Electricity does not exist naturally in any useful form. It has to be generated from any other sources of energy such as solar, wind, hydro, thermal, atom, etc. Photovoltaic cells help us trap energy from the sunlight and generators are used to convert mechanical power available in other forms to electricity. The mechanical power can be obtained from wind, flowing water, and steam using turbines. Motors are used to convert back the electricity to mechanical power. So, collectively, electric machines are devices that convert mechanical energy to electrical energy and vice versa.
Classification of Electric machines Basically electric machines are classified into
Static Electric machines – Transformers
Rotating Electric machines – Motors (convert electrical energy to mechanical energy) and Generators (convert mechanical energy to electrical energy)
Transformers
Any static device that can transfer alternating current from one circuit to another circuit by electro-magnetic induction can be considered as a transformer. Transformers are used to transform alternating current from one voltage level to another voltage level.
A basic transformer consists of two coils that are coupled using a magnetic core. In the case of three-phase transformers, two sets of coil per phase will be present. One set of coils is known as the primary winding and the other is known as the secondary winding. These two windings are insulated from each other and are magnetically coupled through the iron core. The alternating voltage is connected to the primary winding. When connected, an alternating magnetic flux is produced at amplitude proportional to the magnitude of the applied voltage, frequency and the number of turns. This flux links with the secondary winding and induces a voltage proportional to the number of secondary turns.
The ratio of the number of primary turns and the secondary turns is known as the turns ratio of the transformer. Any ratio of voltage transformation is possible and can be achieved by properly proportioning the number of primary and secondary turns.
Rotating electric machines The rotating electric machines used to either convert mechanical energy to electrical energy or vice versa. There are three basic types of rotating electric machines.
DC electric machines – DC motors and DC generators
Synchronous machines – Alternators and Synchronous motors
DC machines are available in different sizes and forms from small stepper motors in printers to huge traction motors. A DC machine consists of field winding at the stator and armature at the rotor.
As you know an electromagnetic conversion requires relative motion between the field and armature windings. To achieve relative motion between the stator and the rotor, the armature is rotated externally using a prime mover (turbines or engines). When the armature is rotated past the excited field poles, EMF is induced in the armature.
The induced EMF is alternating in nature. In order to convert it to DC, the two ends of the armature are connected to commutator. Commutators are metallic bars fitted to the shaft of the machines and connected to armature winding, which shift the direction of current each half turn. A commutator is divided into multiple segments and each segment is insulated from each other. Carbon brushes are used to collect the current from the commutator.
In DC machines armature always stays at the rotor in order to convert the induced alternating voltage to direct voltage. The armature consists of several slots and is mounted on the shaft which is supported on the bearing.
DC motors and DC generators have a same design. Generally a motor can be used as a generator and vice versa. Based on the stator and rotor winding connections, a DC machine can be classified as follows:
