A.C. Machines

These are further classified as transformers, synchronous machines, induction machines and ac commutator machines.

1. Transformers. :- A transformer is not an electromechanical converter. It is used in almost all device energized from ac source. It usually steps up or step down electrical energy from one voltage level to another voltage level. However, it is also employed with unity turns ratio for isolation purposes.

In small sizes, single-phase are used. for large ratings, 3-phase transformers are more common.

2. Synchronous machines.:-In these machines, the field poles may be on stator or rotor. In bigger sizes, field poles are always on the rotor.The cross sectional view of a two pole synchronous machine is given below.

Field winding on rotor is excited with dc whereas stator winding handles three phase ac power. Rotor run at synchronous speed N(s). Frequency f of the emf generated in armature is given by

f = PN(s)/120 Hz

where P =number of field poles.

A synchronous machine is a doubly excited machine. It is employed as an alternator for the generation of three-phase power at all the generating stations. It is also used as a synchronous motor for special purpose.

3. Induction machines.:-These are of two types, three phase induction machines and single phase induction machines. Both types are singly excited in the sense that stator of three-phase ac source whereas stator of single phase induction motor is energized from 1-phase ac source.

(i) Three phase induction machines:-The stator winding of three phase induction motor, connected to 3 phase mains, carries both load current and field producing exciting current.the field produced rotates at synchronous speed in the air gap but rotor speed is always less then synchronous speed in the air gap but rotor speed is always less then the synchronous speed. Two type of 3 phase induction motors are as under:-

 1. SCIM Squirrel cage induction machine.:- Rotor carries copper or aluminium, bars embedded in rotor slots. These bars are short circuited by two end rings, one at each end. SCIM has the most rugged construction and is used where control of starting torque or speed is not needed.

  2. SRIM Wound rotor induction machine.:- Its stator is similar to the stator of SCIM, but rotor carries 3-phase winding with three leads connected to three slip rings mounted on shaft. Brushes pressing on the slip rings allow the insertion of external resistance in the rotor circuit.SRIMs are used where speed, or torque, control is required.

(ii) Single phase induction machines.:-In addition to 3 phase induction machines discussed above, single-phase induction motors are used where single phase low voltage(230 V, 50 Hz) is available, as in homes, offices, classrooms, shops etc.

4. AC Commutator machines.:-As the name suggest these machines are fed from ac source and are fitted with commutators.