OVERVIEW OF ELECTRICAL MACHINES

Electrical Machines plays an vital role in power generation & distribution and especially in Industries because 44% of electricity generated in power plants are utilized by these industries to run high rating and large electrical machines. Now I am to say just a gist of electrical machines used in power plants and industries.

Basically electrical machines are classified into two types. They are:

1.  DC MACHINES
2.  AC MACHINES 

DC MACHINES:

•  In these type of machines, the field winding is wounded around the magnetic material (i.e electromagnet) located in stator and the armature winding is wounded around the rotor slots.
• we can say field winding is stationary and the armature winding is movable for the DC machine.

From the diagram, 

• For DC motor→dc supply is given  to the rotor via commutator(here commutator acts as Inverter) as a result armature flux get induced and due to interaction of this flux with main filed flux, motor starts to rotate.
• For DC generator→ dc supply is extracted from the rotor via commutator & brushes(here commutator acts as Rectifier). The cutting of main field flux due to the rotation of armature conductors in rotor, emf get induced. Here, magnetic field is stationary and the armature conductor is rotating.

(NOTE: The generator always produce sinusoidal voltage(AC) ,through commutator only it is changing to DC supply and vice versa).

AC MACHINES:

There are two types of AC machines.They are:

• Synchronous Machine
• Asynchronous Machine( Induction machine)

SYNCHRONOUS MACHINE:

• The field winding is wounded around the rotor slots and connected to the DC supply.
• The armature winding is wounded around the stator slots.
• Unlike DC machines, the armature winding is wounded either is star connection or delta connection(120 degree displacement) to create a Rotating Magnetic Field(RMF).
• we can say field winding is movable and armature winding is stationary for Synchronous Machine.

From the diagram,

• For synchronous motor→ Three phase AC supply is given to the stator(armature winding) to created rotating magnetic field. DC supply is given to the rotor to magnetize the rotor slots and to create magnetic field.
• The rotor magnetic field get locked with the RMF and rotor rotates in the same speed as that of RMF speed.

(NOTE: The motor always rotates in Synchronous speed which is nothing but RMF speed)

• For synchronous generator→The three phase AC supply is extracted from the stator(armature winding) . The magnetic field is created in the rotor and it tends to rotate. Due to the rotation of magnetic field, which cuts the stationary armature conductor. As a result, emf get induced. 

ASYNCHRONOUS MACHINE:

• Also called as Induction machine, consists of armature winding in the stator and rotor has copper bars which is shorted by the end rings.This is type of rotor is called as Squirrel cage.
• RMF is created when three phase AC supply is given to the stator and this changing RMF induce emf in the rotor . Due to the copper bars shorted ,current tends to flow and magnetic field is created.
• In this machine, the rotor and generator mode is obtained by controlling the slip. Slip is defined as the difference between the synchronous speed(RMF speed) and the rotor speed.

MOTORING MODE:

• If slip value is greater than 1(i.e s>1),it is a motoring mode.
• The rotor speed is always lesser than the RMF speed.

GENERATING MODE:

• If slip value is lesser the 0(i.e s<0),the motor acts as induction generator.

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