ELECTROMAGNETISM

 ELECTROMAGNETISM

Electromagnetic induction

      Electric current in a wire generates a magnetic field .There is also a complimentary phenomenon: if a wire is moved across the lines of a magnetic field, an electric current is generated in it. This production of electric current in a conductor by change of a magnetic field is called electromagnetic induction. This phenomenon is utilized in a dynamo or generator in which a current is induced in a coil rotating in a static magnetic field. The current alternates with each passage through the field and we speak of alternating current. The direction in which the conductor passes the field defines the direction of the current. The commercial supply is ac. Since ac changes polarity, the number of polarity changes in one second is indicated by a term called frequency and it is measured in Hertz

Solenoid operating principle

If a conductor is formed into the shape of one turn of a coil and a current is passed through the conductor magneto motive force is created. This magneto motive force can be increased by increasing the number of turns of the coil or by increasing the magnitude of current.  

 

SOLENOID WORKING PRINCIPLE

For solenoid applications, an electromagnet consisting of simple coils of wire do not generate sufficient mmf to drive the systems of most valves. The mmf is greatly increased for the same current consumption if the wire is formed around an iron core, rather than being air wound.

Types based on construction

• I type solenoid
• T type solenoid

I type solenoid

The figure shows a practical I type solenoid.A coil is wound over the stationary iron and movable armature.When current flows through the coil,magnetic field is generated and the movable armature is attracted towards the stationary iron.This force is utilized to move mechanical parts or as in solenoid valves to alternatively open and close valve seals.  

 

I TYPE SOLENOID

T type solenoid

A T type solenoid generates a greater attracting force.A coil is wound over a E shaped stationary iron. A T type movable armature is pulled towards the stationary part  when the coil is energized.In this design the iron circuit has three equal air gaps to get a maximum attraction force between the two iron parts.  

T type solenoid

Types based on actuation

• DC solenoid
• AC solenoid

DC solenoid valves: DC solenoid valves have a Soft iron core. This ensures a simple and rugged design. The heat losses occurring in operation depend on the resistance of the solenoid coil and therefore on the current. The solid iron also provides optimum conductance for the magnetic field.

Switching: When DC solenoid is switched on the current rises slowly. This is due to the back emf produced by the coil which opposes the applied voltage.    

DC SOLENOID SWITCHING

                        

Advantages of DC Solenoids

• Gentle switching operations
• Easily switched on
• Low turn on power
• Low holding power
• Silent
• Long service life

Disadvantages

• Over voltages can occur during cutoff
• Arc suppression is required
• High induced contact wear
• Rectifier is required if only AC voltage is available
• Longer switching time

 

Arc quenching in DC solenoids

             

                                      

ARC QUENCHING USING R AND C              ARC QUENCHING USING DIODE

AC solenoids: For these types of solenoids the actuation is AC.

AC solenoids have the following advantages and disadvantages.

• Short switching times
• Large pulling force
• No spark suppression required
• No rectifier required

Disadvantages

• High mechanical stress
• Shorter service life
• Buzzing noise