ENERGY STORAGE IN SINGLE EXCITED SYSTEM

Consider once again the attracted armature relay excited by an electric source as in Fig.4. The field produces a mechanical force Ff in the direction indicated which drives the mechanical system (which may be composed of passive and active mechanical elements). The mechanical work done by the field when the armature moves a distance dx in positive direction is

In this form of expression for the mechanical force of field origin,  is the independent variable, i.e. it is a voltage-controlled system as voltage is the derivative of .

In linear systems where inductances are specified it is more convenient to use coenergy for finding the force developed (Eq. (25)). If the system is voltage controlled, the current can be determined by writing the necessary circuit equations.

It is needless to say that the expressions of Eqs (25) and (29) for force in a translatory system will apply for torque in a rotational system with x replaced by angular rotation θ.

Flow of Energy in Electromechanical Devices

Electromechanical energy conversion is a reversible process and Eqs (26)-(29) govern the production of mechanical force. In Fig. 4 , if the armature is allowed to move on positive x direction under the influence of Ff , electrical energy is converted to mechanical form via the coupling field. If instead the armature is moved in the negative x direction under the influence of external force, mechanical energy is converted to electrical form via the coupling field. This conversion process is not restricted to translatory devices as illustrated but is equally applicable to rotatory devices. Electrical and mechanical losses cause irreversible flow of energy out of a practical conversion device. The flow of energy in electromechanical conversion in either direction along with irrecoverable energy losses is shown in (Fig-5)

                     (Fig-5)Flow of energy in electromechanical energy conversion via a coupling field

ENERGY IN MAGNETIC SYSTEM

Energy can be stored or retrieved from a magnetic system by means of an exciting coil connected to an electric source. Consider, for example the magnetic system of an attracted armature relay as shown below.

INTRODUCTION TO PRINCIPLES OF ELECTROMECHANICAL ENERGY CONVERSION

•   The chief advantage of electric energy over other forms of energy is the relative ease and high efficiency with which it can be transmitted over long distances.

• •    Its main use is in the form of a transmitting link for transporting other forms of energy, e.g. mechanical, sound, and light, etc. from one physical location to another.

• •    Electric energy is seldom available naturally and is rarely directly utilized.

• •    Obviously two kinds of energy conversion devices are needed, to convert one form of energy to the electric form and to convert it back to the original or any other desired form.

• •    These devices can be transducers for processing and transporting low- energy signals.

• •    A second category of such devices is meant for production of force or torque with limited mechanical motion like electromagnets, relays, actuators etc.

• •    A third category is the continuous energy conversion devices like motors or generator which are used for bulk energy conversion and utilization.

• •    Electromechanical energy conversion takes place via the medium of a magnetic or electric field the magnetic field being most suited for practical conversion devices.

• •    Because of the inertia associated with mechanically moving members, the fields must necessarily be slowly varying, i.e. quasistatic in nature.

CRYSTAL STRUCTURE OF INTRINSIC SEMICONDUCTOR

• An outermost shell of an atom is capable of holding eight electrons. It is said to be completely filled and stable, if it contains eight electrons.

• But  the  outermost  shell  of  intrinsic  semiconductor  like  silicon  has  only  four electrons.

• Each  of  these  four  electrons  forms  a  bond  with  another  valence  electron  of the neighboring  atoms. This is  nothing but sharing of  electrons. Such  bonds     are called covalent bonds.

• The  atoms  align  themselves  to  form  a  three  dimensional  uniform  pattern called a crystal.

• The crystal structure of germanium and silicon materials consists of repetitive occurrence in three dimensions of a unit cell. This unit cell is in the form of a tetrahedron with an atom at each vertex.

• The  covalent  bonds  are  represented  by  a  pair  of  dotted  lines  encircling  the two electrons forming the covalent bond.

• Both the electrons are shared by the two atoms. Hence the outermost shell of all the atoms is completely filled, and the valence electrons are tightly bound  to the parent atoms.

• No free electrons are available at absolute zero temperature. Hence such an  intrinsic   semiconductor   behaves   as   a   perfect   insulator   at   absolute   zero temperature.

• At  room  temperature,  the  number  of  valence  electrons  absorbs  the  thermal energy, due to which they break the covalent bond and drift to the conduction  band. Such electrons become free to move in the crystal

Microsoft Launches Office 365, a New Google Competitor

Microsoft launched Office 365 in New York City June 28, very conspicuously choosing the same venue it used for Windows 7’s debut in October 2009. And as with Windows 7, Microsoft has a lot riding on this particular launch. If Office 365 succeeds with businesses and consumers, it’ll help validate the company’s choice of an "all in" cloud strategy. If the platform fails, it’ll provide an opening for other cloud-software producers—most notably Google, which already offers a cloud-productivity platform with Google Apps—to establish themselves in the space. Office 365 is a rebranding of Microsoft’s BPOS (Business Productivity Online Suite), and binds Microsoft Office, SharePoint Online, Exchange Online and Lync Online onto a common cloud platform that costs between $2 and $27 per user, per month. On top of that, Microsoft is offering an Office 365 Marketplace with productivity apps and professional services. In sum, Office 365 is meant to provide everything from conferencing to document editing to video editing in one convenient (and accessible) place. There’s also a focus on interoperability across multiple devices, including smartphones running Microsoft’s Windows Phone. For this release, Microsoft is particularly targeting SMBs, claiming Office 365 will give them a competitive edge without the burden of complex on-premises systems. The question is whether those businesses will find Office 365 durable enough, and feature-filled enough, to meet their needs. 

