UNIT-V-Synchronous Machines

1 IFETCE/EEE/ M.SUJITH / III YR/VI SEM/EE2355/DEM/ VER 1.0

Output equations – choice of loadings – Design of salient pole machines – Short circuit ratio – shape of pole face – Armature design – Armature parameters – Estimation of air gap length –Design of rotor –Design of damper winding– Determination of full load field mmf – Design of field winding – Design of turbo alternators – Rotor design.

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TYPES OF SYNCHRONOUS MACHINES It is classified as • Salient pole machines and • cylindrical rotor machines depending upon the construction. SALIENT POLE MACHINES: driven by water wheels or diesel engines. operate at low speeds. requires large no.of poles to produce the required frequency. has projected poles and the field coils are mounted on the poles. 3 IFETCE/EEE/ M.SUJITH / III YR/VI SEM/EE2355/DEM/ VER 1.0

CYLINDRICAL ROTOR MACHINES:  Driven by steam turbines and gas turbines.  Run at very high speeds.  Have slots on the outer periphery of smooth cylindrical rotor.  Field conductors are placed on this slots. • I.

Synchronous machines operating on general power supply networks may be divided into the following categories: HYDRO-GENERATORS:  Prime mover - water wheel.  speed - 100 to 1000rpm.  capacity - 750 MW 4 IFETCE/EEE/ M.SUJITH / III YR/VI SEM/EE2355/DEM/ VER 1.0

II.

III.

IV.

V.

TURBO-ALTERNATORS:  Prime mover- steam turbine or gas turbine  Speed - 3000rpm  Capacity - 1000MW ENGINE DRIVEN:  Prime mover- I.C engine (diesel or petrol)  Speed - 1500rpm  Capacity - 20MW MOTORS:  Motors are manufactured with wide ranging capacity.  They are provided with damper windings. COMPENSATORS:  Speed - 3000rpm  Rating - 100MVAR 5 IFETCE/EEE/ M.SUJITH / III YR/VI SEM/EE2355/DEM/ VER 1.0

CONSTRUCTION • Stationary armature, rotating field type of construction is preferred. • High speed alternators have non-salient pole rotor (Turbo alternators) and they have either 2-pole or 4-pole. • Slow speed alternators have salient pole rotor (water wheel alternators) and they have more than 4 poles.

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• In a synchronous generator, a DC current is applied to the rotor winding producing a rotor magnetic field. The rotor is then turned by external means producing a rotating magnetic field, which induces a 3-phase voltage within the stator winding. • In a synchronous motor, a 3-phase set of stator currents produces a rotating magnetic field causing the rotor magnetic field to align with it. The rotor magnetic field is produced by a DC current applied to the rotor winding. • Field windings are the windings producing the main magnetic field (rotor windings for synchronous machines); armature windings are the windings where the main voltage is induced (stator windings for synchronous machines). 7 IFETCE/EEE/ M.SUJITH / III YR/VI SEM/EE2355/DEM/ VER 1.0

Construction of synchronous machines The rotor of a synchronous machine is a large electromagnet. The magnetic poles can be either salient (sticking out of rotor surface) or non-salient construction.

Non-salient-pole rotor: usually two- and four-pole rotors.

Salient-pole rotor: four and more poles.

Rotors are made laminated to reduce eddy current losses. 8 IFETCE/EEE/ M.SUJITH / III YR/VI SEM/EE2355/DEM/ VER 1.0

OUTPUT EQUATION OF SYNCHRONOUS MACHINES The output equation of A.C machine is given by, Q = Co D2L ns x 10-3 and The output coefficient, Co = 11 Bav ac Kw x 10-3 Where, Q = kVA output for alternator and kVA input for synchronous motor. D = diameter of the stator bore, m L = length of the stator core, m ns = synchronous speed, rps Bav= specific magnetic loading, Wb/m2 Ac = specific electric loading, amp.cond/m Kw = stator winding factor. 9 IFETCE/EEE/ M.SUJITH / III YR/VI SEM/EE2355/DEM/ VER 1.0

CHOICE OF LOADING • Choice of specific magnetic loading depends on: Iron loss Voltage rating Stability Parallel operation Transient short circuit current • Choice of specific electric loading depends on: Copper loss Temperature rise Voltage rating Synchronous reactance Stray load losses 10 IFETCE/EEE/ M.SUJITH / III YR/VI SEM/EE2355/DEM/ VER 1.0

CHOICE OF SPECIFIC MAGNETIC LOADING (Bav ) i) high

Bav→ high flux density in the teeth and core →high iron loss → higher temperature rise.

