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Transcript
ADDIS ABABA UNIVERSITY
ADDIS ABABA INSTITUTE OF TECHNOLOGY
DEPARTEMENT OF ELECTRICAL AND COMPUTER ENGINEERING
Course Title: Electrical Engineering Lab IV
ECEG- 3207
(Electrical Machine)
Tests on one phase transformer Experiment-02
Transformers
Transformers are magnetic circuit elements used in AC circuits for a variety of applications. Transformers are
often used to convert voltages from high to low values, or from low to high values. In its simplest form, a
transformer consists of two closely coupled wire coils that share a common magnetic field. If a voltage is
applied to one of the coils (called the primary), then a voltage will be induced on the second coil (called the
secondary). The actual voltage induced in the secondary coil is proportional to the ratio of the number of turns
in the two coils. That ratio is called the turns ratio, for which we will use the symbol a. If the number of turns in
the primary is N1 and the number of turns in the secondary is N2,
the turns ratio a is
a = N1/N2
EQUIPMENT and COMPONENTS
No
Description
Code/Lab Reference
Quantity
1
DC & AC Power source
-
2
2
Multimeter (VOM)
-
1
3
Ammeter
-
2
4
Voltmeter
-
2
5
Variable Resistor
-
1
6
Conducting Wires
-
10
7
Resistor
8
Transformer
9
Watt meter
10
Circuit Board
R=V/I
Raverage = RTotal/Number of R
η = PO/Pi
η= VsIs/Pi
2
-
1
-

Connect circuit

Find the rated value of voltage, current, frequency and volt-amper

Measuer winding resistance of a transformer
Open circuit taste

Using variac apply different voltage

Measure power, current & secondary voltage
Short circuit taste
1
1

Connect circuit

Using variac apply increase current

Measure power, current & voltage

Calculate the equivalent resistance
Load test

Primary voltage equals with rated voltage

Vary secondary current 0 to rated current

Measure voltage and plot the curve
Vp =110+110 = 220v
Vs = 55+55 = 110v
F = 50Hz
VA=2.5KVA
SHORT CIRCUIT
Vary secondary
Current
,
Factor
=12.5
I=5A
max
I(A)
1
2
2.2
2.5
4.2
5.3
6.3
7.5
8.6
9.9
10
Vs(v)
120
120
120
120
115
115
113
113
112
111
111
Ps (w)
190
300
450
600
900
1150
1350
1600
1850
2100
2190
V= 220v I= 5A Total factor= 100
No load power Vp=220v
Vs=112v
P=0.4w*100
LOAD TASTE All the circuit parameters must be known so that the equivalent circuit can be used for the above
purpose. These parameters can be easily determined by performing tests that involve little power consumption.
Two tests, a no-load test( or open circuit test) and short circuit test will provide information for determining the
parameters of the equivalent circuit.
Open circuit (OC) test
The shunt branch parameters can be determined by performing this test. Since, the core loss and the
magnetizing current depend on applied voltage, this test is performed by applying the rated voltage
to one of the windings keeping the other winding. the secondary terminals are open (no load is connected across
the secondary), current drawn from the source is called as no load current. Under no-load condition the power
input to the transformer is equal to the sum of losses in the primary winding resistance R1, and core loss. Since, no
load current is very small, the loss in winding resistance is neglected. Hence, on no load the power drawn from the
source is dissipated as heat in the core. If Io and Pi are the current and input power drawn by the transformer at
rated voltage V1 respectively
Short circuit (SC) test
Suppose the input voltage is reduced to a small fraction of rated value and secondary terminals are shortcircuited. A current will circulate in the secondary winding. Since a small fraction of rated voltage is applied to the
primary winding, the flux in the core and hence the core loss is very small. Hence, the power input on short circuit
is dissipated as heat in the winding. The transformer are short circuited. The primary voltage is gradually applied
till the rated current flows in the winding. Since, the applied voltage is very small, the magnetizing branch can now
be eliminated from the equivalent circuit.
Effciency
Efficiency of the transformer is defined as:
η = output power / input power
transformers of normal design, the flux in the core varies only a few percent between no-load to full load.
Three Phase Transformers
Three-phase, also written as 3-phase or 3φ supplies are used for electrical power generation, transmission, and
distribution, as well as for all industrial uses. Three-phase supplies have many electrical advantages over singlephase power and when considering three-phase transformers have to deal with three alternating voltages and
currents differing in phase-time by 120 degrees.
A three phase transformer or 3φ transformer can be constructed either by connecting together three single-phase
transformers, thereby forming a so-called three phase transformer bank, or by using one pre-assembled and
balanced three phase transformer which consists of three pairs of single phase windings mounted onto one single
laminated core.
Three Phase Voltages and Currents
The
secondary
Three Phase Transformer Connections
primary and
windings of a transformer can
be
connected in different configuration as shown to meet practically any requirement. In the case of three phase
transformer windings, three forms of connection are possible: "star" (wye), "delta" (mesh) and "interconnectedstar" (zig-zag). The combinations of the three windings may be with the primary delta-connected and the
secondary star-connected, or star-delta, star-star or delta-delta, depending on the transformers use. When
transformers are used to provide three or more phases they are generally referred to as a Polyphase Transformer.
A three-phase transformer generally has the three magnetic circuits that are interlaced to give a uniform
distribution of the dielectric flux between the high and low voltage windings. The exception to this rule is a threephase shell type transformer. In the shell type of construction, even though the three cores are together, they are
non-interlaced.
The three-limb core-type three-phase transformer is the most common method of three-phase transformer
construction allowing the phases to be magnetically linked. Flux of each limb uses the other two limbs for its
return path with the three magnetic flux's in the core generated by the line voltages differing in time-phase by 120
degrees. Thus the flux in the core remains nearly sinusoidal, producing a sinusoidal secondary supply voltage.
1. Percentage efficiency h = (Output power/Input power) *100
2. Percentage Regulation =(V02 –V2)/ V02 * 100
Where,
V02 =Secondary voltage on no load.
V2 = Secondary voltage on load.
Transformer ratio is = Vs/Vp
 Identify the name-palate on the transformer
 Measure the winding resistance of the transformer by the ammeter voltmeter method
 Connect the transformer in Y-Y Apply rated voltage to the primary