Firefox 5 Browser Launches, Boasting Tweaks, Security, Privacy

Mozilla's latest browser includes more than 1,000 improvements and performance enhancements. That being said, the actual user interface doesn't vary much from Firefox 4.

 

 

Mozilla's new Firefox 5.0 for PCs and Firefox for Android aim to offer users a best-of-class combination of security, privacy and speed. Coming a mere three months after the release of Firefox 4, this latest browser supposedly includes more than 1,000 improvements and performance enhancements. However, features like "Do Not Track" may do more to draw users increasingly leery of the Web's rampant data mining. For most consumers, downloading and installing Firefox 5 will only take a few minutes, although larger companies still deploying Firefox 4 could become annoyed at having to switch so soon. Despite those hundreds of improvements, Firefox 5 doesn't seem to offer a radically different experience from its predecessor. Certainly, it addresses security concerns left over from Firefox 4, whose life essentially ends with this release. But for the most part, Mozilla's latest offering embraces the same streamlined design (with an emphasis on putting the Web's content front-and-center, via shrunk icons and an eliminated "status" bar along the bottom of the screen) and features as before. Firefox 5 will face competition not only from Microsoft's still-enduring Internet Explorer franchise, but also upstarts such as Google Chrome and Safari, both of which have gained market share over the past year. Meanwhile, Firefox for Android aims to extend features like "Do Not Track" across multiple platforms.  

 

CLASSIFICATION OF MATERIALS BASED ON ENERGY BAND THEORY

• Based on the ability of various materials to conduct current, the materials are classified as conductors, insulators and the semiconductors.

• A  metal  which  is  very  good  carrier  of  electricity  is  called  conductor.  The copper and aluminium are good examples of a conductor

• A  very poor conductor  of electricity  is termed as  insulator.  The glass,  wood, mica, diamond are the examples of an insulator.

• A  metal  having  conductivity  which  is  between conductor  and  an  insulator  is called  semiconductor.  The  silicon  and  germanium  are  the  examples  of  a semiconductor.  This  does  not  conduct  current  at  low  temperatures  but  as temperature increases these materials behave as good conductors..

ENERGY BAND DIAGRAMS

CONDUCTORS

• In  the  metals  like  copper,  aluminium  there  is  no  forbidden  gap  between valence band and conduction band. The two bands overlap.

• Hence  even  at room temperature, a  large number of  electrons  are  available for conduction.

• So without any additional energy, such metals contain a large number of free electrons and hence called good conductors.

INSULATORS

• In  case  of  such  insulating  material,  there  exists  a  large  forbidden  gap  in between the conduction band and the valence band.

• Practically it is impossible for an electron to jump from the valence band to

• the   conduction   band.   Hence   such   materials   cannot   conduct   and   called insulators.

• The  forbidden  gap  is  very  wide,  approximately  of  about  7  eV  is  present  in insulators. For a diamond, which is an insulator, the forbidden gap is about 6 eV.

                            

• Such  materials  may  conduct  only  at  very  high  temperatures  or  if  they  are subjected to high voltage. Such conduction is rare and is called breakdown of an insulator.

• The other insulating materials are glass, wood, mica, paper etc.

SEMICONDUCTORS

• The  forbidden  gap  in  such  materials  is  very  narrow  as  shown  in  Fig.  Such materials are called semiconductors.

• The forbidden gap is about 1 eV.

• In  such  materials,  the  energy  provided  by  the  heat  at  room temperature  is sufficient to lift the electrons from the valence band to the conduction band.

INTRINSIC SEMICONDUCTORS

• A sample of semiconductor in its purest form is called an intrinsic semiconductor.

• The impurity content in intrinsic semiconductor is very small, of the order of one part in 100 million parts of semiconductor.

AC phase

Things start to get complicated when we need to relate two or more AC voltages or currents that are
out of step with each other. By \out of step," I mean that the two waveforms are not synchronized:
that their peaks and zero points do not match up at the same points in time. The graph in ¯gure 1.24
illustrates an example of this.

The shift between these two waveforms is about 45 degrees, the \A" wave being ahead of the\B" wave. A sampling of di®erent phase shifts is given in the following graphs to better illustrate
this concept: Figure 1.26

Because the waveforms in the above examples are at the same frequency, they will be out of step
by the same angular amount at every point in time. For this reason, we can express phase shift for
two or more waveforms of the same frequency as a constant quantity for the entire wave, and not
just an expression of shift between any two particular points along the waves. That is, it is safe to
say something like, \voltage 'A' is 45 degrees out of phase with voltage 'B'." Whichever waveform
is ahead in its evolution is said to be leading and the one behind is said to be lagging.

