పక్షులు కరెంట్ తిగపై వున్న షాక్ ఎందుకు రాదు?

ఎందుకు

పక్షులు కరెంట్ తిగపై వున్న షాక్ ఎందుకు  రాదు?

పక్షులు కరెంట్ తిగపై వున్న షాక్ ఎందుకు రాదు అనగ, కరెంట్ ఒకె తిగ నుండి ప్రవహించదు.

కరెంట్ ఒక వలయంగ నుండి మాత్రమె  ప్రవహిస్థుంది. కబట్టి పక్షులు ఒకె తిగ పై వున్నపుడు షాక్ రాదు  పక్షి ఒక తిగపై వుండి మరొక తిగను తగిలినపుడు మత్రమె వలయం ఎర్పడి షాక్ వస్థుంధి.

WHY

Why the birds didn't get the shock when the birds stands on the power line?

if the bird entire body lies on one a line, current will not flow through the body because of there is no closed path (circuit).if the bird touches the both lines then the circuit will be formed and current will flow through the bird so bird experiences the shock.  

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.                                      

                                

                                         

TYPES OF SINGLE PHASE INDUCTION MOTORS

INDUCTION MOTOR

Introduction

The induction motor is one kind of AC motor it is mostly used in industries and home appliances so that’s why induction motor is treated as the king of the industry.

                        Mainly induction motor categorized into three types those are a single phase (1 ), three phase (3 ) and poly phase. Poly phase is nothing but two or more phases having the same voltage and same frequency. As a rough estimate 90% of the world using poly phase induction motors. The reasons are simple in construction, good power factor, speed regulation, low cost.

SINGLE PHASE INDUCTION MOTOR

The main difference between the single phase motor and three phase motor is, three phase motor is self-starting but the single phase motor is not self-starting motor because of rotating magnetic field is not produced in the stator, it producing only alternative magnetic field. Here alternative magnetic field is nothing but the direction of magnetic field is changed two times in one cycle. In other words according to double field revolving theory a flux ( ) is resolved into two types

                               m /2+ m/2

Each of these components rotates in opposite directions. That is one m/2 is works on clockwise direction other one rotates in an anti-clockwise direction. These two fluxes having same magnitudes and opposite direction so they cancel each other then the net torque at the starting is zero.

                                Due to this reasons, torque will not develop in the stator. If the rotor is pushed by the hand or by using the other mechanism the motor will be started.

                            To start the motor without using any external mechanism the single phase motor is fed to the two phase supply but since the supply is a single phase  so, two windings are mounted on the stator  due to this 90  degrees of phase angle created between those two windings. Depending upon this mechanism single phase motors can be classified into two types those are split phase motor and shaded pole motor. Split phase motor classified into two types namely resistance split phase, capacitor split phase again this capacitor split phase further classified as capacitor start –induction run, capacitor start capacitor run induction motors.

RESISTANCE SPLIT PHASE INDUCTION MOTOR

                     Resistance split phase induction motor is also called as split phase motor the name itself indicates that it shifts the one phase into two phases as shown in below.

  Resistance split phase induction motor

The stator having two windings namely main winding, auxiliary winding or starting winding. Auxiliary winding has high resistance and low inductance.  Main winding having low resistance high inductance. These two windings are connected in parallel so 90 phase displacement created between two windings. A centrifugal switch connected in series with the auxiliary winding. The rotor is of squirrel cage type. The reason behind to choose the squirrel cage rotor is we needed constant torque i.e., we can’t vary the torque of the motor and maintenance is also easy. Whenever single phase supply is fed to the stator then rotating magnetic field is produced. Due to Faraday's law of electromagnetic induction principle emf is induced in the rotor conductor then the interaction between the stator flux and rotor currents torque is developed motor will run. To change the direction of rotation interchange the auxiliary winding terminals by this we can say the direction of the motor depends on the auxiliary winding or starting winding.

