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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 selfexcited 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