A.C. Drives for industrial application (voltage source, current source inverter drives & cycloconverter drives)

A.C. DRIVES         [POWER ELECTRONICS] ---by V.K. Mourya

# Introduction : The electrical drive system which control the speed of A.C. motors is known as AC Drives.

So , according to types of motors and their industrial application there are two types of A.C. Drive.

i). Induction Motor drive

ii). Sunchronous Motor drive

Control Mechanism of Ac Drives:- Mostly, There are two types of control method of AC drives.

i). Variable Frequency A.C. Drives

ii). Constant V/F Operation

i). Variable Frequency A.C. Drives:- The speed of Induction Motor can be Calculated by given equation                            

                  Speed in RPM,  Ns =  120 f

                                                         P

Where, f = frequency of AC supply, in Hz

             P = Number of Poles in Stator Winding.

And the Voltage Equation of Induction motor is given by               V = 4.44 KfΦNT

Where V = e.m.f. induced in stator, in volt.

             Φ = air – gap flux in stator

             NT = number of turns in stator winding.

As from above equations, It is clear that the  Synchronous speed (Ns) is directly proportional to frequency (f)

           i.e.             Ns α f.

Thus the speed of motor can be controlled by varying supply frequency.

ii).  Constant V/F operation:- As from Voltage Equation, V = 4.44 KfΦNT. If supply frequency (f) is changed, e.m.f induced in stator or voltage (v) also changed to maintain the same air – gap flux (Φ) below or above rated value, because the operation with air – gap flux (Φ) below or above rated value is not desirable.

                             So, to avoid saturation and to minimize losses, the motor is operated at rated air- gap flux by varying terminal voltage with frequency in the same ratio as to keep (V/f) ratio constant. This type of control is also known as “Constant  volt per hertz”.

As shown in figure below, the motor is started through a Variable frequency – Variable Source (VFS) and a frequency command F ' is used to  change the control speed. This flux control block produces a voltage command V ' for VFS to maintain the ratio (V '/ F ') constant.

VFS can be a voltage source inverter or cyclo-converter.

A constant V/F control of Induction Motor is shown in figure:-

This variable frequency supply is generally obtained by the following inverters:-

i.)                  Voltage Source Inverter Drive

ii.)                Current Source Inverter Drive

iii.)              Cycloconverter controlled A.C. Drive.

i.)                  Voltage Source Inverter Drive :-  We know that the inverter converts a fixed voltage dc to a fixed ( or variable) a.c. voltage with variable frequency. Thus a voltage Source Inverter (VSI) allows a variable frequency supply to be obtained from dc supply.

The voltage source inverter feeding an induction motor is shown in figure:-

Each thyristor is to be fired for an angle 180˚. So, to maintain the air-gap flux of Induction Motor, the ratio of voltage and frequency must be kept constant. The output voltage is varied by varying d.c. input voltage. This adjustable d.c. voltage is obtained by using a controlled rectifier and inverter.

i.)                  Current Source Inverter Drive :- In this ac drive, the induction motor is controlled by using an current source inverter. This type of a.c. drive is known as Current source inverter drive as shown in figure :- 

Each thyristor are fired with a phase difference of 60˚ in sequence of their numbers i.e. SCR1, SCR2, SCR3, SCR4, SCR5, SCR6. Alarge inductance is connected in series to maintain a constant current. The diodes (D1- D6) and capacitors (C1-C6) provide commutation of thyristor (SCR1-SCR6).

The fundamental component of motor current (IL) is given by    IL  =  √ 6   .Id

                                                                                                                                                                          ∏

For a given speed, torque is controlled by varying dc link current (Id) and by changing the dc link voltage (Vd). Thus, dc voltage can be varied also by two arrangements.

First, when the ac supply is used , a controlled rectifier is connected between the supply and inverter and thus speed of induction motor can be controlled.

Another method is ,when dc supply is used, a chopper is connected between dc supply and inverter. Hence, in this way, the speed of induction motor is controlled.

i.)                   Cycloconverter controlled AC drive :- The Cyclo-converter allows variable frequency and voltage obtained from a fixed voltage and frequency of a.c. supply. Three- phase Cycloconverter drives are used to control the speed of both 3- phase induction motors and synchronous motor.