Measure the suconday voltage

Determine transformer ratio

Repit  
Y-
 Y connection
 No Load Test circuit

Vary input voltage 0.5v- 1.2v
 Short circuit taste

Connect circuit

Using variac apply increase current secondary about 0.5Irated , 0.75Irated and Irated

Measure power, current & voltage

Calculate the equivalent resistance
 Load test

Apply Primary voltage and Increase gradualy load current on the secondary side

Decreade the resistance of the load resistance

Measure current and secondary voltage

Calculate efficiency
 \Transformer resistance using Digital Ohm metter

Primary = 0.8  in U-X, V-Y, Z-W

Secondary = 0.5 Ua-Xa, Va-Ya, Za-Wa
 Power 2.5/2.17kvA & 50/60Hz
Reduce by half 2.5/2 and 2.17/2
 Ratio value
Transformer ratio is = Vs/Vp
Connection
Primary Voltage
Secondary Voltage
Ratio
Y/Y
Uv
Y/Y
Vy
Y/Y
Zw
Y/
Uv
Y/
Vy
Y/
Zw
/
Uv
/
Vy
/
Zw
/Y
Uv
/Y
Vy
100
88.6
1
/Y
Zw
100
86
1
 No load test
Vin
190
I1
0.25
I2 (mA)
101
W1
10
W2
10
VT
127
VOUT
98
Short circuit No load
V=50v, I = 5A, factor 12.5
Iprimary
Isecondary
Watt
1
0
10
4
2
V=220v, I =5A, total machine factor= 100
When no load power
Vp=220v,
Vs=120v,
W=0.4*100
With load
Ip
10
Watt
21.9
Vs
111
Is (A)
17.5
Load circuit
Vp=380v,
Vs=204v,
I1p=0.8, I2p=0.15, I1s=0, I2s=0, Watt factor= 480
Vp
Vs
380v
197.5
193.5
I1p (A)
1.8
2.05
I2p (A)
1.8
2.05
I1s (A)
3.4
3.8
I2s (A)
3.2
3.6
Power
1200
132
0.22+0.23+0.227
Vary
Primery
Voltage
Rav =
3
V(v)
I(A)
R average = 0.226
V
R
R (Ω)
Vary secondary
Voltage
0.22+0.23+0.227
Rav =
3
V(v)
I(A)
R (Ω)
AVERAGE = 0.08
0.22
Vp =110+110 = 220v
Vs = 55+55 = 110v
F = 50Hz
VA=2.5KVA
OPEN CIRCUITE
Vp(v)
Vs(v)
0.23
0.227
I
Io(A)
Pe(W)
Vary secondary
Current,
Factor=12.5
I=5A max
I(A)
4
2
V(v)
2.6
1
8
6
5.6
4
50
10
P (w)
75
7
V= 220v I= 5A Total factor= 100
No load power Vp=220v
Vs=112v
P=0.4w*100
Current
Vary secondary
,
I(A)
1
Vs(v)
120
Ps (w)
190
Factor
=12.5
I=5A
max
LOAD TASTE
When transformer setep down the voltage reduce but current incrases
Connection is one of the factor of transformer ratio.
1, the factor that determine the transformation ratio of a three phase transformer are Voltage supply,
efficiency of transformer, copper resistance of a wire, Iron core , Number of turns in a winding
2, In ( Yy ), (wye-wye), each transformer has one terminal connected to a common junction, or neutral point
with the three remaining ends of the primary windings connected to the three-phase mains supply.
Phase wiring for “Y-Y” transformer
All the winding ends marked with dots are connected to
their respective phases A, B, and C, while the non-dot ends
are connected together to form the centers of each “Y”.
Having both primary and secondary winding sets
connected in “Y” formations allows for the use of neutral
conductors (N1 and N2) in each power system.
3, Y/Y connection requires the use of three transformers,
and if any one transformer becomes fault or disabled, the
whole group might become disabled. Nevertheless, the
star connected three phase transformer is especially convenient and economical in electrical power distributing
systems, in that a fourth wire may be connected as a neutral point,
4, Three-phase transformer is connected in star–delta, ( Yd ) each Y-connected primary winding will receive the
phase voltage, VP of the supply, which is equal to 1/√3 × VL. Then each corresponding secondary winding will
then have this same voltage induced in it, and since these windings are delta-connected,
5, Single phase power has a sine wave voltage that crosses zero before reversing its polarity. In the region near
the zero-crossing there is not much power. At zero there is none at all. So single phase loads often need some
trickery to deliver output in this area. Often it is just the inertia of the motor or appliance.
6, Under balanced voltage conditions the flux in each leg alternates at supply frequency. The sum of the 3
fluxes is always zero and the fluxes are mutually out of phase with each other by 120 degrees.
A path of magnetic flux in the three leg phase transformer will be magnetic flux connecting the primary and also
secondary windings, that travels almost completely within a iron core.
The magnetic flux linking the primary and secondary windings traveled almost entirely within the iron core,
with no intentional path through air.
Induction Motor
An induction or asynchronous motor is an AC motor in which all electromagnetic energy is transferred by inductive
coupling from a primary winding to a secondary winding, the two windings being separated by an air gap. In threephase induction motors, that are inherently self-starting, energy transfer is usually from the stator to either a
wound rotor or a short-circuited squirrel cage rotor. Three-phase cage rotor induction motors are widely used in
industrial drives because they are rugged, reliable and economical. Single-phase induction motors are also used
