They are broadly classified as: ELECTRICAL MACHINES 1 Electrical Machines – except transformers are electromechanical energy converting device. They are broadly classified as: DC Machines AC Machines DC Generator Transformers DC Motor Induction Machines Synchronous Machines

DIRECT CURRENT MACHINES 1 Depending upon the mode of operation, a DC Machine is either called a DC Generator or DC Motor. Cut view – DC Machine

DC Machines 1 When a machine converts mechanical energy into electrical energy which is DC in nature, it is called as a DC Generator. ME EE Whenever a machine converts electrical energy which is DC in nature, into mechanical energy, it is called a DC Motor. EE ME

DC Generators: DC Generator works on Faraday’s laws of Electro magnetic induction. According to this law, whenever a conductor cuts a magnetic flux, dynamically induced e.m.f is produced in the conductor. This e.m.f causes a current to flow when the conductor circuit is closed. The direction of induced current is given by Fleming’s right hand rule. 1

Thus the essential parts of a generator are: DC Generators 1 Thus the essential parts of a generator are: a magnetic field system (2) a system of conductors (3) relative movement between (1) and (2) Condition (3) can be satisfied in two different ways. The magnetic field system can be arranged in the stationary part of the machine called the Stator, and the system of conductors in the rotating part of the machine, called the Rotor. Alternatively, the system of conductors can be placed in the Stator and the field system in the Rotor.

Constructional Details – DC Machine The various parts of a DC machine can be grouped into two categories. Stator – Stationary part Rotor – Rotating part Airgap between Stator & Rotor 1

Stator Parts Constructional Details – DC Machine Frame / Yoke 1 Constructional Details – DC Machine Stator Parts Frame / Yoke (2) Field Magnets (3) Field or Exciting winding (4) Interpoles (5) Interpole winding

Acts as an outer protective cover for the machine 1 Constructional Details – DC Machine Stator Parts Frame / Yoke Acts as an outer protective cover for the machine Serves path for the magnetic flux (iii) Provides mechanical support for the poles

Stator Parts Constructional Details – DC Machine 1 Constructional Details – DC Machine Stator Parts Field magnets consists of Pole core & Pole shoes. Can be Permanent magnets or Electromagnets. Poles are Salient in nature (ii) Welded or bolted to yoke (iii) Even in number (iv) Forms an Hetrogeneous field system (v) Laminated to reduce Eddy current loss (vi) Made of Silicon steel to reduce Hysterisis loss Portion of the core near the airgap is made wider – Pole shoe Spreads the flux Provides support for the field winding

Constructional Details – DC Machine 1 Constructional Details – DC Machine Stator Parts Field Winding / Exciting Winding Made of Copper Wires When energized with DC, electromagnetize the poles and provide the working flux. (iii) All windings connected in series

Constructional Details – DC Machine 1 Stator Parts Inter Poles / Commutator Poles Provided in between the main poles Fitted on to Yoke No: of Interpoles = No: of poles (iv) Flux produced by the Interpoles provide sparkless commutation

Constructional Details – DC Machine 1 Stator Parts Inter Pole Winding / Commutator field Winding Windings are connected in series which are in turn connected in series with armature (ii) Same electromagnetizes the Interpoles (iii) Made of Copper

Constructional Details – DC Machine 1 Rotor Parts Shaft 2) Armature core & Armature winding 3) Commutator segments 4) Brush holder & Brushes

Constructional Details – DC Machine Rotor Parts Shaft Provides mechanical support to the armature and to the commutator. The rotor assembly is supported and allowed to rotate freely by two bearings fitted between two end covers and the shaft. Shaft transfers mechanical energy to and from the machine. 1

Constructional Details – DC Machine Rotor Parts Armature Core with Armature winding The purpose of armature core is to rotate the conductors in a uniform magnetic field. Core is cylindrical in shape Laminated to reduce Eddy current loss and made of Silicon steel to reduce Hysterisis loss (iii) Slots are cut in the outer periphery of the core to accommodate the armature conductors or winding 1

Constructional Details – DC Machine Rotor Parts Commutator Segments Function of the commutator is to facilitate the collection of current from the armature conductors. The e.m.f. generated in the armature conductors of a DC machine is an alternating e.m.f. The commutator helps in converting this alternating e.m.f. into a direct one. Ends of armature coil terminated to Commutator segments (ii) Made of Copper segments (iii) Arranged side by side to form a ring (iv) Insulated in between with mica 1

Constructional Details – DC Machine 1 Rotor Parts Brushes The brushes collect the armature current from the commutator segments and supply it to the load (in the case of generator) or feed the current into the commutator segment (in the case of motor). Made of graphite or carbon or a mixture of graphite and carbon or even copper and are rectangular in shape Held in Brush holders No: of brushes depend on the magnitude of current

Constructional Details – DC Machine 1 Constructional Details – DC Machine Rotor Parts Brushes (contd.) Usually the no: of Brushes = no: of Poles Desirable properties for brushes: Good Electrical Conductivity Less coefficient of Friction

1 Cut view - DC Machine