Phase shift, like voltage, is always a measurement relative between two things. There's really no
such thing as a waveform with an absolute phase measurement because there's no known universal
reference for phase. Typically in the analysis of AC circuits, the voltage waveform of the power
supply is used as a reference for phase, that voltage stated as \xxx volts at 0 degrees." Any other
AC voltage or current in that circuit will have its phase shift expressed in terms relative to that
source voltage.


This is what makes AC circuit calculations more complicated than DC. When applying Ohm's
Law and Kirchho®'s Laws, quantities of AC voltage and current must re°ect phase shift as well
as amplitude. Mathematical operations of addition, subtraction, multiplication, and division must
operate on these quantities of phase shift as well as amplitude. Fortunately, there is a mathematical
system of quantities called complex numbers ideally suited for this task of representing amplitude
and phase.

Because the subject of complex numbers is so essential to the understanding of AC circuits, the
next chapter will be devoted to that subject alone.

MEASUREMENTS OF AC MAGNITUDE

So far we know that AC voltage alternates in polarity and AC current alternates in direction. We
also know that AC can alternate in a variety of di®erent ways, and by tracing the alternation over
time we can plot it as a \waveform." We can measure the rate of alternation by measuring the time
it takes for a wave to evolve before it repeats itself (the \period"), and express this as cycles per
unit time, or \frequency." In music, frequency is the same as pitch, which is the essential property
distinguishing one note from another.

However, we encounter a measurement problem if we try to express how large or small an AC
quantity is. With DC, where quantities of voltage and current are generally stable, we have little
trouble expressing how much voltage or current we have in any part of a circuit. But how do you
grant a single measurement of magnitude to something that is constantly changing?

One way to express the intensity, or magnitude (also called the amplitude), of an AC quantity
is to measure its peak height on a waveform graph. This is known as the peak or crest value of an
AC waveform: Figure 1.14

Unfortunately, either one of these expressions of waveform amplitude can be misleading when
comparing two di®erent types of waves. For example, a square wave peaking at 10 volts is obviously
a greater amount of voltage for a greater amount of time than a triangle wave peaking at 10 volts.
The e®ects of these two AC voltages powering a load would be quite di®erent: Figure 1.16

One way of expressing the amplitude of di®erent waveshapes in a more equivalent fashion is to
mathematically average the values of all the points on a waveform's graph to a single, aggregate
number. This amplitude measure is known simply as the average value of the waveform. If we
average all the points on the waveform algebraically (that is, to consider their sign, either positive
or negative), the average value for most waveforms is technically zero, because all the positive points
cancel out all the negative points over a full cycle: Figure 1.17

This, of course, will be true for any waveform having equal-area portions above and below the
\zero" line of a plot. However, as a practical measure of a waveform's aggregate value, \average" is
usually de¯ned as the mathematical mean of all the points' absolute values over a cycle. In other
words, we calculate the practical average value of the waveform by considering all points on the wave
as positive quantities, as if the waveform looked like this: Figure 1.18

Polarity-insensitive mechanical meter movements (meters designed to respond equally to the
positive and negative half-cycles of an alternating voltage or current) register in proportion to
the waveform's (practical) average value, because the inertia of the pointer against the tension of
the spring naturally averages the force produced by the varying voltage/current values over time.

Conversely, polarity-sensitive meter movements vibrate uselessly if exposed to AC voltage or current,
their needles oscillating rapidly about the zero mark, indicating the true (algebraic) average value of
zero for a symmetrical waveform. When the \average" value of a waveform is referenced in this text,
it will be assumed that the \practical" de¯nition of average is intended unless otherwise speci¯ed.

Another method of deriving an aggregate value for waveform amplitude is based on the wave-
form's ability to do useful work when applied to a load resistance. Unfortunately, an AC mea-
surement based on work performed by a waveform is not the same as that waveform's \average"
value, because the power dissipated by a given load (work performed per unit time) is not directly
proportional to the magnitude of either the voltage or current impressed upon it. Rather, power is
proportional to the square of the voltage or current applied to a resistance (P = E2/R, and P =
I2R). Although the mathematics of such an amplitude measurement might not be straightforward,
the utility of it is.

Consider a bandsaw and a jigsaw, two pieces of modern woodworking equipment. Both types of
saws cut with a thin, toothed, motor-powered metal blade to cut wood. But while the bandsaw uses
a continuous motion of the blade to cut, the jigsaw uses a back-and-forth motion. The comparison
of alternating current (AC) to direct current (DC) may be likened to the comparison of these two
saw types: Figure 1.19

The problem of trying to describe the changing quantities of AC voltage or current in a single,
aggregate measurement is also present in this saw analogy: how might we express the speed of a
jigsaw blade? A bandsaw blade moves with a constant speed, similar to the way DC voltage pushes
or DC current moves with a constant magnitude. A jigsaw blade, on the other hand, moves back
and forth, its blade speed constantly changing. What is more, the back-and-forth motion of any two
jigsaws may not be of the same type, depending on the mechanical design of the saws. One jigsaw
might move its blade with a sine-wave motion, while another with a triangle-wave motion. To rate
a jigsaw based on its peak blade speed would be quite misleading when comparing one jigsaw to
another (or a jigsaw with a bandsaw!). Despite the fact that these di®erent saws move their blades

