3-2- Theory

-

Republic of Iraq

Ministry of Higher Education

and Scientific Research

University of Technology-Electromechanical

Department

1435 2014

September 14 2014 Lab. of DC Electrical Machines Electromechanical Eng. Dept

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Experiment No.( 1 )

Anatomy of D.C. Machines 1.1-Construction:

An electro-mechanical energy converter is a rotating electrical machine that converts mechanical energy into electrical energy or electrical energy into mechanical energy. The two kinds of electro-mechanical energy converters are: therefore, electric generator and electric motors. These machines maybe either direct current or alternating current machines. A D.C. machine has two ports stationary part called stator and rotary part-called rotor. The most important part of the stator is the field with its customary laminated steel care and field windings. The field poles are usually bolted to the field yoke or frame upon which the whole structure rests. The rotor is built up of a laminated steel sheets slotted to receive the insulated copper armature windings. The armature coil ends are soldered to the risers of the commutator segments, carbon-brushes in the brush holders rest upon the commutator and collect the current. Spring tension is applied to the brushes so that good uniform contact is made between them and the commutator segments.

Each one of the field-pole cores is built up of stack of steel lamination about 0.4 to 0.6 mm thick per lamina on, having good magne c proper es and riveted together. The entire pole is bolted to the yoke frame, the pole core beors the field windings:

a- A shunt field in which there are many turns of fine wire.

b- A series field in which there are comparatively few turns of their wire

c- Compound field in which both a shunt and series windings and used.

Fig(1.1) shows the parts of dc machine.

1.2-Interpole's for dc machines: One of the most important developments in the design of d.c

machines was the use of interpoles to correct the objectionable effects of armature-reaction and commutation. These are narrow poles placed in the magnetic neutral are always permanently connected in series with the armature winding. See fig. ( 1.2 ) and g (1.4)


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1.3-Compensating windings: Compensating winding are used for the purpose of neutralizing the

effect of armature-reaction in the zones outside the influence of the interpoles and particularly to maintain an uniform flux distribution under the faces of the main poles. They are suitably connected in series with the armature and interpoles windings. They are located in the slots provided in the pole-shoes.

1.4-Experimental investigation 1.4.1-Part A: 1- You are given a disassembled four poles d.c machine. Observe the constructional mentioned above.

2- On each pole two windings are placed. Find out by avometer which one is series winding and which one is shunt winding. 3- Measure the resistance of both the windings by ammeter-voltmeter method. Circuit diagram is given in fig. (1.1.b).

4- Now connect all four series or shunt windings in series (fig.1.4.a) and give dc supply. By a magnetic indicator check the south and north poles. By a magnetic material check the field effect. Move a coil connected to a voltmeter inside the machine to see the induction effect.

5- Reverse the current flow in the field windings and see its effect by using the magnetic pole indicator.

6- Connect all four series windings in series and all four shunt winding in series in all you have four terminals.

7- Connect the machine as a series machine, shunt machine, separately excited machine and compound machine either cumulatively connected or differentially connected (fig.1.4.b&c) machine show the connection for each machine to your teacher.

1.4.2-Part B: Go to machine panel No.2. you will nd the symbolic diagram of each field winding and armature:

1- Repeat point 7 of part A as in g. (1.5 ).


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2-Connect the given machine as a shunt generator and using the dc shunt motor as prime moves. Start the set. See the field-flux distribution under a pole by an oscilloscope. Connect the oscilloscope to the search coil.

1.5-Discussion: 1- Why resistance of series and shunt field windings difference give reason.

2- Draw the diagram of a pole and winding on the pole, for a certain current direction show the direction of the flux and the polarity of the pole. How do you change the direction of flux?

3- How do you connect the interpoles to armature winding? Give reason.

4- When you move a coil in a magnetic field, voltage is induced in the coil. Explain the law.

Fig(1.1.a)


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Fig(1.1.b) Fig(1.1.c)

Fig(1.2.a) Fig(1.2.b)

Fig. (1.3)

Fig. (1.4.a)


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Fig. (1.4.b) commutative Fig. (1.4.c) differential

Fig(1.5.a) shunt generator Fig(1.5.b) series generator

Fig(1.5.c) short compound Fig(1.5.d) long compound


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Fig(1.5.e) separately excited generator


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Experiment No.( 2 )

No - Load Characteristics of a D.C. Shunt Generator

2-1- Objective:

To obtain the no load characteristics of a d.c. shunt generator.

2-2- Theory:

The magnetization curve of a generator shows the relationship between the no-load terminal voltage (which equals to the armature induced e.m.f) and the field current (that flows in the shunt field circuit).

