EXPERIMENT N0 - faculty.yu.edu.jofaculty.yu.edu.jo/mohammadh/Lists/Taught Courses... · of a dc Generator. EXPERIMENT N0.1 ... characteristic of a shunt dc generator . Figure

1/16

Figure (1-1): Basic Operation

of a dc Generator.

EXPERIMENT N0.1

DC GENERATORS

I. Objectives:

To investigate the principles of operation of dc generators by

measure some of the characteristics of a separately excited dc generator,

dc shunt generator, dc series generator, and dc compound generator such

as, no-load characteristics, and terminal characteristics. Also to be

familiar with the main advantages and disadvantages of these types of dc

generators by comparing the main features among them, specially the

voltage regulation and control methods of the output voltage.

II. Equipment: 1. D.C Machine (as a Generator) MV 1006 / TERCO 1

2. D.C Machine (as a Motor) MV 1028 / TERCO 1

3. Torque measuring unit MV 1051 / TERCO 1

4. Power supply MV 1300 / TERCO 1

5. Shunt regulator MV 1905 / TERCO 2

6. Resistor load MV 1100 / TERCO 1

7. Load switch MV 1500 / TERCO 1

8. Multimeters 5

III. Theory:

Direct current machines are energy transfer devices. These machines

can function as either a motor or a generator. DC motors and generators

have the same basic construction, differing primarily in the energy

conversion. In DC generators, the three conditions that necessary to

induce voltage are: magnetic field, conductor, and relative motion

between them. So the basic DC generator has four basic parts: magnetic

field, single conductor or loop, commutator and brushes, as shown in

Fig.(1-1).

EXPERIMENT NO.(1) DC GENERATORS

2/16

Figure (1-2): Left-Hand Rule for

Generators.

A single conductor, shaped in the form of a loop, is positioned

between the magnetic poles. As long as the loop is stationary, the

magnetic field has no effect (no relative motion). If the loop is rotated,

there will be a relative motion between the magnetic field and the

conductor, so an EMF (voltage) will induce into it. The magnitude of the

induced EMF depends on the field strength and the rate at which the flux

lines are cut, as given by the following equation.

Eg = K (1-1)

Where:

Eg = generated voltage

K = constant, depending on the construction of the machine

= magnetic flux strength

= angular speed

The direction of the induced current flow can be determined

using the "left-hand rule" for generators, as shown in Fig.(1-2).

Applying the left-hand rule to both sides of the loop will show that

current flows in a counter-clockwise direction in the loop

Commutator converts the AC voltage generated in the rotating

loop into a DC voltage. In a simple one-loop generator, the commutator

is made up of two semi cylindrical pieces of a smooth conducting

material, usually copper, separated by an insulating material. Each half of

the commutator segments is permanently attached to one end of the

rotating loop.

Brushes usually made of carbon, slides along the commutator as it

rotates so the brushes make contact with each end of the loop. Each brush

slides along one half of the commutator and then along the other half.

Every time the ends of the loop reverse polarity, the brushes

switch from one commutator segment to the next. This means that one

brush is always positive with respect to the other. The voltage between

the brushes fluctuates in amplitude between zero and some maximum

value, but it is always of the same polarity as shown in Fig.(1-3).


3/16

%100

fl

flnl

V

VVVR

Figure (1-4): The magnetization curve of a

generator for a fixed speed.

The magnetic field may be supplied by either a permanent magnet

or an electromagnet. The magnetic fields in DC generators are most

commonly provided by electromagnets. The current which flow through

the electromagnet conductors to produce the magnetic field is known as

the field excitation current and can be supplied to the field winding in one

of two ways; it can be fed from a separate DC source (Separately excited

generator) or it can come directly from the output of the generator, in that

it is called a self-excited generator. In a self-excited generator, the field

winding is connected directly to the generator output. The field may be

connected in parallel with the output (Shunt generator), in series with the

output (Series generator), or a combination of the two (Compound

generator).

The field current produces a flux in the machine core in accordance

with its magnetization curve, and since EA is directly proportional to the

flux at constant speed, the relation between EA and If (no-load

characteristic) will be as shown in Fig.(1-4).

Dc Generators are compared by their voltages, power ratings,

efficiencies, and voltage regulations. Voltage regulation (VR) is defined

by the equation

(1-2)

EA

If

= o

Figure (1-3): Commutation in a DC Generator.


