Understanding Solid State Frequency Converters

If solid state is determined to be the solution to your needs, then the rectifier design must be considered.

In the early days of aviation, aircraft used onboard DC batteries to power their radios and other electrical equipment. However, as planes started carrying more electrical equipment, the demand for more power capacity at the lowest weight possible led to the adoption of engine-driven DC generators and with the jet age the need for even more power and lighter weight became necessary, which spawned the development and use of 400 Hz AC power systems.

Originally, ground support operations used diesel motor-generator sets to provide the power while the plane was on the ground. While these mg-sets provided pure sinusoidal power and high overload capabilities, they did so at a fairly high capital and operating/maintenance cost not to mention the noise and pollutants to the environment. Today, with the development of the new diesel engines, which make the pollutants much less, the higher operating and capital costs are still a consideration factor.

The development of the solid state frequency converters in the mid 1980s significantly reduced all the costs (capital/operating/maintenance/weight/size) and they produced no pollutants. The only drawback to choosing solid state units was the fact they needed a 60 or 50 Hz 380/480volt input source instead of diesel fuel. However, in remote areas (where diesel units are normally required), many times it would be much cheaper to run electrical cable to where a solid state unit could be used than to pay the added capital expenditure difference and constantly spend a lot more for the diesel fuel and maintenance costs associated with diesel GPUs. The key as to which to use, is determined by the operational needs (military and extremely remote sites aside) is really the overall cost of ownership and reliability factor.

Today, the vast majority of frequency converters sold in the United States are solid state rather than rotary, which is again not always the best or most economical way to provide 400 Hz power. The decision to go with solid state or motor generators is one that should not be decided by the available annual capital budget set aside but by a study by knowledgeable and experienced 400 Hz engineers who do take into account all the factors involved in cost of ownership which includes, maintenance, reliability, equipment life, quality of power (in and out) esthetics, reputation of service by the airport or airline, noise, and pollution to the environment.

The decision of what to use or buy when it comes to aircraft power systems must be made by a truly knowledgeable, and responsible individual or body, based on all the factors and not by the initial capital cost or a budget, but by the overall cost and total operational needs.

Solid State Considerations
If solid state is determined to be technically the solution to your needs, then the following must be considered.

Most solid state frequency converters continue to use the original design which consisted of silicon controlled rectifiers (SCR) to convert AC voltage into DC voltage and then use an insulated-gate-bipolar-transistor (IGBT) equipped inverter to form the 400 Hz waveform for the output voltage. The output of the inverter section is then filtered and smoothed to meet the requirements of the aircraft, which has been published in MIL-STD 704F.

The advantage of this design is that it uses easily controlled, inexpensive components. The disadvantages are that the units can be electrically and audibly “noisy,” as well as needing frequent adjustments of their analog control circuits due to environmental conditions. The less expensive units will also need more frequent replacement of the power capacitors that are used to smooth the output waveform.

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