- Magnetic frame or Yoke
- Pole Cores and Pole Shoes
- Pole Coils or Field Coils
- Armature core
- Armature Winding
- Brushes and Bearings
The diagram given below represents the various parts of a DC machine.
1. Yoke (Magnetic Frame)
- It provides mechanical support for the poles and acts as a protecting cover for the whole machine
- It carries the magnetic flux produced by the poles.
In small generators where cheapness rather than weight is the main consideration, yokes are made of cast iron.
But for large machines usually cast steel or rolled steel is employed.
The modern process of forming the yoke consists of rolling a steel slab around a cylindrical mandrel and then welding it at the bottom.
The feet and the terminal box etc. are welded to the frame afterward. Such yokes possess sufficient mechanical strength and have high permeability.
2. Pole Cores and Pole Shoes
- they spread out the flux in the air gap and also, being of larger cross-section, reduce the reluctance of the magnetic path
- they support the exciting coils (or field coils)
- The pole core itself may be a solid piece made out of either cast iron or cast steel but the pole shoe is laminated and is fastened to the pole face by means of countersunk screws
- In modern design, the complete pole cores and pole shoes are built of thin laminations of annealed steel which are riveted together under hydraulic pressure. The thickness of laminations varies from 1 mm to 0.25 mm.
3. Field system
The function of the field system is to produce a uniform magnetic field within which the armature rotates.
The m.m.f. developed by the field coils produces a magnetic flux that passes through the pole pieces, the air gap, the armature, and the frame.
Since armature and field systems are composed of materials that have high permeability, most of the m.m.f. of field coils is required to set up flux in the air gap.
4. Armature core and Laminations
The armature core is keyed to the machine shaft and rotates between the field poles.
It consists of slotted soft-iron laminations (about 0.4 to 0.6 mm thick) that are stacked to form a cylindrical core as shown in the figure.
The purpose of laminating the core is to reduce the eddy current loss. Thinner the lamination, greater is the resistance offered to the induced e.m.f., smaller the current and hence lesser the I²R loss in the core.
5. Armature Winding
The slots of the armature core hold insulated conductors that are connected in a suitable manner. This is known as armature winding.
This is the winding in which “working” e.m.f. is induced. The armature conductors 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.
- (a) lap winding
- (b) wave winding.
A commutator is a mechanical rectifier which converts the alternating voltage generated in the armature winding into a direct voltage across the brushes.
Depending upon the manner in which the armature conductors are connected to the commutator segments, there are two types of armature winding in a DC machine viz.,
(a) lap winding
(b) wave winding.
Great care is taken in building the commutator because any eccentricity will cause the brushes to bounce, producing unacceptable sparking. The sparks may bum the brushes and overheat and carbonise the commutator.
Multipole machines have as many brushes as they have poles. For example, a 4-pole machine has 4 brushes. As we go round the commutator, the successive brushes have positive and negative polarities. Brushes having the same polarity are connected together so that we have two terminals viz., the +ve terminal and the -ve terminal