DC Generators

 

Generator Principle

An electric generator is a machine that converts mechanical energy into electrical energy. An electric generator is based on the principle that whenever flux is cut by a conductor, an e.m.f. is induced which will cause a current to flow if the conductor circuit is closed. The direction of induced e.m.f. (and hence current) is given by Fleming's right-hand rule.

Therefore, the essential components of a generator are :

    I.         a magnetic field 

    II.        conductor or a group of conductors  

    III.       motion of conductor w.r.t. magnetic field.

As an energy converter, the d.c. generator is not 100 percent efficient because there are energy losses in the machine.

 

Construction of D.C. Generator (or Motor)

The d.c. generators and d.c. motors have the same general construction. When the machine is being assembled, the workmen usually do not know whether it is a d.c. generator or motor. Any d.c. generator can be run as a d.c. motor and vice-versa. All d.c. machines have five principal components viz (1) field system (2) armature core (3) armature winding (4) commutator (5) brushes

(1) Field system: The function of the field system is to produce a uniform magnetic field within which the armature rotates. It consists of several salient poles (of course, even number) bolted to the inside of the circular frame (generally called yoke). The yoke is usually made of solid cast steel whereas the pole pieces are composed of stacked laminations. Field coils are mounted on the poles and carry the d.c. exciting current. The field coils are connected in such a way that adjacent poles have opposite polarity.

(2) Armature winding: The slots of the armature core hold insulated conductors that are connected properly. 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 to increase the voltage and in parallel paths to increase the current. The armature winding of a d.c. machine is a closed-circuit winding, the conductors being connected in a symmetrical manner forming a closed-loop or series of closed loops.

(3) Armature core: 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 do not 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 face and the armature “teeth”.

(4) Commutator: A 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 properly to give rise to the armature winding. Depending upon the manners in which the armature conductors are connected to the commutator segments, there are two types of armature winding in a d.c. 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 burn the brushes and overheat and carbonise the commutator.

(5) Brushes: The purpose of brushes is to ensure electrical connections between the rotating commutator and stationary external load circuit. The brushes are made of carbon and rest on the commutator. The brush pressure is adjusted through adjustable springs.