DC MACHINE By Amandeep Kaur Asstt Professor Types

DC MACHINE By Amandeep Kaur Asstt. Professor

Types of DC Machine • D. C. Generator: Converts mechanical to electrical power • D. C. Motor: Converts dc electrical power to mechanical power

Definition • An electric motor is an electromechanical device that converts electrical energy into mechanical energy. • All electric motors operate through the interaction of magnetic fields and currentcarrying conductors to generate force.

Classification of DC motors • Electrical motors are broadly classified on the availability of type of supply. They are, • DC motors • AC motors • DC/AC motors (Universal Motor)

Main Construction Components • • • Magnetic frame or Yoke Pole-cores and Pole shoes Pole coils or field coils Armature core Armature windings or conductors Commutator Brushes Bearings End Housings Shaft

Construction of DC Motor

Magnetic Frame or Yoke Ø The outer cylindrical frame to which main poles and inter poles are fixed and by means of which the machine is fixed to foundation is called the Yoke. Ø Yoke serves basically two purposes: i. It provides mechanical protection to the inner parts of the machine ii. It carries the magnetic flux produced by the poles and provides a low reluctance path for the magnetic flux Ø Yoke is made of cast iron for smaller machines and for larger machines, it is made up of cast steel. Cast steel have better magnetic properties as compared to cast iron.

Pole Core and Pole Shoes • • • The magnetic poles of DC motor are structures fitted onto the inner wall of the yoke with screws The pole core is of small cross sectional area and its function is to just hold the pole shoe over the yoke, whereas the pole shoe having a relatively larger crosssectional area spreads the flux produced over the air gap and also, being of larger cross section reduce the reluctance of the magnetic path. The pole shoe also carries slots for the field windings that produce the field flux. Usually the pole core and pole shoes are made up of thin cast steel or wrought iron laminations.

Field or Exciting Coils Ø The field winding of dc motor are made with field coils (copper wire) wound over the slots of the pole shoes in such a manner that when Direct Current (DC) flows through the field Winding, it magnetizes the poles which produces the required flux. Ø The field coils of all the poles are connected in series in such a way that when current flows through them, the adjacent poles attain the opposite polarity

Armature Core Ø It is in Cylindrical shape and keyed to the rotating shaft. The outer periphery slots are cut which accommodate the armature winding. Ø The armature core serves the following purposes. • It houses the armature conductors or coils and causes them to rotate and hence cut the magnetic flux of the field magnets. • Its most important function is to provide a path of very low reluctance to the flux through the armature from a N-pole to a S-pole • Armature Core is basically made up of Silicon Steel.

Armature Winding q The armature winding is the heart of D. C machine. It is the place where conversion of power takes place. q Armature Windings are arranged in such a manner that electric current are induced in these wires when the armature is rotated in the magnetic field Ø The armature winding are connected in series - parallel § The conductors being connected in series so as to increase the voltage and in parallel paths so as to increase the current. § The armature winding of a d. c. machine is a closed-circuit winding and the conductors being connected in a symmetrical manner forming a closed loop or series of closed loops.

Types of Armature Winding Ø On the basis of connections, there are two types of armature winding named as q Lap winding § The number of parallel paths between conductors A is equal to the number of poles P i. e A = P q Wave Winding § In wave Winding, the number of parallel paths between conductors A is always equal to 2 irrespective of the number of poles. Hence the machine designs are made accordingly.

Commutator Ø It is placed between the armature and the external circuit Ø It basically converts the alternating current induced in the armature conductors in unidirectional current Ø The commutator is of cylindrical shape and is made up of wedge shaped hard copper segments. The segments are insulated from each other by a thin sheet of mica.

Brushes • The brushes are pressed upon the commutator and form the connecting link between the armature winding and the external circuit • The brushes of dc motor are made with carbon or graphite structures, making sliding contact over the rotating commutator. • The function of brushes is to collect current from the commutator and supply it to the external load circuit

Commutator and Brush Assembly

End Housing • These are attached to the ends of the main frame and support bearings • The front housing supports bearing and brush assemblies whereas the rear housing usually supports the bearing only

End Housing

Bearings • The ball or roller bearings are fitted in the end housings. • The function of the bearings is to reduce friction between the rotating and stationary parts of the machine. • Mostly high carbon steel is used for the construction of bearings as it is very hard material

Bearings

Shaft • The shaft is made of mild steel with a maximum breaking strength. • The shaft is used to transfer the mechanical power from or to the machine. • The rotating parts like armature core, commutator, cooling fan etc are keyed to the shaft

4 - Pole DC Motor

Applications of DC Motors

Demonstration of Construction of DC Motor

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