the machines project

8/7/2019 The Machines Project

http://slidepdf.com/reader/full/the-machines-project 1/21

Design of Electric Machines

EE271: Electronics and Electrical Techniques and Design 2

Jeswin Mathew Louise Morran Maria Orr Richard Morrison Ewan Moyes

1:1:1:1:1

Group EME 7

Completed under the supervision of Dr.C Booth

Date Completed: 10/3/2011



Page | 1

Declaration

´We declare that the contents of this report are our own work under the supervision of Dr.C Boothµ

Signed Date

................................ Richard Morrison ......................................

................................. Jeswin Mathew ...... .................................

................................ Louise Morran ..........................................

.................................. Ewan Moyes .... ........................................

................................ Maria Orr ...............................................



Page | 2

Abstract

The primary objective of this team project was to develop an understanding into the principles that

govern the operation of electrical machines mainly synchronous and induction machines and their

primary function: converting electrical power into useful motion. This was done by conductingexperiments with the stator and the synchronous and induction motors that were provided in the

laboratory. The project was divided into two segments: in the first four sessions the team

experimented with a small scale stator and rotors, and in the last session, a relatively large scale stator

and rotors were experimented with. The project was undertaken by a team of five students and was

conducted over a period of five weeks.



Page | 3

Contents

Abstract««««««««««««««««««««««««««««««««««2

Introduction««««««««««««««««««««««««««««««««..4

Principles of Electromagnetism used in electric

machines««««««««««««.4

Magnetic field in a current carrying

solenoid«««««««««««««««««.4

Fleming·s Right hand and left hand

rule«««««««««««««««««««.5

Two Pole and Four Pole Machines««««««««««««««««««««««.6

Two pole Machines««««««««««««««««««««««««««««6

Four Pole Machines««««««««««««««««««..«.«««««««..6

Synchronous Machines«««««««««««««««««««««««««««.9

Introduction to Synchronous

Machines«««««««««««««««««««....9

Operation of a SynchronousGenerator«««««««««««««««««««...9

Operation of a Synchronous

Motor«««««««««««««««««««««.11

Induction

Machines«««««««««««««««««««««««««««««12

Introduction«««««««««««««««««««««««««««««««..12

Operation of Three Phase

Motors««««««««««««««««««««««.14

Experimentation with the induction motor in the

Laboratory«««««««««««.15



Page | 4

Conclusion«««««««««««««««««««««««««««««««««1

7

References«««««««««««««««««««««««««««««««««1

8



Page | 5

1. Introduction

Electromagnetism is one of the most exploited principles in engineering: The laws of

electromagnetism which stipulate state current flowing in a given direction around a coil of wire induces

a magnetic field due to the moment of electrons, when the electrons are static the magnetic field iseradicated and there is only an electric field.

Synchronous and Induction motors are widely used as they are an efficient way of converting

electrical power to mechanical energy and therefore some useful mechanical work is done by the

electricity supplied. The motor consists of a rotor, which spins due to a torque induced by the magnetic

field and a stator which is the stationary part of the motor system and consists of coil windings

contained in an iron core with an air gap in the centre as shown in figure 1 for the rotor to be inserted.

The coils are wound in such a way that coils adjacent to each other are out of phase i .e. one of them is

wound clock wise and the other anti-clockwise so that the current passing through them will be in the

opposite direction, making the direction of the magnetic poles induced in these coils in the opposite

direction of each other as it will be discussed in the report .

Synchronous motors as the name indicates contain a rotor that rotates at the same rotational speed

as the frequency of the AC supply to the stator windings i.e. the higher the frequency, the higher the

rotational speed. When an AC excitation is given to the stator windings, it interacts with the armature of

the synchronous rotor to produce rotational motion. But the operation of the induction motor is

different; the speed of the rotor is not in synchronous with the rate of change of magnetic fieldof the

stator; the induction motor operates due to e.m.f induced in its rotor, therefore if the rotor and stator

were synchronous there won·t be any e.m.f induced.

