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Electric
Generator
Introduction
An electric generator converts mechanical energy into
electrical energy. When an
electrical conductor experiences an external
magnetic field
of changing
intensity, the changing magnetic flux induces
an
emf
(1) in the conductor (
Faraday's
law). The emf drives a current in the conductor
that sets up a magnetic field that opposes the change in the flux it
sees (
Lenz's
law,
Ampere's
law). If the change in flux is the result of motion,
conversion from mechanical to
electrical energy occurs.
Conventional generators use coils of
copper
wire for the conductor and
magnets to supply
the magnetic
field. In one configuration, the magnet poles rotate around an
axle
and the coils are
fixed in proximity. When a pole rotates near a
coil,
its magnetic field penetrates the center (core) of the coil and loops
back
around. The concentration of flux through the coil core changes
with the rotation and induces the emf in the coil (see also
inductance,
inductor).
The magnetic field setup by the current that is driven by the emf
opposes the flux
change and so
the
coil opposes the pole's motion, i.e. the coil repels the
approaching pole and attracts the departing pole through
Lorentz force.
If the coil is
open-circuit, the induced emf cannot cause a continuous current/field
to oppose the pole's motion. The coil merely instruments
with its open-circuit voltage the velocity of the free-spinning
pole. But if the coil is
close-circuit,
it absorbs energy from the pole's momentum and stores it in its field
while its current circulates.
The
energy
stored in the coil is related to its inductance and its
current. The coil current depends on the induced emf, the coil
inductance and coil and load
resistances. The induced emf is related to the flux rate of
change seen, i.e. the magnet power and its angular velocity. The
resulting coil
energy is available for transfer to an electrical
load. Energy transfer is related to load
resistance. If the
load
resistance is decreased, then the current flow, the opposition to
motion,
and the energy transfer are all increased (2).
The dipole nature of magnets is very convenient for creating a
constantly changing magnetic field. When a magnet's north and
south poles
alternately rotate near a fixed coil, the alternating field intensity
seen by
the coil is
sinusoidal, and the field intensity time rate of change and the
induced voltage/current are sinusoidal too. The sinusoid is the
preferred power waveform because its energy is concentrated in a narrow
frequency band to which the generator design may be optimized for
efficiency.
(1) Emf, or electromotive force, with units of volts, is actually an
energy, not a force.
(2) In an application with a fixed coil resistance and adjustable load
resistance, maximum energy transfer with 50% efficiency occurs when the
load
resistance equals the coil resistance. If coil resistance can be
changed, then the lower the coil
resistance the better for both energy transfer and efficiency.
Coil
and load reactances should aways be matched, if possible, to minimize
reflection for maximum energy dissipation in the load resistance and
minimum dissipation in the coil resistance.
Permanent Magnets and Electromagnets
Magnets are objects that supply magnetic fields useful for
mechanical/electrical energy conversion. A simple electric
generator consists of a
permanent
magnet
rotating on an axle and a nearby fixed-position
coil.
Electromagnets
may be used in
place of permanent magnets in a generator when the cost of permanent
magnets is
prohibitive. But electromagnets are less efficient because they
draw
power, they are more bulky, less reliable, and they require
commutator brushes on the axle and a priming battery and voltage
regulator.
Ferrite (ceramic) is currently
the most widespread and cost-effective permanent
magnet material. But recent advances have produced permanent
magnet materials with very high magnetization. Among the
advantages are
reduced generator size, reduced rotor mass and reduced centrifugal
forces at
high speeds. Currently the highest-magnetization material is
Neodymium. Its drawbacks include a license fee, brittleness,
flammability, and permanent loss of magnetization with heat
(170°F-350°F / 20°C-80°C).
