20. DC Generator

*D.C. GeneratorsPrepared and presented by Doren Nedrick

*Principle of Operation When a conductor cuts, or is cut by, lines of magnetic force, an e.m.f. is induced into that conductor.

*Principle of Operation

*The magnitude of Induced EMF The magnitude of the e.m.f. induced into the loop depends on:Strength of the magnetic field per pole.Speed at which lines of force are cut by a moving conductor.Number of active conductors connected in series.Number of pairs of poles used.

*Direction of Induced E.M.F. The direction of the induced e.m.f. in a generator can be found by using Fleming's right-hand rule:Index finger - Direction of the main field (N to S)Thumb - Direction of rotationSecond finger - Direction of current flow in the rotating conductors.

*Flemings Right Hand Rule

*Construction of a DC Generator

*Pictorial Diagram

Four Pole DC Generator*

*Construction of a D.C. Generator The main parts of a d.c. generator are as follows:Armature: This consists of four basic parts:(a) Copper coils fixed into slots in the armature core.(b) The core, made up of laminated silicon steel sheets, insulated from one another to minimize the effects of eddy currents.(c) The commutator, which consists of a series of copper segments insulated with mica. The armature coils, which generate alternating current, are connected to the risers on the commutator segments.

*Construction of the DC GeneratorThe commutator is a device which is used, in the case of the generator, to draw uni-directional current from the rotating armature coils.(d) The armature shaft: both the armature core and the commutator are keyed on to the steel shaft. The shaft may also contain a fan for cooling the generator windings.

*Construction of the DC GeneratorThe yoke is that part of the generator which forms the outer casing of the generator and supports the main field system inside the generator. It is made of wrought iron or steel as it makes up the magnetic circuit of the main poles.The main poles are either moulded with the yoke or bolted to the yoke. Pole shoes are screwed on to the ends of the poles to hold the field coils in place and also to increase the efficiency of the magnetic path.

*Construction of the DC GeneratorThe field coils are usually cotton-insulated copper conductors and are wedged on to the pole pieces.Brush Gear. The purpose of the brushes is to collect the current from the armature conductors, through the commutator. The brushes are made of carbon and are housed in a brush box fitted with a spring, to ensure good contact on the commutator. The brush boxes are fixed on a rocker arm which allows movement of the brush positions.

*Armature Reaction in DC GeneratorTo understand how armature reaction is minimized, we must first understand two terms:Geometric neutral axis (Fig. a). This is a line drawn at right angles to the main poles.Magnetic neutral axis (Fig. c). This is a line drawn at right angles to the resultant field.NOTE: The magnetic neutral axis in a generator is always an angle of lead

*Armature Reaction in DC Generator

*Armature Reaction in DC GeneratorThis distortion of the main field by the field due to the current flowing in the armature conductors is termed armature reaction. Armature reaction causes sparking at the brushes, particularly on heavy loads, and a drop in output voltage.

*Armature Reaction in DC GeneratorPerfect (or sparkles) commutation is achieved by placing the brushes on the magnetic neutral axis, that is, in a position where the brushes are not shorting commutator segments which are connected to armature coils cutting lines of force (i.e., generating an e.m.f.).

*InterpolesInterpoles are small poles fitted between the main poles and are connected in series with the armature conductors. When the field due to the armature current attempts to distort the main field the interpoles produce a field in opposition to it (i.e., N to N). If the interpoles were not fitted, the brush position would have to be altered with differing armature currents because the magnetic neutral axis changes with changes in armature current.

*Interpoles

Polarity of Interpoles: An interpole has the same polarity as the pole in front of it, in the direction of rotation.

*D.C. Generator Field SystemsShunt Generator Operation. This is as follows:1. The prime mover (petrol engine, electric motor, etc.) runs the armature up to the required speed.2. The armature conductors cut lines of force due to residual magnetism in the main poles. The shunt generator is a 'self excited' machine.3. This initial flux cutting induces an e.m.f. into the armature conductors.

*Schematic diagram of the shunt Generator

*Shunt Generator Operation4. The e.m.f. induced into the armature conductors is applied across the field since the field is connected in parallel with the armature.5. A current flows in the field coils causing a field which strengthens the residual field.6. The armature conductors are now cutting a stronger field and the induced e.m.f. builds up until maximum voltage is attained.

*ControlThe output voltage of the d.c. shunt generator is controlled by connecting a variable resistor in series with the field (Fig. 11.6). Control is obtained by varying the field strength (e.g., weak field gives a low voltage). Generated Voltage and Output Voltage.

*Shunt Generator EquationThere are two main sources of voltage drop in a d.c. shunt generator (Fig. 11.6):(a) The voltage drop due to the current flowing through the armature conductors. This is termed an Ia x Ra drop where Ia = armature current and Ra = armature resistance.(b) The voltage drop due to the contact resistance of the brushes (Vb). This is generally about 2V.

*Shunt Generator Equation

*Shunt Generator EquationVT = Eg - (Ia x Ra + Vb)EG - generated voltageVT - terminal voltage

NOTE: All the current in a shunt generator, that is field current (If) and line current (IL), flows from the armature.IA (armature current) = IL + IF

*ExampleA DC shunt generator supplies a current of 28A at 400V. The armature resistance is 0.5 and the field resistance is 200. Assume a brush drop of 2V. Calculate (a) armature current and (b) generated voltage.Ia = If + ILand If = VT Rf If = 400V/200 = 2A Ia = 2A + 28A = 30

*Generated VoltageVT = Vg (IaRa + Vb)Vg = VT + (IaRa + Vb)Vg = 400V + (30A x 0.5 + 2V)Vg = 400V + (15V + 2V) = 417V

*ActivityA d.c. shunt generator is used to supply a current 20 A at 30V to a charging panel. The armature resistance is 0.7 and the field resistance 60. Assuming a brush drop of 2 V, find (a) armature current, (b) generated voltage.

*Characteristics of Shunt Generator The output voltage of the shunt generator drops approximately 25 per cent from no-load to full-load. Applications. The shunt generator is used for battery charging and motor car generators.

*Series Generator The series generator (Fig. 11.9) has both the field (Y-YY) and the armature (A-AA) connected in series.

*Characteristics The output voltage is dependent on the load applied: voltage increases with load.

*Application This type of generator is seldom used but can be used as a 'booster' on d.c. transmission lines.

*Compound Generator This type of generator (Fig. 11.10) contains both a shunt and a series field.

*TypesThere are two types of compound generator:(a) Cumulative compound. Shunt and series fields act together.(b) Differential compound. The shunt and series field are counter-wound to oppose one another.

*CharacteristicsThe output of the compound generator depends on the relative strength of the two fields and whether they act together (cumulative) or oppose one another (differential). The fields can be compounded to give a practically constant output voltage on all loads.

*Applications Lighting sets, marine equipment

*Activity