DC MACHINES

DC MACHINES

DC Machine is nothing but it is an electro mechanical conversion device. Machines are classified into two types. Those are motor and generator where motor converts electrical energy into mechanical energy, generator converts mechanical energy to electrical energy. For small rated generator is also called as dynamo. Machine is a electro mechanical conversion device i.e., machine is link between mechanical and electrical system. Main advantage of DC machine a DC machine can operates as a motor and generator, it is depends on the input.

  

Motor:

The very basic DC motor contains a current carrying armature which is connected to the supply end through commutator segments and brushes and placed within the north south poles of a permanent magnet as shown in the diagram below. Now to go into the details of the operating principle of DC motor its important that we have a clear understanding of Fleming's left hand rule to determine the direction of force acting on the armature conductors of dc motor.

                                         Fleming’s left hand rule

              Fleming's left hand rule says that our left hand, if we extend the index finger, middle finger and thumb of in such a way that the current carrying conductor is placed in a magnetic field  is perpendicular to the direction of  then the conductor experiences a torque or  force in the direction of rotation mutually perpendicular to both the direction of field and the current in the conductor.

                    Torque is very important parameter for the every motor .torque is nothing but the rotational force.

Fleming's left hand rule says                 

Thus DC motor is used where there is need of speed control of motor by means of controlling its field. DC motors are mainly used high torque applications.

CONSTRUCTION  

YOKE

Yoke is the outer frame of the motor which is made up of cast iron for the small rated machines and cast steel for large machines.  This is also called as casting. it protects the inner part of the motor and it is useful for housing the magnetic field so , that’s way it is also called as magnetic frame.

                                                 

                                             YOKE OR CASTING

Permanent magnets

We are familiar with the magnets. Due the magnets magnetic lines are created around the magnet i.e., magnetic field.  So, magnets produces magnetic field in there all over life. So that’s way we are treated as permanent magnets. These permanent magnets are mounted in the stator. Here these permanent magnets are producing the magnetic flux (magnetic lines of force). These permanent magnets are used in small rated machines. Because of which cannot produce much more flux and construction is difficult.

                                           4 pole stator                  2 pole stator  

FIELD WINDING

Where we can’t able to use the permanent magnets we are using these field windings. Field winding is nothing but magnetic field is produced by using windings. These windings are mounted on the pole core.                                                                                                                                                                                          

ARMATURE

Armature is the rotating part of the machine. This is mounted inside of the stator by using bearings. This armature is made up of high permeable silicon steel to avoid magnetic losses. This armature having the slots around the armature, these slots are wounded by the copper conductors. In windings only copper material is used because of it is having high conductance.  

COMMUTATOR

Commutator is connected to the armature and external circuit with the help of brushes. Commutator is cylindrical and which is made up of copper. It has two main functions those are it collecting the alternating currents from the armature and these alternating currents are converted into direct currents.

BRUSHES

These brushes are mounted on the Commutator and which is made up of carbon or graphite. Brushes have the vital role in the machine. Because of currents flowing to the internal parts of the machine is through these brushes only so the brushes has to keep in contact with proper manner.

ARMATURE WINGDINGS

A dc machine is always be in a  double layer and connected in series because of to make closed circuit. So one slot having two coil sides, these consecutive coils are mounted in commutator segments. commutator segments are made up of copper material. Each commutator segments are insulated with the mica insulation. Two types of windings are used in armature winding those are lap, wave windings.

LAP WINDING

Finishing end of the one coil and starting end of the next coil  is connected to one commutator segment. Mainly lap winding is used for the high current , low voltage rated machines.

wave WINDING

one end of the coil is connected to starting end of the other coil of same polarity of first coil. Wave winding is mainly  used for low current, high voltage machines.

                                      

Emf induced in a coil

Let us consider a rectangular coil is rotating in anti clockwise direction with the angular velocity in a uniform magnetic field

                       

From the above figure, rectangular coil is set up between the two permanent magnets so the coil is lies on the main magnetic field here, main magnetic field is nothing but magnetic field is created by the permanent magnets. When the coil is connected to the load then the coil sides are formed like SP and RQ as we can see  in the above figure. Whenever the coil experiences the mechanical force by the engine, prime mover, turbine etc, then the coil sides cut the main field flux so dynamically emf is induced in the coil due to Faraday's electromagnetic induction. Dynamically induced emf is nothing but magnetic field is stationary and conductor (coil) is moving.

                         when the coil is perpendicular to the main field then the induced emf is in the coil is zero because of coil does not able to cut main magnetic field but, whenever coil is parallel to the magnetic field then the emf induced in the coil is maximum because coil cuts the main magnetic field as shown in below figure

                           

TYPES OF DC MOTORS

DC machines are widely used in the various places in the world with the different  specifications those are as follows

                                          

                                        

PERMANENT MAGNET DC MOTOR

In this type of motor Magnets  are used to produce magnetic field in the stator. This magnetic field constant in nature so we con’t able to control. By this we can say this permanent magnet DC motor is used in where we no need control the speed.

                                               

SEPARATELY EXCITED MOTOR

in order to start the motor basically we need two fluxes those are field winding and armature winding it may be AC or DC motor. Here in this separately excited DC motor field winding is energized externally i.e., may be batteries. By raving the supply of field winding we can control speed of the motor. This kind of DC motor is used where the speed has to control.

                                  

                                     

SERIES MOTOR

In this motor field winding and armature windings are connected in series that’s way we treated this series motor.

This motor having the highest starting torque then all the motors. When the load on the motor is increases then the speed of the motor decreases and torque is increases. When the motor delivers high torque it draws lo current from the source.

                                                       

                                             

SHUNT MOTOR

In this shunt motor field winding and armature windings are connected in parallel and also parallel to the source. Due to this shunt winding, winding having high resistance so current drawn by the source is also low. we can change the speed widely by changing the field flux. But, it does not having starting torque that's why  these kind of motors are used in high speed and low torque applications.                                                                      

                                  

                                                

COMPOUND MOTOR

Compound DC motor is nothing but the combination of series and shunt DC motor. Compound motor having two field windings. In this two field windings one winding is connected series with the armature winding and another one is connected parallel with the armature winding. By this series winding is useful for producing high torque and parallel winding is used to change the speed of the motor.                                                                               

                                               

                 As we can understand this compound motor is modified from DC series and shunt motors.this compound motor classifieds into two types those are as follows. 

DIFFERENTIAL COMPOUND MOTOR

Differential compound motor is having large series field winding then the shunt field winding. So the series field opposes the shunt field. This  kind of motors having high torque than the cumulative compound motor.this is also called as short compound shunt         

                                   .

                                   

                                 

CUMULATIVE COMPOUND MOTOR

cumulative compound motor is having large shunt field winding then the series field winding. So the shunt field opposes the series field. This  kind of motors having high speed than the differential compound motor.this is also called as long shunt compound motor.