The Basic diagram of cycloconverter drives is shown in figure:-

However, the main disadvantage of cycloconverter based control drives is that the output frequency is one-third of the input frequency. Hence ,Cycloconverter are used to low speed induction motor drives.  

Moddes of Propagation of electromagnetic waves

# MODES OF PROPAGATION OF ELECTROMAGNETIC WAVES:→

The communication is done from one radiating antenna to receiving antenna with the help of propagation of radio waves in that region. So, some methods of propagation of radio waves are given below:-  

(i)                 Surface Wave Propagation.

(ii)               Sky Wave propagation

(iii)             Space Wave propagation.

(i.) SURFACE WAVE PROPAGATION:-   An electromagnetic wave that travels over the surface of the earth, this propagation of wave is known as surface wave propagation.

                         It is propagated by the charge induced in the earth when the waves move over earth surface. These charges move with wave and produce the current. But the earth offers a resistance to the flow of current, so energy is dissipated and the wave gets attenuated and propagates towards the earth’s surface .Surface wave used at frequencies as low as 15 KHz.

At high frequency, surface wave can not be used for long distance transmission because attenuation is more.

APPLICATIONS→ 1.) Surface wave used for ship to ship and ship to shore communication.

2.)    It is used for radio navigation and maritime mobile communication.

DISADVANTAGES→

(1)   Surface wave required a high power transmission.

(2)   These waves are limited to very low level and medium frequencies requiring large antennas.

(3)These losses in these waves may be increase or decrease with surface material.

ADVANTAGES→

(1)   Surface waves are not affected by changing weather condition.

(2)   At enough power it can be used to communicate between any two locations in the world.

(ii.) TROPOSCATTER PROPAGATION  or  SPACE WAVE PROPAGATION:-  In space wave propagation, the electromagnetic wave travels of earth’s atmosphere. Space waves may be both ground and reflected waves.

Direct wave travel in straight line between the transmitting and receiving antennas.

                      Space wave propagation with direct waves is called line-of-sight transmission (LOS). Therefore direct space wave is limited by the curvature of earth.

Ground reflected waves are the waves reflected by earth’s surface as they propagate between transmit and receive antenna.

The space wave propagation between two antenna is shown in figure:-

The field intensity at the receiver antenna depends upon the distance between the two  antennas. The curvature of earth presents horizon to space wave propagation called radio horizon.

                 The  refraction of wave is caused by the troposphere because of changes in its density, temperature ,water vapour content and relative conductivity,

        The region of radio horizon can be increased by elevating the transmit and receive antenna above the earth’s surface with tower or by placing them on the top of mountain or high building.

The space wave and radio horizon is shown in figure .

Let   d=distance to radio horizon(miles)

h=height of antenna above sea level.

         =>      d = √ 2h

        as        d = dt +dr

                        d = √ 2ht   +√ 2hr

where, dt = distance to radio horizon for transmit antenna

              dr = distance to radio horizon for receiving antenna

            ht = height of transmit antenna ,  &    hr = height of receiving antenna.

Duct Propagation:-  When the density of lower atmosphere is such that electromagnetic wave are trapped between lower atmosphere and earth’s surface then this type of propagation of wave is known as duct propagation.

The layer of atmosphere acts as duct, and hence the electromagnetic waves can propagate for a large distance around the curvature of earth with in this duct as shown in figure.

(iii) SKY WAVE PROPAGATION or  IONOSPHERIC PROPAGATION :- When the electromagnetic wave propagates in ionosphere region of earth’s atmosphere and these wave either reflected or reflected back to earth  by the ionosphere, then this  type of wave propagation is known as sky wave or ionospheric  propagation.

The Sky Wave Propagation is shown in fig.

The ionosphere located approximately 50km to 400km above the earth’s surface.

When a radio passes through ionosphere the electric field of the wave exerts a force on the free electrons causing then to vibrate.

These vibrating electrons decrease the current which is equivalent to reducing the dielectric constant.

By reducing the dielectric constant, the velocity of propagation is increases and hence the electromagnetic wave bent away from ionosphere toward the earth’s surface.

Basic Microwave Communication System/Link

Microwave communication system  [ECE]  --- by V.K. Mourya

# Basic Microwave Communication Link :-

                                                                       Microwave signal are used for communication over long distance continental or intercontinental. Microwave is the communication link which make the communication possible. The basic block diagram of microwave communication system is shown in figure.