extensively for smaller loads. Although most AC motors have long been used in fixed-speed load drive service,
they are increasingly being used in variable-frequency drive (VFD) service, variable-torque centrifugal fan, pump
and compressor loads being by far the most important energy saving applications for VFD service. Squirrel cage
induction motors are most commonly used in both fixed-speed and VFD applications.
An induction motor has two main parts:-Stator and Rotor
a) stationary stator:- The Stator is made up of a number of stampings with slots to carry three phase
windings. It is wound for a definite number of poles. The stator windings are connected directly to the
three phase power source. The stator is connected to a 3-phase AC power supply.
b) A rotor composed of punched laminations, stacked to create a series of rotor slots, providing space for the
rotor winding.
Two types of rotors are used in Induction motors
c) 1) Wound rotor and
d) 2)Squirrel-cage rotor
Tranfsfer ratio = VP/VS
= VStator /VRotor
R=V/I
Raverage = RTotal/Number of R
Rotor resistor = Ra + Rb – Rc
2
η = PO/Pi
PROCEDUR
-using variac and voltmeter identify the stator winding wires
- identify the stator starting and ending turn of the wire
Give ac supply volt and identify the flux dirction using there volt difference or sum
1, Stator resistance
Xu=Yv=Zw=0.7Ω
Rotor resistance
Rab=1.6
Rac=1.1
Rbc=0.6
Ra= (1.6+1.1-0.6)/2 =1.05
Rb= (1.6+0.6-1.1)/2 =0.55
Rc= (1.1+0.6-1.6)/2 =0.05
Raverage=(1.05+0.55+0.05)/3 =0.55
transformer ratio = Vp/Vs
Vstator is Vuv = 100
Vuw = 100
average = 100
Vvw =100
Vrotor is Vkl = > 34
Vkm= > 33.45
average = 34.75
Vlm= > 36.8
An induction motor is an alternating current motor where power to the rotor is supplied by electromagnetic
induction. The magnetic field produced in the stationary coils is changed in order to get the current required to
run the rotor. They use shorted wire loops on rotating armatures to obtain their torque from induced currents.
When flux oppose the voltage reading is zero or Flux pass same direction voltmeter reads the sum of the two
fluxs.
Conversion of electrical power into mechanical power takes place in the rotating part of an electric motor
_ In Induction machines the rotor receive power by induction in exactly the same way as the secondary of the two
winding transformer receives the power from primary.
_ An Induction motor can be treated as a rotating transformer, i.e. one in which primary
Induction Motor
An induction or asynchronous motor is an AC motor in which all electromagnetic energy is transferred by
inductive coupling from a primary winding to a secondary winding, the two windings being separated by an air
gap. In three-phase induction motors, that are inherently self-starting, energy transfer is usually from the stator
to either a wound rotor or a short-circuited squirrel cage rotor. Induction motor is one in which alternating
current is supplied to the stator directly and to the rotor by induction or transformer action from the stator. As
in the synchronous machine, the stator winding produce a magnetic field in the air gap rotating at synchronous
speed as determined by the number of stator poles. The rotor of an induction machine may be one of two
types. Three-phase cage rotor induction motors are widely used in industrial drives because they are rugged,
reliable and economical. Single-phase induction motors are also used extensively for smaller loads. Although
most AC motors have long been used in fixed-speed load drive service, they are increasingly being used in
variable-frequency drive (VFD) service, variable-torque centrifugal fan, pump and compressor loads being by far
the most important energy saving applications for VFD service. Squirrel cage induction motors are most
commonly used in both fixed-speed and VFD applications.
An induction motor has two main parts:-Stator and Rotor
Stationary (stator):- The Stator is made up of a number of stampings with slots to carry three phase windings. It
is wound for a definite number of poles. The stator windings are connected directly to the three phase power
source. The stator is connected to a 3-phase AC power supply.
Rotor composed of punched laminations, stacked to create a series of rotor slots, providing space for the rotor
winding.
Two types of rotors are used in Induction motors
A wound rotor is built with a polyphase winding similar to, and wound with the same number of poles as, the
stator. The terminals of the rotor winding are connected to insulated slip rings mounted on the shaft. Carbon
brushes bearing on these rings make the rotor terminals available external to the motor,
A squirrel-cage rotor with a winding consisting of conducting bars embedded in slots in the rotor iron and shortcircuited at each end by conducting end rings. The extreme simplicity and ruggedness of the squirrel-cage
