100 Most Important MCQ on Electric Drive-2021 | Industrial Drive MCQ

Ques 71. In the case of contactor the rated current can carry upto ______ hours without exceeding temperature.

  1. 8 Hours
  2. 2 Hours
  3. 1 Hours
  4. 5 Hours

Answer 1. 8 Hours


The rated thermal current is the maximum current the contactor can carry for 8 h without exceeding temperature rise limits.


Ques 72. The saturable core reactor is generally is a

  1. Variable Resistor
  2. Step Down Transformer
  3. Variable Impedance
  4. Thermal Relay

Answer. 3. Variable Impedance


A saturable core reactor acts as a variable inductance connected in series with a load across an input ac power supply. It looks like a transformer having two windings wound on a core of steel as shown in the figure. One of these windings named control winding (CW) receives a small dc current controlled by a potentiometer and a fixed dc voltage source. This acts as an input bias signal that controls the amount of alternating current that can flow through the other winding, namely the gate winding (GW) and the load in series. The alternating current in the gate winding produces an objectionable ac voltage in the control winding that mad disturb the fixed dc bias in it.

100 Most Important MCQ on Electric Drive-2021 | Industrial Drive MCQ 1


Therefore most saturable reactors include two identical core! A and B as shown in Figure each steel cork has its own ac winding named GW1 and GW2 while the dc coil surrounds one leg of each core Here the two ac windings GW 1 and GW2 art connected in parallel and in phase opposition to each other. Thus within the dc coil, that is, CW two flux movements are in opposite directions a the same instant of time; therefore, no ac voltage is superimposed on the dc coil.

100 Most Important MCQ on Electric Drive-2021 | Industrial Drive MCQ 2

Saturable core reactors are relatively inefficient (90% large in size, and require water or air cooling, depending upon their power rating. Their main advantage is their immunity t modest transients and overloads. Well-designed SCRs ca offer similar reliability at greater efficiency and reduced size

The main advantage of saturable core reactors is that it is not possible to burn out the unit. Even if the controlled electric heater is short-circuited, it will not allow enough current to flow to burn out the reactor. The disadvantages of the saturable core reactors, in addition to their size and weight, is that the cannot adjust the line voltage from 0 to 100%.


Ques 73. The application of suitable core reactor is

  1. Overload protection of transformer
  2. Plugging of Induction Motor
  3. Used for voltage Regulation
  4. All of the above

Answer.3. Stepless AC voltage variation


A saturable core reactor is a device consisting of an iron core, an AC or gate winding, and a control winding (Figure). The impedance of the gate winding can be changed b means of the magnetic flux produced by the DC current in the control winding. A DC control signal of a few watts can, therefore, control hundreds of kilowatts of AC power. Since the control signal requires watts of power a magnetic amplifier is always used to boost the control signal from the process controller up to the power level required by the control winding of the saturable core reactor.

100 Most Important MCQ on Electric Drive-2021 | Industrial Drive MCQ 2


The most common saturable reactor consists of a three-legged core where each coil is wound on separate magnetic “leg”. The two AC coils in Fig. should have relatively low and equal number of turns and the middle coil usually has the much higher number of turns. This gives the possibility of amplification through lower control current to obtain control over powerful circuits. Because of this in many cases, the (regulated) saturable reactor is used as a “Magnetic Amplifier”. The main difference between a magnetic amplifier and a saturable core reactor is that the magnetic amplifier uses a half of the magnetic loop and the amplifier uses the complete magnetic loop. Because of this, the term “Saturable Core” is often substituted by the term “Magnetic Amplifier”.

Basically, a saturable reactor consists of three essential elements: Direct current source, the Magnetic core with windings and Alternating current source.

  • The magnetic amplifiers have the following advantages
  • Easy and effective amplification of weak DC signals.
  • Complete conductive isolation of input and output circuits of the amplifier for DC signals.
  • The magnetic amplifier is sturdy, mechanically shock proof.
  • it practically needs no maintenance.
  • It can be used as a device to operate as a DC to AC converter.
  • It can amplify several signals simultaneously;
  • The output of the device responds to the algebraic sum of the separate input signals. magnetic amplifiers are thus used for voltage regulation and other control.