Construction:

Antenna:- Mostly a parabolic refractor types of antenna are used which is used to transmit and receive the signal.

Circulator: A circulator is used to isolate transmitter with the receiver input and to couple transmitter to antenna and antenna to receiver input.

Protection Circuitry: It provides safety to the mixer from overloads.

Mixer (Receiver):  It has two outputs. One is the incoming signal and other is the signal from lower band pass filter (BPF).The mixer gives an IF signal of 70Mhz.

Band pass filter (BPF): It provides the necessary selectivity to the receiver and it prevents the interference.

IF amplifier and AGC:- It amplifies the signal up to a intermediate frequency of 70Mhz. and its gain is controlled through AGC (automatic gain control)

Amplitude limiter: As the signal is frequency modulated one so as amplitude limiter is used to avoid unwanted amplitude variations.

Mixer (Transmitter): It is used to convert IF frequency to transmitting microwave frequency band to pass through it and hence prevent interference.

POWER AMPLIFIER:-This amplifier amplifies the transmitted power from a repeater section in the range of 0.2W to 10W.

MICROWAVE SOURCE:- Klystron & Gunn Oscillators were used as microwave source. Now, V H F transistor crystal oscillators are used for microwave source.

POWER SPLITTER:- It divides the output power from a microwave source and feeds a large portion to the transmitter mixer, which converts it into transmitting microwave frequency.

SHIFT OSCILATOR:- It provides one of the inputs to the balanced mixer so that it produces 70MHz IF at the output of receiver mixer.

 

This microwave link communicates with 600 to 2700 channels per carrier. Thus the number of carriers in each direction can be four to twelve.

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Machine Drawing next Important questions[mech. engg.]

Important Definitions

31. Roughness width cut off :  The distance used to find the arithmetical average deviation is called the roughness width cut off.

32.Waviness :  It may be defined as the surface irregularities in the form of waves having larger wavelength.

33. Waviness height : It is the distance from the peak of the wave to its valley. It is measured in millimeters.

34. Waviness width : It is the spacing between the successive waves. It is measured in millimeters.

35.  Lay : Lay is the primary direction of the surface pattern prepared by machine tool marks.

36.  Flaws : They occur at random places on a machine component. These are cracks & scratches.

 37.  Ideal surface : It is a hypothetical perfect surface without any micro irregularity.

38.  Actual surface : It is defined as surface of a part which actually is received after machining surface.

39.  Nominal surface : A nominal surface is theoretical, geometrically perfect surface which is not to exist in the general practice.

40. Actual Profile :  It is the actual surface obtained by a manufacturing process.

41. Datum Profile : The profiles which passes through the lowest points of actual profile is called Datum profile.

42. Mean Profile : The profiles which passes  through the actual profile in such a way that the filled up areas between mean profile & actual profile is equal to the areas of voids between mean profile & actual  profile within sampling length.

43. What is reference profile?

Ans 43: The profile which passes through the highest points of actual profile is called reference profile.

44. What do you mean by peak to valley height?

Ans 44: It is the distance between reference profile & datum profile.

45. Define mean roughness index.

Ans45: It is the arithmetic mean of absolute values of heights of peaks or depth of valleys.

46. Define surface roughness number.

Ans 46: It is the average departure of machined surface over a sampling length which is generally taken as 800 microns.

47. Draw the basic symbols of surface roughness.

48. Define coupling.

Ans 48: It may be defined as the joining of two or more pieces of shafts in – order to obtain large length.

49. Where is Oldham’s Coupling best suited?

Ans 49: It is best suited to connect the shafts whose axes are parallel, but not in alignment.

50. Which  material is used for Oldham’s Coupling?

Ans 50: Shafts – Mild Steel

              Flanges & Disc – Cast Iron

              Keys – Mild Steel

51. Draw roughness symbols with values & grade numbers.

VFD (Variable Frequency Drive) [power electronics]

Variable-Frequency Drive (VFD)            [Power Electronics]

#  Variable Frequency Drive (VFD):-

                                                                           A variable- frequency  (VFD) is a system for controlling the rational speed of an A.C motor by controlling The frequency   of electric power supplied to the motor. 

