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Chapter: Electrical Drives & Control : Introduction to Electrical Drives

Important Short Questions and Answers: Introduction to Electrical Drives

Electrical Drives & Control - Introduction to Electrical Drives - Important Short Questions and Answers: Introduction to Electrical Drives


Define Drive and Electric Drive?


Drive: A combination of prime mover, transmission equipment and mechanical Working load is called a drive


Electric drive: An Electric Drive can be defined as an electromechanical device for converting electrical energy to mechanical energy to impart motion to different machines and mechanisms for various kinds of process control.


List out some examples of prime movers?

Hydraulic Engine,


Steam engine,


Turbine or electric motors.


List out some advantages of electric drives?


Availability of electric drives over a wide range of power a few watts to mega watts.

Ability to provide a wide range of torques over wide range of speeds.

Electric motors are available in a variety of design in order to make them compatible to any type of load.


Give some examples of Electric Drives?

Driving fans, ventilators, compressors and pumps.

Lifting goods by hoists and cranes.

Imparting motion to conveyors in factories, mines and warehouses

Running excavators & escalators, electric locomotives trains, cars trolley buses, lifts & drum winders etc.


What are the types of electric drives?

Group electric drives (Shaft drive),


Individual Drives,


Multi motor electric drives.


What is a Group Electric Drive (Shaft Drive)?


This drive consists of single motor, which drives one or more line Shafts supported on bearings.


The line shaft may be fitted with either pulleys & belts or gears, by means of which a group of machines or mechanisms may be operated.


Classify electric drives based on the means of control?







What are the advantages and disadvantages of Group drive (Shaft drive)?


A single large motor can be used instead of a number of small motors.


The rating of the single motor may be appropriately reduced taking into account the diversity factor of loads.



There is no flexibility; Addition of an extra machine to the main Shaft is difficult.


The efficiency of the drive is low, because of the losses occurring in several transmitting mechanisms.


The complete drive system requires shutdown if the motor, requires Servicing or repair.


The system is not very safe to operate


The noise level at the work spot is very high.


What is an individual electric drive? Give some examples. (UQ)


In this drive, each individual machine is driven by a separate motor. This motor also imparts motion to various other parts of the machine.


Single spindle drilling machine, Lathe machines etc.


What is a multi motor electric drive? Give some examples.


In this drive, there are several drives, each of which serves to activate on of the working parts of the driven mechanisms.


Metal cutting machine tools, paper making machines, rolling mills, traction drive, Traveling cranes etc.,


Write about manual control, semiautomatic control & Automatic control?


Manual control: The electric drives with manual control can be as simple as a room fan, incorporating on switch and a resistance for setting the required speed.


Semiautomatic control: This control consists of a manual device for giving a certain command (Starting, braking, reversing, change of speed etc.,) and an automatic device that in response to command operates the drive in accordance with a preset sequence or order.


Automatic control: The electric drives with automatic control have a control gear, Without manual devices


What are the typical elements of an Electric Drive?

What is a load diagram? What are its types? What are required to draw a load

A load diagram is the diagram which shows graphically the variation of torque acting on the electric drive. The motor of the electric drive has to overcome the load torque expressed as a function of time.


One for the static or steady state process


Other for the dynamic process, when the dynamic components of torque are induced by the inertia of the motor & load.


(Instantaneous speed, acceleration, Torque & power) as a function of time are required to draw…..


What are the types drive systems?(UQ)

Electric Drives


Mechanical Drives


Electromechanical Drives


Hydraulic drives.


Give an expression for the losses occurring in a machine?

The losses occurring in a machine is given by

W = Wc + x2 Wv

Where Wc = Constant losses

Wv = Variable losses at full load

X = load on the motor expressed as a function of rated load.


What are the assumptions made while performing heating & cooling calculation of an electric motor?


The machine is considered to be a homogeneous body having a uniform temperature gradient. All the points at which heat generated have the same temperature. All the points at which heat is dissipated are also at same temperature.


Heat dissipation taking place is proportional to the difference of temperature of the body and surrounding medium. No heat is radiated.


