ELECTRICAL MACHINES AND DRIVES - COURSE NOTES FOR UNITS I TO V

(FOR III SEM MECHANICAL ENGINEERING STUDENTS)

UNIT-I
INTRODUCTION TO ELECTRICAL DRIVES
robots, pumps, machine tools, etc. Prime movers are required in drive systems to provide the
sources: di esel engines, petrol engines, hydraulic motors, electric motors etc.
There are several advantages of electrical drives:
a. Flexi ble control characteristic – This is particularly true when power electronic
converters are employed where the dynamic and steady state characteristics of the motor
can be controlled by controlling the applied voltage or current.
b. Available in wide range of speed, torque and power
c. High efficiency, lower noi se, low maintenance requirements and cleaner operation
d. Electric energy is easy to be transported.
A typical conventional electric drive system for variable speed application employing multi-
machine system is shown in Figure 1. The system is obviously bul ky, expensive, inflexible and
require regular maintenance. In the past, induction and synchronous machines were used for
constant speed applications – this was mainly because of the unavailabili ty of variable frequency
supply.
Drives are employed for systems that require motion control – e.g. transportation system, fans,
movement or motion and energy that is used to provide the motion can come from various
Drives that use electric motors as the prime movers are known as electrical drives

W ith the advancement of power electronics, microprocessors and digital electronics, typical
electric drive systems nowadays are becoming more compact, efficient, cheaper and versatile –
this is shown in Figure 2. The voltage and current applied to the motor can be changed at will
by employing power electronic converters. AC motor is no longer limited to application where
only AC source is available, however, it can also be used when the power source available is DC
or vice versa.












Electric drives is multi-disciplinary field. Various research areas can be sub-divided from electric drives as shown in Figure 3.


COM PONENTS OF ELECTRICAL DRIVES
The main components of a modern electrical drive are the motors, power processor, control unit and electrical source. These are briefly discussed bel ow
a) Motors
Motors obtain power from electrical sources. They convert energy from electrical to mechanical - therefore can be regarded as energy converters. In braking mode, the flow of power is reversed. Depending upon the type of power converters

2 marks (Questions & Answers)

UNIT – I

INTRODUCTION

                                             

1.      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.

2.   List out some examples of prime movers.

I.C Engines, Steam engine, Turbine or electric motors.

3.  List out some advantages of electric drives.

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

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

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

4.     Give some examples of Electric Drives.

                     i.        Driving fans, ventilators, compressors and pumps.

                   ii.        Lifting goods by hoists and cranes.

                 iii.        Imparting motion to conveyors in factories, mines and warehouses

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

5.     What are the types of electric drives?

Group electric drives (Shaft drive),              I

ndividual Drives,

Multi motor electric drives.

6.   Classify electric drives based on the means of control.

Manual,      Semiautomatic,   Automatic.

7.   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.

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

Advantages:        

·         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.

Disadvantages:

·         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.

2.   What is an individual electric drive? Give some examples.

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.

3.   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.,

4.   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

12. What are the Typical elements of an Electric Drive?

 1. Power Supply 2. Speed & Torque Control
3. Motor 4. Geared coupling 5. Load

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

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.

Types:

·         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…..

14.     What are the types Drive systems?

Electric Drives                                   Mechanical Drives

Electromechanical Drives                  Hydraulic drives.

15.     Give an expression for the losses occurring in a machine.

The losses occurring in a machine is given by 

W = W_c + x² W_v

                   Where Wc = Constant losses

                             Wv = Variable losses at full load

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

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

i.             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.

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

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

17.   What are the factors that influence the choice of electrical drives?

1.  Shaft power & speed                              11.   Speed range

2.  Power range                                           12.   Efficiency

3.  Starting torque                                                13.      Influence on the supply network

4.  Maintenance                                          14.   Special competence

5.  Total purchase cost                                15.   Cost of energy losses

6.  Influence on power supply                     16.   Environment

7.  Availability                                             17.   Accessibility 

8.  Nature of electric supply                        18.   Nature of load

9.  Types of drive                                         19.   Electrical Characteristics

10.Service cost                                            20.   Service capacity & rating

18.       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 consideration is the heating effect  

                of load. In this connection various forms of loading or duty cycles have to be considered.

19.       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.

20.   What are the classes of duties?

1.   Continuous duty

2.   Short time duty operation of motor        Main classes of duties

3.   Intermittent periodic duty

4.   Intermittent periodic duty with starting

5.   Intermittent periodic duty with starting & braking

6.   Continuous duty with intermittent periodic loading

7.   Continuous duty with starting & braking

8.   Continuous duty with periodic load changes

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

1.   Reversible &non reversible in controlled constant speed

2.   Reversible and non reversible step speed control

3.   Reversible and non reversible smooth speed control

4.   Constant predetermined position control

5.   Variable position control

6.   Composite control.

22.  List out some applications for which continuous duty is required.

          Centrifugal pumps, fans, conveyors & compressors

23.  Why the losses at starting is 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 starting it is started only once in its duty cycle and the losses during starting do not have much influence on heating.

24.   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 (P_sh) that is much higher than P, if the temperature rise is the consideration.

25.   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.

26.   How a motor rating is determined in a continuous duty and variable load ?

1.   Method of Average losses

2.   Method of equivalent power

3.   Method of equivalent current

4.   Method of equivalent Torque

27.   Define heating time constant & Cooling time constant?

          The time required to heat the machine parts to 63.3% of its final temperature rise is called as heating time constant.

The time required to cool the machine parts to 36.6% of its final temperature fall is called as cooling time constant.

28.   Draw the heating & Cooling curve of an electric motor.

 

29.   What are the various function performed by an electric drive?

1.    Driving fans, ventilators, compressors & pumps etc.,

2.    Lifting goods by hoists & cranes

3.    Imparting motion to conveyors in factories, mines & warehouses and

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

30.   Write down the heat balance equation.

Heat balance equation is given by

          Ghd0 + S0  .dt  =  p.dt