It is also known as Adjustable- frequency drives (AFD) variable – speed drives (VSD), AC drives ,inverter drives.

Operating  Principle:

                                        The synchronous speed of  an AC motor is determined by the frequency of AC supply and number of poles in the stator winding. Thus the Synchronous motor operates at the synchronous speed determined by this equation.

                     Speed in RPM  =  120  X   f

                                                           P

Where, f  be the frequency of AC supply.

                      P = Number of poles in stator winding.

Hence , speed of motor can be controlled by varying frequency of power supplied to motor.

VFD S ystem Description:

                                                 A VFD system consists of an AC motor, a controller and an operator interface as shown in figure:-

VFD Motor:  VFD system consists of 3-Φ induction motor. Mostly, Induction motors are preferred than synchronous motor. Because induction motor are suitable for controlling and is cheap.

VFD Controller :-  It is a solid state  electronic power conversion  device which first convert  AC power in to DC intermediate by using bridge rectifier. This DC power  is then converted in to quasi- sinusoidal AC power using an inverter circuit.

 VFD Operator Interface:-    An operator interface is used to start and stop the motor and to adjust the operating speed.

                              This VFD operating interface  consists of an keypad, display, indication lights and meters to provide information about the operation of the drive. Motor can be  controlled and monitored on computer by using an serial communication part on VFD operator interface .

 VFD Operation :-  When VFD starts ,the applied frequency  and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current.

               This starting method allows a motor to develop 150% of its rated torque while drawing only 50% of its rated current.

        When a motor is switched ON at full voltage, initially it have at least 300% of its rated current.  While it produce less then 50% of its rated torque.

        As load accelerates, the  available torque usually decrease very little and then increases to a peak while the current remains very high until the motor get full speed.

              By using VFD system, the frequency & voltage applied to the motor  are ramped down at a controlled rate.

              When frequency becomes zero, the motor is shut-off. A small amount of breaking torque is available to help decelerate the load.

                Thus the speed of AC motor can be controlled by varying  frequency of power applied to the motor.

Machine Drawing Notes (Mechanical Engineering)

Machine Drawing               (Mechanical Engineering)

Very-Very important questions to get sure success which covered complete syllabus

Q1: Define basic size?

Ans1: Basic size is that size of a components on which great or less size of a component can be measured.

Q2: Define Limit?

Ans2: It is the higest  and lower limit of a size of a component . The higest size is called high limit of the lowest size is known as low limit.

Q3:

Ans3:  Basic size +20mm

          Maximum  limit of size =20+0.005 +20.005mm(Max. limit)

          Minimum Limit of size =20-0.004=19.996mm(Mini. Limit)

Q4: Define Tolerance?

Ans4: It is the difference between maximum & minimum size of a machine part .Tolerance is the difference between high limit & low limit of a size of a component.

Q5: Calculate the tolerance of the following?

                   Basic size  =  80+0.002

Ans5:  Tolerance= High limit - Low limit

                             = 80.002-79.998 = 0.004 mm

Q6: What are the types of tolerance?

Ans6: Tolerances are of two types:-

i)                    Unilateral Tolerance:- This tolerance is given on one side of the basic size.

 For example-

i)                    Bilateral Tolerance:- In this system the tolerance is given on both the sides of the basic size.

For example-

Q7: Define allowance?

Ans7:  The difference between the dimension of two mating ports is called allowance. It is of two types

i)                    Maximum Allowance:- It is the difference between the  maximum size of the hole & minimum size of the shaft.

ii)                  Minimum Allowance:-  It is the difference between the  maximum size of the shaft & minimum size of the hole.

Q8: Given, i) The maximum hole size =  30.099 mm and minimum shaft size = 30.050 mm. calculate the maximum allowance?

(ii). The maximum hole size =  30.075 mm and minimum shaft size = 30.060 mm. calculate the minimum allowance?

Ans8:  i)  Maximum allowance = 30.099 – 30.050 = 0.049 mm

ii). Minimum Allowance = 30.075 – 30.060 = 0.015 mm

Q9: Define FIT.

Ans9: The assembly of two mating parts is called Fit.

Q10: Write different type of Fits?

Ans10: 1) Clearance Fit:- In clearance fit , shaft is smaller than hole. This type of fits gives loose fitting. There must be some degree of freedom between a shaft & a hole.