In the two circuits above, we have the same amount of load resistance (2 ­) dissipating the same
amount of power in the form of heat (50 watts), one powered by AC and the other by DC. Because
the AC voltage source pictured above is equivalent (in terms of power delivered to a load) to a 10 volt
DC battery, we would call this a \10 volt" AC source. More speci¯cally, we would denote its voltage
value as being 10 volts RMS. The quali¯er \RMS" stands for Root Mean Square, the algorithm used
to obtain the DC equivalent value from points on a graph (essentially, the procedure consists of

squaring all the positive and negative points on a waveform graph, averaging those squared values,
then taking the square root of that average to obtain the ¯nal answer). Sometimes the alternative
terms equivalent or DC equivalent are used instead of \RMS," but the quantity and principle are
both the same.

RMS amplitude measurement is the best way to relate AC quantities to DC quantities, or other
AC quantities of di®ering waveform shapes, when dealing with measurements of electric power. For
other considerations, peak or peak-to-peak measurements may be the best to employ. For instance,
when determining the proper size of wire (ampacity) to conduct electric power from a source to
a load, RMS current measurement is the best to use, because the principal concern with current
is overheating of the wire, which is a function of power dissipation caused by current through the
resistance of the wire. However, when rating insulators for service in high-voltage AC applications,
peak voltage measurements are the most appropriate, because the principal concern here is insulator
\°ashover" caused by brief spikes of voltage, irrespective of time.

Peak and peak-to-peak measurements are best performed with an oscilloscope, which can capture
the crests of the waveform with a high degree of accuracy due to the fast action of the cathode-
ray-tube in response to changes in voltage. For RMS measurements, analog meter movements
(D'Arsonval, Weston, iron vane, electrodynamometer) will work so long as they have been calibrated
in RMS ¯gures. Because the mechanical inertia and dampening e®ects of an electromechanical meter
movement makes the de°ection of the needle naturally proportional to the average value of the AC,
not the true RMS value, analog meters must be speci¯cally calibrated (or mis-calibrated, depending
on how you look at it) to indicate voltage or current in RMS units. The accuracy of this calibration
depends on an assumed waveshape, usually a sine wave.

Electronic meters speci¯cally designed for RMS measurement are best for the task. Some in-
strument manufacturers have designed ingenious methods for determining the RMS value of any
waveform. One such manufacturer produces \True-RMS" meters with a tiny resistive heating ele-
ment powered by a voltage proportional to that being measured. The heating e®ect of that resistance
element is measured thermally to give a true RMS value with no mathematical calculations whatso-
ever, just the laws of physics in action in ful¯llment of the de¯nition of RMS. The accuracy of this
type of RMS measurement is independent of waveshape.

For \pure" waveforms, simple conversion coe±cients exist for equating Peak, Peak-to-Peak, Av-
erage (practical, not algebraic), and RMS measurements to one another: Figure 1.21

  REVIEW:

•  The amplitude of an AC waveform is its height as depicted on a graph over time. An amplitude
• measurement can take the form of peak, peak-to-peak, average, or RMS quantity.
• Peak amplitude is the height of an AC waveform as measured from the zero mark to the highest
• positive or lowest negative point on a graph. Also known as the crest amplitude of a wave.
• Peak-to-peak amplitude is the total height of an AC waveform as measured from maximum
• positive to maximum negative peaks on a graph. Often abbreviated as \P-P".
• Average amplitude is the mathematical \mean" of all a waveform's points over the period of
• one cycle. Technically, the average amplitude of any waveform with equal-area portions above

and below the \zero" line on a graph is zero. However, as a practical measure of amplitude,
a waveform's average value is often calculated as the mathematical mean of all the points'
absolute values (taking all the negative values and considering them as positive). For a sine
wave, the average value so calculated is approximately 0.637 of its peak value.

• \RMS" stands for Root Mean Square, and is a way of expressing an AC quantity of voltage or

current in terms functionally equivalent to DC. For example, 10 volts AC RMS is the amount
of voltage that would produce the same amount of heat dissipation across a resistor of given
value as a 10 volt DC power supply. Also known as the \equivalent" or \DC equivalent" value
of an AC voltage or current. For a sine wave, the RMS value is approximately 0.707 of its
peak value.

• The crest factor of an AC waveform is the ratio of its peak (crest) to its RMS value.

• The form factor of an AC waveform is the ratio of its RMS value to its average value.

•  Analog, electromechanical meter movements respond proportionally to the average value of

an AC voltage or current. When RMS indication is desired, the meter's calibration must be
\skewed" accordingly. This means that the accuracy of an electromechanical meter's RMS
indication is dependent on the purity of the waveform: whether it is the exact same waveshape
as the waveform used in calibrating.

