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

Introduction The stator of a synchronous motor is identical to that of an

induction motor. However, unlike an induction motor, a magnetic field is created by the rotor either through the use of permanent magnets or through a rotor winding with slip rings and brushes. The presence of the magnetic field on the rotor allows the rotor to move at synchronous speed with the stator field.

Introduction (cont’d) The rotor shape of a synchronous motor may be salient or

non-salient, i.e. the air gap may be non-uniform or uniform, respectively.

Introduction (cont’d) Synchronous motors are more expensive than induction

motors but offer the advantage of higher efficiency, an important advantage at high power.

Thus, synchronous motors are used for power generation and large motor drives.

A comparison of a 6MW induction motor and a wound field synchronous motor is shown on the next slide.

Introduction (cont’d)

Introduction (cont’d) Salient pole synchronous generators are used in low speed

applications, such as in hydroelectric power stations where they are used to match the low operating speed of the hydraulic turbines.

Non-salient pole synchronous generators are used in high-speed applications such as steam-power stations to match the high-speed steam turbines.

Introduction (cont’d)

In addition to the field winding, the rotor of a wound field synchronous motor usually also contains a second winding. This armortisseur, or damper winding, is like the short-circuited squirrel cage bars in an induction motor.

Also, additional damper windings in the rotor can be used to represent the damping effects of eddy currents in the solid iron of the rotor poles.

Introduction (cont’d) The de-axis is aligned with the North pole of the rotor and the

qe-axis is aligned orthogonally to the de-axis.

Note: In Ong’s book the qe-axis leads the de-axis whereas in Bose’s book the qe- axis lags the de-axis.

•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 (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 Faraday’s law an emf will be induced in the stator (armature) which by Lenz’s 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 Farday’s and Lenz’s 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.

Machine

Machine

Machine

Machine

Equivalent circuit of CRSM (2)

Machine Machine

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

Vt

Ef1

jIa1Xs

jIa2XsEf2

Ia1

Ia2

Power

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

-3 -2 -1 0 1 2 3-1

-0.5

0

0.5

1

Delta(Radians)

Nor

mal

ized

Tor

que,

Pow

er

Generator

Motor Tmax,Pmax

Torque versus Speed

Power Angle Characteristics of SPSM

Equivalent Circuit of Non-Salient Pole Wound Rotor Motor

A simple per-phase equivalent circuit for a round rotor synchronous motor can be developed in a manner similar to the per-phase equivalent circuit of the induction motor.

The figure on the next slide shows a transformer-coupled circuit linking the stator and the moving rotor winding.

Equivalent Circuit of Non-Salient Pole Wound Rotor Motor (cont’d)

The rotor is supplied with a current If

produced by a voltage Vf.

Equivalent Circuit of Non-Salient Pole Wound Rotor Motor (cont’d)

The rotor circuit can be replaced by an ac current source whose amplitude is If’(=nIf) and frequency is e. That results in the equivalent circuit below:

Equivalent Circuit of Non-Salient Pole Wound Rotor Motor (cont’d)

Neglecting Rm and replacing the current source and parallel inductance with a Thevenin equivalent, we get the equivalent circuit shown below:

Permanent Magnet TechnologyThe use of permanent magnets (PMs) in construction of

electrical machinesbrings the following benefits:no electrical energy is absorbed by the field excitation

system and thus there are no excitation losses which means substantial increase in the efficiency,

higher torque and/or output power per volume than when using electromagnetic excitation,

better dynamic performance than motors with electromagnetic excitation (higher magnetic flux density in the air gap),

simplification of construction and maintenance,reduction of prices for some types of machines.

Permanent Magnet Classification

Permanent Magnet Classification

IntroductionPM synchronous motors are widely used in industrial

servo-applications due to its high-performance characteristics.

PMSM Nick-name : Sine-wave brushless DC motorGeneral characteristics

Compact High efficiency (no excitation current) Smooth torque Low acoustic noise Fast dynamic response (both torque and speed) Expensive

Application industrial drives, e.g., pumps, fans, blowers, mills, hoists, handling

systemselevators and escalators, people movers, light railways and

streetcars (trams), electric road vehicles, aircraft flight control surface actuation

ConstructionGeneral features about the

layout- Sinusoidal or quasi sinusoidaldistribution of magnet flux in

theair-gap- Sinusoidal or quasi sinusoidalcurrent waveforms- Quasi sinusoidaldistribution of stator

conductors

Classification based on rotor technologyMerrill’s rotor-Classical

configurationThe laminated external ring has

deep narrow slots between each of the PM poles.

The leakage flux produced by the PM

can be adjusted by changing the width of

the narrow slots. The PM is mounted on the

shaft with the aid of an aluminum or zinc alloysleeve.

Classification based on rotor technologyInterior-MagnetThe interior-magnet rotor has radially magnetized and alternately poled magnets. Because the magnet pole area is smaller

than the pole area at the rotor surface, the air

gap flux density on open circuit is less than the flux density in the magnet. The magnet is

verywell protected against centrifugal forces.

Such a design is recommended for high

frequency high speed motors.

Classification based on rotor technologySurface-Magnet RotorThe surface magnet motor can have

magnets magnetized radially or sometimes circumferentially. An external high conductivity non-ferromagneticcylinder is sometimes used. It protects

the PMs against the demagnetizing action

of armature reaction and centrifugal

forces, provides an asynchronous starting

torque, and acts as a damper.

Classification based on rotor technologyInset-Magnet RotorIn the inset-type motors PMs are

magnetized radially and embedded in shallow

slots. The rotor magnetic circuit can be

laminated ormade of solid steel. In the first case a

starting cage winding or external non-

ferromagnetic cylinder is required. The q-axis

synchronous reactance is greater than that in the

d-axis.

Classification based on rotor technologyThe synchronous reactance in q-axis is greater than that in d-

axis. A starting asynchronous torque is produced with the aid of both a cage

winding incorporated in slots in the rotor pole shoes (laminated core) or

solid salient pole shoes made of mild steel sleeve.

Comparison between surface and buried magnet PMSMSurface MagnetsSimple motor

constructionSmall armature

reaction fluxPermanent magnets

not protected against armature fields

Eddy-current losses in permanent magnets

Expensive damper

Buried Magnets Relatively

complicated motor construction

High armature reaction flux

Comparison between surface and buried magnet PMSM

New Trends in PMSM

Concentrated windings - Short end-turns - Compact winding - High inductance

New Trends in PMSMConcentrated windings - Short end-turns - Compact winding - High inductance

New Trends in PMSMSpecial winding configuration for ”fault tolerant” PM

drivesElectric, magnetic and thermal decoupling of phases.High inductance can be used to limit a short-circuit

Role of Magnet Thickness in PMSM

Thicker magnets gives higher flux and thus more torque per amp.

But higher flux also means higher core losses.Thicker magnets gives lower inductancesFaster respond, but higher PWM current rippleThicker magnets makes the motor more resistant to

demagnetizationThicker magnet also increases the cost significant.Doubling the thickness will typically only give 5-10%

more flux

Operation Principle

TheoryPhase Resistance R

The resistance in the copper used in the phase winding

Phase emf or peak flux-linkage from the PM

Phase inductance Lph

Typically the sum of air-gap, slot and end-turn inductance

Mutual inductance M

The flux linkage coupling from one phase to another with sinusoidal

windings on a three phase machine 1/2 of the airgap flux will couple to the

other phase.

TheoryA three phase PMSM can be modeled by the equivalent

diagram shownin the figure