*AC Machines Mainly two typesInduction Similar to a transformer- Stator windings excited; Rotor winding only has induced currents-Rugged; less costly

Synchronous Similar to a DC machine; Magnets located in the rotor; Armature located in the stator; 3-phase windings; EMF induced in armature because of the rotation of the rotor

*A four-pole alternatorA Simple AC Generator

*The elementary 3-phase 2-pole round rotor synchronous generator has a stator equipped with 3 coils displaced 120ofrom each other; although shown as concentrated, they actually are distributed. When the rotor is excited with dc and rotated, the resultant field will also rotate so that sinusoidal voltages are generated in the 3 stator phases.

*Relationship between speed and induced EMF frequencyIf there are 2 poles, positive to negative half cycle excursion takes place in one revolution.So, if n is the revolutions per sec, then, frequency of the induced EMF f=n

If there are p poles, then, within one revolution p/2 cycles of voltage is induced.So, frequency f=pn/2 where n is rotational speed in rps.

If the speed increases, then induced emf magnitude and frequency will increase (similar to transformer equation)

*Revolving Magnetic FieldThree phase space distributed winding fed with three-phase time distributed current causes a revolving magnetic field to be set up whose speed is 2f/p rps.

*Revolving Magnetic FieldAt t=to, t4 At t=t1 At t=t2At t=t3

*AC MotorsAC motors can be divided into two main forms:synchronous motorsinduction motorsHigh-power versions of either type invariably operate from a three-phase supply, but single-phase versions of each are also widely used particularly in a domestic setting

*A squirrel-cage induction motor

*Induction motor

Initially Rotor stationary

So, EMF induced in the rotor windings/conductors.

Already short circuited.

So, Current flows and flux is set up.

Interaction between stator and rotor flux; torque generated.

Torque opposes its own cause; induced flux in the rotor;

So, rotor starts rotating in the same direction as that of stator magnetic field.

*Synchronous Motor

No difference between the stator of induction and synchronous motors.

Rotor has a permanent magnet or electro magnet; so, has independent flux.

This magnet is dragged along with the stator rotating magnetic field.

So, rotates at 2f/p rps OR does not rotate at all.

*SUMMARY

Topics covered1. Introduction2. Circuit solutions using KVL, KCL and network theorems- DC circuits3. Transients4. Sinusoidal AC circuits- single-phase and 3-phase, power and power factor, Resonance5. Electronics topicsDiodesTransistorsOp-Amps6.Mgnetic circuits7. Transformers8.DC Machines9. AC Machines