                                V-I characteristics

Where

           im  is the current flowing through main winding

                            is  is the current flowing through starting winding

           V is the voltage

           I is resultant current             

Due to the resistance and reactance ratios of two windings large phase angle created in between them. im  lags behind the v by a much larger than ia. When the rotor reaches to the 75% of rated torque then the centrifugal switch is opened. By this, we can say starting winding only used for starting purpose but it doesn’t effect the running of the motor. Then the motor will run as a single phase.

             Applications of this motors are centrifugal pumps, blowers etc. These kinds of motors are available in the range of 0.05 to 0.5 HP

CAPACITOR START INDUCTION MOTOR

Capacitor start induction motor is quite similar to the resistance split phase motor except for the capacitor. Here the Capacitor is an electrolytic capacitor. This capacitor is connected in series with the starting winding. Due to this capacitance large angle (in practice 90 ) created in between starting and auxiliary windings.

                                 CAPACITOR START MOTOR

Due to this phase angle more starting torque developed as compared to the split phase motor. Operation of centrifugal switch is same as in the resistance split phase it is also connected in series with the capacitor. When motor reaches to 75% of rated torque then the centrifugal switch open from the circuit.

           These types of motors are used in centrifugal pumps, fans etc. these types are rated in the range of 0.125 to 0.75HP

CAPACITOR START CAPACITOR RUN MOTOR

Capacitor start capacitor run induction motor is also one kind of split phase induction motor. These kind of motors are used when the high starting is needed. This type of motors is having high starting torque than the capacitor start motor.

             These types of motors having two electrolytic capacitors. Those are starting capacitor and running capacitor name itself that starting capacitor is works at starting but, running capacitor will works both starting and running conditions. Which are properly chosen by the ratings. We already discussed in previous, centrifugal switch is opened when the motor reaches to the 75% of rated torque. So the starting capacitor and auxiliary winding is disconnected from the circuit. Running capacitor is useful for the improving the power factor. These kinds of motors having high efficiency than the other motors.

                              Which are used in high starting applications like compressors, conveyors and centrifugal pumps etc.

PERMANENT SPLIT CAPACITOR MOTOR

These types of motors modified from the capacitor start-capacitor run motors. This type of motors is also having starting, running windings and capacitor but there is no centrifugal switch is used. So the capacitor and starting winding is permanently closed. In other words starting winding and capacitor is used at the starting and running conditions.

                     As compared to the other type of motors this permanent split capacitor motor having starting torque is quite low, the motor does not run with the heavy load and it has improved power factor. such motors are available with the range of 0.05 to 0.75HP and which are used in blowers, fans, pumps and many other low torque applications.   

        

         

Difference between permanent magnet and field winding

Permanent magnet vs field winding in machines

Machines are mainly classified into two types those are motor and generator which may be of AC or DC type. Permanent magnet and field windings are used to produce a magnetic field around the armature or conductor. A permanent magnet is nothing but the magnet which  is used for the small rating of the machines mostly which is used in dynamos. Dynamo is nothing but a machine which converts electrical energy to mechanical energy as well as mechanical to electrical energies i.e., motor and generator respectively .The advantage of the dc machine over the AC machine is that dc motor will  work as a motor and as well as a generator.

                            The field winding is nothing but, a winding is wounded on the conductor and which winding is energizes, it produces the magnetic field as the same way of permanent magnets. Usually, field windings are used for greater magnetic field strength as compared to permanent magnet and we can’t able to design the permanent magnet for the large rated machines. By using this field winding we can control the speed of the machines in some cases. But, it consumes energy while working. In permanent magnets we cannot  control the speed of the machines but the risk of operation is very less and losses are negligible.

                          

From the above figure (a) , where  the north pole (N),  south pole(S) are the two permanent magnets. so the no of poles in the machine is 2.those are permanent magnets .

                                   From the above figure (b) , the north pole (N),  south pole(S) are the  permanent magnets. so the no of poles in the machine is 4.this poles are made by the field windings.