Construction details of DC Motor

Dc Motor


Generally rotating machine contains two parts the annular stationary part known as stator & the cylindrical rotatry part is known as rotor, separated by a least possible air gap. The stator and rotor are both made of ferromagnetic material (steel) which conducts the magnetic flux upon which depends the process of energy conversion. Slots are cut on the inner periphery of the stator and outer periphery of rotor conductors are placed in the slots of stator and rotor. They are interconnected to form winding. In dc machine field winding (concentrated type) is mounted on stator and armature winding (distributed type) along with commutator is mounted on rotor. The field poles are symmetrical and even in number, alternately north and south. 


Dc Machines

There are several parts of Dc machine

Yoke

Poles

Armature core

Commutator

Brush

Bearings

Shaft


Yoke

The yoke of a dc machine is an annular ring which provides the mechanical support for poles and it also provides the path for flux. In small dc motor yoke made from fabricated steel and in special application (power electronics converter) laminated steel is used.


Poles

The main field flux is created by poles excited with direct current. There are two types of poles permanent magnet poles and electromagnet poles. Steel laminations are provided to reduces pulsation losses i.e. eddy current loss.


Pulsation losses

Due to teeths and slots on the outer periphery of rotor flux distribution is non uniform i.e. flux is pulsating with high frequency which creates eddy currents (called pulsation losses) in the pole shoes


Armature core

Armature is center for electromechanical energy conversion hence it should have high permeability. The armature is made up of silicon steel with slots cut out on the periphery to accommodate the insulated armature winding. The ends of each armature coil are connected to the commutator segments to form a closed winding. To reduce eddy current losses thin laminations are provided. Laminations are cut parallel to flux direction

 

Brush

Brush is used to collect current from rotating commutator and winding. The brushes are alternatively positive and negative (electrical angle between adjacent pair being 1800). These are placed in brush Holder with the spring for good mechanical conditions. DC motor brushes are made up of copper, carbon, electro-graphite, cu-graphite.

In order to have successful commutation the brushes are physically placed along d axis to short circuit the coil along q axis but schematically brushes are shown at q axis/MNA.

 

Bearing

The function of the bearings is to reduce friction between the rotating and stationary parts of the machine. Bearings support the armature which is mounted on the shaft.

 

Shaft

The shaft is made of mild steel. The rotating parts like armature core and commutator are mounted and keyed to the shaft.


Commutator

Commutator is a mechanical switch which converts alternating voltage and current which are induced in armature in Dc and it is also called mechanical rectifier. The commutator is cylindrical in shape and comprises several wedge shapped copper segments bound together while they are insulated from each other.




# DC Machines





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