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Write shorts notes on the types of dc machines.
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TYPES OF DC MACHINES:

i) DC generator

ii)DC motor


i) DC generator 

 An electrical generator is a rotating machine which usually converts mechanical energy into electrical energy for doing work. The energy changing is based on the principle of electromagnetic induction.  According to Faraday's laws of electromagnetic induction, Whenever a conductor is feel motion in a magnetic field, emf induced dynamically in the conductor. When an external load is connected to the conductor this induced emf make a current flow in the load.  Thus the mechanical energy which is given in the form of movement to the conductor is converted into electrical energy.  Dc generators can be classified as per their methods of field excitation. There are two types of dc generators on the basis of excitation.

Power delivered to the source is V*I(a).


Separately excited Dc generators:- If the field winding is excited by a separate dc supply from the external source, then the generator is called separately excited dc generators.


Self -excited Dc generators:- If the field winding energy is supplied from the armature of the generator itself, then it is called self- excited dc generators. Self -excited dc generators are further classified as


Series generator:-

In series generator field winding is connected series to the armature itself.

The voltage generated in series field generator is E(generated)=V(terminal voltage)+I(a)*R(a)+I(se)*R(se)+V(brush).

Power generated is E(g)*I(a).


Shunt generator:

In shunt generator field winding is connected across the armature or parallel to the armature.

The generated emf in shunt field generator is E(generated)=V(terminal voltage)+I(a)*R(a).

Power generated is E(g)*I(a).

Power delivered to the source is V*I(a).


Compound generator 

The compound generator consists of both shunt field and series field winding on its structure. One winding is series and other is in parallel with the armature of the generator. 

 

ii) DC Motor: Electric motors are everywhere around us. Almost all the electro-mechanical movements we see around us are caused either by a AC or a DC motor. Here we will be exploring DC motors. This is a device that converts DC electrical energy to a mechanical energy.


 Principle of DC Motor


 This DC or direct current motor works on the principal, when a current carrying conductor is placed in a magnetic field, it experiences a torque and has a tendency to move. This is known as motoring action.  If the direction of current in the wire is reversed, the direction of rotation also reverses. When magnetic field and electric field interact they produce a mechanical force, and based on that the working principle of DC motor is established.  The direction of rotation of a this motor is given by Fleming’s left hand rule, which states that if the index finger, middle finger and thumb of your left hand are extended mutually perpendicular to each other and if the index finger represents the direction of magnetic field, middle finger indicates the direction of current, then the thumb represents the direction in which force is experienced by the shaft of the DC motor.  Structurally and construction wise a direct current motor is exactly similar to a DC generator, but electrically it is just the opposite. Here we unlike a generator we supply electrical energy to the input port and derive mechanical energy from the output port. We can represent it by the block diagram shown below.


image 

Here in a DC motor, the supply voltage E and current I is given to the electrical port or the input port and we derive the mechanical output i.e. torque T and speed ω from the mechanical port or output port.


The input and output port variables of the direct current motor are related by the parameter K. 

image 

So from the picture above we can well understand that motor is just the opposite phenomena of a DC generator, and we can derive both motoring and generating operation from the same machine by simply reversing the ports. 


Detailed Description of a DC Motor  

 The direct current motor is represented by the circle in the center, on which is mounted the brushes, where we connect the external terminals, from where supply voltage is given.  On the mechanical terminal we have a shaft coming out of the Motor, and connected to the armature, and the armature-shaft is coupled to the mechanical load. On the supply terminals we represent the armature resistance Ra in series.

image

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