Synchronous machines

Question 1

What are the properties of Synchronous machines?

Answer 1

1. They are mostly used as generators
2. They supply constant voltage at constant frequency
3. They can deliver active or reactive power
4. They have a field winding on the rotor supplied by an external DC source and rotated by an external prime mover

Question 2

What are the two types of synchronous generators?

Answer 2

1. None salient round or cylindrical rotor
2. Salient rotor

Question 3

Describe the construction of Round Rotor machines?

Answer 3

1. The rotor is cylindrical shaped laminated iron core fitted with slots.
2. Single phase coils are placed in the slots of the field winding
3. DC current is supplied through the brushes and slip rings to the rotor windings to create the magnetic field.

Question 4

What are the uses for round rotor machines?

Answer 4

They are used for high speed applications such as the steam turbine generators

Question 5

Describe the construction of the salient rotor machines?

Answer 5

1. The rotor has Salient poles excited by the DC source
2. DC current is supplied to the rotor through the slip rings and brushes

Question 6

What are the uses of salient rotor machines?

Answer 6

They are used for low speed applications like Hydroelectric generators

Question 7

Describe the theory of operation of the Synchronous machines?

Answer 7

• A DC source supplies current to the rotor (Field winding) to produce flux
• An external prime mover is used to rotate the rotor at synchronous speed and so rotating magnetic fields at the same speed are produced
• This induces a voltage on the terminals of the armature winding the frequency of theses voltages is in synchronism with thr rotor speed
• The magnitude of the produced voltage is: E_A=4.44NØF ​

Where F=N*no.ofpoles/120

Question 8

What are the typical rotor speeds?

Answer 8

• 3600 rpm for 2 poles
• 1800 rpm for 4 poles
• 450 for 16 poles

Question 9

Why is the voltage E_A not equal VT?

Answer 9

1. The distortion of the air gap magnetic field represented by X
2. The armature wire resistance represented by R_a
3. The self inductance of the armture coil represented by X_A

Question 10

What is the EQN. of voltage regulation?

Answer 10

V.R= (E_A-V_T/V_T )*100

Question 11

E_A=?

Answer 11

E_A=V_T+I_A(jX_S+R_A)

Question 12

Prove the rule P=3V_phE_Asinδ/X_S

Answer 12

Question 13

Pout=

Answer 13

P_out= 3VEsin(δ)/Xs

and

P_out=3*V_ph*I_A

Question 14

Qout=

Answer 14

Q_out=3V_phI_Asin(Ø)

and

Qout=3V_phE_Acos(δ)/X_S - 3V_ph/X_S

Question 15

What are the factors that affect the behaviour of the synchronous generator?

Answer 15

1. The load PF
2. Wether or not it is acting alone or in parralel operation

Question 16

What are the assumption made to study the affect of changing the load on a synchronous generator acting alone?

Answer 16

1. The armature resistance is zero
2. The Rotor flux is constant
3. The generator speed is constant

Question 17

Describe what happens when the load increases to the speed, flux and voltage of the synchronous generator?

Answer 17

When the Load increases the Load Current increases, however since the field current, speed and flux are kept constant according to this EQN. (E_A=KØN) so the volt is constant.

Question 18

What are the variables of the lagging PF and what is its drawing?

Answer 18

I’_A>I_A

E’_A=E_A

δ’>δ

V’_phph

Question 19

What are the variables of the unity PF and its drawing?

Answer 19

I’_A>I_A

E’_A=E_A

δ’>δ

V’_phph

Question 20

What are the variables of the leading PF and its drawing?

Answer 20

I’_A>I_A

E’_A>E_A

δ’>δ

V’_ph>V_ph

Question 21

What are the conclusions of a synchronous generator acting alone?

Answer 21

1. If a lagging Load (+Q) is added V_ph will decrease drastically
2. If a unity PF Load is added then V_{<strong>ph</strong>} will decrease slightly
3. If a leading Load is added (-Q) then V_{<strong>ph</strong>} will increase

Question 22

How can we keep V_T constant? Explain your answer.

Answer 22

To keep V_T constant we must vary E_A and since (E_A=KØN) and because the frequency is constant then we can change E_A by changing the flux.

1. We can increase the field current by decreasing the field resistance
2. Increasing the field current will increase the flux
3. Increasing the flux will increase E_A
4. Increasing E_A will increase V_T

Question 23

What are the advantages of synchronous generators parralel operation?

Answer 23

1. Can supply bigger loads
2. More reliable
3. One or more generators can be removed for maintanence
4. Generators can operate ate higher effeciancy

Question 24

What are the conditions for paralleling generators?

Answer 24

1. Rms line voltages for 2 generators must be equal
2. All three generators must be in phase
3. The phase angles must be equal
4. The oncoming generator must have slightly higher frequency

Question 25

What are the steps for the parallel operation procedure?

Answer 25

1. The field current of the oncoming generator must be adjusted to be equal to the running system
2. The phase sequence must adjusted using the 3 bulb test to be the same as the running system
3. The frequency must be adjusted to be slightly higher than the system
4. If all three light bulbs go out at once then the system is in phase and voltage differance is zero and the switch connecting the two generators will shut.

Question 26

What is the maximum power determined by and at which angle δ?

Answer 26

It is determined by the maximum torque that can be applied without losing synchronism and it occurs at δ=90

Question 27

What is the EQN. of |E|?

Answer 27

Question 28

Draw the power flow diagram of the synchronous machine?

Answer 28