ii) high B av → low Tph → low leakage reactance (Xl ) → high short circuit current iii) In high voltage machines slot width required is more to accommodate thicker insulation→smaller tooth width → small allowable Bav iv) Stability : Pmax =VE/Xs . Since high Bav gives low Tph and hence low Xl Pmax increases and improves stability. 11 IFETCE/EEE/ M.SUJITH / III YR/VI SEM/EE2355/DEM/ VER 1.0

v) Parallel operation : Ps = (VE sinδ)/Xs ; where δ is the torque angle. So low Xs gives higher value for the synchronizing power leading stable parallel operation of synchronous generators. GUIDE LINES : Non-salient pole alternator : 0.54 – 0.65 Wb/m2 Salient – pole alternator : 0.52 – 0.65 Wb/m2

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CHOICE OF SPECIFIC ELECTRIC LOADING (ac) •

COPPER LOSS AND TEMPERATURE RISE:

High value of ac → higher copper loss leading high temperature rise. So choice of depends on the cooling method used. •

OPERATING VOLTAGE :

High voltage machines require large insulation and so the slot space available for conductors is reduced. So a lower value for ac has to be chosen. •

SYNCHRONOUS REACTANCE (Xs) :

High value of ac results in high value of Xs , and this leads to, a) poor voltage regulation b) low steady state stability limit. •

STRAY LOAD LOSSES:

Increase with increase in ac. Guide lines : Non-salient pole alternators : 50, 000 – 75,000 A/m Salient pole alternators : 20,000 – 40,000 A/m 13 IFETCE/EEE/ M.SUJITH / III YR/VI SEM/EE2355/DEM/ VER 1.0

DESIGN OF SALIENT POLE MACHINES  Diameter of stator bore  Outer diameter Dr = D length of air gap  The selection of diameter D depends upon (i) the type of poles used (ii) permissible peripheral speed  Two types of poles (i) Round poles (ii) Rectangular poles Round poles – ratio of bs is between 0.6 to 0.7 Length of poles = Width of pole shoe (or) L= bs Rectangular poles – Ratio of pole arc to pole pole pitch varies between 1 to 5 . Not should exceed for normal machines  Otherwise the design of field system becomes not economical  Ratio L / τ = 1 to 5 14 IFETCE/EEE/ M.SUJITH / III YR/VI SEM/EE2355/DEM/ VER 1.0

 The deciding factor is the peripheral speed with circular poles larger than with rectangular poles  The value of allowable peripheral speed  Bolted on pole construction = 50m/s  Dovetailed and T head constructions - 80 m/s Bolted Constructions

Dovetailed Constructions

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SHORT CIRCUIT RATIO (SCR)

• SCR = Field current required to produce rated voltage on open circuit / Field current required to produce rated current on short circuit = 1/ direct axis synchronous reactance = 1/Xd • Thus SCR is the reciprocal of Xd , if Xd is defined in p.u. value for rated voltage and rated current. But Xd for a given load is affected by saturation conditions that then exists, while SCR is specific and univalued for a given machine. • For Salient pole Hydroelectric generators , SCR varies from 1.0 - 1.5 • Modern turbo alternators SCR varies from 0.5 to 0.7 16 IFETCE/EEE/ M.SUJITH / III YR/VI SEM/EE2355/DEM/ VER 1.0

O.C.C and S.C.C Characteristics

op SCR  os

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EFFECT OF SCR ON MACHINE PERFORMANCE: i) Voltage regulation : A low SCR → high Xd → large voltage drop → poor voltage regulation.. ii) Parallel operation : A low SCR → high Xd → low synchronizing power → parallel operation becomes difficult. iii) Short circuit current : A low SCR → high Xd →low short circuit current. But short circuit current can be limited by other means not necessarily by keeping a low value of SCR.

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iv) Self Excitation : Alternators feeding long transmission lines should not be designed with small SCR as this would lead to large terminal voltage on open circuit due to large capacitance currents. Summarizing ,high value of SCR leads to, i) high stability limit ii) low voltage regulation iii) high short circuit current iv) large air gap The present trend is to design machines with low value of SCR, this is due to the recent development in fast acting control and excitation systems.

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SHAPE OF POLE FACE • The ratio of pole arc to pole pitch varies Ψ between 0.67 and 0.75. • If value of Ψ is too large Ψ >0.75  Interpolar flux leakage becomes excessive leading to high value of flux density in pole body and improper flux distribution over the armature • If the value Ψ