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presentation for college student

1. Name - Deepak Kumar Giri Roll. No – 2420EW12 Regd. No – 2001201076 Dept. – Electrical Engineering SEEMANTA ENGINEERING COLLEGE JHARPOKHARIA, MAYURBHANJ DC Machine

2. Content Introduction Different part of dc machine Principle of operation of DC machine Armature reaction and its effect Commutation Methods of improving commutation

3. Introduction A DC Machine is an electro-mechanical energy conversion device . There are two types of DC machines; one is the DC generator, and another one is known as DC motor. A DC generator converts mechanical power ( ωT ) into DC electrical power (EI), whereas, a DC motor converts d.c electrical power into mechanical power.

4. Different part of dc machine Stator Assembly Yoke Field poles Field windings Rotor Assembly Armature Armature Winding Commutator Brushes

5. Stator Assembly Yoke: Act as a protecting cover to the DC machine It is made up of cast iron or steel Poles It consist of pole core & pole shoe It support the field coils It spread out the flux in air gap uniformly Field windings Forms a electromagnet Current passing through the field coils produces the necessary magnetic flux

6. Rotor Assembly Armature It houses the Armature conductors and Provides a Low-reluctance path to the flux. Cylindrical/Drum-shaped Armature Winding the conductors which are housed and protected within the armature slots are connected properly There are two basic types of windings i.e. lap and wave windings Commutator Facilitates the collection of Current from the Armature conductors Converts AC-DC (Generator) or DC-AC (Motor) Brushes The brushes usually made of carbon or graphite. Function is to collect current from commutator

7. Principle of DC Generator DC generator converts mechanical energy into electrical energy, when a conductor move in a magnetic field in such a way conductors cuts across a magnetic Cux of lines and e.m.f . produces in a generator and it is defined by faradays law of electromagnetic induction e.m.f . causes current to flow if the conductor circuit is closed. The direction of induced current is given by Fleming's right hand rule.

8. Principal of DC Motor In any electric motor, operation is based on simple electromagnetism.• When a current-carrying conductor is placed in an external magnetic field, it will experience a force i.e. Lorentz force .Due to this force torque is produced which rotates the rotor of motor and hence a motor runs. The direction of this force is given by Fleming's left-hand rule. its magnitude is given by F = BIL.

9. Armature Reaction When a current flow in the armature conductor then it will produces as armature filed flux, Which is directly disturb to the main field flux either change the direction of field or reduce the magnitude. That is called armature reaction. Effect Of Armature Reaction The armature magneto motive force ( mmf ) produces two undesirable effects on the main field flux and these are:- Cross magnetizing effect :- It distorts the main field flux called as cross magnetizing effect De-magnetizing effect :- It reduce the magnitude of the main field flux called as demagnetizing effect

10. Commutation The commutation in DC machine or more specifically commutation in DC generator is the process in which generated alternating current in the armature winding of a dc machine is converted into direct current after going through the commutator and the stationary brushes. Again in DC Motor, the input DC is to be converted in alternating form in armature and that is also done through commutation in DC motor.

11. Methods to improve commutation Improving commutation means to make current reversal in the short- circuited coil as sparkless as possible. The following are the two principal methods of improving commutation: Resistance Commutation :- . In the resistance commutation method of improving commutation, the high resistance carbon brushes are used. This increases the resistance between the contact of commutator segments and brushes. The additional advantages of carbon brushes are that self lubricating and polish the commutator (ii) should sparking occur, damage the commutator less than when Cu brushes are used

12. 2. E.M.F Commutation In this method, a reversing e.m.f is produced to neutralize the reactance voltage in the short- circuited coil. If reversing e.m.f. is made to be exactly equal and opposite to reactance voltage, the current in the short-circuited coil will reverse quickly, which results in sparkless commutation The reversing e.m.f. may be produced in two ways Brush Shift- -- either by giving the brushes a forward lead sufficient enough to bring the short-circuited coil under the influence of next pole of opposite polarity Using interpoles.

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