Lecture Notes ME 269
Ayman El-Hag Synchronous Machine 2/16/2004 ME 269. Synchr. Machine · 1 Classification of AC Rotating Machines
· · · Synchronous Machines: Synchronous Generators: A primary source of electrical energy. Synchronous Motors: Used as motors as well as power factor compensators (synchronous condensers). Asynchronous (Induction) Machines: Induction Motors: Most widely used electrical motors in both domestic and industrial applications. Induction Generators: Due to lack of a separate field excitation, these machines are rarely used as generators.
ME 269. Synchr. Machine · · · · 2/16/2004 · 2 SYNCHRONOUS MACHINES
Round Rotor Machine
· The stator is a ring shaped laminated ironcore with slots. Three phase windings are placed in the slots. Round solid iron rotor with slots. A single winding is placed in the slots. Dc current is supplied through slip rings. Concept (two poles)
A Stator with laminated iron-core C
Slots with winding · · · B A
+ - C
+ N Rotor with dc winding S A B
+ - C
- B 2/16/2004 ME 269. Synchr. Machine · 3 Round Rotor Machine 2/16/2004 ME 269. Synchr. Machine · 4 SYNCHRONOUS MACHINES
Salient Rotor Machine
· The stator has a laminated iron-core with slots and three phase windings placed in the slots. The rotor has salient poles excited by dc current. DC current is supplied to the rotor through slip-rings and brushes.
2/16/2004 ME 269. Synchr. Machine · Concept (two poles)
BN C+ AS A+ · · C- B+ · 5 Salient Rotor Machine 2/16/2004 ME 269. Synchr. Machine · 6 SYNCHRONOUS GENERATOR
Operation concept · The field winding is supplied with a DC current -> excitation. Rotor (field) winding is mechanically turned (rotated) at synchronous speed (ns). The RMF (rotating magnetic field) produced by the field current induces voltages in the outer stator (armature) winding.
2/16/2004 ME 269. Synchr. Machine Operation (two poles)
ns
BC+
N Flux f · A+
S A- · C- B+ · 7 SYNCHRONOUS MACHINES
Operation concept
· · · The frequency - speed relation is p is the number of poles. f = (p / 2) ns = p ns / 2 Typical rotor speeds are 3600 rpm for 2-pole, 1800 rpm for 4 pole and 450 rpm for 16 poles. The rms. value of the induced voltages is: EA = 4.44 N BA f , (BA = )
· where: N = number of turns, B= flux density, A = cross sectional area of the magnetic circuit, f = frequency, and = flux per pole 2/16/2004 ME 269. Synchr. Machine · 8 Comparisons Between DC and Synchronous Machines 2/16/2004 ME 269. Synchr. Machine · 9 Synchronous Generators Equivalent Circuit (round rotor)
1) DC current in the field winding produces the main flux, Nf. 2) Nf induces an emf, EG, in the armature winding. 3) Depending on the load condition, the armature current IA is established. In the following discussions, it is assumed to be a lagging power factor. 4) IA produces its own flux due to armature reaction, EAR is the induced emf by NAR. 5) The resulting phasor, Eresultant = EG + EAR is the "true" induced emf that is available.
2/16/2004 ME 269. Synchr. Machine · 10 Synchronous Generators Equivalent Circuit (round rotor) Vt 2/16/2004 ME 269. Synchr. Machine · 11 Synchronous Generators Equivalent Circuit (round rotor)
Phasor Diagrams: V Vt V Vt = E A - I A jX A - I A jX AR - I A RA = E A - jX s I A - I A RA = E A - I A ( RA + jX s ) where, (XAR + XA) = synchronous reactance, Xs. 2/16/2004 ME 269. Synchr. Machine · 12 Synchronous Generators Equivalent Circuit (round rotor)
Inductive Load Resistive Load * = power angle
2/16/2004 ME 269. Synchr. Machine · 13 Power Supplied by a Synchronous Generator
S = P + jQ = 3Vt I A
* Ea (- ) - Vt 0 S = 3Vt 0( ) - jX s EaVt sin( ) P=3 Xs
2/16/2004 ME 269. Synchr. Machine · 14 SYNCHRONOUS MACHINES
Power angle Characteristics
· The P() curve shows that the increase of power increases the angle between the induced voltage and the terminal voltage. The power is maximum when =90o The further increase of input power forces the generator out of
P( ) Round Rotor Machine Pmax
100 80 60 40 20 0 0 30 60 90 120 150 180 · · synchronism. This generates large current and mechanical forces. · The maximum power is the static stability limit of the system. Safe operation requires a 1520% power reserve. · 2/16/2004 ME 269. Synchr. Machine · 15 Examples
A 25 kVA, 230 V three phase, four pole, 60 Hz, Yconnected synchronous generator has a synchronous reactance of 1.5 /phase and a negligible resistance. The generator is connected to an open circuit of constant voltage (230 V) and frequency (60 Hz), find: a) The generated EMF (EG) when the machine is delivering rated kVA at 0.8 power factor lagging. b) If the field current If is increased by 20 % without changing the power input find the stator current Ia.
2/16/2004 ME 269. Synchr. Machine · · 16 Voltage Regulation
· As the load on the generator increases, the terminal voltage drops. But, the terminal voltage, must be maintained constant, and hence the excitation on the machine is varied, or input power to the generator is varied. That means, EG has to be adjusted to keep the terminal voltage Vt constant. · Voltage Regulation, V. R. = VNL - VFL × 100% VFL 2/16/2004 ME 269. Synchr. Machine · 17 Measuring the Equivalent Circuit Parameters 2/16/2004 ME 269. Synchr. Machine · 18 Measuring the Equivalent Circuit Parameters
Measurement of RA: The winding resistance is measured by applying a DC voltage across the generator terminals under "stand-still" conditions. An equivalent value of the RA can then be calculated using the measured DC value. 2/16/2004 ME 269. Synchr. Machine · 19 Measuring the Equivalent Circuit Parameters
Measurement of Xs:
· · (i) Open circuit test: With the generator terminals open; ­ Bring the generator to its rated speed. ­ Increase the field current, IF, gradually from zero. ­ Measure the open circuit terminal voltage, Vt(L-L.) ­ Plot Vt(L-L.) as a function of IF. -> (OCC) Open Circuit Characteristic
2/16/2004 ME 269. Synchr. Machine · 20 Measuring the Equivalent Circuit Parameters
Measurement of Xs:
· · (ii) short circuit test: Bring the generator up to its rated speed. .Increase IF gradually from zero and record ISC.Plot IF vs ISC on the original graph from test i. .This is the SCC Short Circuit Characteristic.
ME 269. Synchr. Machine · · 2/16/2004 · 21 Measuring the Equivalent Circuit Parameters
Isc Measurement of Xs: EA Z s = RA + X s = IA
2 2 OC SC cons tan t I F In many cases Rs can be ignored.
2/16/2004 ME 269. Synchr. Machine IF · 22 SYNCHRONOUS MACHINES
Power System Operation
· · · · In a network several hundred synchronous generators operate in parallel. Each generator operates with the same speed. The load increase is achieved by increasing the input power, that increases the power angle . The speed remain constant. The power angle must be less than 90 degrees. The load should be 30-20% less than the maximum power ( = 90o) 2/16/2004 ME 269. Synchr. Machine · 23