construction are outstanding advantages of this type of induction motor and make it by far the most commonly
used type of motor in sizes ranging from fractional horsepower on up.
Tranfsfer ratio = VS/VP
=VRotor /VStator
R=V/I
η = PO/Pi
-
Autotransformer
Stator
variac
A
V
A
Supply 120v at starting time A1 = 12A & A2 =2.75A
150v at starting time A1= 16A & A2 = 3.75A
Rotor
We observe the current the induction motor at starting time the delta connection three times greater than the
star connection
Using Inductor in the stator
variac
Stator
A
v
Rotor
Inductor
A
Using Resistor in the rotor
Variable Resistor
Stator
variac
Rotor
A
v
A
An induction motor is an alternating current motor where power to the rotor is supplied by electromagnetic
induction. The magnetic field produced in the stationary coils is changed in order to get the current required to
run the rotor. They use shorted wire loops on rotating armatures to obtain their torque from induced currents.
In Induction machines the rotor receive power by induction in exactly the same way as the secondary of the two
winding transformer receives the power from primary.
An Induction motor can be treated as a rotating transformer, i.e. one in which primary winding (the stator) is
stationary but the secondary (Rotor) is free to rotate by induction.
An Induction motor should be started in star connection to reduce the starting current and to save the motor from
damage then change to delta connection.
Direct Current Generator
Direct Current Generator is a rotating electric machine which delivers a unidirectional voltage and current. An
armature winding mounted on the rotor supplies the electric power output. One or more field windings mounted
on the stator establish the magnetic flux in the air gap. A voltage is induced in the armature coils as a result of the
relative motion between the coils and the air gap flux. Faraday's law states that the voltage induced is determined
by the time rate of change of flux linkages with the winding. Since these induced voltages are alternating, a means
of rectification is necessary to deliver direct current at the generator terminals. Rectification is accomplished by a
commutator mounted on the rotor shaft. Carbon brushes, insulated from the machine frame and secured in brush
holders, transfer the armature current from the rotating commutator to the external circuit.
The field windings of dc generators require a direct current to produce a magnetomotive force (mmf) and
establish a magnetic flux path across the air gap and through the armature. Generators are classified as series,
shunt, compound, or separately excited, according to the manner of supplying the field excitation current. In the
separately excited generator, the field winding is connected to an independent external source. Separately excited
generators are among the most common of dc generators, for they permit stable operation over a very wide range
of output voltages. Using the armature as a source of supply for the field current, dc generators are also capable of
self-excitation. Residual magnetism in the field poles is necessary for self-excitation. Series, shunt, and compoundwound generators are self-excited, and each produces different voltage characteristics. The armature winding and
field winding of a series generator are connected in series. The field winding of a shunt generator is connected in
parallel with the armature winding. A compound generator has both a series field winding and a shunt field
winding. Both windings are on the main poles with the series winding on the outside. The shunt winding furnishes
the major part of the mmf. The series winding produces a variable mmf, dependent upon the load current, and
offers a means of compensating for voltage drop.
Direct-Current Generator that functions as a generator. Its operation is described by the following equations: P =
UIAr, where P is the output power, U the terminal voltage, and IAr the armature current; U = E – IArRAr, where E is
the armature electromotive force and RAr is the resistance of the armature circuit; and RAr = rAr + rCo + rSe (Figure 1).
Figure 2. Emf generated in a steadily rotating DC
generat
Figure 1. Excitation methods for DC generators: (a) separately excited generator, (b) self-excited generator, (c)
compound generator; (Ar) armature, (Co) commutating-pole windings, (Sh) shunt-field winding, (Se) series-field
winding, (IAr) armature current, (I) load current, (Rex) resistance for regulating the exciting current, (Iex) exciting
current, (rAr) resistance of armature winding, (rCo) resistance of commutating-pole windings, (rSh) resistance of
shunt-field winding, (rSe) resistance of series-field winding, (rL) load. A terminal voltage that remains constant