1. Principles of Electromagnetism Used inElectric Machines

Figure 1: A stator and its rotor



Page | 6

2.1 Magnetic field in a solenoidWhen a dc current is passed through a coil winding or solenoid, the magnetic flux lines generated

are very similar to that of a magnet i.e. the flux lines originate from the North Pole and links back at the

South Pole and also the flux density decreases with increase in distance from the magnet. The solenoid

with DC current passing through portrays the flux pattern as shown in figure 2 and hence is rendered

with a north pole and a south pole; the direction of the poles is determined by the use of the right hand

grip rule [1] (For conventional Current direction). But when the DC supply is replaced with an AC source,

the current changes direction at the frequency of the source and thus the magnetic field lines change

direction which consequently fluctuates the direction of north and south poles .

2.2 Fleming·s Right Hand and Left Hand RuleFleming·s right hand rule as shown in figure also known as the generator principle is a mnemonic to

investigate the direction of induced current when a coil of wire is rotated in a magnetic field .[1]

Faraday·s law of electromagnetic induction state that ¶The e.m.f induced in a coil is proportional to the

rate of change of the magnetic flux· as illustrated by the right hand rule and electric generators exploit

this concept to produce AC current. It·s not necessary that the coil should be moving butinstead the

coil can be stationary and a magnetic can be rotated close to the field windings. This principle was

evaluated by rotating a synchronous rotor which, was turned into a permanent magnet, in the air gap in

the stator and the e.m.f induced in the coil was seen in the oscilloscope as shown in figure.

Fleming·s left hand rule is a mnemonic used to encapsulate the electric motor principle and is shown

in Figure 4.[1] The motor principle states that when a current carrying conductor is placed in a magnetic

field, it experience a force or thrust due to the opposing magnetic field and this can cause rotational

motion when a stator rendered with an AC excitation is interfering with the magnetic f ield. The rule is

also a viable method to determine the direction the rotor is going to spin initially.

Figure 2: The similarity between the magnetic flux lines of a current carrying solenoid and a magnet



Page | 7

3. Two Pole and Four Pole Machines

3.1 Two Pole MachinesA two pole machine is a basic type of motor which runs using a rotating permanent magnet as a

rotor inside a stator made up of two repelling poles, as shown in Figure 5. This arrangement causes the

permanent magnet in the centre to be repelled as north meets north. As the central magnet then rotates

and starts getting attracted by the South Pole, the poles in the stator switch because of AC current

supply (This principle is discussed in section 2.1). The momentum of the permanently magnetic rotor

pushes it past the stage of being parallel with the now southern electromagnet in the stator which then

repels it, initiating the whole cycle again.

When setting up this arrangement, it is best to use an electromagnetic core. This can be done

through coils of wire, wound so that their electric field lines are in phase with each other, and having a

direct current flowing through, thus forming the same effect as a permanent magnet. This will be

discussed in more detail in the Four Pole Machines section.

Figure 5: Two Pole Machine [2]

Figure 3: Flemings Right hand rule (The

generator principle) [2] Figure 4: Flemings left hand rule (The

motor principle) [2]



Page | 8

There are many advantages of using an electromagnetic core in a two pole system rather than a

permanent magnet. The primary advantage is being able to switch off the magnetic core by simply

stopping the flow of direct current. This is a great benefit over permanent magnets as they cannot be

manipulated as easily, which means that the electromagnetic core is much safer as if any problems

arise, the machine can be isolated.

The polarity of the core can easily be changed by simply swapping the inputs to the windings inthe core. This again is a feature in which a permanent magnet cannot compete with as it has a set

direction of magnetic field. Finally, a permanent magnet, over a period of time, will lose its magnetism,

whereas an electromagnet does not only keep its magnetism, but it can change it depending on the value

of the input power .

3.2 Four Pole Machines

Four pole machines are similar to that of two pole machines in that they too use a permanently

magnetic core and outside alternating current stator coils. The difference is that instead of the one pole

pair arrangement shown in Figure 5, a four pole machine consists of two pole pairs in the stator, and two

pole pairs in the core. The arrangement of this is shown in Figure 6.

In the four pole machine, an iron core is used in which the windings are attached to make the

permanent magnet. The high permeability of iron meant that it was very fitting for enhancing the

magnetic field by confining and guiding the magnetic field lines.

This addition to the system made the motor run as an induction machine rotor . This is a type of

motor which runs on alternating current, and has power supplied to it through electromagnetic induction

in the coils as discussed in section 2.3

To design this in the laboratory, coils of copper wire, made up of forty turns, were wrapped

around the iron core as shown inFigure 7. A direct current supply is passed into the windings of the

core using slip rings. Through the use of carbon brushes, permanentcontact can be made, which

means a constant DC supply can be produced.

