DC MACHINES

DC MACHINES

INTRODUCTIONDC machines, also known as the electromechanical energy converter, which is functioning from a DC source, then generating the mechanical energy or converting the mechanical energy into a DC energy. DC machines can be divided into two types which are DC generator and DC motor.

CONSTRUCTION

Yoke : It provide mechanical Support for poles & protectionPole Shoe : It is extended part of pole & enlarge area of poleField Winding : When Current is passed through field winding it electro magnetize the poles which produce necessary fluxArmature Core & Winding : It provide path of low reluctance to the flux produced by field winding & when armature winding is rotated using prime mover. the magnetic flux and voltage gets induced in itCommutator : It Convert alternating current, collects the current form armature conductor and pass it load with the help of brushes & provide unidirectional torque for dc motor

WORKING PRINCIPLE

There are mainly three types of electric motor. DC MotorInduction Motor Synchronous Motor.

In a basic dc motor, an armature is placed in between magnetic poles. If the armature winding is supplied by an external dc source, current starts flowing through the armature conductors. As the conductors are carrying current inside a magnetic field, they will experience a force which tends to rotate the armature. Suppose armature conductors under N poles of the field magnet, are carrying current downwards (crosses) and those under S poles are carrying current upwards (dots). By applying Flemings Left hand Rule, the direction of force F, experienced by the conductor under N poles and the force experienced by the conductors under S poles can be determined. It is found that at any instant the forces experienced by the conductors are in such a direction that they tend to rotate the armature.

TYPE OF DC MACHINES

Separately Excited DC Generator

The field winding or coil is energized by a separate or external DC source.

The flux produced by the poles depends upon the field current with the unsaturated region of magnetic material of the poles. i.e. flux is directly proportional to the field current. But in the saturated region, the flux remains constant.

Self Excited DC Generator

The current to the field winding is supplied by the generator itselfIn this the field coils mat be connected in parallel with the armature in the series, or it may be connected partly in series and partly in parallel with the armature windings.

Series Excited

Compound Excited

APPLICATION DC Shunt Motors is a constant speed motor. (Lathes, Drills, Boring mills, Shapers, Spinning and Weaving machines) DC Series motor is a variable speed motor. (Electric traction, Cranes, Elevators, Air compressor, Vacuum cleaner, Hair drier, Sewing machine)DC Compound motor, thedifferential compound motors are rarely used because of its poor torque characteristics. (Presses Shears, Reciprocating machine)

CONCLUSIONAdvantages

Speed control over a wide range both above and below the rated speedHigh starting torqueAccurate steep less speed with constant torqueQuick starting, stopping, reversing and accelerationFree fromharmonics, reactive power consumption and many factors which makes dc motors more advantageous compared to ac induction motors.

Disadvantages

High initial costIncreased operation and maintenance cost due to presence of commutator and brush gearCannot operate in explosive and hazard conditions due to sparking occur at brush ( risk in commutation failure)

SIMPLE DC MOTOR