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Concentric belts and PWM
Winding belts may take on an alternate concentric form as shown in
Figure below.
Concentric belts.
For a 2-phase motor, driven by a sinewave, the torque is constant
throughout a revolution by the trigonometric identity:
sin2θ + cos2θ = 1
The generation and synchronization of the drive waveform requires a
more precise rotor position indication than provided by the hall effect
sensors used in brushless DC motors. A resolver, or optical or
magnetic encoder provides resolution of hundreds to thousands of
parts (pulses) per revolution. A resolver provides analog angular
position signals in the form of signals proportional to the sine and
cosine of shaft angle. Encoders provide a digital angular position
indication in either serial or parallel format. The sine wave drive may
actually be from a PWM, Pulse Width Modulator, a high efficiency
method of approximating a sinewave with a digital waveform. (Figure
below) Each phase requires drive electronics for this wave form
phase-shifted by the appropriate amount per phase.
PWM approximates a sinewave.
Synchronous motor efficiency is higher than that of induction motors.
The synchronous motor can also be smaller, especially if high energy
permanent magnets are used in the rotor. The advent of modern solid
state electronics makes it possible to drive these motors at variable
speed. Induction motors are mostly used in railway traction. However, a
small synchronous motor, which mounts inside a drive wheel, makes it
attractive for such applications. The high temperature
superconducting version of this motor is one fifth to one third the
weight of a copper wound motor. The largest experimental superconducting
synchronous motor is capable of driving a naval destroyer class ship. In
all these applications the electronic variable speed drive is essential.
The variable speed drive must also reduce the drive voltage at low
speed due to decreased inductive reactance at lower frequency. To
develop maximum torque, the rotor needs to lag the stator field
direction by 90o. Any more, it loses synchronization. Much
less results in reduced torque. Thus, the position of the rotor needs to
be known accurately. And the position of the rotor with respect to the
stator field needs to be calculated, and controlled. This type of
control is known as vector phase control. It is implemented with
a fast microprocessor driving a pulse width modulator for the stator
phases.
The stator of a synchronous motor is the same as that of the more
popular induction motor. As a result the industrial grade electronic
speed control used with induction motors is also applicable to large
industrial synchronous motors.
If the rotor and stator of a conventional rotary synchronous motor
are unrolled, a synchronous linear motor results. This type of motor is
applied to precise high speed linear positioning.
A larger version of the linear synchronous motor with a movable
carriage containing high energy NdBFe permanent magnets is being
developed to launch aircraft from naval aircraft carriers.
Synchronous motors load the power line with a leading power factor.
This is often useful in canceling out the more commonly encountered
lagging power factor caused by induction motors and other inductive
loads. Originally, large industrial synchronous motors came into wide
use because of this ability to correct the lagging power factor of
induction motors.
This leading power factor can be exaggerated by removing the
mechanical load and over exciting the field of the synchronous
motor. Such a device is known as a synchronous condenser.
Furthermore, the leading power factor can be adjusted by varying the
field excitation. This makes it possible to nearly cancel an arbitrary
lagging power factor to unity by paralleling the lagging load with a
synchronous motor. A synchronous condenser is operated in a borderline
condition between a motor and a generator with no mechanical load to
fulfill this function. It can compensate either a leading or lagging
power factor, by absorbing or supplying reactive power to the line. This
enhances power line voltage regulation.
Since a synchronous condenser does not supply a torque, the output
shaft may be dispensed with and the unit easily enclosed in a gas tight
shell. The synchronous condenser may then be filled with hydrogen to aid
cooling and reduce windage losses. Since the density of hydrogen is 7%
of that of air, the windage loss for a hydrogen filled unit is 7% of
that encountered in air. Furthermore, the thermal conductivity of
hydrogen is ten times that of air. Thus, heat removal is ten times more
efficient. As a result, a hydrogen filled synchronous condenser can be
driven harder than an air cooled unit, or it may be physically smaller
for a given capacity. There is no explosion hazard as long as the
hydrogen concentration is maintained above 70%, typically above 91%.
The efficiency of long power transmission lines may be increased by
placing synchronous condensers along the line to compensate lagging
currents caused by line inductance. More real power may be transmitted
through a fixed size line if the power factor is brought closer to unity
by synchronous condensers absorbing reactive power.
The ability of synchronous condensers to absorb or produce reactive
power on a transient basis stabilizes the power grid against short
circuits and other transient fault conditions. Transient sags and dips
of milliseconds duration are stabilized. This supplements longer
response times of quick acting voltage regulation and excitation of
generating equipment. The synchronous condenser aids voltage regulation
by drawing leading current when the line voltage sags, which increases
generator excitation thereby restoring line voltage. (Figure below) A
capacitor bank does not have this ability.
Synchronous condenser improves power line voltage
regulation.
The capacity of a synchronous condenser can be increased by replacing
the copper wound iron field rotor with an ironless rotor of high
temperature superconducting wire, which must be cooled to the liquid
nitrogen boiling point of 77oK (-196oC). The
superconducting wire carries 160 times the current of comparable copper
wire, while producing a flux density of 3 Teslas or higher. An iron core
would saturate at 2 Teslas in the rotor air gap. Thus, an iron core,
approximate µr=1000, is of no more use than air, or any other
material with a relative permeability µr=1, in the rotor.
Such a machine is said to have considerable additional transient ability
to supply reactive power to troublesome loads like metal melting arc
furnaces. The manufacturer describes it as being a “reactive power shock
absorber”. Such a synchronous condenser has a higher power density
(smaller physically) than a switched capacitor bank. The ability to
absorb or produce reactive power on a transient basis stabilizes the
overall power grid against fault conditions.
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