DEFINITIONS ABOUT AC WAVEFORMS

When an alternator produces AC voltage, the voltage switches polarity over time, but does so in a
very particular manner. When graphed over time, the \wave" traced by this voltage of alternating
polarity from an alternator takes on a distinct shape, known as a sine wave: Figure 1.8

If we were to follow the changing voltage produced by a coil in an alternator from any point
on the sine wave graph to that point when the wave shape begins to repeat itself, we would have
marked exactly one cycle of that wave. This is most easily shown by spanning the distance between
identical peaks, but may be measured between any corresponding points on the graph. The degree
marks on the horizontal axis of the graph represent the domain of the trigonometric sine function,
and also the angular position of our simple two-pole alternator shaft as it rotates: Figure 1.9

Since the horizontal axis of this graph can mark the passage of time as well as shaft position in
degrees, the dimension marked for one cycle is often measured in a unit of time, most often seconds
or fractions of a second. When expressed as a measurement, this is often called the period of a wave.

The period of a wave in degrees is always 360, but the amount of time one period occupies depends
on the rate voltage oscillates back and forth.

A more popular measure for describing the alternating rate of an AC voltage or current wave
than period is the rate of that back-and-forth oscillation. This is called frequency. The modern unit
for frequency is the Hertz (abbreviated Hz), which represents the number of wave cycles completed
during one second of time. In the United States of America, the standard power-line frequency is
60 Hz, meaning that the AC voltage oscillates at a rate of 60 complete back-and-forth cycles every

An instrument called an oscilloscope, Figure 1.10, is used to display a changing voltage over time
on a graphical screen. You may be familiar with the appearance of an ECG or EKG (electrocardio-
graph) machine, used by physicians to graph the oscillations of a patient's heart over time. The ECG
is a special-purpose oscilloscope expressly designed for medical use. General-purpose oscilloscopes
have the ability to display voltage from virtually any voltage source, plotted as a graph with time
as the independent variable. The relationship between period and frequency is very useful to know
when displaying an AC voltage or current waveform on an oscilloscope screen. By measuring the
period of the wave on the horizontal axis of the oscilloscope screen and reciprocating that time value
(in seconds), you can determine the frequency in Hertz.

Voltage and current are by no means the only physical variables subject to variation over time.
Much more common to our everyday experience is sound, which is nothing more than the alternating
compression and decompression (pressure waves) of air molecules, interpreted by our ears as a phys-
ical sensation. Because alternating current is a wave phenomenon, it shares many of the properties
of other wave phenomena, like sound. For this reason, sound (especially structured music) provides
an excellent analogy for relating AC concepts.

In musical terms, frequency is equivalent to pitch. Low-pitch notes such as those produced by
a tuba or bassoon consist of air molecule vibrations that are relatively slow (low frequency). High-
pitch notes such as those produced by a °ute or whistle consist of the same type of vibrations in
the air, only vibrating at a much faster rate (higher frequency). Figure 1.11 is a table showing the
actual frequencies for a range of common musical notes.

Astute observers will notice that all notes on the table bearing the same letter designation are
related by a frequency ratio of 2:1. For example, the ¯rst frequency shown (designated with the
letter \A") is 220 Hz. The next highest \A" note has a frequency of 440 Hz { exactly twice as many
sound wave cycles per second. The same 2:1 ratio holds true for the ¯rst A sharp (233.08 Hz) and
the next A sharp (466.16 Hz), and for all note pairs found in the table.

Audibly, two notes whose frequencies are exactly double each other sound remarkably similar.
This similarity in sound is musically recognized, the shortest span on a musical scale separating such
note pairs being called an octave. Following this rule, the next highest \A" note (one octave above
440 Hz) will be 880 Hz, the next lowest \A" (one octave below 220 Hz) will be 110 Hz. A view of a
piano keyboard helps to put this scale into perspective: Figure 1.12

As you can see, one octave is equal to seven white keys' worth of distance on a piano keyboard.
The familiar musical mnemonic (doe-ray-mee-fah-so-lah-tee) { yes, the same pattern immortalized
in the whimsical Rodgers and Hammerstein song sung in The Sound of Music { covers one octave
from C to C.

While electromechanical alternators and many other physical phenomena naturally produce sine
waves, this is not the only kind of alternating wave in existence. Other \waveforms" of AC are
commonly produced within electronic circuitry. Here are but a few sample waveforms and their
common designations in ¯gure 1.13

These waveforms are by no means the only kinds of waveforms in existence. They're simply a
few that are common enough to have been given distinct names. Even in circuits that are supposed
to manifest \pure" sine, square, triangle, or sawtooth voltage/current waveforms, the real-life result
is often a distorted version of the intended waveshape. Some waveforms are so complex that they
defy classi¯cation as a particular \type" (including waveforms associated with many kinds of musical
instruments). Generally speaking, any waveshape bearing close resemblance to a perfect sine wave
is termed sinusoidal, anything di®erent being labeled as non-sinusoidal. Being that the waveform of
an AC voltage or current is crucial to its impact in a circuit, we need to be aware of the fact that
AC waves come in a variety of shapes.