2) Interference Fit:- In interference Fit, there is always a negative allowance between the largest hole size & the smallest shaft size. In this fit, the shaft is always larger than the hole.

3) Transition Fit:- In this type of Fits, the fit may be either clearance fit or interference fit. Any pair of parts mating with transition fit may fit with interference while on other pair with the same fit have a clearance fit.

Q11: Define Deviation.

Ans11: It is defined as the difference between actual size or Limit sizes, either maximum or minimum, and the corresponding basic size.

Q12:Define actual deviation?

Ans12: It is the algebraic difference between the actual measured size & corresponding basic size.

Q13: Define upper deviation?

Ans13:  The algebraic difference between the maximum limit of a size & the corresponding basic size.

Q14: Define lower deviation?

Ans14:  The algebraic difference between the minimum limit of a size & the corresponding basic size.

figure:deviation

Q15: Define zero line.

Ans15: Zero line is the line of zero deviation and represents the basic size.

Q16: Define tolerance Zone.

Ans16: Tolerance Zone is the algebraic difference between maximum limit of size & minimum limit of size.In other words, tolerance zone is the algebraic difference between upper deviation & lower deviation.

Q17: Define functional Deviation.

Ans17: Functional deviation is the upper or lower deviation which is closest to basic size.

Q18: Define hole basis system.

Ans18: It is the standard system of  limits & fits. The size of the hole is kept constant & the size of the shaft is kept variable to get the different types of fits is known as hole basis system. The symbol ‘H’ is a fundamental deviation is selected for the holes, when the hole basis system is follows.

Q19: Define shaft basis system.

Ans 19: When the sizes of the shaft is kept sonstant in a standard system of limits & fits, and variations given to the hole for obtaining different types of fits, is known as shaft basis system. The fundamental deviation symbol ‘H’ is selected for shaft basis.

Q20: Define fundamental tolerance .

Ans20: This is also known as grade of Tolerance in Indian Standard System, there are 18 grade of tolerances represented by number symbols both for hole and shaft such as ITOI, ITO, IT1 to IT16. A high number gives a large tolerance zone . fundamental tolerance are indicated for sizes ranging upto 525 mm.

Q21: Define Limit System?

Ans21: The system in which deviations are accepted are called limit system.

Q22: Define standardization.

Ans22:  Standardisation means the size of a machine element accepted nationally or internationally.

Q23: What do you mean by H7/g6?

Ans23: It is sliding fit. Its application are in clutches & shafts.

Q24: What do you mean by φ25 H8 u7?

Ans24:  Basic size = 25 mm

H8 means an h hole to tolerance grade 8.

 u7 means a u shaft to tolerance grade 7.

Q25: Define clearance.

Ans25: It is defined as the difference between the dimensions of  the hole & the shaft assigned intentionally to obtain a particular type of fit. It may be positive or negative when the shaft size is smaller than the hole size, it will be positive & when the shaft size is bigger than the hole , it will be negative.

 Maximum Clearance:- It is the difference between maximum hole & minimum shaft.

Minimum clearance:- In clearance fit, the minimum clearance is the difference between the minimum hole & the maximum shaft.

figure: Clearance

Q  26:Define Interferance.

Ans26: The  negative difference between the size of the hole & shaft before its assembly in made.

Q27: If the size of the hole is 40.02,  40.00 and the size of shaft is 39.96,  39.94 . Find (i) Hole tolerance   (ii) shaft tolerance   (iii) allowance?

Ans27:  (i) Hole tolerance = Maximum limit – Minimum limit

                                            = 40.02 - 40.00 = 0.02 mm

(ii) shaft tolerance =  max. limit – min.limit= 39.96 – 39.94 = 0.02 mm

(iii) Allowance = min. size of hole – max size of shaft = 40.00 – 39.96 = 0.04 mm

Q28: Define surface roughness.

Ans28:  Surface roughness may be defined as the fine irregularities in the surface texture.

Q29: Define Roughness height?

Ans29: It is also known as roughness average (Ra) is an arithmetical average of the roughness deviation.

Q30: Define roughness width.

Ans30: It is the distance parallel to the nominal surface between the two successive peaks, which constitute the predominant pattern of the roughness. It is measured in millimeter.