from increasing the productivity of various undertakings, has enhanced the reliability and safety of operation.

 

Ques 10.  What is the total annual cost of a group drive with a motor costing Rs.18000  with that of 10  individual motors, each costing Rs. 5000. The annual consumption is 80000 kWh. Electrical energy costs 20 paise per kWh. Depreciation, maintenance, and other fixed charges amount to 10 percent.

  1. Rs. 16800
  2. Rs.1800
  3. Rs. 18000
  4. Rs. 17800

Answer.4. Rs 17800

Explanation:-

⇒ The capital cost of group drive = Rs.18000

⇒ Annual Depreciation, maintenance, and other fixed charges = 10% i.e

10% of 18000 = 18000 × 10 ⁄ 100 = Rs. 1800

Energy consumption per annum = 80000 kWh

Annual energy charges = 0.20 × 80000 = Rs. 16000

Total annual cost of Group drive = Annual fixed charge + Annual energy charge

= 1800 + 16000 = Rs. 17800.00

2 Responses

  1. srinivasa rao says:

    Nice questions intrusting..

  2. tamboli says:

    learn new methode

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