This curve is obtained by driving the generator at constant speed with its terminals left open-circuited.

This curve is often called the open circuit characteristics curve (OCC) or no load curve.

The total induced EMF of the armature is given by:-

E =a60

volt

Where:-

Z = total number of conductors in the armature.

= useful flux per pole in Weber.

N = speed in rpm.

a = number of parallel paths in the armature windings.

P =number of poles.

For any given machine the terms Z P and a are constants and the EMF equation can be written as:-

E = Ke.N.


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With the shunt generator, self excitation takes place as follows:

With the machine runs up to full speed, and the field circuit is opened. The machine acts as every feeble generator giving about 2% to5% of normal voltage and this is due to residual magnetic flux of the magnetic poles of the machine. When the filed circuit is completed the EMF due to the flux circulates a current around the circuit of the armature and the field.

If the produced flux is in the same direction of residual flux the induced (EMF) will be increased.

The small induced (EMF) will be applied to field resistance Rf of the filed winding, and filed current will increase the flux again, causing induced (EMF) to climb up the OCC. But this increased value of E is again applied to (Rf) and will increase (If), which in turn increase, (E) some more.

The process continues with climbing up the OCC and (If, V) climbing up the Rf- line until the two points coincide at the intersection point. We say that the shunt generator voltage (V) has built up to (V0). Fig. (2.1) Shown the (OCC) of d.c shunt generator.

The first part of the curve will be linear and the tangent to this curve represents the critical resistance of the field circuit. The curve starts to bend when the iron begins to saturate. For a given magnetic flux, the (EMF) generated varies in direct proportional to the speed of rotation. If the number of magnetization curves for different speeds be plotted on the same base and to the same scale, then the ordinates of the various curves for any particular value of the excitation will vary in direct proportion to the speed, i.e. the critical resistance varies directly with speed.

If the resistance of the field circuit is kept constant so that it gives rated voltage, then any reduction of the sped will cause a reduction of the induced (EMF) and hence flux per pole. This means that there is a certain speed (critical speed), below which the machine will not excite.

The process of voltage build-up requires the following conditions to succeed:

1- There must be residual flux to start the process. 2- The flux produced by (If) should aid the residual flux.


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3- The resistance of the field circuit (Rf) should be small enough to intersect the (OCC) in the saturation region (Rf < R critical).

4- Proper relation ship between direction of rotation and direction of flux in the main magnetic poles.

5- The speed should be greater than the critical speed (N > N critical).

2-3- Test procedures:

1- Connect the circuit as shown in fig. (2.2). 2- Run the machine at speed of 1500 r.p.m. 3- The speed must be kept constant through out the test. 4- Take the readings of (If) and (V). 5- Repeat steps (3, 4) for speed of 1700r.p.m.

2-4- Graphs:

Plot the open circuit characteristics for two speeds.

2-5- Discussion:

1- What is the no-load characteristic of d.c. generator? 2- Discuss the saturation (magnetic, no-load) characteristic? 3- In the no-load test of d.c. generators what is the parameter must be

kept constant? 4- What are the conditions foe self-excited d.c. generators or the

conditions for voltage build up?


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Figure. (2.1)

Figure. (2.2)


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Experiment No.( 3 )

Load Characteristics of a D.C Shunt Generator

3-1- Objective:

To obtain the load characteristics of d.c shunt generator.

3-2- Theory:

The principal advantage of d.c shunt generator is that the generator can be self-excited and this eliminates need for an external source of excitation.

3-2-1- Build up of the emf:

When an unloaded shunt generator is started the only flux available in the field is that due to residual magnetism left from previous operation. That is sufficient to induce a small electromotive force in the armature winding. The resulting current in the filed coils increases the flux and the field gradually builds up until a stable output is reached dependent on the magnetization characteristic curve and field circuit resistance.

The armature may not develop its rated voltage for any of the following reasons:

1-|Residual magnetism is absent in the poles.

2- The total filed resistance is greater than the critical resistance for the speed used.

3- The speed is less than the critical speed for the total field resistance used.

4- Improper connection of the field circuit for the direction of armature rotation.

5- Brushes improperly used.

6- External load resistance lower than some critical value when the machine is started on load.


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7- Direction of rotation of armature is not correct.

3-2-2- Generator under load:

The terminal voltage of the generator drops when the load current is increase this drop is due to armature reaction and armature resistance drop.

Because of the drop in voltage the field current is decreased consequently, hence the flux and the generated emf are reduced. There by causing further reduction in terminal voltage when the load resistance drop to such a low value as to shunt the armature current away from the field windings not only the output voltage but also the load current will decrease. The characteristics will be then as shown in g.(3.1).

