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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 the - RMF caused by stator
- Non-excited rotors are also possible as in case

of reluctance motors

Synchronous Machine (2)

- 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 thanequivalent induction machines

Synchronous Machine Construction (a)CRSM

(b) SPSM

Concept of synchronous reactance (1)

- Like dc machines synchronous machines will also

have armature - reaction. However unlike dc machine we do not

like to eliminate - it, 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 original - field 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 proved - as follows Suppose the original field flux is

?f ?m cos?t. - By Fardays and Lenzs law this would produce a

voltage - Ef Em sin?t. This voltage produce a current and

hence flux that - opposes ?f, under short-circuit. This current

then has to of the form - Isc-Im cos?t. Clearly Ef/jXar Em sin?t/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 flows - out of it, in case of a motor it flows into it.

Equivalent circuit of CRSM (2)

Motor(Exact)

Generator (Exact)

XsXarXal (Synchronous reactance) Zs RajXs

(Synchronous impedance) Xal is leakage

Reactance Ra is armature resistance

Phasor diagram of CRSM

Note ? is ve for (a) generator and ve for (b)

motor

Derivation of power equation for CRSM on 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)

Note Er same as Ef Va same as

Vt Ra has been neglected

Question 1)Why is the loci of stator current and

excitation voltage moves on a straight line? 2)

What is happening to power factor as field is

changed?

V curves

Effect of Field Change (Load constant) for a

generator

a Power

Ia2

Ef2

Ef1

jIa2Xs

jIa1Xs

a Power

Vt

Ia1

Conclusion for effect for field change

with constant 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 used - to 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 developed power of 5 hp and a torque angle

of 5o. Find the speed and developed torque.

Suppose that the load increases such that the

developed torque doubles. 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 machine - varies 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)

- XdXadXal(d)irect axis synchronous reactance)
- XqXaqXal (q)uadrature axis synchronous

reactance) - Xad d(irect) axis armature reactance wLad
- Xaq (q)uadrature axis armature reactancewLaq
- Xal leakage reactance
- FadLadId
- FaqLaqIq
- Id d(irect) axis component of the armature

current - Iq (q)uadrature axis component of the armature

current - IaIqjId

Explaining d-q axes using diagrams

Equivalent circuits of SPSM

Power Angle Characteristics of SPSM

Examples on SPSM

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