Chapter 16

DC Generators

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

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

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

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

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

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

Generator Construction (cont’d.)

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

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

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

Generator Operation

• Effect of armature current– 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

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

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

Generator Voltage

• Equation used for average emf produced by a generator

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

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

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

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

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

Parallel Operation of Generators

• Shunt generators in parallel

Parallel Operation of Generators (cont’d.)

• Compound generators in parallel

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

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

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

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