Dc Machines

DC Machine

Dc machine is a rotating Electrical machine in which electro-mechanical energy conversion takes place. Dc machine can be classified in two types i.e. Dc Motor and Dc Generator. A dc generator converts mechanical energy into dc electrical energy. A dc motor converts dc electrical energy to mechanical energy.

 

Dc generator

A dc generator converts mechanical energy into electrical energy.

 

Working Principle of Dc Generator

Dc Generator works on the principle of Faraday law of electromagnetic induction.

 

Faraday's Law

According to Faraday law of electromagnetic induction when a conductor cuts a time varying magnetic flux a dynamically induced emf (voltage) is produced in it. The magnitude of induced emf is directly proportional to rate of change of flux linkages.

      λ   =   N.ɸ

     N  = number of conductors

     ɸ  = flux

     λ  = flux linkages.

     e  =   dλ/dt    =    N.dɸ/dt

If flux is time invarying in nature then for effective rate of change of flux linkages relative motion between conductor and flux required for effective rate of change of flux linkages.

 

 

In dc generator field winding is mounted on stator, which produces the main working flux/ magnetic field. The distribution of main working flux with respect to space is trapezoidal means it is time varying. When armature conductors cut this time varying flux an emf is induced in armature conductors. Armature conductors provided with closed path, hence the induced current circulates with in the path.

The direction of induced current is Fleming Right Hand Rule. According to Fleming Right Hand Rule, if we spread the fore finger, middle finger and thumb of our Right Hand in such a way that these are mutually perpendicular then if fore finger is pointed in the direction of flux/ magnetic field and thumb is pointed in the direction of motion/ rotation then middle finger represent the direction of induced current with in the conductors. The direction of induced current changes whenever the direction of motion changes.

Commutator Action in dc generator

In the above discussion we show that the direction of current changes for every half cycle of rotation. Means dc generator reflect ac current to load terminals. Commutator helps to maintain the direction of current same for every half cycle of rotation. Let us consider armature rotates in clockwise direction, then current in the conductor AB is in upward direction (X represent current goes inward) and current in the conductor DC is downward (dot shows the current goes outward) for next half cycle the direction of current get reversed means direction of induced current is alternating in armature conductor. But with a split ring commutator connection of armature conductors get reversed in every half cycle which help to maintain unidirectional current at load terminals. Split ring commutator works by virtue of its rotation with and also called “Mechanical Rectifier”.

Types of Dc Generator

Dc Generator are classified in two categories based on the method of Excitation

1       Separately Excited Dc Generator.

2       Self Excited Dc Generator.

 

Separately Excited Dc Generator

As the name implies separately excited it requires separate dc source for excitation given to field as well as armature winding. In This type of generator excitation is independent of load variations. It is a doubly excited generator.

 

Self Excited Dc Generator

In this type of generator field winding and armature winding is excited by a single source. It does not require separate dc source for excitation purpose.

 

Self excited generator are further classified as:

 

Shunt Generator

Series Generator

Compound Generator

 

Compound Generator are further classified as :

 

Over Compound Dc Generator

Flat/ Level Compound Dc Generator

Under Compound Dc Generator

Dc motor

A dc motor converts electrical energy into mechanical energy.

 

Working principle of dc motor

In dc motor field winding is mounted on stator, which produce the main working flux and armature winding is mounted on rotor. When armature is connected to direct current voltage source the current drawn by armature conductor establishes a magnetic field which interacts with the main working flux and creates electromagnetic torque whose direction is given by Fleming Left Hand Rule.

According to Fleming Left Hand Rule when if we spread the fore finger, middle finger and thumb of our Left Hand in such a way that these are mutually perpendicular then if fore finger is pointed in the direction of flux/ magnetic field and middle finger is pointed in the direction of current then thumb represents the direction of torque / direction of rotation. The direction of rotation changes whenever the direction of current changes.

 

Need of Commutator in Dc Motor

The function of commutator in dc motor is to produce unidirectional torque, while converting dc to ac virtue of rotation with coil. Commutator is also known as Mechanical Inverter.

 

Types of Dc Motor

1. Permanent Magnet Dc Motor

2. Separately Excited Dc Motor

3. Shunt Motor

4. Series Motor

5. Compound motor

 

Compound Motor is further classified as

 

Cummutatively Compound

Differentially Compound

 

 

EMF Equation of Dc Machine

 

Let,

P = Number of Poles

ɸ = Flux per pole (Wb)

Z = Number of conductors

N = Speed in RPM

A = Number of parallel path

Z/A = Number of conductors per path

 

According to Faraday’s Law

e = Ndɸ/dt           ( Here N is number of turns)

average induced emf in a conductor

e = dɸ/dt

In one revolution of armature flux cut by conductor is = Pɸ

And time taken to complete one revolution = 60/N

e = PɸN/60    ………….. Induced emf in a conductor.

Number of conductors = Z/A

             

In Dc  generator e = eg which is known as generated emf.

While in dc motor e = eb which is known as back emf.

related tags

#_Induction Machine

#_Transformer