| GEORATOR FREQUENCY
CONVERTERS |
Guide to Power Frequency Converters
This article is intended as a basic guide for an end user
who needs to specify an appropriate power frequency converter
for an application. Georator Corporation is in a unique position
to provide an unbiased guide to Power Frequency Converters
because we provide both solid state and rotary converters.
In most situations either type may provide the desired power
but there are usually clear reasons why one type is superior
to the other for a given application.
A Power Frequency Converter is a machine that takes electrical
input power at one frequency and voltage and provides electrical
output power at a different frequency and usually at a different
voltage. An example of this is to take United States 120/208
VAC 3 Phase 60 Hertz power as an input and provide 220/380
VAC 3 Phase 50 Hertz as an output, which is nominal power
in many European countries.
As a refresher to those who are unfamiliar with terms such
as frequency and voltage, AC (Alternating Current) power
is provided as a voltage sine wave of a specified frequency.
Frequency is the number of cycles (called Hertz or Hz) of
the voltage sine wave that occur each second. AC Voltage
is defined as the potential between a point and some given
reference. For example a L-N (Line to Neutral) voltage uses
Neutral as the reference whereas a L-L (line to Line) voltage
references one line against another.
Although some devices can be and are designed to operate
on both 60 and 50 Hz input power, many are not. A synchronous
motor, for example, will run at a different speed, which
can cause problems in many systems. Other systems may overheat,
provide reduced capability, and/or have increased failure
rate if operated with a power frequency different than that
for which they were designed. Additionally, many regulatory
agencies require testing of products at the power frequency
of the country where the products will be used.
Types of Power Frequency Converters
A common misconception is that a power frequency converter
is similar to and therefore should cost about the same as
a transformer used to change line voltage.
A transformer changes the amplitude of the voltage waveform
but has no effect on the frequency. It is, by closest comparison,
only a single-stage "converter" of voltage but not frequency.
An inverter is also a single stage converter, converting
DC into AC.
All true power frequency converters provide two stages of
conversion. Implementations of this conversion scheme can
be categorized into two general groups:
- Rotary (motor-generators)
- Solid State (electronic)
The major technical difference between the two is the method
used to convert input power at one frequency into outgoing
power at a different frequency. Rotary units use input power
to run a motor, which produces mechanical energy to spin
a generator, which in turn produces the required output power.
Solid state units convert incoming AC power into DC, and
then convert the DC into the required output power. Either
method of conversion supplies an output that is acceptable
for most applications but there are differences in the capabilities
and features that make one type of converter more suitable
for a particular application.
Comparative Study
The table below outlines the strengths and weaknesses of
each approach. By analyzing the specific needs relative to
the application, the user can make an educated choice regarding
the converter type that is best for their specific application.
|
| Comparative
Features |
| Rotary |
Solid State |
| Less costly
per kW (or KVA) |
More costly
per kW (or KVA) |
| Costs do not
increase linearly with power; e.g., 3x power costs 1.5x dollars |
Costs are more
linear, e.g., 3x power costs 3x dollars (because hardware
expansion is linear). |
| More attuned
to larger applications 10 KVA plus |
More attuned
to smaller applications 1-5 KVA |
| Rugged floor
mount construction |
Generally in
equipment racks or rack mountable |
| Generally fixed
output frequency |
Highly variable
output frequency, typically 45-500 Hertz |
| MTBF: 20,000
to 32,000 Hrs. (belted) 30,000 to 60,000 Hrs. (single shaft) |
MTBF: 10,000
to 20,000 Hrs. |
| Preventive maintenance
is required, e.g., bearing maintenance, belt replacement
(except single shaft units), cleaning air intakes and exhausts |
Little or no
preventive maintenance other than cleaning fans, exhausts |
| Some installation
and setup is required, e.g., concrete pad, power circuits |
Some installation
and setup may be required, but usually less than rotary alternative |
| Some environmental
objections, e.g., audible noise, unit weight, space factor,
etc. |
Fewer environmental
objections, e.g., generally quieter, lighter weight, etc. |
| Input to the
converterís motor has lagging power factor that increases
with load. |
Input current
has high crest factor that also causes leading power factor
that increases with load. |
| Harmonic distortion
and noise on the input power is not passed to the output |
Harmonic distortion
and noise on the input power is not normally passed to the
output, some high frequency noise may be passed to output. |
| Output harmonic
distortion is moderately low, typically <4 to 5% |
Output harmonic
distortion is lower, <0.05%, |
| Low output source
impedance |
Very low output
source impedance |
| Can source heavy
overload currents 2-4 X for short periods of time, depends
upon generator windings and momentum of rotating components.
