The magnetic field always chooses the low reluctance path hence to achieve a strong magnetic field we need to maintain a low reluctance path. It can be achieved by keeping a smaller air gap between the yoke and armature of an electric motor.
The air gap between the pole pieces and the armature is kept very small to keep the field strength at its highest value. This is due to the reason the flux lines can pass through iron very easily than the air gap. The small air gap generates high voltage.
The larger the air gap means stronger must be the magnetizing force to produce the required voltage hence the air gap is kept very small.
Important Short Notes:
Yoke:
The outer frame of a dc machine is called as the yoke. It is made up of cast iron or steel. It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.
Armature core:
The armature core is the rotor of a dc machine. It is cylindrical in shape with slots to carry armature winding. The armature is built up of thin laminated circular steel disks for reducing eddy current losses.
The armature core is made of silicon steel laminations that are insulated from each other by an insulating varnish coating. These laminations are used to reduce eddy current losses. It may be provided with air ducts for the axial airflow for cooling purposes. The armature is keyed to the shaft.
Armature winding:
It is usually a former wound copper coil that rests in armature slots. The armature conductors are insulated from each other and also from the armature core. Armature winding can be wound by one of two methods: lap winding or wave winding.
Double-layer lap or wave windings are generally used.
A double-layer winding means that each armature slot will carry two different coils.