Synchronous Machine The stator is similar in construction that of a induction motor

The rotor can be Salient or Non-Salient (cylindrical rotor)

Field excitation is provided on the rotor by either permanent or electromagnets with number of poles equal to the poles of theRMF caused by stator

Non-excited rotors are also possible as in case of reluctance motors

The rotor gets locked to the RMF and rotates unlike induction motor at synchronous speed under all load condition

All conventional power plants use synchronous generators for converting power to electrical form They operate at a better power factor and higher efficiency than equivalent induction machinesSynchronous Machine (2)

Synchronous Machine Construction (a)CRSM (b) SPSM

Concept of synchronous reactance (1) Like dc machines synchronous machines will also have armaturereaction. However unlike dc machine we do not like to eliminateit, but try to use it to our benefit.

Essentially, this armature reaction will determine how much power can be transferred to or from the synchronous machine and limits the current that is flowing in the synchronous machine and hence provides inherent short-circuit protection: a great boon when we are talking about zillions of megawatts of power flow!

Concept of synchronous reactance (2)Suppose we short-circuit a synchronous generator with the field circuit excited. By Faradays law an emf will be induced in the stator (armature) which by Lenzs law has to oppose the originalfield on the rotor. It means the resulting armature reaction will induce an opposing emf to the one produced by the main field.

One way to represent this is the following circuit where Xar conjures up the effect of armature reaction. This can be provedas follows: Suppose the original field flux is f= m cost.By Fardays and Lenzs law this would produce a voltage Ef= Em sint. This voltage produce a current and hence flux thatopposes f, under short-circuit. This current then has to of the form Isc=-Im cost. Clearly Ef/jXar= Em sint/jXar would give such a current.

Equivalent circuit of CRSM (1)Generator (Appx.)Motor(Appx.)Generator (Exact)Motor(Exact)Only difference is in current direction; in a generator it flowsout of it, in case of a motor it flows into it.

Equivalent circuit of CRSM (2)Xs=Xar+Xal (Synchronous reactance)Zs= Ra+jXs (Synchronous impedance)Xal is leakage ReactanceRa is armature resistance Generator (Exact)Motor(Exact)

Phasor diagram of CRSM Note: is +ve for (a) generator and ve for (b) motor

Derivation of power equation for CRSMon the green board

Effect of Load Change (Field constant) Note: Er same as Ef Va same as Vt Ra has been neglected

Effect of Field Change (Load constant) Question: 1)Why is the loci of stator current and excitation voltagemoves on a straight line?2) What is happening to power factor as field is changed?

Note: Er same as Ef Va same as Vt Ra has been neglected

V curves

Effect of Field Change (Load constant)for a generator VtEf1jIa1XsjIa2XsEf2Ia1Ia2a Powera Power

Conclusion for effect for field change withconstant load on power factor For motor with increased (decreased)excitation power factor becomes leading (lagging)

For generator with increased (decreased) excitation power factor becomes lagging (leading)

Unloaded overexcited synchronous motors are sometimes usedto improve power factor. They are known as synchronous condensers

Torque versus Electrical Load Angle

Torque versus Speed

Example 1 A six-pole 60 Hz synchronous motor is operating with a developedpower of 5 hp and a torque angle of 5o. Find the speed and developed torque. Suppose that the load increases such that the developed torquedoubles. Find the new torque angle. Find the pull-out torque and maximum developed power for this machine.

Example 2 An eight-pole, 240 V-rms, 60 Hz, delta connected synchronous motor operates with a constant developed power of 50 hp and a torque angle of 15o and unity power factor. Suppose the field current is increased by 20%. Find the new torque angle and power factor. Is the new power factor lagging or leading? Assume linear magnetic characteristics.

SPSM and the concept of Direct and Quadrature Axes Since in the salient pole machine the reluctance of the machinevaries with the position of the pole, flux due to armature reaction varies with power factor. Thus Xar alone is no longer sufficient for the equivalent circuit.

Reluctance is minimum along polar (direct) axis. Hence component of the armature reaction acting along this axis produce maximum flux. Let this component be Fad.

Reluctance is maximum along the inter-polar (quadrature )axis. Hence the component of the armature reaction acting along this axis produce minimum flux. Let this component be Faq.

SPSM and the concept of Direct and Quadrature Axes (2) Xd=Xad+Xal=(d)irect axis synchronous reactance) Xq=Xaq+Xal= (q)uadrature axis synchronous reactance) Xad= d(irect) axis armature reactance =wLad Xaq = (q)uadrature axis armature reactance=wLaqXal = leakage reactance

Fad=LadId Faq=LaqIq Id= d(irect) axis component of the armature current Iq = (q)uadrature axis component of the armature currentIa=IqjId

Explaining d-q axes using diagrams

Equivalent circuits of SPSM

Power Angle Characteristics of SPSM

Examples on SPSM