REVIEW:

• AC produced by an electromechanical alternator follows the graphical shape of a sine wave.
• One cycle of a wave is one complete evolution of its shape until the point that it is ready to
• repeat itself.
•  The period of a wave is the amount of time it takes to complete one cycle.
• Frequency is the number of complete cycles that a wave completes in a given amount of time.
• Usually measured in Hertz (Hz), 1 Hz being equal to one complete wave cycle per second.
• Frequency = 1/(period in seconds)
• 1.3 Measurements of AC magnitude
• So far we know that AC voltage alternates in polarity and AC current alternates in direction. We
• also know that AC can alternate in a variety of di®erent ways, and by tracing the alternation over

What Is Alternating Current (AC) ?

Most students of electricity begin their study with what is known as direct current (DC), which is
electricity °owing in a constant direction, and/or possessing a voltage with constant polarity. DC
is the kind of electricity made by a battery (with de¯nite positive and negative terminals), or the
kind of charge generated by rubbing certain types of materials against each other.

As useful and as easy to understand as DC is, it is not the only \kind" of electricity in use. Certain
sources of electricity (most notably, rotary electro-mechanical generators) naturally produce voltages
alternating in polarity, reversing positive and negative over time. Either as a voltage switching
polarity or as a current switching direction back and forth, this \kind" of electricity is known as
Alternating Current (AC): Figure 1.1

Whereas the familiar battery symbol is used as a generic symbol for any DC voltage source, the
circle with the wavy line inside is the generic symbol for any AC voltage source.

One might wonder why anyone would bother with such a thing as AC. It is true that in some
cases AC holds no practical advantage over DC. In applications where electricity is used to dissipate
energy in the form of heat, the polarity or direction of current is irrelevant, so long as there is
enough voltage and current to the load to produce the desired heat (power dissipation). However,
with AC it is possible to build electric generators, motors and power distribution systems that are

Notice how the polarity of the voltage across the wire coils reverses as the opposite poles of the
rotating magnet pass by. Connected to a load, this reversing voltage polarity will create a reversing
current direction in the circuit. The faster the alternator's shaft is turned, the faster the magnet
will spin, resulting in an alternating voltage and current that switches directions more often in a
given amount of time.

While DC generators work on the same general principle of electromagnetic induction, their
construction is not as simple as their AC counterparts. With a DC generator, the coil of wire is
mounted in the shaft where the magnet is on the AC alternator, and electrical connections are
made to this spinning coil via stationary carbon \brushes" contacting copper strips on the rotating
shaft. All this is necessary to switch the coil's changing output polarity to the external circuit so
the external circuit sees a constant polarity: Figure 1.3

problems of spark-producing brush contacts are even greater. An AC generator (alternator) does
not require brushes and commutators to work, and so is immune to these problems experienced by
DC generators.

The bene¯ts of AC over DC with regard to generator design is also re°ected in electric motors.
While DC motors require the use of brushes to make electrical contact with moving coils of wire, AC
motors do not. In fact, AC and DC motor designs are very similar to their generator counterparts
(identical for the sake of this tutorial), the AC motor being dependent upon the reversing magnetic
¯eld produced by alternating current through its stationary coils of wire to rotate the rotating
magnet around on its shaft, and the DC motor being dependent on the brush contacts making and
breaking connections to reverse current through the rotating coil every 1/2 rotation (180 degrees).

So we know that AC generators and AC motors tend to be simpler than DC generators and DC
motors. This relative simplicity translates into greater reliability and lower cost of manufacture.

But what else is AC good for? Surely there must be more to it than design details of generators and
motors! Indeed there is. There is an e®ect of electromagnetism known as mutual induction, whereby
two or more coils of wire placed so that the changing magnetic ¯eld created by one induces a voltage
in the other. If we have two mutually inductive coils and we energize one coil with AC, we will
create an AC voltage in the other coil. When used as such, this device is known as a transformer:
Figure 1.4

As useful as transformers are, they only work with AC, not DC. Because the phenomenon of
mutual inductance relies on changing magnetic ¯elds, and direct current (DC) can only produce
steady magnetic ¯elds, transformers simply will not work with direct current. Of course, direct
current may be interrupted (pulsed) through the primary winding of a transformer to create a
changing magnetic ¯eld (as is done in automotive ignition systems to produce high-voltage spark
plug power from a low-voltage DC battery), but pulsed DC is not that di®erent from AC. Perhaps
more than any other reason, this is why AC ¯nds such widespread application in power systems.


 REVIEW:

• DC stands for \Direct Current," meaning voltage or current that maintains constant polarity

or direction, respectively, over time.

• AC stands for \Alternating Current," meaning voltage or current that changes polarity or

direction, respectively, over time.

• AC electromechanical generators, known as alternators, are of simpler construction than DC

electromechanical generators.

• AC and DC motor design follows respective generator design principles very closely.
•  A transformer is a pair of mutually-inductive coils used to convey AC power from one coil tothe other. Often, the number of turns in each coil is set to create a voltage increase or decrease
• from the powered (primary) coil to the unpowered (secondary) coil.
• Secondary voltage = Primary voltage (secondary turns / primary turns)
• Secondary current = Primary current (primary turns / secondary turns)

Electromagnetic Theory

The most important equations for present purposes are:


e = Nd/dt; e = Blv; and Force = Bli;
 
most practical machines having the directions of B, v and f at right angles
to one another.