4/16

AAAT RIEV

Figure (1-5): A separately excited dc

generator.

The Separately Excited DC Generator

The equivalent circuit of such a machine is shown in Fig.(1-5). In

this figure the armature circuit is represented by an ideal voltage source

(EA) and a resistor (RA). The field coils are represented by inductor (Lf)

and resistor (Rf). It is clear that the armature current (IA) is equal to the

load current (IL) in a separately excited generator. By Kirchhoff's voltage

low, the terminal voltage is:

(1-3)

According to the previous equation the terminal characteristic (VT

versus IL) of a separately excited generator is a straight line as shown in

Fig.(1-6-a). In generators without compensating windings, an increase in

IA causes an increase in armature reaction and armature reaction causes

flux weakening. This flux weakening causes a decrease in EA= K

which further decreases the terminal voltage of the generator as show in

Fig.(1-6-b)

There are two possible ways to control the output voltage:

1. Change the speed of rotation. If then EA = K increases, so VT =

EA-IARA increases too.

2. Change the field current. If If increases then the flux in the machine

increases. Therefore EA = K increases and VT = EA-IARA

increases too.

RA

+

-

RF

LF

Load

IL IA IF

EA VT

+

-

VF

AR drop

IL

VT

EA RAIA

IL

VT

EA

RAIA drop

Figure (1-6): The terminal characteristic of a separately excited dc

generator with(a) and without(b) compensating windings.

(a) (b)


5/16

Figure (1-7): The equivalent circuit

of a shunt dc generator.

Figure (1-8): Voltage buildup on starting

in a shunt dc generator.

The Shunt-Wound DC Generators

When the field windings is connected in parallel with the armature

of the generator, the generator is called a shunt-wound generator (Fig.(1-

7)). In this circuit the armature current supplies both the field current and

the load, so

The Kirchhoff's voltage low equation for the armature circuit is:

The voltage buildup in a dc generator depends on the presence of a

residual flux in the poles of the generator as shown in Fig.(1-8)

There are several possible causes for the voltage to fail to buildup

during starting. Among them are:

1- There may be no residual flux

2- The direction of rotation of the generator or the field current may have

been reversed which produces a flux opposing the residual flux instead

of adding to it.

3- The field resistance may be adjusted to a high value so the field

current will be very small and the voltage never buildup.

AAAT RIEV

RA

+

-

RF

LF

Load

IL IA

IF

EA VT

+

-

EA/VT

If

VT versus If

Ifnl

VTnl

EA versus If

Rf =VT/If

EResid

ual

LFA I I I


6/16

Figure (1-9): The terminal

characteristic of a

shunt dc generator

.

Figure (1-10): The equivalent circuit of

a series dc generator.

As the load is increased, IA increases and then armature resistance

voltage drop increases, causing VT to decrease. However, when VT

decreases, the field current decreases and so the flux also decreases with

it. This causes EA to decrease which causes a further decrease in the

terminal voltage. By the effect of the armature reaction the terminal

voltage will decrease more and more as shown in Fig.(1-9)

As in the separately exited generator, there are two ways to control

the voltage of a shunt generator:

1. Change the speed of rotation.

2. Change the field current.

The Series-Wound DC Generators

When the field winding of the DC generator is connected in series

with the armature, the generator is called a series-wound generator

(Fig.(1-10)).

According to this equivalent circuit, it can be noted that

LFA III

and

(1-4)

Since the excitation current in a series-wound generator is the

same as the load current, the curve represents the relation between the

generated voltage (EA) and the load current, looks like the magnetization

)( SAAAT RRIEV

IL

VT

RAIA drop

Field weakening effect

A.R.

RA

+

-

RS LS

Load

IL IA IF

EA VT

+

-


7/16

Figure (1-11): A series generator

terminal characteristic.

Figure (1-12): the equivalent circuit

of a compounded dc

generator.

curve. The IA(RA+RS) drop goes up by increasing load current which

reduces the terminal voltage as shown in Fig. (1-11).

The Compound DC Generators

Series and shunt generators have a disadvantage in that changes in

load current cause change in generator output voltage. One means of

supplying a stable output voltage is by using a compound generator.

A compound dc generator is a dc generator with both series and

shunt fields, connected so that the magnetomotive forces from the two

fields are additive (cumulatively compound) or subtractive (differentially

compound). Fig (1-12) shows the equivalent circuit of a compound dc

generator.