Ques 74. The application of the Magnetic amplifier is

  1. Used for servo motor drive
  2. Control shunt Motor
  3. Control series Motor
  4. None of the above

Answer 1.Use for servo Motor drive


The magnetic amplifier is an electromagnetic device for amplifying electrical signals.

Applications of Magnetic Amplifiers

Following are the applications of the magnetic amplifier:

  • Saturable reactors are particularly well suited for airborne systems and are used in guided missiles.
  • These are also used in controllers and relays directly operated from thermocouple photocells, mechanical relays, electronic and square wave generators due to their flip-flop characteristics.
  • These are also excellent DC amplifiers without any drift problem.
  • They are widely used for servomotor drive in devices subjected to high environmental stress.
  • They can be used in voltage regulating DC power supply.


Ques 75. The losses in electric drive systems is/are

  1. Electrical Transmission Losses
  2. Mechanical Transmission Losses
  3. Load Losses
  4. All of the above

Answer.4  All of the above



Energy conservation in an electrical drive is achieved by the reduction of losses in its various parts. Typical losses include the following:

  1. Electrical transmission losses: These losses depend on the drive power factor and harmonics in the line current.
  2. Conversion losses in the power modulator (or converter): The semiconductor converter usually has low conversion losses.
  3. Electric motor losses to convert electric power into mechanical power: These are determined by choice of the motor (quality of its design and selection of right rating) and quality of supply (voltage variations, unbalance, frequency variations and harmonics).
  4. Mechanical losses:- It is part of the transmission system such as bearings, gears, clutches, and belts.
  5. Losses in the load: A load in a machine required to perform a specified task such as fan, pump, and train.
  6. Losses caused by throttling or by other means that control material flow by absorbing or bypassing excess output.
  7. Mechanical transmission losses, such as friction losses to move material from one location to another. Losses in the pipeline carrying fluid are one such example.

The efficient operation of an electrical drive, with minimum consumption of energy, requires that the drive and the driven machinery (or load) operate efficiently. This calls for the careful design of both electrical and mechanical systems. Only those measures are considered here which can lead to efficient operation of electric drive systems.


Ques 76. Which of the following pair is used for frequency converter?

  1. Squirrel cage induction motor and Wound rotor induction motor
  2. Wound rotor induction motor and synchronous motor
  3. Squirrel cage induction motor and synchronous motor
  4. Any of the above

Answer.1. Squirrel cage induction Motor and wound rotor induction Motor


 Frequency converter

A conventional wound-rotor motor along with a squirrel cage induction motor may be used as a frequency converter to generate a frequency different from that of the utility company. The stator of the wound-rotor machine is connected to the utility line, and the rotor is driven at an appropriate speed by a motor M i.e Squirrel Cage Induction motor. The rotor supplies power to the 3-phase load at a voltage E2 and frequency f2 both of which depend upon the slip.

f2 = sf


100 Most Important MCQ on Electric Drive-2021 | Industrial Drive MCQ 4

In general, the desired frequency is two or three times that of the utility company. According to above equation in order to attain this frequency, the slip must be positive and greater than 1. As a result, the shaft must be rotated against the direction of the revolving flux.

The operation of the frequency converter b then identical to that of an induction motor opera^ling as a brake. However, the power P usually dissipated as heat in the rotor is now available to supply power to the load. The converter acts as a generator.


Ques 77. The torque Motor design is very similar to that of

  1. Induction Motor
  2. DC Motor
  3. Single Phase Motor
  4. Rotary Synchronous Motor

Answer.4. Rotary Synchronous Motor


Torque motor

A torque motor can be thought of as being a linear motor that is folded to make a cylinder and whose ends are connected to each other. It is similar to a rotary synchronous motor with a very high number of pole pairs and has a large diameter.

100 Most Important MCQ on Electric Drive-2021 | Industrial Drive MCQ 5


The windings in the stator are arranged offset by 120° per pole division to each other and fed with three-phase AC current. The stator current creates a high-pole rotating field which rotates slowly. The rotor follows accordingly. Due to its large diameter, the motor has an accordingly high torque.

