A DC generator produces direct power, while an AC generator produces alternating power.
DC Generator principle
An Electric generator is a machine that converts mechanical energy into electrical energy. It is based on the principle that whenever flux is cut by a conductor, an emf is induced which will cause a current to flow if the conductor circuit is closed. The direction of induced emf is given by Fleming`s right-hand rule.
The essential components of generators are:
- A Magnetic field
- Conductor or a group of conductors
- The motion of the conductor with respect to the magnetic field
Construction of DC Generator
Dc generators and dc motors have the same general construction. Dc generator can be run as a dc motor and vice versa. All dc machines have five principal components:
- Filed system
- Armature core
- Armature winding
The field system consists of a number of salient poles bolted to the inside of a circular frame(Yoke). The yoke is usually made of solid cast steel whereas the pole pieces are composed of stacked laminations. Filed coils are mounted on the poles and carry the dc exciting current. The field coils are connected in such a way that adjacent poles have opposite polarity.
- To produce a uniform magnetic field within which the armature rotates.
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. The laminations are individually coated with a thin insulating film so that they don’t come in electrical contact with each other. The purpose of laminating the core is to reduce the eddy current loss. The laminations are slotted to accommodate and provide mechanical security to the armature winding and to give a shorter air gap for the flux to cross between the pole and the armature “teeth”.
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” emf 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. The armature winding of a dc machine is a closed-circuit winding, the conductors being connected in a symmetrical manner forming a closed loop or series of closed loops.
Types of Armature Windings
Simple wave windings
The armature coils are connected in series through commutator segments in such a way that the armature winding is divided into two parallel paths irrespective of the number of poles of the machine. If there are Z armature conductors, then Z/2 conductors will be in series in each parallel path. Each parallel path will carry a current Ia/2 where Ia is the total armature current.
Points to be noted for wave winding
- There are two parallels path irrespective of the number of poles of the machine.
- Each parallel path has Z/2 conductors in series, Z being the total number of armature conductors.
- Emf generated =emf/parallel path.
- Total armature current, Ia=2*current/parallel path
Simple lap winding
The armature coils are connected in series through commutator segments in such a way that the armature winding is divided into as many parallel paths as the number of poles of the machine. If there are Z conductors and P poles, then there will be P parallel paths, each containing Z/P conductors. Each parallel path will carry a current of Ia/P where Ia is the total armature current.
points to be noted for Lap winding
- There are as many parallel paths as the number of poles (P) of the machine.
- Each parallel path has Z/P conductors in series where Z and P are the total numbers of armature conductors and poles.
- E.m.f generated=e.m.f/parallel path
- Total armature current, Ia=P*current/parallel path
Armature Resistance (Ra)
The resistance offered by the armature circuit is known as armature resistance(Ra) and has
- resistance of armature winding
- resistance of brushes
Armature resistance depends upon the construction of the machine. Except for small machines, it has a value of less than 1Ω.
Read Also: AC vs DC system
The commutator is a mechanical rectifier that converts the alternating voltage generated in the armature winding into a direct voltage across the brushes. The commutator is made of copper segments insulated from each other by mica sheets and mounted on the shaft of the machine. The armature conductors are soldered to the commutator segments in a suitable manner to give rise to the armature winding.
The brushes are made of carbon and rest on the commutator. The brush pressure is adjusted by means of adjustable springs. If the brush pressure is very high, the friction produces heating of the commutator and the brushes. If it is low, causes sparking on the commutator.
Function of brush
- to ensure the electrical connection between the rotating commutator and stationary external load circuit.
- Lubricate and polish the commutator.
- If sparking occurs, they damage the commutator less than with the copper brushes.
- provide good commutation.
- The contact resistance is high and causes a loss of about 2 V and hence is unsuited for low voltage machines in which this forms a larger percentage loss.
- Due to high voltage drop, the commutator must be made larger than for copper brushes.
- Their low current density necessitates large brush holders.