Figure 6: Four Pole Machine [2]



Page | 9

These were then tested with a compass to make sure that the poles had the correct polarity.

Due to the North-South-North-South arrangement that the coils were wound in, with a direct current

flowing through it, the core works like a permanent magnet, with two set North poles and two set South

poles.

As the iron core is now set up as a permanent magnet, and with the coils in the stator being

polarised in a North-South-North-South fashion as the central rotor turned, the North opposing poles

would repel as the South poles were repelling. This became continuous as the machine rotated, whichkept the rotations constant.

The problem with this is that as the iron core is now a permanent magnet. Without a starting

rotation the north would just attract to the south of the stator, and the system would not move .

Therefore, the core needs an initial rotational acceleration to get the initial momentum to overcome this

pull, and run as a motor . This is easily resolved through a handle being attached, and the core being

revolved manually. It will also be discussed how capacitors can be used to get the machine up to speed .

When this speed is reached, the capacitor circuit can be disconnected, and the machine will run at

synchronous speed.

Although the four poles in the machine do cause a slower rotational speed than that of the

previously described two pole machine, the rotations will be a lot smoother due to the more constantinteraction within an electric field. This means that higher efficiencies can be present, which is

something that is highly sought after when used in industry.

A simple continuity test, consisting of making sure each coil and core were insulated correctly

was put in place, and with this being proved, the system was taken to the next stage, in which it was

inserted into the outer chamber, as illustrated in Figure 8.

Figure 7:Coil Arrangement

Figure 8: System with iron core insert



Page | 10

4. Synchronous Machines

4.1 Introduction to Synchronous MachinesSynchronous machines are a type of electric machine that can be used as a generator or motor. They are

common in power generation and synchronous generators with ratings of hundreds of Megavolt-

amperes are widespread in the generation business, they are favoured for their stability and higher

efficiency. The frequency of the AC current produced by the generator is proportional to the speed of the

generator, and this works visa-versa when this type of machine is used as a motor. Hence they can also

be used as constant speed motors or, if a variable frequency generator is available, they can operate at

different speeds. This makes synchronous motors useful for machinery that is rarely shut down and

need to run at an exact speed, as the speed is fixed to the frequency of the AC supply.[3]

4.2 Operation of a Synchronous Generator Like all electrical generators the function of synchronous machines is based on Faraday·s law of

electromagnetic induction, and has two basic components; the field windings and the armature coils.

The field coils within the rotor are energised with a DC current to create a permanent magnetic field; this

can be done using car bon brushes. A permanent magnet can be used but does not create as strong a

magnetic field. Then when these coils are rotated within the armature coils fixed to the stator, as show in

Figure 9, the changing magnetic field induces an AC current in the armature coils.[3]



Page | 11

The rotor can have more than one set of poles and the number of poles directlyaffects the frequency

of the current produced by the generator . When considering a synchronous machine with only two

poles, or one pole pair, as shown in Error! Reference source not found.; when the rotor goes through

one full rotation of 2 radians, one full sinusoidal waveform through 2 radians is induced in the

armature coils, and the angle of rotation matches the phase angel of the sinusoidal waveform. In thecase of a two pole pair motor, as shown in Error! Reference source not found., when the rotor goes

through one full revolution the sinusoidal waveform produced will have gone through two full cycles, so

4 radians.

This leads on to the general relation between the angle of the rotor and the angle of the sinusoidal waveThis leads on to the general relation between the angle of the rotor and the angle of the sinusoidal wave

form, show as Equation 1 where ´Pµ is the number of poles.

Equation 1

This brings us the relation shown by Equation 2 where ¶ns· is the speed of the generator in rpm, ¶f· is thefrequency of the AC waveform produced in Hz and ¶P· is the number of poles on the rotor .

Equation 2

This results in the design parameters of the rotor depending on the speed at which the rotor is intended

to turn at. There are two main types of rotor: cylindrical rotors (Figure 10) used for high speed machines,

such as steam turbines and Salient pole rotors (Figure 11), used for low speed machines such as

hydroelectric and diesel-electric engines. During the lab a four pole salient rotor was constructed. [3]

Figure 9: Simplified cross-sectional diagram of a two pole

synchronous machine [4]



Page | 12

Figure 10 Figure 11

During the lab a synchronous machine with a single pole pair was also constructed, the wiring diagram

for which is shown in Figure 12. This was first used a generator then a motor.