under changing load conditions is the principal requirement imposed upon a DC generator. The relationship
between the terminal voltage and the load current U = f(I) is called the external characteristic of a DC generator
and is determined by the type of field excitation used in the generator.
A simple DC generator consists of the same basic elements as a simple AC generator: i.e., a multi-turn coil rotating
uniformly in a magnetic field. The main difference between a DC generator and an AC generator lies in the manner
in which the rotating coil is connected to the external circuit containing the load. In an AC generator, both ends of
the coil are connected to separate slip-rings which co-rotate with the coil, and are connected to the external
circuit via wire brushes. In this manner, the emf seen by the external circuit is always the same as the emf
generated around the rotating coil. In a DC generator, the two ends of the coil are attached to different halves of a
single split-ring which co-rotates with the coil. The split-ring is connected to the external circuit by means of metal
brushes.
Sparkless current commutation is another important requirement that must be satisfied by a DC generator.
Sparking can be reduced if the stator of the machine is equipped with additional commutating poles. High-power
DC generators are sometimes built with compensating windings, which are inserted in the slots of the pole shoes
and are connected in series with the armature winding. The purpose of the compensating winding is to
compensate for the armature field in the zone under the main poles. The winding provides an automatic
compensation at all load conditions and a uniform distribution of magnetic flux density under the pole arc. The
maximum voltage between adjacent commutator bars is thus lowered, and potential sparking outside of the
commutation zone is eliminated.
Thre are general-purpose DC generators and DC generators for special purposes are produced, special-purpose
machines are DC generators for electric welding which are driven by an induction motor and operated at currents
of 125 to 500 amperes and at voltages of 60 to 70 volts. DC generators are used in automatic control systems; they
operate with greater accuracy than do AC generators.
Most common electrical appliances (e.g., electric light-bulbs, and electric heating elements) work fine on AC
electrical power. However, there are some situations in which DC power is preferable. For instance, small electric
motors (e.g., those which power food mixers and vacuum cleaners) work very well on AC electricity, but very large
electric motors (e.g., those which power subway trains) generally work much better on DC electricity.
DC generators will act as DC motors if connected to a DC power source and not spun at a sufficient speed. This is a
problem in DC power systems, as the generator will act as a load.
Direct Current Motors
A DC motor is a mechanically commutated electric motor powered from direct current (DC). The stator is
stationary in space by definition and therefore its current. The current in the rotor is switched by the commutator
to also be stationary in space. This is how the relative angle between the stator and rotor magnetic flux is
maintained near 90 degrees, which generates the maximum torque.
Direct current motors are unsurpassed for adjustable-speed applications, and for applications with severe torque
requirements. Millions of fractional horsepower DC motors are used by the transportation industries in
automobiles, trains and aircraft where they drive fans and blowers for air conditioners, heaters and defrosters;
they operate windshield wipers and raise and lower seats and windows. One of their most useful functions is for
the starting of gasoline and Diesel engines in autos, trucks, buses, tractors and boats.
The DC motor contains a stator and a rotor, the latter being more commonly called an armature. The stator
contains one or more windings per pole, all of which are designed to carry direct current, thereby setting up a
magnetic field. The armature and its windings are located in the path of this magnetic field, and when the winding
also carries a current, a torque is developed causing the motor to turn. A commutator associated with the
armature winding is actually a mechanical device, to assure that the armature current under any given stator pole
will always circulate in the same direction irrespective of position. If a commutator were not used, the motor
could not make more than a fraction of a turn, before coming to a halt. In order for a DC motor to run, current
must flow in the armature winding and the stator must develop a magnetic field (flux), either by means of a shunt
winding or a series winding (or both). The torque developed by a DC