Torque motors are available as built-in or complete motors. In the case of built-in motors, the motors do not have their own bearings. These motors must, therefore, be fitted on a machine shaft which has its own bearings. The stator and the rotor are supplied as separate components. As with linear motors, assembly requires special knowledge and skills.

Due to their large diameter, torque motors have a hollow shaft. This simplifies their integration into the driven machine. Torque motors are available in both air-cooled and water-cooled versions.

In many cases, the position encoder must be mounted externally. The connection to the motor shaft can be made, for example, using a toothed drive belt. The position encoder must be in line with the magnetic poles of the rotor.

Torque motors can deliver torques from 100 Nm up to approximately 5 kNm and have speeds ranging from 10 rpm to 300 rpm.


Ques 78. The status Indicator of the Motor is useful when

  1. When the Motor is located in the Remote Place
  2. Help us to determine the On/Off condition of the Motor
  3. Both of the above
  4. None of the above

Answer.3. Both of the above


Status Indicators

An indication of whether or not the motor is running is important. The current status of a motor, running or stopped, is particularly useful if a start switch is located remotely from the motor, such as on a motor control center or on the main control panel somewhere. The most common way to indicate a motor’s status is to use a small colored light. Usually, a red light is used to indicate the motor is running and a green light is used to indicate that the motor is stopped. Status indicating lights are shown diagrammatically in the Figure

100 Most Important MCQ on Electric Drive-2021 | Industrial Drive MCQ 6


The letter in the circle indicates the color of the light, such as R = red, G = green, A = amber, and W = white. The running light can simply be wired across the starter contactor’s coil so if the starter coil is energized, the running light is also. The stopped or off light can be energized through a small auxiliary normally closed contact of the motor contactor.


Ques 79. Fans, pumps, Blowers are the example of _________ drive

  1. Fixed speed Drive
  2. Variable Speed drive
  3. Servo Drive
  4. Any of the above

Answer. 1. Fixed speed drive


Fixed-speed drives are operated at a single, constant speed. They possess only the equipment necessary for switching on and off, as well as for protecting against overload. It is not possible to vary the speed of the drive, which means that depending on the load the speed may fluctuate. Typical applications for fixed-speed drives are fans and pumps which are operated with an asynchronous motor direct-on-line.


Ques 80. Switchable Speed drive, Open Loop speed drive, closed-loop speed drives are the example of

  1. Fixed speed Drive
  2. Variable Speed drive
  3. Servo Drive
  4. Any of the above

Answer. 2. Variable Speed drive


Variable Speed Drive

The speed of variable-speed drives is adjustable and the drives can b operated at least two different speeds. In addition to an electric motor, these drives have a controller which is responsible for adjusting the speed. Depending on the application, the controller is accordingly complex and allows for different speed-ranges and accuracy.

The variable speed drive can be divided into three parts

  1. Switchable-speed drives
  2. Open-loop variable-speed drives
  3. Closed-loop variable-speed drives

Switchable-speed drives allow operation at least two different speeds. Example applications are switchable-speed fans and pump or traversing drives with forward and reverse operation. Asynchronous motors together with the necessary contactor controller art typically used.

Open-loop variable-speed drives have a continuously variable speed, However, there is no feedback of the actual speed, meaning that, depending on the drive type, there may load dependent deviation from the speed setpoint. A controller with an electronic power set lion is necessary for the speed control. Examples of this type of drive are asynchronous motors with frequency controllers and V/f control.

Closed-loop variable-speed drives also have a continuously variable speed but, in addition, measure the actual speed of the motor. In this. way, deviations in the speed from the speed setpoint can be recognized and corrected. To enable this, it is necessary to have a controller with the appropriate control algorithms. A very widespread implementation of the closed-loop variable-speed drive is the asynchronous motor with frequency controller and vector control.

8 thoughts on “100 Most Important MCQ on Electric Drive-2021 | Industrial Drive MCQ”

  1. This multiple choice questions is best. This explanation is show good . These MCQ is very important for my prepration. These is very short and easy .

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