Figure 12

Synchronous generators can also be configured to produce more than one AC waveform, or phase.

Figure 13 shows a three phase generator that will produce three waveforms of the same magnitude and

frequency but 120° apart, shown in Figure 14. This application of synchronous generators it verycommonly used to generate on a national scale.[6]

Figure 13² Three phase machine [8]



Page | 13

Figure 14 ² Three Phase Waveform [7]

4.3 Operation of a Synchronous Motor The synchronous motor operates under the same principles as the synchronous generator; if an AC

voltage is applied to the stator winging then the rotor will rotate at the synchronous speed dictated by

Equation 2. The main disadvantage to a synchronous motor is that is cannot just be switched on; it must

be started with a very low frequency supply, then the frequency must be slowly increased to increase the

speed of the motor . It was observed in the lab that the rotor, at a low frequency, would rotate less than

180 degrees, and then turn back on itself a further half turn . The rotor did not complete a full rotation

until the frequency was increased a certain amount, creating enough momentum for it to rotate past the

maximum magnet strength point. It was also observed that making the rotor turn in a particular direction

was difficult.

Although, when performed in the lab, this did not seem to be a disadvantage, variable frequency

generators on a larger scale can be larger and more expensive than single frequency generators. There

is however advantages to using synchronous motors, the speed of the motor will not change when a

load is applied.

The synchronous motor is dictated by the frequency of the alternating current because the alternating

current creates a rotating magnetic field that ¶pulls· round the energised coils on the rotor, as show by

15. The speed that the field rotates at is dictated by the frequency of the alternating current.

Figure 15

Rotating Field

Rotating Rotor



Page | 14

When the motor is loaded the rotor turns slightly behind the rotating field, the angle is referred to as

the toque angle. The rotor still runs at synchronous speed but as the load increases the torque angle

increases. [5]

5. Induction Motors

5.1 IntroductionAs mentioned previously in the four pole machine section, the motor during the second session

of the project was running on an AC supply as an induction motor . This section will focus and explain in

more detail just what this denotes. Induction motors consist of two main parts, a rotor and a stator . The

rotor is made of slotted cylindrical iron cores . The slots in the iron cores are to receive the conductors.

In the most up to date motors, the most common type of rotor has cast iron conductors with short

circuiting end rings. Figure 16 helps to illustrate and convey the steps taken for the induction motor to

function.

The two common types of induction motors are three phase and single phase as shown in

figure 17 below. For the purposes of this experiment however, three phase was chosen as the most

suitable type.

Simple AC two pole induction motor [7]

Figure 16: Two pole Induction Motor



Page | 15

From their outer appearance, the single phase and three phase motors look very similar and it

could be difficult to distinguish between the two. However, taking the inner workings of the motors into

account, it is clear they are very different. To distinguish between the two we can refer back to theprevious section on two pole and four pole machines. Single phase would be found in a two pole

machine. The field in a single phase machine does not rotate on its own but in fact reverses 180Ü (Figure

18) and so in order for the motor to rotate it must be started manually - commonly by using capacitive

circuits which switch off once the appropriate speed has been reached . The capacitor is placed in series

with the primary winding which induces a phase shift of 90. for the starting current, giving a much better

starting torque. The capacitor and primary winding can then be disconnected by a centrifugal switch.

Single phase motors are used for smaller objects such as fans or small household appliances as these

do not require a high power input.

Three phase motors have no requirement for manual start up. The three phases in a three phasemotor are separated by 120Ü (figure 19) and so a good quality rotating field is produced. These motors

tend to be smaller, cheaper and more efficient than single phase which is evidently the reason why 90%

of all industrial motor applications use three phase motors.

Figure 17: Single and Three Phase Motors [8]

Fi ure 18: Sin le Phase AC current 9

Fi ure 19: Three Phase AC current which are 1200 a art



Page | 16

5.2 Opera

i¡ ¢ ¡

f Three phase m ¡

¡ rs

The three phase induction motor was invented by Nikola Tesla- a pioneer in electromagnetism.

