Synchronous machines Flashcards
What are the properties of Synchronous machines?
- They are mostly used as generators
- They supply constant voltage at constant frequency
- They can deliver active or reactive power
- They have a field winding on the rotor supplied by an external DC source and rotated by an external prime mover
What are the two types of synchronous generators?
- None salient round or cylindrical rotor
- Salient rotor
Describe the construction of Round Rotor machines?
- The rotor is cylindrical shaped laminated iron core fitted with slots.
- Single phase coils are placed in the slots of the field winding
- DC current is supplied through the brushes and slip rings to the rotor windings to create the magnetic field.
What are the uses for round rotor machines?
They are used for high speed applications such as the steam turbine generators
Describe the construction of the salient rotor machines?
- The rotor has Salient poles excited by the DC source
- DC current is supplied to the rotor through the slip rings and brushes
What are the uses of salient rotor machines?
They are used for low speed applications like Hydroelectric generators
Describe the theory of operation of the Synchronous machines?
- 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: EA=4.44NØF
Where F=N*no.ofpoles/120
What are the typical rotor speeds?
- 3600 rpm for 2 poles
- 1800 rpm for 4 poles
- 450 for 16 poles
Why is the voltage EA not equal VT?
- The distortion of the air gap magnetic field represented by X
- The armature wire resistance represented by Ra
- The self inductance of the armture coil represented by XA
What is the EQN. of voltage regulation?
V.R= (EA-VT/VT )*100
EA=?
EA=VT+IA(jXS+RA)
Prove the rule P=3VphEAsinδ/XS
Pout=
Pout= 3VEsin(δ)/Xs
and
Pout=3*Vph*IA
Qout=
Qout=3VphIAsin(Ø)
and
Qout=3VphEAcos(δ)/XS - 3Vph/XS
What are the factors that affect the behaviour of the synchronous generator?
- The load PF
- Wether or not it is acting alone or in parralel operation
What are the assumption made to study the affect of changing the load on a synchronous generator acting alone?
- The armature resistance is zero
- The Rotor flux is constant
- The generator speed is constant
Describe what happens when the load increases to the speed, flux and voltage of the synchronous generator?
When the Load increases the Load Current increases, however since the field current, speed and flux are kept constant according to this EQN. (EA=KØN) so the volt is constant.
What are the variables of the lagging PF and what is its drawing?
I’A>IA
E’A=EA
δ’>δ
V’phph
What are the variables of the unity PF and its drawing?
I’A>IA
E’A=EA
δ’>δ
V’phph
What are the variables of the leading PF and its drawing?
I’A>IA
E’A>EA
δ’>δ
V’ph>Vph
What are the conclusions of a synchronous generator acting alone?
- If a lagging Load (+Q) is added Vph will decrease drastically
- If a unity PF Load is added then V<strong>ph</strong> will decrease slightly
- If a leading Load is added (-Q) then V<strong>ph</strong> will increase
How can we keep VT constant? Explain your answer.
To keep VT constant we must vary EA and since (EA=KØN) and because the frequency is constant then we can change EA by changing the flux.
- We can increase the field current by decreasing the field resistance
- Increasing the field current will increase the flux
- Increasing the flux will increase EA
- Increasing EA will increase VT
What are the advantages of synchronous generators parralel operation?
- Can supply bigger loads
- More reliable
- One or more generators can be removed for maintanence
- Generators can operate ate higher effeciancy
What are the conditions for paralleling generators?
- Rms line voltages for 2 generators must be equal
- All three generators must be in phase
- The phase angles must be equal
- The oncoming generator must have slightly higher frequency
What are the steps for the parallel operation procedure?
- The field current of the oncoming generator must be adjusted to be equal to the running system
- The phase sequence must adjusted using the 3 bulb test to be the same as the running system
- The frequency must be adjusted to be slightly higher than the system
- 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.
What is the maximum power determined by and at which angle δ?
It is determined by the maximum torque that can be applied without losing synchronism and it occurs at δ=90
What is the EQN. of |E|?
Draw the power flow diagram of the synchronous machine?