Frequency Conversion Guide

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  • Frequency Conversion Guide

Frequency Converter Comparison

Georator Corporation is in a unique position to provide an unbiased guide to Power Frequency Conversion because we provide both solid state and rotary frequency converters.

All true power frequency converter units provide two stages of conversion. Implementations of this frequency conversion scheme can be categorized into two general groups:

Rotary frequency converter 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 frequency converter units convert incoming AC power into DC, and then convert the DC into the required output power.

Comparative Study of Rotary Frequency Converter vs. Solid State Frequency Converter

The table below outlines the strengths and weaknesses of each conversion approach. By analyzing the specific needs relative to the application, the user can make an educated choice regarding the frequency converter type that is best for their specific application.

Comparative Features of Frequency Converter Units

Rotary Frequency Converter

Solid State Frequency Converter

> 5 KVA, Less costly per kW (or KVA)

> 5 KVA, More costly per kW (or KVA)
(in 1-3 KVA sizes, solid state tends to be less expensive)

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 converter's 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 2-4X 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

The Final Analysis

Solid State frequency converter units are available in sizes from 1KVA or less to over 300 KVA. That range provides a large overlap between the solid state frequency converter and rotary units that are also built from 1KVA to well above 300 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 frequency conversion system is more appropriate.

Georator Corporation has over 45 years experience in engineering electric power frequency converters for specific customer requirements.

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