OC is the emf due to the residual magnetism, OD is the armature current and OC / OD = armature resistance. The emf OC is absorbed by the armature voltage drop and armature reaction effect.

3-2-3- Construction of internal characteristic from external characteristic:

A curve (AB) g.(3.1) is plo ed between the load current and terminal voltage. This is the external characteristic. Tow straight line (OR) and (OS) are drawn through the origin to represent the armature and field resistances respectively. The field current (EF) corresponding to an point (P) on (AB) is added to the corresponding load current (OG) to give the armature current (OH)

A perpendicular (HK) , erected to meet (OR) gives the armature voltage drop, which when added to (PQ = KH) gives a point Q on the internal characteristic. This construction is performed for a number of points, and the internal characteristic (AL) is obtained.

Applying KVL on the generator circuit, the terminal voltage of a d.c generator can be written as:

V= E – ( IR + Vb + E )


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Where

E =KeN

IR = armature résistance voltage drop.

Vb = brushes voltage drop.

E = voltage reduction due to armature reaction .

Hence PQ = IR + Vb + E


1- Connect the circuit diagram shown in Fig. (3.2). 2- Run the generator at its rated speed which is maintained constant

through out the experiment. 3- Adjusted the field regulator until the terminal voltage equals to the

rated voltage of the machine with its terminals open circuited . 4- Load the generator gradually from (0 – 125%) full load current . 5- Record all the instrument reading in each step.

H. G. J

3-4- Graph:-

Plot the external and internal characteristics.

3-5- Discussion:-

1- Why the output voltages drop when the load current is increased? 2- Explain the build up of the emf for d.c shunt generator? 3- What are the reasons made the armature not develop it rated

voltages


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Fig. (3.1)


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Fig. (3.2)


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Experiment No.( 4 )

Load Characteristics of a Separately – Excited D.C Generator

4-1- Objective:

To obtain the load characteristics of separately excited d.c generator.

4-2- Theory:

In a separately excited d.c generator the field winding is connected in series with a variable resistor (know as a field regulator).and an ammeter to a separate d.c source such as battery or d.c generator. There fore the field current is independent of the machine it self and is unaffected by the load change. The generator is driven at rated speed and is kept constant. There fore the armature voltage tends to remain steady as the load changes. Armature reaction and Ia Ra drops in the armature cause the voltage to drop as the load current increases. For rough calculation to obtain the internal characteristic; armature reaction drop is neglected and the equation become

E = V + IaRa

In other word, the internal characteristic is obtain from the IaRa ordinates in the external characteristic curve.

A curve AB in g (4.1) is plo ed between the load current and the terminal voltage in the external characteristic.

A straight line OC is drawn through the curve to represent the armature resistance drop and ordinates of OC are added to those AB ( PQ = DE ) to give the internal characteristic AF .


1 – Connected the circuit diagram as shown in g. (4.2)

2 – Run the machine at rated speed and keep it constant throughout the experiment.


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3 – The field regulator is adjusted until the terminal voltage equals the rated voltage.

4 – The machine is gradually loaded from (0 – 125%) full load current.

5 – Record all the instruments.

4-4- Graph:

Plot the external and internal characteristics.

4-5- Discussion:

1 – Why did the field current of this generator un affected by the load change?

2 – How can we obtained the internal characteristics from the external characteristics?


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Fig. (4.1)

Fig. (4.2)


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Experiment No. ( 5 )

Load Characteristics of a D.C Series Generator

5-1- Objective:

To obtain the load characteristics of d.c series excited generator.

5-2-Theory:

The machine can not produce any voltage when it is disconnected from the load except that generated due to residual magnetism in the pole pieces. Consequently at no load the voltage of a series generator approximately zero as against the shunt generator in which the voltage is at a maximum value at no load. Both the armature and the series field carry the same load current so the voltage increases with the load current. Without the effect of armature – reaction the emf characteristic would be similar to the magnetization curve. Armature reaction effect and the IL (Rf+Ra) drop both reduce the terminal voltage. The net result is generally rising characteristic like that shown in curve AC in g .(5.1) but if satura on is approached the characteristics are drop and the extern becomes nearly vertical . Neglecting the armature reaction effect, the output voltage.

V = E – IL (Ra + Rf) = IL RL

Where: V = open circuit voltage

IL = load current

Rf = series field resistance

Ra = armature and interpole resistance

RL = load resistance

Internal characteristics (curve AE) is obtained by adding ordinates of (Ra + Rf) IL to external (curve AC). The magnetization characteristic is represented by (curve AB). See fig.(5.1).