Permanent magnets are made by applying a magnetic field to a magnetic
material. The strength of the applied field should be high enough
to ensure saturation in the material. The material's remanence
describes how well it retains its magnetization after the external
field is removed. The material's coercivity determines how strong an
external field is needed to remove the magnetization. These are
sometimes quoted in units of Teslas. These together describe the
quality of the resulting permanent magnet. There is also a
quality factor sometimes specified in kJ/m^3. Magnetic materials
suitable for permanent magnets are referred to as "hard" while those
suitable for coil cores are referred to as "soft".
Permanent
Magnets
Oersted Technology:
About Magnets
Methods of Magnetizing
Permanent Magnet (pdf)
Coercivity
and Remanence in Permanent Magnets
Multiple Magnets and Coils
Multiple magnets and coils reduce cyclic mechanical
stresses by spreading around the axle and balancing the magnetic forces
between
the rotor and the stator.
Odd/staggered phases reduce
starting/
cogging torque by reducing the
number of magnets that are aligned with coils at any rotational
angle. Staggered phases also reduce wire thickness/weight needed
in transmission and application
by 50% or more.
When multiple magnets and
coils are placed
around an axle it is necessary to alternate the polarity of the magnets
so the coils see a continuous change of flux polarity during axle
rotation. This requires that the number of magnets
be even. In a
single phase
generator, the number of coils equals
the number of magnets (north/south pole pairs) (see
figure). Multiple coils around
the generator are wired in series or parallel but with alternating
polarities
consistent with the magnets so that the induced voltages or currents
sum
together. In a
staggered phase
generator with N phases and M
magnet pole pairs, N groups of M coils are formed by wiring together
coils that are offset
by M around the
generator. N should be odd to
minimize starting/cogging torque. Wiring phase group coils in
series
sums their voltages and wiring in parallel sums their currents.
The N phase groups may be connected in a series loop with the two ends
of each phase group comprising one phase tap. The loop should run
in a
continuous direction around the generator, selecting phase group ends
that
preserve current direction in the loop. This loop configuration,
called
delta, gives phase tap
current equal to the square root of N times the phase group
current. In a different configuration, the N
phase groups are terminated together at associated ends consistently
spaced
in one
direction around the generator. Successive phase taps are between
successive free group
ends
spaced similarly around the generator. This configuration, called
star, gives phase tap voltage
equal to the square root of N times the phase group voltage.
Further characteristics of star and delta are discussed
here.
In any configuration, connecting wire lengths should provide precise
resistance matching for all the coils
when measured at the generator terminals. Symmetry is
important throughout the generator design.
Dimensions
By Faraday's law, the emf induced in a coil is equal to
the number of
coil turns times the magnetic flux time rate of
change seen. The flux rate of change is the product of the
magnet's power and its
tangential velocity,
which is proportional to the axle rotation rate times the magnet's
radius, or distance from the axle center. In the case of
sinusoids, the coil current magnitude is
related to the emf by the coil
inductance, and the
load voltage magnitude is calculated from the current magnitude and the
coil and load resistances. The output power is the
voltage times the current, or the product of the coil turns, magnet
power and radius, rotation rate, and load resistance divided by the sum
of coil and load resistances.
Practical coils have finite resistance but this
should
be kept as low as possible to minimize energy loss and maximize
efficiency of energy transfer. For a given energy
transfer rate, or range thereof, and source and load impedances, the
best compromise in coil resistance may be chosen.
The larger the core
area,
the smaller the core flux density, resistance and heat. Of
course,
with heat comes power loss and material degradation. The
generator
circumference
should be filled out by coils to maximize power
output and by cores to reduce core flux density.
The coils' second dimension (either radial or axial) is then extended
to approximate the magnet's second dimension. The coil should be
long in the dimension perpendicular to the rotation as almost all of
the coil's magnetic force interaction with the rotor is concentrated in
this dimension. The coils' radius
of curvature should be maximum to minimize stress on the
insulation.
If available mechanical power comes at low rpm, the generator radius
can be
increased toward maximum power output until dimensional or stability
limits are reached. Then coil and core can extend in the coil's
third dimension to fully exploit the
power source.
