1. Transformers :-
A transformer is not an electromechanical converter. It is used in almost all the devies energised from ac source. It usually steps up or steps 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 transformers 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. Field windig on rotor is excited with D.C . whereas stator winding handles three-phase A.C. power. Rotor runs at synchronous speed Ns . Frequency f of the e.m.f. generated in armature is given by
f = PNs / 120 Hz
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 applications.
3. Induction machines:-
These are of two types, three-phase induction machiase and single phase induction machines. Both types are singly excited in the sense that stator of 3-phase induction motor is fed from 3-phase A.C. source whereas stator of single phase induction motor is energised from 1-phase A.C. source.
A). Three phase induction machines :-
The stator winding of a 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 than synchronous
speed. Two types of 3-phase induction motor is energised from 1-phase A.C. source.
a) . SCIM:-
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.
b) . SRIM:-
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 the 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. A 3-phase induction motor, connected to 3-phase source, can operate as a 3-phase induction generator when driven above synchronous speed. This, however has not become commercially popular due to certain limitation.
B). Single phase induction machines:-
In addition to 3-phase induction machines discussed above, single phase induction motors are used where single phase low voltage is available, as in homes, offices, class rooms, shops, etc.
4. A.C. Commutator machines:-
As the name suggests, these machines are fed from A.C. source and are fitted with commutators. A large variety of A.C commutator machines are reported in the literature. Only three phase schrage motors and single phase series motors have withstood the onslaught of time . For wide speed control, 3-phase schrage motor is reliable and introduces no harmonics into the supply system as are produced by inverter-fed three phase induction motors.
Single phase series motors are indispensable where high starting torque and light weight is desirable as in sewing machines, food processor, vacuum cleaners and tools. In small sizes, this motor is gives the highest power output per cost, though it makes more noise. In addition, this motor is easily adaptable to speed or torque control though the use of power electronics based 1 phase voltage controllers.
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