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H. G. J

5-3-Test procedures:

1- Connect the circuit diagram shown in g .(5.2)

2- Run the machine at rated speed and record the no load voltage . The speed is maintained constant.

3- Reduced the load resistance RL gradually this increase IL load current until this current reach about 20 % over it rated

4- Record all the instrument reading at each step .

5-4-Graph:

Plot the internal and external characteristics .

5-5-Discussion: 1- Compared between d.c shunt and series generator for their internal and

external characteristics.

2- What is the critical field resistance for this generator?


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Fig. (5.1)

Fig. (5.3)

Fig. (5.2)


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Experiment No.( 6 )

Load Characteristics of a Compound Generator

6-1-Objective:

To obtain the load characteristics of d.c compound generator.

6-2-Theory: The characteristics of a compound generator are a combination of the

shunt and series characteristics. The shunt generator has a drooping characteristic of output voltage generator has a rising characteristic (for small value of load current). However there are two possibilities, depending upon the method of connecting the series field relative to the shunt field in one connection, the series field assists the shunt field and is known as commutative connection, the other, series field opposes the shunt field and is known as differential connection. The compound generator that develops the same voltage at full load as at no load (for the same speed) is said to be flat or level compound fig (6.1). Under compound gives full-load voltage less than the no-load value while over compound causes the full-load voltage to exceed that at no-load. Proper amount of compounding requires proper number of turns in the series winding. Usually a diverter is used across the series field through which any degree of over-compound may be obtained up to the maximum by increasing the resistances of the diverter compound generators are either short-shunted and long-shunted. Since the shunt field current is only a very small percentage of the total load of the machine. It is evident that it makes little difference which connection is used, the choice being more a matter of convenience than any thing else.

6-3-Procedure: 1- The compound generator and DC shunt motor coupled to the shaft

of the generator are connected according to fig (6.2). Switches S1 and S2 are open with the series field connected to assist the shunt field, the generator is run up to normal and its voltage is adjusted to the rated voltage by means of a shunt field regulator and kept unaltered. The speed is maintained constant. Switch S1 is closed and the generator is loaded gradually up to 20% overload readings of the voltage and load current are noted.


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2- Repeat (1) with switch S2 closed and diverter placed in minimum point.

3- Repeat (1) with switch S2 closed and the diverter placed in mid point.

4- Repeat (1) with switch S2 open and the series field reversed.

6-4-Conclusions and graphics: 1- Draw the external characteristics on the same graph paper.

2- Explain and discuss the shapes of all curves and the differences that exist between them.

3- Comment on the possible industrial application of the over, level, under and differential compound generator.

4- What are the basic differences, if any between under and differential compound generator?

H. G. J


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Fig. (6.1)

Fig. (6.2)


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Experiment No.( 7 )

Characteristics and Speed Control of Shunt Excited Motor

7-1-Object:

In this experiment a survey of the main characteristics and control of dc shunt motor is made. Due to particular feature of these motors, a study of the speed control is very important or useful.

7-2-Theory:

A dc shunt motor consists of two parts, armature and the main field poles. The circuits of these two parts are connected in parallel and supplied from a common dc source. Resistances are connected in series with both circuits to provide for the necessary speed control.

Excluding the effect of armature reaction and resistance drop, the main two variables in speed control are the supply voltage and the flux. Speed can be calculated from the following equation:

KIaRaVaN

Where

N = the speed

Va = the armature voltage

Ia Ra= armature resistance drop

K=constant

=flux

It is clear that speed varies directly with (Va) and inversely with flux or field current.

Therefore if speeds above rated speed are desired, field control would be suitable and for speed below rated speed the armature control is used.


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7-3-Procedure:

The circuit diagram is connected as shown in g.(7.1). The motor is started insuring that the starting resistance is maximum and the field resistance is minimum.

7.3. -A- Starting from the rated speed, the motor is loaded by means of the coupled dc generator and readings of speed N, armature current Ia and torque T are noted. The torque readings are taken at each load by balancing the lever arm of the torque measuring unit by putting the weights.

H. G. J

Through out the test the field current of the motor is kept constant at the value Ifo needed to run the machine on no-load rated speed, IL and Vt are noted.

7.3. -B-

Adjust the load of the machine so that Ia is kept constant a (4 A). The eld resistance is varied and speeds up to 1800 rpm are obtained. For each

variation of field resistance readings of speed N, torque T and If are noted.

