Frequency Converter Comparison

Frequency converters provide two stages of conversion. They fall into the category of either rotary, solid state (fixed frequency and adjustable frequency converter), uninterruptible power supply, or permanent magnet generators.

Rotary frequency converters use input power to run a motor.  This provides energy to spin a generator, which then produces the output power. Solid state frequency converters convert AC power into DC.  The DC is then converted to the required output power.

Rotary vs. Solid State Frequency Converters

There are unique benefits to rotary and solid state frequency converters.  Analyzing your specific application and needs prior to choosing your approach will allow you to choose the best fit.

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)Text

MTBF: 10,000 to 20,000 Hrs.Text

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 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

Additional Benefits of Solid State 400Hz Ground Power Units vs. Rotary

  • Diesel motor generators have high ambient noise
  • Diesel motor generators burn fuel
  • Diesel motor generators create undesired emissions
  • Diesel motor generators are mechanical and require regular lubrication of the rotating parts.
  • Solid state frequency converters are often a less expensive alternative for diesel motor generators
  • Operating costs are significantly lower for solid state frequency converters than motor generators.  The savings occur because fuel is not required and mechanical maintenance is not required