Permanent Magnet Synchronous Motor

 Shown in Fig. 6.3 is a surface-mounted magnet machine with an airgap,
or surface armature winding. Such machines take advantage of the fact that modern, permanent magnet materials have very low  permeabilities and that, therefore, the magnetic field produced is relatively insensitive to the size of the air-gap of the machine. It is possible to eliminate the stator teeth and use all of the periphery of the air-gap for windings.

Not shown in this figure is the structure of thee armature winding.
This is not an issue in “conventional” stators, since the armature is    contained in slots in the iron stator core. The use of an air-gap winding


gives opportunities for economy of construction, new armature winding
forms such as helical windings, elimination of “cogging” torques and
(possibly) higher power densities.

High Speed Applications of Linear motors

In the magnet high-speed train Transrapid wheels and rail are replaced by a contact-less
working electromagnetic float and drive system. The floating system is based on attractive forces of the electromagnet in the vehicle and on the ferro-magnetic reaction rails in the railway. Bearing magnets pull the vehicle from below to the railway, guide magnets keep it on its way. An electronic control system makes sure, that the vehicle always floats in the same distance to the railway. Transrapid motor is a long-stator linear motor. Stators with moving field windings are installed on both sides along the railway. Supplied three-phase current generates an electromagnetic moving field within windings. The bearing magnets, and so also the vehicle are pulled by this field. Long-stator linear motor is divided into several sections. The section, in which the vehicle is located, is switched on. Sections, that make high demands on thrust, motor power is increased as necessary. Drive integrated in the railway and cancelling of mechanical components make magnet high-speed vehicles technical easier and safer. Transrapid consists of two light weight constructed elements.

Capacity of the vehicles can be adjusted to certain requirements. Operating speed is between 300 and 500 km/h. A linear alternator supplies floating vehicle with required power. Advantages of magnet highspeed train are effective in all speed areas. After driving only 5 km Transrapid reaches a speed of 300 km/h in contrast to modern trains needing at least a distance of 30 km. Comfort is not interfered with jolts and vibrations. Since vehicle surrounds the railway Transrapid is absolutely safe from derailment. Magnet high-speed train makes less noise than conventional railway systems because there is no rolling noise. Also energy consumption is reduced
compared with modern trains. This high-speed system is tested in continues operation at a
testing plant in Emsland in Germany and some commercial routes in Germany are planned. A high-speed train route is currently under construction in Shanghai, China, further projects areeither in progress or under review.

Linear motors

Since linear motors do not have any gear unit it is more simple converting motion in electrical drives. Combined with magnet floating technology an absolutely contact-less and so a wear resistant passenger traffic or non-abrasive transport of goods is possible. Using this technology usually should enable high speed. So Transrapid uses a combination of synchronous linear drive and electromagnetic floating. Linear direct drives combined with magnet floating technology are also useful for non-abrasive and exact transport of persons and goods in fields as transportation technology, construction technology and machine tool design. Suitable combinations of driving, carrying and leading open new perspectives for drive technology.

9.5.1 Technology of linear motors

In the following function, design, characteristic features, advantages and disadvantages are demonstrated shortly. In principle solutions based on all electrical types of machines are possible unrolling stator and rotor into the plane

Linear motor then corresponds to an unrolled induction motor with short circuit rotor or to permanent-magnet synchronous motor. DC machines with brushes or switched reluctance machines are used more rarely. Depending on fields of usage linear motors are constructed as solenoid, single-comb or double-comb versions in short stator or long stator implementation. It is an advantage of long stator implementations that no power has to be transmitted to passive, moved secondary part, while short stator  implementations need the drive energy to be  transmitted to the moved active part. For that reason an inductive power transmission has to be used to design a contact-less system. In contrast to rotating machines in single-comb versions the normal force between stator and rotor must be compensated by suitable leading systems or double-comb versions must be used instead. This normal force usually is one order of magnitude above feed force. In three-phase windings of synchronous or induction machines a moving field is generated instead of three-phase field. This moving field moves at synchronous speed.

As in three-phase machines force is generated by voltage induction in the squirrel-cage rotor of the induction machine or by interaction with permanent-magnet field of the synchronous machine.

Three-phase machine supply is made field-oriented by frequency converters to achieve high dynamic behavior. For that induction machines need flux model and speed sensor, but synchronous machines just need a position sensor. For positioning jobs high dynamic servo drives with cascade control consisting of position control with lower-level speed and current control loop are used. This control structure is usual in rotating machines. Depending on the place the position measurement is installed a distinction is made between direct and indirect position control. Since many movements in production and transportation systems are translatory, linear drives are useful in these fields. In such motors linear movements are generated directly, so that gear units such as spindle/bolt, gear rack/pinion, belt/chain systems are unnecessary. As a result from that rubbing, elasticity and play are dropped, which is positive for servo drives with high positioning precision and dynamic. In opposition to that there are disadvantages such as lower feed forces, no self-catch and higher costs.