The rate of dissipation of heat is constant at all temperatures.


What are the factors that influence the choice of Electrical drives?

Shaft power & speed

Power range

Starting torque


Total purchase cost

Influence on power supply


Nature of electric supply

Types of drive

Service cost

Speed range


Influence on the supply network

Special competence

Cost of energy losses



Nature of load

Electrical Characteristics

Service capacity & rating


Indicate the importance of power rating & heating of electric drives.


Power rating: Correct selection of power rating of electric motor is of economic interest as it is associated with capital cost and running cost of drives.


Heating: For proper selection of power rating the most important considerations the heating effect of load. In this connection various forms of loading or duty cycles have to be considered.


How heating occurs in motor drives?


The heating of motor due to losses occurring inside the motor while converting the electrical power into mechanical power and these losses occur in steel core, motor winding & bearing friction.


What are the classes of duties?

Continuous duty

Short time duty operation of motor Main classes of duties

Intermittent periodic duty

Intermittent periodic duty with starting

Intermittent periodic duty with starting & braking

Continuous duty with intermittent periodic loading

Continuous duty with starting & braking

Continuous duty with periodic load changes


How will you classify electric drives based on the method of speed control?

Reversible &non reversible in controlled constant speed

Reversible and non reversible step speed control

Reversible and non reversible smooth speed control

Constant predetermined position control

Variable position control

Composite control.


List out some applications for which continuous duty is required.

Centrifugal pumps, fans, conveyors & compressors


Why the losses at starting are not a factor of consideration in a continuous duty motor?


While selecting a motor for this type of duty it is not necessary to give importance to the heating caused by losses at starting even though they are more than the losses at rated load. This is because the motor does not require frequent start nit is started only once in its duty cycle and the losses during starting do not have much influence on heating.


What is meant by “short time rating of motor”?


Any electric motor that is rated for a power rating P for continuous operation can be loaded for a short time duty (Psh) that is much higher than P, if the temperature rise is the consideration.

What is meant by “load equalization”?

In the method of “load Equalization” intentionally the motor inertia is increased By adding a flywheel on the motor shaft, if the motor is not to be reversed. For Effectiveness of the flywheel, the motor should have a prominent drooping characteristic so that on load there is a considerable speed drop.


How a motor rating is determined in a continuous duty and Variable load?

Method of Average losses, equivalent power, equivalent Torque


Define heating time constant & cooling time constant?


Heating time constant is defined as the time taken by the machine to attain 0.623 of its final steady temperature rise.


Cooling time constant is, therefore, defined as the time required cooling the machine down to 0.368 times the initial temperature rise above ambient temperature.


What are the various function performed by an electric drive?

Driving fans, ventilators, compressors & pumps etc.,

Lifting goods by hoists & cranes

Imparting motion to conveyors in factories, mines & warehouses and

Running excavators & escalators, electric locomotives, trains, cars, Trolley buses and lifts etc.


Write down the heat balance equation?


Heat balance equation is given by Ghd0 + S0 .dt = p.dt


What is ingress protection code? (UQ)


The protection code deals with the methods employed for safeguarding the motor against the entry of external agents like dust, water etc.


For example IP 21 deals with safeguarding motor against foreign bodies like water. IP stands for ingress protection code.


What are the mechanical considerations to be considered in Selection of motor (UQ)

Types of enclosures


Types of bearings


Types of mounting


Types of drive


Noise emitted


Mention four types of mechanical load?

Load torque remaining constant irrespective of the speed


Load torque increasing with the square of the speed


Load torque increasing with speed, Load torque decreasing


Define continuous duty of a motor?


Continuous duty: This type drive is operated continuously for a duration which is long enough to reach its steady state value of temperature. This duty is characterized by constant motor torque and constant motor loss operation. This type of duty can be accomplished by single phase/ three phase induction motors and DC shunt motors.




Paper mill drives



Centrifugal pumps and



Draw the block diagram of electric drive?

Draw the heating and cooling curve?

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