7.4-Results:

Watts 60

2 NT Wo = Power output =

Where T = torque [N.m] = (9.81) (weight in Kg) ( L in meter)

WI=input power = (Vt) (Iin) =Vt (Ia+If) = [w]

WiWo Efficiency= =

1-plot N against T for test (A)

2-plot Ia and efficiency against output power for test (A)


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3-plot N and T against If for test (B)

7.5-Conclusions:

1-Comment on the curves obtained in view of the theoretical performance.

2- Discuss the effect of a possible break (open circuit) in the shunt field circuit while the motor is running.

3- Mention some of the practical applications of the shunt motor in view of its characteristics and give reasons for that.


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Fig.(7.1)

Shunt excited DC motor

Experiment No.( 8 ) Swinburne's Test

8.1-Objective: Determination of no- load losses and estimation of efficiency of a d.c shunt

machine by indirect methods.

8.2-Theory: The losses during no load conditions of a d.c shunt machine will be iron loss,

friction and wind age loss, field copper loss. If those losses are measured during no. load condition, then efficiency of a d.c shunt machine either as a generator or as a motor no. load losses can be measured by a Swinburne's test in this test the given d.c shunt machine is driven as a motor. The method cannot be used in the case of a d.c series motor.

The given shunt machine is run on no. load as D.c shunt motor and the input current, shunt field current and applied voltage are noted. (See Fig.8.1). The resistance of armature (Ra) cold= Ra1 and the resistance of the shunt field (R shunt=R sh1) are measured by ammeter and volt. Meter at the ambient temperature. (Ra hot= Ra2) and (Rsh hot=Rsh2) are calculated for an assumed temperature rise of (750 c ) during loaded condition (t2=t1+75c ) by using the formula

Ra2=Ra1 (1+75 t1) ………………………………… 1 Rsh2=Rsh1 (1+75 t1) ………………………………. 2 Where ( ) is the coefficient of increase of resistance with temperature,

t1 is ambient temp.

t1= 0 / (1+ 0 t1) ……………………………….. 3 t2= 0 / (1+ 0 t1) ………………………………… 4 0=0.00427 for pure copper

Estimation of :-

Let the measured values be:

V = applied voltage


[2]

I = no- load line current

If= field current

Ra1= armature resistance at ambient temperature then Ia= (I -If)= armature current armature copper loss at temperature t1= V² / Rsh1

= If² Rsh1

Then stray losses

P stray = Iron loss+ Friction loss

= V I - (Ia² * Ra 1+ Is²h * Rsh1)

P constant= constant losses

= stray losses + shunt copper loss at temperature t2

P constant = P stray + V² / Rsh2

From the cold resistance of the armature, copper loss I2 x Ra1 is calculated and from the cold resistance of the shunt field, copper loss r2/Rsh1 is calculated the sum of these two losses deducted from the input power V I gives the friction and iron losses i.e. the stray losses.

If now the calculated field copper loss v2/Rsh2 using the hot temperature is added to the stray losses. The result will be constant losses Pc.

1) Efficiency ( ) as d.c shunt motor :

Let the line current on load be (I) then

Ia= I – IshF = I – V /Rsh2

Pcu 2=armature copper loss at the given

Load current = 2Ia *Ra2

Input power = v*I


[3]

The Efficiency ( ) = {1-(Pcu2 + Pconst.) / V*I} * 100%

2) Efficiency as d.c shunt motor:

Let the load current be (I) then Ia = I + IshF = I + V / Rsh2

Armature copper loss at the given

Load current = pcu2 = Ia2 x Ra2

Output power = V * I

Efficiency ( ) = V*I / (V*I+Pcu2 + Pconst.) x 100%

8.3-Test procedures:

1-Connect the machine as in the fig. (8.1). Start the machine as shunt motor with full start resistance in circuit, gradually reduce the starter resistance and adjust the rated speed by record input current (I) field current (I shf) and supply Voltage (Va). 2-Stop the machine by opening the D.C main supply switch. 3-Measure (Rat1), the resistance of armature at room temperature (t1) by ammeter-voltmeter method.

8.4-Graph:

Plot the efficiency Vs percentage load current for cases motor and generator operation.

8.5-Discussion:

1-Comment on the curves option. 2-Note when the maximum efficiency occurs. 3- Is this method applicable for testing a series machine? If not

why? 4- What are the advantages and disadvantages of Swinburne's test? 5- Find out the condition of maximum efficiency by mathematical derivation.


[4]

H

M

DC Supply

V

Ia

Starter

A

Rf

ShuntfieldWdg

A

Fig (1)

Swinburne test circuit diagram

If

ZZ

S2

,

Z

S1

T

HH

N


[5]

G D F C B T(s)

N(rpm)

N1

Without field

With field

FIG.(8.2 )


[6]

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3-2- Theory