Synchronous Generator

Name : P.D.M. Chandrasekara Index No : 110072F Group : 03 Submission Date : 2014/07/18 Instructed By : Mr. L.P.M.I. Sampath

OBSERVATION SHEETName : P.D.M. ChandrasekaraIndex No : 110072FField : EEGroup : G-03Date : 2014/07/04Instructed By : Mr. L.P.M.I. Sampath

01) Open Circuit Test Open circuit voltage (V)Field current (A)

200.01

400.03

600.05

800.07

1000.10

1200.12

1400.14

1600.17

1800.20

2000.25

2200.30

2400.39

2600.52

2700.60

02) Short Circuit Test

Short circuit current (A)Field current (A)

00.00

10.01

20.04

30.07

40.10

50.13

60.15

03) Load Test Inductive Load TestLoad voltage (V)Load current (A)

105.6

205.2

304.9

404.5

504.1

603.8

703.4

803.0

902.5

1002.1

1061.9

Resistive Load TestLoadLoad voltage (V)Load current (A)

Load 01200.00

Load 11190.22

Load 21190.44

Load 31180.64

Load 41170.84

04) SynchronisationName plate dataSynchronous GeneratorDC motorPhase3

Frequency50 Hz

Voltage240 V

Current6.6 A

Speed1500rpm

V220 V

A15 A

Speed1500rpm

Open circuit characteristics

Open circuit voltage (V)Field current (A)

200.01

400.03

600.05

800.07

1000.10

1200.12

1400.14

1600.17

1800.20

2000.25

2200.30

2400.39

2600.52

2700.60

Short circuit characteristics

Short circuit current (A)Field current (A)

00.00

10.01

20.04

30.07

40.10

50.13

60.15

Calculations01) Synchronous reactance, Xs

whereVOC - Open circuit voltage ISC - Short circuit current

Calculation for field current 0.1 A, VO/C= 116 V (from graph) IS/C= 4 A(from graph)XS= 120 / 4 = 29

Similarly, calculate synchronous reactance for given field currents .

Field Current(A)Open circuit voltage(V)Short circuit current(A)Synchronous reactance()

0.10116429

0.20178822.25

0.302201218.33

0.402481516.53

0.502651913.94

0.602752212.5

0.702842610.92

02. Synchronous Reactance (XS) vs. Field Current (If)

Field Current (A)Synchronous reactance ()

0.1029

0.2022.25

0.3018.33

0.4016.53

0.5013.94

0.6012.5

0.7010.92

02)

From the name plate data,Rated Voltage= 240 VRated Current= 6.6 Afrom the open and short circuit characteristics,Field current for rated open circuit voltage= 0.375 AField current for rated short circuit current= 0.14 A

Saturated synchronous reactance (XS(sat)) in per unit,

Saturated synchronous reactance (XS (sat)) in ohms, (Using open and short circuit characteristics curves)

03. Load Voltage vs. Load Current for Inductive Load (Cos = 0)

Load Current (A)Load Voltage (V)

5.610

5.220

4.930

4.540

4.150

3.860

3.470

3.080

2.590

2.1100

1.9106

Load Voltage vs. Load Current for Resistive Load (Cos = 1)

Load Current (A)Load Voltage (V)

Load 00.00120

Load 10.22119

Load 20.44119

Load 30.64118

Load 40.84117

03. V 2 (2 XS I Sin ) V + (XS 2 I 2 - E 2) = 0

calculation for load current 1 A E = 240 V XS = 17.14 WhenCos = 0 = 900 = - 900 V 2 (2 XS I Sin ) V + (XS 2 I 2 - E 2) = 0V 2 (2 XS I Sin ) V + (XS 2 I 2 - E 2) = 0 V = 257.14V = 222.86

When Cos = 1 = 0 V 2 (2 XS I Sin ) V + (XS 2 I 2 - E 2) = 0 V = 239.39

When Cos = 0.9

= 25.840 = - 25.840 V 2 (2 XS I Sin ) V + (XS 2 I 2 - E 2) = 0V 2 (2 XS I Sin ) V + (XS 2 I 2 - E 2) = 0 V = 246.97V = 232.03

Load Current (A)Terminal Voltages (V)

1222.86232.03239.39246.97257.14

2205.72223.07237.54252.95274.28

4171.44202.05230261.81308.56

6137.16176.61216.85266.26342.84

8102.88146.97196.97265.61377.12

1068.6109.15167.99258.56411.4

1234.3263.10123.68242.34445.68

Terminal Voltage Vs Load Current

Load Current (A)Terminal Voltages (V)

1222.86232.03239.39246.97257.14

2205.72223.07237.54252.95274.28

4171.44202.05230261.81308.56

6137.16176.61216.85266.26342.84

8102.88146.97196.97265.61377.12

1068.6109.15167.99258.56411.4

1234.3263.10123.68242.34445.68

Discussion1. Importance of the SCR with respect to the generator performance.

The ratio of the field current for rated open circuit Voltage and field current for rated short circuit current give SCR. In addition, SCR is given by inverse of per unit saturated synchronous reactance. Therefore, if we know SCR value we can have an idea about the Xs.The synchronous reactance is higher when the SCR is low. So it becomes comparatively very high value than armature resistance. This results in a significant phase angle (lagging) between the induced Voltage and the armature current.As the value of SCR gets higher and higher, the Synchronous reactance reduces considerably causing a good voltage regulation at the output terminals.In the cases of parallel operation of generators, Synchronous reactance is essential in determining the circulating current within the two generators. As Xs is a function of SCR we can have an idea about the circulating current by knowing the SCR.

2. Comment on the variation of synchronous reactance with field current.

When we examine the graph of Synchronous Reactance vs. Field Current Graph, It can be seen that the graph can be divided in to two parts a linear region and a non linear region. First, the synchronous reactance linearly reduced and then it started to reduce in non-linear fashion.Linear region is due to the fact that Xs reduces when rotor frequency is reduced. In this region machine has not been saturated. But this Xs is not only the winding reactance it also includes the armature reaction to the field produced. This is high at low values of field current and it reduces afterwards. This is the reason for the non linear behavior

3. Synchronous generator has characteristic of a current transformer. Explain.

Lets take that the rotor of the Synchronous generator represents the primary winding and stator represents the secondary winding. Then the field current is the primary current and the armature current is the secondary current.Now as the armature current is much higher than the field current, this looks like a step up current transformer.

In a current transformer when the secondary is short circuited, that short circuit current is proportional to the primary current.

From the graph of Ia Vs If it is visible that short circuited armature current is proportional to field current. Therefore in that sense also the synchronous generator is acting as a current transformer.

When a load is connected to the secondary of a current transformer, its primary current depends on the load.

In the same way, field current of a synchronous generator is also depend on the load while it is connected to the armature.

4. Comment on the variation of the terminal voltage with load current for various power factor loads.

When there are loads with lagging power factor terminal voltage tends to reduce rapidly, but that reduction is close to being linear.

With resistive loads terminal voltage reduces slower but the non linear manner of the curve is more visible than the inductive loads.

When capacitive loads are used terminal voltage tends to get higher than the original Voltage.

Value of Xs is essential in finding the terminal Voltage.