1. Chapter 16
DC Generators

2. Objectives After studying this chapter, you will be able to:
Explain the theory of electromagnetic induction
Describe the basic operation of the AC generator
Describe the construction and operation of various types of DC generators

3. Objectives (cont’d.)
Describe the uses and operating characteristics of various types of DC generators
Discuss the methods of connecting DC generators and the basic troubleshooting procedures

4. Electromagnetic Induction
Takes place when a conductor moves across a magnetic field or when a magnetic field moves across a conductor
Electromotive force induced in the conductor
Direction of emf determined by using left hand rule for a generator

5. Generator Construction
Basic generator
Consists of a permanent magnet mounted on a frame (yoke)
Coil of insulated wire mounted on iron core (rotor or armature) is positioned between magnet poles
Armature rotates through magnetic field

6. Generator Construction (cont’d.)
Amount of emf produced depends on
Strength of main field flux
Number of loops of wire on armature
Angle at which armature coils move across lines of force (right angles produce most voltage)
Speed of coil rotation

7. Generator Construction (cont’d.)
All rotating generators produce an alternating emf
Armature construction
Two types of windings: lap and wave winding
Lap winding is used to obtain high current capacity
Wave winding is used to obtain high voltage output

8. Generator Construction (cont’d.)

9. Generator Construction (cont’d.)
Core is made of soft iron or steel disks called laminations
Disks are dipped in insulating varnish and mounted on the rotor shaft
Armature core slots are lined with insulation called fish paper
Commutator made of copper segments

10. Generator Construction (cont’d.)
Brushes
Connect the commutator to load conductors
Made of graphite and carbon
Frame and field poles
Field cores are attached to the frame which provides support and forms part of circuit
Field coils wound with cotton covered wire with a baked enamel insulation

11. Generator Construction (cont’d.)
Field excitation
In all generators field flux is produced by current flowing in coils placed on the field cores
Except magnetos
Separately excited generator
Field is excited from a separate source
Self-excited generator
Current is obtained from machine’s own armature

12. Generator Operation Effect of armature current Neutral plane
Armature reaction results when main field is distorted as a result of the interaction between the two fields
Neutral plane
When the armature coils are moving parallel with the lines of force
Moving through the neutral plane
No emf is induced

13. Generator Operation (cont’d.)
Armature self-induction
When current increases, magnetic field increases, expands and moves across coil loops, inducing an emf into the coil
Voltage of self-induction opposite to the applied voltage
Interpoles (commutating poles)
Method of adjustment to changing loads

14. Generator Operation (cont’d.)
Compensating for armature reaction
Compensating windings placed in main pole faces to eliminate armature reaction
Other effects of armature current
Magnetomotive force that opposes the main field flux is produced, decreasing the generated emf
Reversed torque developed in the rotor

15. Generator Voltage
Equation used for average emf produced by a generator

16. Generator Voltage (cont’d.)
Saturation curve
Beyond a certain number of ampere-turns, all electromagnets become saturated
Near saturation point
Large increase in current causes only a slight increase in voltage

17. Self-Excited Generator
Three types: shunt, series and compound
Difference is how the armature and field windings are connected
Self-excited generator may fail to build up a voltage, causes include
Loss of residual magnetism
Break or opening in the field
Loose brush connections or contacts

18. Self-Excited Generator (cont’d.)
Shunt generators
Field and armature connected in parallel
Series generators
Field connected in series with the armature
Compound generators
Combines certain features of shunt and series generators into one machine
Better for varying loads

19. Separately Excited Generator
Magnetization current for field coils is supplied externally
From DC generator, batteries or rectifier
Field current is independent of armature emf
Field flux is less affected by load changes than in the self-excited generator
High cost and large physical size

20. Voltage Control Versus Voltage Regulation
Voltage regulation determined by machine design
How well the generator maintains constant output voltage under changes in load
Voltage control takes place outside the generator
Rheostat controls the current through the shunt field

21. Parallel Operation of Generators
Shunt generators in parallel

22. Parallel Operation of Generators (cont’d.)
Compound generators in parallel

23. Generator Efficiency
Three major losses: mechanical, electrical and magnetic
Mechanical losses
Friction at bearings and between brushes and commutator, and winding losses
Electrical losses
Resistance of the field and armature conductors

24. Generator Efficiency (cont’d.)
Magnetic losses
Reluctance in the magnetic circuit: eddy currents and hysteresis
Hysteresis can be reduced by selecting core materials with good permeability

25. Summary
A generator is constructed of a magnet, yoke, coil, and core (armature)
Shunt, series and compound generators are types of self-excited generators
A separately-excited generator is less impacted by load changes than a self-excited generator
Armature current affects the voltage

26. Summary (cont’d.)
Generator voltage may reach a saturation point in which large increases in current produce only small increases in voltage
Generators may be connected in parallel to increase the generated voltage
Generator efficiency is influenced by mechanical, electrical and magnetic losses