Heat
from coil electrical resistance limits the number of turns but the
wire gauge can be increased to accommodate more turns, limited by the
additional flux path length creating more core resistance and heat.
If available mechanical power comes at high rpm, the generator radius
can be reduced to minimize centrifugal stress and tangential velocity
and associated frequency-dependent electromagnetic losses. The
generator axial length can then be extended to fully exploit
the
power source.
If available mechanical power comes at both high and low rpm then the
dimensions may be chosen for the average rpm. This means
additional coil/core heat at high rpm and additional magnetic drag at
low
rpm. Reducing the core density/continuity mitigate both issues
simultaneously while additional rotor fanning and mass mitigate them
individually.
If the flux through the coil cores is parallel to the axle, the
arrangement is called axial flux, and the other arrangement is radial
flux. Axial flux can exert axial forces on the bearings and
should
only be used if the force is to be canceled by other axial
forces. See
Axial Loads.
Wire Gauge
In light of the above, to achieve maximum power transfer while
minimizing space and materials,
a
smaller wire gauge is used to enable more coil turns and thus deliver
more
power in the same space. But wire resistance, even with copper,
can create
significant heat in
longer lengths of smaller gauge wire, wasting
energy and stressing materials. (see
magnet wire)
If the load resistance is high during
maximum
sustained power transfer then high
voltage is needed to transmit available power and the coil turns should
be greater, the
wire gauge smaller and the coil insulation strong to withstand the high
voltage. If the load
resistance is low during maximum sustained power transfer then high
current is needed to transmit available power and the coil
turns should be fewer and the wire gauge larger to withstand the high
current.
If the time rate of flux change is high then the wire gauge should be
small to minimize the
skin effect.
But if the load resistance
is low, more coils with
fewer turns may be configured in parallel to better match the load
resistance for maximum power transfer.
Wire
Gauge Table
Magnet Wire
and Coil Winding
Magnet wire
is usually made from copper with a coating of enamel insulation.
The
insulation is vulnerable to various stresses and contaminations which
deteriorate the insulation and lead to eventual coil failure.
Commercial priority has been on compact coil size which may compromise
insulation thickness and longevity of the
coils. To maximize the
longevity of the coils, the
most reputable wire vendor should be found and the highest
thermal classes and insulation builds should
be considered.
The energy transfer in the generator involves a
force between
the
rotating magnets and the coil wires. This force is a sinusoidal
push/pull in the direction of rotation on the wire lengths that are
perpendicular to the rotation. The spacing and tension of coil
winding
should be consistent to prevent concentrated areas of stress in the
coil from this force. The coil should be supported on the two
appropriate sides by the core that surrounds it and
epoxy potting
may
be
the best way to ensure the coils maintain rigid support.
The coils should be carefully wound to ensure consistent
winding tension, minimizing both vibrations
and premature insulation failure. Winding should take place in a
dust
and humidity-free environment. Winding tension specifications should be
heeded. Avoid touching or
otherwise contaminating the wire.
Dust, dirt, metal filings, oils,
humidity and
external
vibration, heat and light decrease heat dissipation and/or react with
or stress the enamel and contribute
to insulation deterioration. Options for mitigating these
include potting coils in epoxy or
varnish. Potting also
reduces induction of high frequency harmonics produced by individual
windings of smaller gauge wire
prone to independent vibration. Potting also supports the coils
against the forces exerted by the magnets.
Femco
Magnet Wire
Coil Cores
Iron and other ferrous materials have very high magnetic
permeability,
which allows their magnetization by external fields to add greatly to
those fields. To maximize magnetic induction in the coils, iron
cores may be built up through and around the coils and magnets, to form
a dense magnetic flux circuit when a magnet and
coil align through rotation.
