Tuesday, December 24, 2013

Electrical Machine

Motors convert electrical energy into mechanical energy, and generators do the opposite. Both can be either connected in three-phase or single-phase.

Note that: 1 horse power (hp)= 750 W = 0.75 kW approximately.

Above 10hp, single phase motors are very rare, which are replaced by 3-phase motors because of the size and efficiency.

Common terminology associated with rotating machinery are,

Rotor:
The rotating part of a machine.

Stator:
The stationary part of a machine.
Armature:
This is the part from which useful output is taken. In the case of a motor, it is usually the rotor , and in the case of an alternator it is the stator.
Field:
This is the part to which exciting input is given. In the case of a motor, it is usually the stator , and in the case of an alternator it is the rotor.

Motors
Classified into  AC motors and DC motors


AC Motors:

Induction Motor (IM)
This is the mostly used motor in the world. This works on the principle of induction. That is, when an AC is applied to stator, it produces a rotaing magnetic field in the rotor. These two fields interact, and as a result it yields a rotating motion according to Faraday's Law.
3 Phase induction motor technology was invented by not-so-popular Austrian scientist  Nicola Tesla.

Three phase induction motors are classified as;
1. Squirrel cage IM : rotor is made up of laminated steel bars, not by coil windings.
2. Wound rotor IM : rotor has coil windings.

In order to produce torque, an IM needs a slip - a deviation from the nominal synchronous speed. Slip is defined as;
Slip, s= (Ns-Nr) / Ns


Rotor and wound stator of a small motor disassembled












The main difference between a power transformer and an Induction motor is , a power transformer has its rotor virtually locked. i.e. a transformer is equivalent to an IM with its rotor locked.

An IM does not need any special starting mechanism. The main disadvantage of an IM is the lack of control of its rotating speed. Even if we control its speed, it is in a very narrow range.

Synchronous Motor
In a synchronous motor,  stator is supplied by AC, and rotor is supplied by DC. This makes the motor to rotate in a speed called 'synchronous speed'.

Synchronous speed, Ns = (120*f )/ p;
where f = supply frequency such as 50 Hz
          p= no of poles

DC Motors:

Conventional DC Motor (DCM)
DC motors have brushes to supply a rotating magnetic field to obtain continuous rotation. But the main advantage of them is the ability to control their speed over a wide range.  They are classified as;
1. Series excited motors
2. Shunt excited motors
3. Compoundly excited motors

Principle of operation of a DC motor: How direction is commuted?









BrushLess DC Motor (BLDCM)
This type of motors are of modern nature which utilise the semi-conductors for providing excitation without the use of brushes. This has resulted in lesser maintenance.

Servo Motor
These are precise position control applications. This motor has an inbuilt arrangement for a closed loop position control of the rotor.

A servo motor


Stepper Motor
This is special type of DC motor which can rotate in steps (not continuous). Normally seen in open loop positioning such as in computer CD ROMs.

A stepper motor can rotate in step angles,e.g. 15'


Permanent Magnet Motor (PMM)
These are widely found in toys where less power is enough. Instead of a DC winding, a permanet magnet is used to create a fixed field.

Alternators:

DC Generator
This generator gives a DC output. Normally limited in applications.

Synchronous Generator (SG)
This is the most widely used AC generator, such as hydropower plants and thermal power plants.
Here the exciting DC is provided to the rotor windings and, AC output is taken from the stator windings.





A cross-sectional model of a synchronous generator


Permanent Magnet Generator (PMG)
A PMG's either rotor/ stator is a permanent magnet used for excitation.

A permanent magnet generator disassembled, showing a wound stator and PM rotor

Induction Generator (IG)
When an induction motor is rotated more than the synchronous speed, it enters the zone of generator. IG can be used in wind-turbine-generators. As an IG needs external excitation, it has to be supplied by other means.



















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