DC Generators

Simple Loop Generator

Construction Single turn rectangular copper coil ABCD rotating about its axis in magnetic field. Magnetic field provided by either permanent magnets or electro magnets. Ends of coil are joined by two slip-rings ‘a’ and ‘b’. Two collecting brushes press against slip-rings. Function is to collect the current induced in the coil and to convey to external load R. Rotating coil is “ARMATURE” and the magnets as “FIELD MAGNETS”

Working Let coil rotate in clock-wise direction. As coil assumes successive position in the field, the flux linked with it changes. Hence EMF is induced in it proportional to rate of change of flux linkages. When the plane of coil is at right angles to the lines of flux, that is at position 1, the flux linked with the coil is maximum but the rate of change of flux linkage is minimum. Coil sides AB and CD do not cut the flux, rather they slide along them, that is, they move parallel to them.

EMF Induced

Working As the coil continues rotating, the rate of change of flux linkage increases. At position 3, theta =90. Coil plane is horizontal, that is, parallel to the lines of flux. The flux linking the coil is minimum but the rate of change of flux linkage is maximum. Hence EMF induced in the coil is maximum.

EMF Induced

Working In the next quarter, from 90-180 degree, flux linking the coil gradually increases, but the rate of change of flux linkage decreases. Hence, induced EMF decreases till position 5. Here the EMF induced is zero. The current flows from A to B and C to D. Flow of current is from ABMLCD. In load resistance current flows from M to L during 1st half revolution of the coil. In the next half revolution that is, from 180-360 degree variations in the magnitude of the EMF are similar to those in first half revolution. EMF value is maximum in position 7 and minimum in position 1. The direction of induced current is from D to C and A to B. Path of flow of current is DCLMBA which is just reverse of previous current direction.

EMF Induced

Flow of Current Thus we find that current from a simple generator alternates its direction every half cycle. Hence we have Alternating Current. For making the flow of current unidirectional in the external circuit, the slip rings are replaced by split rings. Split rings are made up of conducting cylinder which is cut into two halves or segments insulated from each other by thin sheet of mica or other insulating material.

Unidirectional Flow

Operation of Split Ring In the first half revolution current flows from ABMLCD that is brush 1 is in contact with segment ‘a’. Acts as positive and ‘b’ as negative. In the next halve revolution the direction of induced current has reversed. But the position of segments ‘a’ and ‘b’ have also reverses with the result that brush No1 comes in touch with that segment which is positive. Hence, current in load resistance flows from M to L. This current is unidirectional but not continuous like pure direct current.

Parts of DC generator Magnetic Frame or Yoke Pole Cores and Pole Shoes Pole coils or Field Coils Armature Core Armature winding or Conductors. Commutator Brushes and Bearings

Parts of Generator

Yoke

Pole Shoe

Armature & Commutator

Armature Core Consists of slotted soft iron laminations. Purpose of laminations is to reduce eddy current loss. Laminations are slotted to accommodate and provide mechanical security to armature winding and give shorter air gap flux to cross pole face and the armature teeth.

Armature Winding Slot of armature core hold insulated conductors that are connected in suitable manner, known as armature winding. Here EMF is induced. The armature conductors are connected in series-parallel; the conductor being connected in series so as to increase the voltage and in parallel so on to increase the current. Windings are classified as i) Lap winding and ii) Wave Winding.

Lap Winding Here connection starts from conductors in first slot then connections overlap each other as winding proceeds, till starting point is reached again Number of Parallel Paths = Number of Poles

Wave Winding Here the winding always travels a head avoiding overlap. Number of Parallel Paths = 2

Commutator Acts as mechanical rectifier that converts AC induced EMF in the armature winding into DC across the brushes. Made of copper segments insulated from each other by Mica sheets. Mounted on shaft of the machine.