But iron conducts
electricity so
eddy
currents are induced in the core perpendicular
to
the magnetic flux. Iron's relatively high electrical resistance
converts a lot of
this current into heat. The heat draws energy directly but also
by decreasing the iron's magnetic permeability, e.g., 25% loss
from
20°C to 80°C. So it is necessary to slice the core up
into
thin laminates, electrically isolated from each other by a coat of
lacquer. Higher rates of magnetic
flux change (higher rotor frequency) produce smaller
eddy paths so then it becomes necessary to reduce the laminate
thickness or
consider tape-wound cores, or
powdered
iron cores in which dielectric
between the iron
granules provides gaps that block the smaller eddy
current loops. Increasing the core
area can reduce eddy currents by reducing the flux density but
mechanical issues usually limit
this approach.
Further heat is produced in the core as the core
molecules resist changes in their magnetization. This resistance
to change, known as
magnetic
hysteresis, is greatly reduced by mixing of silicon in the iron and
annealing
it.
The core would ideally extend the entire loop between the magnet pole,
through the coil and back around to the magnet's opposite pole
except for the gaps that allow the
magnet to rotate. The smaller the gaps the greater the magnetic
flux density but other
parameters may be increased to compensate for larger gaps that allow
for magnetic balancing adjustments on the rotor. If power volume
density (compact size) isn't a priority the loop
may have further discontinuities that simplify fabrication with power
requirements being met by other means. The optimum sum total gap
in a core loop aligns the core's magnetic saturation point with the
generator's
maximum rated speed.
Mechanical
& Thermal
The generator mechanics must be rigid and moving parts balanced to
minimize vibration. Vibration wastes energy and produces excess
heat, wear and noise, reducing the efficiency and life of components,
especially the coils and bearings. The materials must be
chosen to withstand
thermal and mechanical stress to maintain rigidity and balance.
The axle should be carefully machined/balanced, the magnet assembly
(rotor) should be securely attached and the rotor/axle assembly
carefully
balanced.
For high speeds the magnets must be tightly secured
against centrifugal (outward radial) force. For high
power, the magnets as well as the coils and cores must be tightly
secured in the magnetic flux direction (radial or axial) and
tangentially. Most magnets are too brittle to be bolted and must
be glued with strong
epoxy.
Care should also be taken to ensure
electromagnetic balance with consistency among the magnets and coils,
in materials, construction and position. The coil assembly should
have 3
dimensional setscrew adjustment to secure
it at the composite mechanical/magnetic equilibrium
position at the most prevalent operating conditions.
For high speeds the rotor surfaces should be continuous and
smooth to minimize
windage
loss. This can
be accomplished with epoxy filler around the magnets, mixing
other non-magnetic materials to reduce the quantity of epoxy.
If
running at high speed the axle bearings must be well chosen,
aligned
and lubricated. When magnets/coils face radially, most bearing
force is radial. When magnets/coils face axially, bearing forces
are both
axial and radial. See
Axial
Loads.
Excess heat may be created by ambient conditions, electrical
resistance in the coils
and cores, eddy currents in the cores, and vibration. Excess heat
weakens the insulation
on the coil wire and core laminates, and with vibration, causes
it to shed and the coil to weaken or fail.
More on
magnet wire. Excess heat
prematurely breaks down bearing lubricant and thus bearings too.
Heat adversely affects the power of magnets. Internal heat should
be
minimized by design but can be mitigated by
rotor-induced air
turbulence.
The bearings and lubricant
must remain cool and
clean. Excess volume or density of
lubricant can create excess load and heat. Proper seals are very
important for keeping out contaminants
because
contamination is a leading cause of premature bearing failure.
To maximize
the generator life, inspect and replace seals, bearings and
lubricant in a timely
fashion and avoid contamination.
More on bearings.
Motor
Temperature Ratings
Axial Loads
In an axial flux configuration, the interaction
between the magnets and the coils/cores presents axial loads on the
magnet rotor, coil stator and the bearings. The interaction
varies with the electrical load. To cancel the
axial loads,
either a pair of rotors on either side of
the stator, or a pair of stators on either side of the rotor will
suffice.