motor is directly proportional to the armature current and the stator
flux. On the other hand, motor speed increases when the voltage
applied to the armature increases. Motor speed will also increase when
the stator flux is reduced. As a matter of fact, the speed can attain
dangerous levels if,
A: shunt , B: series ,
f = field coil
Ro = Vo
Ri
Vi
Vo= RoVi/Ri
n = Vt –Eb –IaRa
Kφ
Read the Name plate
Current =9.1A
Voltage= 220v
Power =1.65kw
Rpm =1500
Frequency = 50Hz
Measure the no load losses, the field current must be rated current
CircuitCircuit
Diagram
Circuit
Diagram
Diagram
ForCircuit
Determination
For
For
Diagram
Determination
Determination
of
For
RaDetermination
of R
ofaRa
of Ra
+
+
+
+
_ +
+
+
A
A
220 V
DC
Supply
D
D
D
P220 V220 V P
P
SD C D C
S
S
TSupply
T
Supply T
Switch
Switch
Switch
220 V
DC
Supply
D
P
S
T
Switch
+
V
_
_
A
_
A
+
A1
+
V
V
M_
_
A2
_
_
_
_
_
+
A1
M
M
A2
A1+
V
A2
_
A1
M
A2
C: compound
Start the motor applying rated voltage
Perform speed contral of the DC motor
Suppose that a DC motor is subject to a constant, but relatively light, load, the motor will spin up until its back emf
almost matches the external emf. If the external emf is increased then the motor will spin up further, until its
back-emf matches the new external emf. Likewise, if the external emf is decreased then the motor will spin down.
It can be seen that the rotation rate of a DC motor is controlled by the emf of the DC power supply to which the
motor is attached. The higher the emf, the higher the rate of rotation. Thus, it is relatively easy to vary the speed
of a DC motor, unlike an AC motor, which is essentially a fixed speed motor.
DC motor is started up, the coil does not initially spin fast enough to generate a substantial back-emf.
The rotational speed of a DC motor is proportional to the EMF in its coil (= the voltage applied to it minus voltage
lost on its resistance), and the torque is proportional to the current. Speed control can be achieved by variable
supply voltage, resistors or electronic controls. The direction of a wound field DC motor can be changed by
reversing either the field or armature connections but not both. This is commonly done with a special set of
contactors (direction contactors).
The effective voltage can be varied by inserting a series resistor or by an electronically controlled switching device.
If one of the hooked wire disconnected the flux become 0, then the speed of the DC moter high that might be
damage the motor.
Sychronous Generator
The term synchronous means it is in step with the grid frequency, so changes in load don't affect speed or
frequency. The synchronous Generator is a synchronous machine it is quite similar to that of the induction
machine. The stator core and windings of a three-phase synchronous machine are practically identical to that of a
three-phase induction machine. The function of the synchronous machine stator is to provide a rotating mmf to
the rotor, just as the stator of the induction machine. The synchronous machine rotor, on the other hand, is
different than that of the induction machine.
The rotor of the synchronous machine is a rotating electromagnet with the same number of poles as the stator.
The poles of the synchronous machine rotor are created by the rotor windings which carry DC currents. Thus, the
synchronous machine requires simultaneous AC and DC excitation of the stator (armature) windings and the rotor
(field) windings, respectively. The magnetic moments associated with the poles of the rotor follow the magnetic
moments of the stator-generated mmf which rotates at the synchronous speed. In other words, the magnetic
fields of the stator and the rotor tend to align themselves. Therefore, under steady state conditions given a
constant frequency AC source, the machine speed (n) of a synchronous machine is equal to the synchronous speed
(ns) defined by where f is the frequency of the AC signal at the stator, and p is the number of poles in the
synchronous machine. Thus, the fundamental difference between a synchronous machine and an induction
machine is that the rotor currents of the induction machine are induced while those of the synchronous machine
are not.
In synchronous generators principle either keep the armature stationary and rotate the field or keep the field
stationary and rotate the armature. Depending upon the speed of rotation and the strength of the magnetic field
EMF is induced in the armature conductors. a synchronous generator delivering power at constant frequency and
rated terminal voltage to a load whose power factor is constant. The curve showing the field current required to
maintain rated terminal voltage as the constant-power-factor load is varied is known as a compounding curve.
Armature reaction for unity power factor