As load current increases the armature current increases which tends

to cause a decrease in the terminal voltage VT = EA- IA(RA+RS). In

cumulatively compound generator, increasing IA cause an increase in the

net flux which increases EA and VT. These two effects oppose each other,

one tending to decrease VT and the other tending to increase VT.

In case of few series turns the first effect will be dominant so the

terminal voltage decreases. This type of construction is called

undercompounded

If there are a few more series turns, then at first the flux

strengthening effect wins but as the load continues to increase, the

resistive drop becomes stronger. If VT at no load is equal to VT at full

load the generator is called flatcompounded.

IL

EA/VT

IA(RA+ RS) drop

+ Armature reaction

RA

+

-

RS LS

Load

IL IA

EA VT

+

-

RF

LF IF


8/16

Figure (1-13): Terminal characteristic of

a compounded dc generator.

If the number of series turns is large, the effect that increases the

terminal voltage will be predominant and the generator is called

overcompounded. Fig.(1-13) shows the terminal characteristic of the

compound dc generator with all previous cases compared with the shunt

generator.

In differentially compounded generator, as the load is increased, IA

increases, causing decreasing in the net flux which in turn decreases the

induced voltage and VT. The resistive voltage drop will also reduce the

terminal voltage and so it will drops drastically by increasing the load

current as shown in Fig.(1-13).

IL

VT

Overcompounded

Flatcompounded

Undercompounded

Shunt

IFL

Differentially

compounded


9/16

IV. Procedures:

Part 1: Separately excited dc generator

No-Load Characteristic

1.1 Connect the dc machine MV 1006 as a generator, and the dc machine

MV1028 as a motor in accordance with circuit diagram (1).

1.2 Make sure that the switch S is off and the two regulators are turned

to minimum value.

1.3 Set the field current of the dc motor to 0.7 A, increase the armature

voltage whereupon the motor starts rotating. Adjust the speed to

1400 rpm (this speed must be kept constant during the experiment).

1.4 Increase the field current of the generator in steps of 0.1 A up to

maximum (see ratings) for each step measure the terminal voltage.

1.5 Repeat step 1.4 after adjusting the speed to 1200 rpm

Load characteristic

1.6 Readjust the speed to 1400 rpm (this speed must be kept constant

during this part).

1.7 Adjust the shunt regulator of the generator to bring the voltage up to

220 V. Record the excitation current, this value must be kept

constant during this part of the experiment.

1.8 Turn the load resistor toward minimum load current. Turn on switch

S. By varying the load resistor increase the armature current in steps

of 1A up to maximum (see ratings), for each step read the terminal

voltage and the shaft torque.

1.9 Turn off the switch S, variable dc voltage, fixed dc voltage, and the

main power supply switches (don’t open the field circuit of the dc

motor while motor is running)


10/16

Part 2: Shunt dc generator

2.1 Leave the dc machine MV1028 as it was, and reconnect the dc

machine MV1006 as a shunt dc generator as shown in circuit

diagram (2).

2.2 Restart the dc motor as in the previous part (step 1.3)

2.3 Adjust the shunt regulator of the dc generator to bring the terminal

voltage up to 220 V.


S. By varying the load resistor increase the load current in steps of

1A up to 5A, for each step read the terminal voltage, the field

current and the shaft torque.




Part 3: Series dc generator


machine MV1006 as a series dc generator as shown in circuit

diagram (3).




0.5A up to 5A, for each step read the terminal voltage and the shaft

torque.





11/16

Part 4: Compound dc generator


machine MV1006 as a compound dc generator with 100% of the

series windings (Overcompounded) as shown in circuit diagram (4).


4.3 Adjust the shunt regulator of the dc generator to bring the terminal

voltage up to 220 V.



1A up to 5A, for each step read the terminal voltage and the shaft

torque.



motor while motor is running).

4.6 Modify the circuit by using 70% of the series windings

(Flatcompounded) and then repeat step 4.3 to 4.5.