If the turbine is on the same shaft (highly preferable) then its steam
ejection might also
present an axial
load on the bearings, depending mostly on the outlet design. Even
if the outlet cancels the axial load this may
vary with the fluid throughput. The average load may be
canceled by making the generator
stator-rotor gap on one
side wider than the other. This will reduce the
generator's power compared to minimizing both gaps.
But this is a legitimate trade-off because the additional
materials required to make up the difference will be paid for easily in
savings to the bearings.
Another option is to use the stator pair configuration while having the
power electronics vary the electrical loads on the two stators to
maintain the axial load balance as the speed changes.
Construction
Forms should be made that enable consistent tension and spacing
of coil wire during winding. The wire should remain as clean as
possible, no touching, no excess humidity or temperature extremes, no
dust/dirt.
Cogging
Torque
Cogging torque is created by the magnetic attraction between the
magnets and coil cores of a motor/generator. These disruptions to
the smooth rotor motion can make the motor/generator difficult
to start and run at low speeds. At higher speed, the rotor
momentum overcomes the cogging torque. The coils tend to
cancel the cogging torque during rotation by presenting an opposing
torque. For easiest startup, cogging torque may be reduced by
an odd/staggered-phase design. This divides the total power
between the phase groups and limits the number of
groups aligned with magnets at any time to one. Cogging torque
might be
further reduced by up to 50% by arching the magnet surface facing the
coil.
Permanent
Magnet Motor
The
permanent
magnet (PM) AC generator
becomes a PM AC
motor,
or synchronous motor, when power
sinusoids are applied to its electrical terminals. This duality
is
utilized in electric vehicle drive motors/regenerative brakes.
Most
AC
motors are induction motors, with
coil stators and squirrel-cage rotors. These rotors consist of
heavy
conductors and laminates. A magnetic field is setup in the rotor
by induction from the stator. The induction motor suffers
significant losses in
creating the rotor magnetic field compared to the PM rotor, which
utilizes the potential energy in permanent magnets. Typical
efficiency of an induction motor is 86%. Typical efficiency of a
synchronous motor is 93%.
Synchronous and induction AC motors are
brushless and therefore more reliable/efficient than
DC motors
and
universal
motors. This is due to non-linear power
waveforms
created by brushed commutators, which also produce ozone
and noise, but allow a lighter, more compact motor.
The synchronous motor is
commonly used in servo
applications and is usually driven by a solid-state
variable
frequency drive (VFD) that enables dynamic control over the motor's
torque and speed, allowing smooth starts/stops that can
greatly extend the life of
components.
Many synchronous motors/drives use an rpm sensor to maintain speed
under changing loads, but sensor reliability can be an issue.
Another
approach is
to sense the phase of the
back-emf
from the motor. When the load changes,
the speed changes and back-emf phase changes with
it. Sensing the phase change allows the control
system to correct the speed or optimize the speed/voltage as the load
changes. A similar approach is
Direct
Torque Control.
It may seem that the power supply must work
against the motor's back-emf. But back-emf draws no power because
there is no associated current. The back-emf simply changes the
reference for the applied forward voltage. The current draw is
derived from the voltage difference and the circuit resistance.
Most large
AC motors are three-phase. Multi-phase
motors are self-starting, quieter and more efficient. Most small
AC motors are
single-phase mainly due to relatively high wiring/connector
costs. Odd/staggered
phase provides similar benefits to both motors and
generators. For efficiency and
reduced stress, the power drive should be well-balanced
electrically and the motor balanced thermally, magnetically,
electrically and mechanically.
Permanent Magnet Synchronous Motor
Appliance motors turn green
References
All About
Circuits: Volume II - AC
Three-phase
power systems
Motor
Maintenance
Coil
Winding Options
Updated: FILEDATE
Copyright (c) 2005, 2009
Robert Drury
Permission is granted to
copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts.
See "GNU
Free Documentation License".
Disclaimer: This information may contain inaccuracies and is
provided
without warranty. Safety first when working with high
temperatures,
pressures, potentials, speeds, energies, various
tools and materials.