There are different types of three phase motors available, such as the squirrel cage design or the slip

ring design, but the main principle of how they work are all the same. Like a general induction motor the

three phase motor is made up of a stator and rotor. When the stator of the three phase motor is

connected to an appropriate AC supply, a rotating magnetic field will be established. The number of

magnetic poles of the rotating field will be equal to the number of poles for which each phase of the

stator winding is wound. This rotating magnetic field created from the AC current supply will induce a

voltage over the rotor conductors as it moves across them. As the rotor winding is directly shorted, the

voltage induced by the rotating field in the secondary winding causes a current to flow in the rotor

conductors. The interaction between the currents in the rotor conductors and the stators· magnetic field

produces a torque which causes the rotor to rotate in the direction of the magnetic field. In practice the

rotor·s speed always falls slightly behind the speed of the rotating magnetic field, this difference is

referred to as ¶slip·. ¶Slip· is the difference between the synchronous and asynchronous speeds. The

Synchronous machines are discussed in detail in section 4.

Advantages of three phase motors [7]

y Low cost

y Robust and sturdy

y Relatively simple maintenance

y No need to start manually as it starts on its own

y Good power factor

Disadvantages of three phase motors [7]

y The efficiency would decrease if speed was to be altered

y Its speed decreases if it experiences an increase in loady When lightly loaded it will run at a low lagging power factor

5.3 Experime £

¤

a¤

i¥ £

i £

¤

he Lab ¥ ra¤

¥ ry wi¤

h¤

he¦ £

duc¤

i¥ £

m ¥

¤

¥ r5 .31 Three phase induction motor

The outer coils were connected as shown in figure A41-4 [5 ]. This circuit created a 3-phase stator which

effectively created a 3-phase rotating field. This means three pairs of magnets are rotating in the stator.

When the 3-phase connections were made, a small compass was placed in the centre of the stator which

then rotated. The magnet shows the created rotating magnetic field.

As the machine was set -up with 3-phases , each 120 de gr ees apart f rom each other , and the po larit y o f

the coi ls chan ges se quentia lly. Fi gure 1 sho ws the set up o f the coi ls in the stator .

Each po le pair has a greater current than the other t wo at a certain point o f time , creatin g the greater

ma gnetic f ie ld accordin g to Biot Sa vart La w as indicated in e quation [1] be lo w:



Page | 17

Where,

R is a set radius,

I is the magnitude of current in A,

H is the strength of the magnetic field (Am-1).

This states that for a set radius, the magnetic field depends on the magnitude of current and therefore

since the currents were out of phase, the strength of the magnetic field varied and thus instead of a

pulsating magnetic field a rotating magnetic field was produced.

The diagram shows the moment in time when pole pair 2 create the strongest magnetic field, therefore

forcing the compass in the direction of that particular pole pair . The magnetic flux across pole 2 would

then decrease, and increase across pole 3, attracting the compass needle to point in that direction . As

the phase continues to change the north pole in this pattern, the compass continously rotates . The three

arrows represent the magnetic flux created by the pole pair .

Figure 21 shows the Stator Pole Pairs. Pole pair 2 has greatest current. Arrow shows rotation of the most

north point as it changes.

The induction rotor used was a ¶squirrel cage· rotor . The iron bars embedded in the core are shorted out

as shown in figure 21. As the voltage from the stator is induced in the bars, a large current is created in

the bars. This current flow in the rotor creates a magnetic field around the bars . The magnetic flux cuts

the bars, creating a north and south pole on each side of the rotor . With this the opposite poles are

attracted, meaning the rotor is attracted to the rotating magnetic field from the stator .

Figure 21 ² Layout of squirrel cage if removed from rotor .

E uation 3

Fi ure 20 7



Page | 18

5 .32 Running the unit as a single phase induction motor

The stator was connected as sho wn in f igure A2 7-3 [5 ]. This set up created 4 poles all in phase with each

other and included a capacitor starting s ystem . The capacitor start s ystem was also connected in a 4

pole la yout ; ho we ver each pole onl y in vol ved one coil .

The capacitor start circuit causes the single phase to be ¶split ·, meaning the 4 capacitor start poles are out o f phase with the running poles creating a rotating magnetic f ield , similar to that f ound in the 3-

phase set up .

This rotating magnetic f ield allo ws the induction motor start f rom stationar y. When a certain speed is

reached , the centri f ugal s witch disconnects the starting circuit and circuit .