4.7 Again modify the circuit but now with only 30% of the series

windings (Undercompounded) and then repeat step 4.3 to 4.5


12/16

Circuit diagram (1):


+ - + - + - L1L2L3N L1L2L3N

220 Vdc 0-220 Vdc 0-220/127 220/127

TG

r.p.m

A

F2

F1

M

Resistive load

S

A

A

Speed (n)

(VT )

If

(IL )

D3

D2

D1

M

F2

F1

A

V

+ - + - + - L1L2L3N L1L2L3N

220 Vdc 0-220 Vdc 0-220/127 220/127

TG

r.p.m

A

F2

F1

M

Resistive load

S

A

A

Speed (n)

(VT )

If

(IL )

D3

D2

D1

M

F2

F1

A

V


13/16



A

+ - + - + - L1L2L3N L1L2L3N

220 Vdc 0-220 Vdc 0-220/127 220/127

TG

r.p.m

A

F2

F1

M

Resistive load

S

A

Speed (n)

(Vt )

(IL )

V

D2

D1

G

D3

F2

F1

+ - + - + - L1L2L3N L1L2L3N

220 Vdc 0-220 Vdc 0-220/127 220/127

TG

r.p.m

A

F2

F1

M

Resistive load

S

A

A

Speed (n)

(VT )

If

(IL )

D3

D2

D1

M

F2

F1

A

V


14/16

V. Results:

Part 1: Separately excited dc generator

No-Load Characteristic

Load characteristic

Part 2: Shunt dc generator

n=1400 rpm

IF (A) 0.0 0.1 0.2 0.3 0.4 0.5

Eg (V)

n=1400 rpm

IF (A) 0.0 0.1 0.2 0.3 0.4 0.5

Eg (V)

N=1400 rpm IF =

Measu

red IL (A) 0

Vt (V) 220

(Nm)

Ca

lcu

late

d Pin(w)

Pout(w)

(%)

n=1400 rpm

Mea

sure

d IL (A) 0 1 2 3 4 5

Vt (V) 220

IF (A)

(Nm)

Ca

lcu

late

d

Pin(w)

Pout(w)

(%)


15/16

Part 3: Series dc generator

Part 4: Compound dc generator

n=1400rpm

Measu

red

IL (A) 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Vt (V)

(Nm)

Calc

ula

ted

Pin(w)

Pout(w)

(%)

N=1400 rpm 100% of the series winding (Overcompounded)

Mea

sure

d

IL (A) 0

Vt (V) 220

(Nm)

Ca

lcu

late

d

Pin(w)

Pout(w)

(%)

n=1400 rpm 70% of the series winding (Flatcompounded)

Mea

sure

d

IL (A) 0

Vt (V) 220

(Nm)

Ca

lcu

late

d

Pin(w)

Pout(w)

(%)

n=1400 rpm 30% of the series winding (Undercompounded)

Measu

red

IL (A) 0

Vt (V) 220

(Nm)

Ca

lcu

late

d

Pin(w)

Pout(w)

(%)


16/16

VI. Tasks:

a. In the same graph, draw the magnetization curve (Eg versus IF) for

the two different speeds according to the measured data in part 1.

b. Complete the tables in parts 1, 2, 3, and 4 by calculating the input

power (Pin = ), the output power (Pout =VtIL), and the efficiency

(in

out

P

P ) for each electrical load.

c. In the same graph, draw the terminal voltage versus the load

current of the separately excited, shunt, and series dc generator.

d. In the same graph, draw the terminal voltage versus the load

current for the different cases of the cumulatively compounded dc

generator and the shunt dc generator.

e. In the same graph, draw the efficiency () as a function of the load

current for the separately excited, shunt, series, and

flatcompounded dc generator.

f. According to the terminal characteristics drawn in C, is there any

effect of the armature reaction in this machine? How can this

problem be remedied?

g. Using the terminal characteristic of the separately excited dc

generator, determine approximately the internal resistance of the

armature of the dc generator (RA).

h. Calculate the voltage regulation of this dc generator in case of

separately, shunt, and series excitation. Calculate the voltage

regulation for the different cases of the cumulatively compounded

dc generator.

i. What is the critical resistance?

j. How can the different cases of the cumulatively compounded dc

generator (over, under, and flat) be gotten without changing the

number of series windings?

k. Explain how to control the output voltage for the different types of

the dc generator and how to reverse the voltage polarity?

Documents

EXPERIMENT N0 - faculty.yu.edu.jofaculty.yu.edu.jo/mohammadh/Lists/Taught Courses... · of a dc Generator. EXPERIMENT N0.1 ... characteristic of a shunt dc generator . Figure