 Industrial application opportunities

Two different opportunities to implement linear drives are shown at the following pictures

Most promising application fields of linear drives for industrial applications:

• machine tools: machining center, skimming, grinding, milling, cutting, blanking and high speed machines.

• automation: handling systems, wafer handling, packing machines, pick-and-place

machines, packaging machines, automatic tester, printing technology

• general mechanical engineering: laser machining, bonder for semiconductor industry,

printed board machining, measurement machines, paper, plastic, wood, glass machining.

APPLICATIONS OF STEPPER MOTORS

•     Due to the digital circuit compatibility of the stepper motors, they are widely used in computer peripherals such as serial printers, linear stepper motors to printers, tape drives, floppy disc drives, memory access mechanisms etc.

•     The stepper motors are also used in serial printers in typewriters or word processor systems, numerical control of machine tools, robotic control systems, number of process control systems, actuators, spacecrafts, watches etc.

•     X-Y recorders and plotters is another field in which stepper motors are preferred.

IMPORTANT DEFINITIONS IN SINGLE PHASE INDUCTION MOTORS AND SPECIAL MACHINES

Holding Torque


It is defined as the maximum static torque that can be applied to the shaft of an excite motor without causing a continuous rotation.

Detent Torque

It is defined as the maximum static torque that can be applied to the shaft of an unexcited motor without causing a continuous rotation.

Under this torque the rotor comes back to the normal rest position even if excitation ceases. Such positions of the rotor are referred as the detent positions.

Step Angle:

It is defined as the angular displacement of the rotor in response to each input pulse.

Critical Torque:

It is defined as the maximum load torque at which rotor does not move when an exciting winding is ener This is also called pullout torque.

Limiting Torque:

It is defined for a given pulsing rate or stepping rate measured in pulses per second, as the maximum load torque at which motor follows the control pulses without missing any step. This is also called pull in torque.

Synchronous stepping rate:

It is defined as the maximum rate at which the motor can step without missing steps. The motor can start, stop or reverse at this rate.

Slewing rate:

It is defined as the maximum rate at which the motor can step unidirectionally. The slewing rate is much higher than the synchronous stepping rate. Motor will not be able to stop or reverse without missing steps at this rate.

Stepper Motor Characteristics

The Stepper motor characteristics are classified as

1. Static characteristics and
2. Dynamic characteristics


The static are at the stationary position of the motor while the dynamic are under running conditions of the motor.

Static Characteristics

These characteristics include

1. Torque displacement characteristics
2. Torque current characteristics

Torque-displacement characteristics: 

This gives the relationship between electromagnetic torque developed and displacement angle 0 from steady state position. These characteristics are shown in the Fig. 7.16

Torque-current characteristics:

The holding torque of the stepper motor increases with the exciting current. The relationship between the holding torque and the current is called as torque-current characteristics. These characteristics are shown in the Fig.7.17

DYNAMIC CHARACTERISTICS

•     The stepping rate selection is very important in proper controlling of the stepper motor.
•     The dynamic characteristics give the information regarding torque stepping rate. These are also called torque stepping rate curves of the stepper motor. These curves are shown in the Fig. 7.18.
•     When stepping rate increases, rotor gets less time to drive the load from one position to other.
•     If stepping rate is increased beyond certain limit, rotor can not follow the command and starts missing the pulses.
•     Now if the values of load torque and stepping rate are such that point of operation lies to the left of curve I, then motor can start and synchronize without missing a pulse.

•     For example, for a load torque of TL, the stepping rate selection should be less than f so that motor can start and synchronize, without missing a step.

•     But the interesting thing is that once motor has started and synchronized, then stepping rate can be increased e.g. upto f for the above example.
•     Such an increase in stepping rate from f to f is without missing a step and without missing the synchronism. But beyond f if stepping rate is increased, motor will loose its synchronism.

•     So point A as shown in the Fig.7.18 indicates the maximum starting stepping rate or maximum starting frequency.
•     It is defined as the maximum stepping rate with which unloaded motor can start or stop without loosing a single step.
•     While point B as shown in the Fig. 7.18 indicates the maximum slewing frequency. It is defined as the maximum stepping rate which unloaded motor continues to run without missing a step.
•     Thus area between the curves I and II shown hatched indicates, for various torque values, the range of stepping rate which the motor can follow without missing a step, provided that the motor is started and synchronized.
•     This area of operation of the stepper motor is called slew range. The motor is said to be operating in slewing mode.
•     It is important to remember that in a slew range the stepper motor can not be started, stopped or reversed without losing steps.
•     Thus slew range is important for speed control applications. In position control, to get the exact position the motor may be required to be stopped or reversed.

•     But it is not possible in a slew range. Hence slew range is not useful for position control applications.
•     To achieve the operation of the motor in the slew range motor must be accelerated carefully using lower pulse rate.
•     Similarly to stop or reverse the motor without loosing acceleration and deceleration of the stepper motor, without losing any step is called Ramping