Overloads generally cause voltage reduction but not large
waveform distortion |
Can source
overloads for generally shorter periods of time, depends
upon capacitive storage in unit. Overloads may cause a sharp
rise in distortion. |
| Full load efficiency
60 to 65% on smallest units (<6.25 KVA) up to 85 to 92%
on large units |
Full load efficiency
60 to 92 % all sizes |
| Efficiency varies
with load, better with heavy loads |
Efficiency
varies with load, better with heavy resistive loads and lower
output frequencies |
|
Rotary Power Frequency Converters
Rotary power frequency converters can be further categorized
as Belt Driven, Coupled Inline and Single Shaft, each of which
have features and characteristics that make them suited for
specific applications. The next few sections describe each
of the types and their characteristics.
Belt Drive Rotary Power Frequency Converters
Belt drive units consist of a motor, generator, base, belts,
pulleys and controls. The motor power is transferred to the
generator via the belts and pulleys. The ratio of the sizes
of the pulleys determines the speed ratio between the motor
and the generator. The use of high-efficiency induction motors
and precision manufacturing can yield a low-cost, highly accurate
(± 1%) output frequency. This high degree of accuracy
is provided by properly sizing the motor such that the RPM
stays within tolerable limits and by the use of precision turned
V-belt pulleys to accurately set the pulley ratio. Synchronous
motors combined with timing belts and pulleys provide even
higher frequency accuracy but are much more costly.
A variation on the induction motor frequency converter uses
variable pitch pulleys to allow a variable output frequency.
Although this method does allow frequency variation, the range
of adjustment cannot approach that of solid state converters.
Another method of providing variable frequency output is by
using a Variable Speed Drive to control the motor speed. This
method is much more costly but does allow wider variability
and is more precise.
Control systems for belt drive converters vary widely depending
upon type and application. The minimum is usually just a motor
starter and an output voltage regulator. More complex systems
may include metering for input and output current and output
voltage and frequency; output overload protection; output control;
thermal protection; over and under voltage protection, and
many other forms of protection designed to prevent damage to
the converter and or the user's load.
Coupled Inline Rotary Power Frequency Converters
Coupled inline rotary power frequency converters are much
like the belted units described above except that power is
transferred from motor to generator via an inline coupling
attached to the motor and generator shafts. This coupling generally
requires less maintenance than belts. The inherent difficulty
with this type of unit is that no pulley ratio exists and therefore
the motor and generator will rotate at the same speed. This
limits the ability to convert frequency. There is, however,
a common application of this type of machine for line isolation
instead of frequency conversion.
Coupled inline units used as line isolators prevent noise
or other disturbances on the input from reaching the output.
Similarly, noise or disturbances on the output will not reach
the input. Many times units such as these are used to separate
delicate electronics or precise test systems from motor drives
or other devices that may cause significant power disturbances.
Single Shaft Rotary Power Frequency Converters
Single shaft rotary power frequency converters have both the
motor and generator rotors on the same shaft. These units are
manufactured as a single assembly for the sole purpose of power
frequency conversion. Instead of a pulley ratio being used
to achieve a speed differential, the single shaft unit applies
a ratio to the number of poles in the motor and generator components.
Single shaft units are generally more efficient and smaller than other rotary
converters. The increased efficiency is achieved since there are fewer moving
parts and no losses associated with belts. These units are also generally more
reliable for the same reasons. Single shaft units are more costly than similarly
rated belted units.
Solid State Power Frequency Converters
Solid state power frequency converters have very few moving
parts (usually only cooling fans) therefore they have lower
requirements for preventive maintenance. The increased complexity
of solid state units does, however, reduce their reliability
over rotary converters.
The output stages of solid state units generally use high power transistors.
Although there are a few systems that use linear output stages, most use digital
methods to produce the output power. In either case the output power usually
has very low distortion although linear output stages may be slightly better
in this respect. Digital output stages are much more efficient and therefore
are almost always used for higher power solid-state units.
The Final Analysis
Solid State units are available in sizes from 1KVA or less
to over 200 KVA. That range provides a large overlap between
the solid state and rotary units that are also built from 1KVA
to well above 200 KVA. This means that for most applications,
there are sizes in both categories that will work. The user
must then decide based upon the requirements of the specific
application which type of system is more appropriate. |
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