P NS
2 60
f  the Name
NS
Read
plate
Voltage= 400/231v

120 f
P
Current rated =1.7A
Power =2.8kw
p.f = 0.8
r.p.m = 1500
Frequency = 50Hz

Measure the resistance of a synchronous Generator
u-x = 2
v-y = 2
w-z = 2
f1-f2 =3. 25
Vrated = 300v
DC supply = 260v

Give excitation current & make a measurments
I excitation = 0.2A
I armature = 0.15A
I generator = 0

make V constant for speed control
V Supply Motor
armature
I(A) excitation motor
I(A) field
(SG)
I(A) armature
V Synchronous
Generator
V Supply Motor
armature
I(A) Synchronous
Generator
I(A)
I(A) armature
V Synchronous
Generator
150v
0
0.25
0.15
75
( No Load)
field (SG)

Switch ON , Load
I(A) armature
I(A) Synchronous
Generator
Start the motor applying rated voltage
I(A)
field (SG)
V Synchronous
Generator
V Supply Motor
armature
Perform speed control of the DC motor
Conclusion
commonly Synchronous Generators especially usefor large power systems, such as turbine generators
and hydroelectric generators in the grid power supply.
The magnetic field created by the armature currents rotates at the same speed as that created by the
field current on the rotor, which is rotating at the synchronous speed, and a steady torque results.
Synchronous Generator is a doubly excited machine .
The dc power required for excitation is supplied by the excitation system.
When a synchronous generator (G) is connected to a large interconnected system containing many
other synchronous generators, the voltage and frequency at its armature terminals are fixed by the
system. G cannot significantly affect the system voltage or frequency. It is usual to represent the
remainder of the system as a constant-frequency, constant-voltage source, referred to as an infinite bus.
Synchronous reactance is a combination of armature winding leakage reactance and a reactance used to
represent armature reaction. The induced voltage and the current when A
armature reaction is
greatest, for a range of field current values. This data can be used to
approximate the
v
synchronous reactance of the machine. It is important to note that this is
only an
SG
approximation as XS is dependent on load condition and phase angle of
armature current
relative to the induced open circuit voltage.
The relationship between armature current & the field current gives the
Sychronous Generator
DC motorconstant line
characterstics
short circuit
The Power factor of a utility depends on the type of load it supplies. Usually the Loads on utility are
inductive loads (where the current lags behind the Voltage) and hence the power factor at which the
utility operates is lagging. Most loads connected to utility are inductive loads like motors, fans, rolling
mills which cause the power factor of the utility to be of lagging in nature.
The effect of armature reaction can be neutralized by use of compensating winding. It is always placed
in series with armature winding. The armature ampere conductors under pole shoe must be equal to
compensating winding ampere conductors which will compensate armature m.m.f. perfectly.