When the induction motor is used as a capacitor run machine , with the capacitor included the measured

rotational speed was 1470rpm . When the capacitor circuit was s witched o ff, the speed decreased to

145 8rpm . Since when the capacitor circuit is in vol ved and 4 e xtra coils are used in the split phase stator ,

nearl y 90r out of phase, there is a greater magnetic force ¶pulling· the stator around. This reduces the

slip in the induction motor creating a greater rotational speed when the capacitor circuit is connected.

The rotational speed of the rotor is influenced by the strength of the magnetic flux. If the rotor was to

speed up to synchronise with the stator·s magnetic rotation, the magnetic flux cutting the bars would

decrease slowing the rotor down.

A greater force is required to rotate the rotor when it is loaded. As the rotor is trying to slow down under

the load, the magnetic flux cutting the iron bars increases, keeping the rotor up to speed.

5 .33 Autotransformer Starter

This method is usually used where a capacitor start system will not provide enough torque to move the

rotor. Each phase of this starting circuit involves a single winding on a laminated core. The coils are

supplied by the mains supply, and tapping points are used to supply a reduced voltage output. This

voltage output is used to supply the motor, and when the current has fallen to the running value, themotor leads are switched over to the full voltage supply. This provides the same torque to current ratio

as for a direct start.

5 .34 Resistance or Reactance Starter

By using resistors or inductors of certain values in series with the motor, the starting current can be

reduced. However the starting torque is reduced with the current making this method less useful.

Conclusion

The laboratory project was overall a success. It helped the group to facilitate the necessary skills

such as teamwork, technical thinking and sourcing information, which are all extremely paramount for

future endeavours. The project facilitated the understanding of synchronous and induction machines

and also the different manners the stator can be configured such as polyphase or single phase and

multiple pairs of poles or a single pair These machines are used in many differing areas in the modern



Page | 19

world. They can range in sizes and although here only two and four pole motors have been discussed,

many more coils and stator windings can be added to lower the uncertainty and improve the overall

performance.

In saying this, it is all relative to the application. If a motor is only to be used in a simple

children·s toy, for example, a straightforward two or four pole machine would work perfectly. If, on the

other hand, it was to be used in a factory, where the motor is driving some form of shaft that requires amore precise system, a higher number of poles would be necessary.

During the project, the notion of electric machines was only evaluated at a rudimentary level relative

to the theoretical background of electric machines: There were several factors relating to dynamics that

were neglected and therefore not calculated such as:

Moment of inertia around the centre of mass for the rotors

The initial direction of rotation of the rotors when the stator was given an AC or DC excitation

The thrust provided by the repelling magnetic fields of the rotor and stator.

Induction motors are widely used as generators in electrical power systems. Their robust design

and simple construction, i.e. no slip rings or rotor windings, make them reliable motors for generation

systems. The induction motor is also normally completely enclosed, protecting the motor against dirt

and water.

Generation in an induction motor occurs when the speed of the rotor exceeds the speed of the

stator frequency. When the stator is supplied the output is controlled by using a capacitive load, so

changed in the power being generator do not surge instantly. However a drawback to this is that the

range of power which the induction machine can produce is reduced significantly.

References

1. Sibley J.,1995 , Introduct ion to Electromagn et ism , 2nd

Ed it ion , Ess ent ia l Electron ics 2. htt p://s km -s yst ems .blogs pot .com /2010/02/pract ica l-po wer -s yst em .htm l, Data Acc ess ed on

4/3/2011

3. Nasar A.S., Electr ic Mac hin es and Po wer Syst ems , Vo lum e 1

4. htt p://sound .west host .com /c loc ks /motor -f5.g if Dat e Acc ess ed on 08.03.2011

5. Laborator y Instruct ion s heet Electron ics and Electr ic : Tec hn iqu es and Des ign , Des ign o f Electr ic

Mac hin es

6. AC Pow § ̈

Systems Handbook , (Page 30)



Page | 20

7. http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indmot.html#c1 , Data Accessed

5/3/2011

8. http://www.acpd.co.uk/ac-induction-motors.html , Data Accessed 5/3/2011

9. http://www.pumped101.com/motorintro.pdf , Data Accessed 5/3/2011

the machines project

Documents