First-Hand:Electrical Power Conversion
Electrical Power Conversion
Submitted by Harold T. Adkins
Soon after graduation in 1950 I joined the Army Research and Development Command and formed and ran a laboratory that had the job of introducing electronics, servo mechanisms, computers, pulsed light rangefinder's (Optar—Lidar), stabilized tank turrets, infrared detectors and suppression, gun fire control systems, nuclear science and weapons, etc. into a comparatively non technical military operation. One of the biggest problems I found was the low efficiency and power level of servo mechanisms and power circuits. I developed a new technology of bang- bang (full on, off and reverse) switching circuits and presented a paper to IEEE on that subject. Previously all most all electrical and electronic circuits were low power, Class A and had an efficiency of about 50% since all they had was low power transistors, vacuum tubes or ignitrons which could not switch fast enough. With full on full off switching of a high enough frequency or pulse width modulation you could achieve proportional control at close to 100% efficiency and a high power level. Developing Tank Stabilization so that you could aim the main guns while traveling at full speed across country involved a lot of finely controlled high power and the opportunity to apply the higher efficiency servos really paid off.
When I later went to Curtiss Wright on the Dart missile as chief engineer I had the opportunity to apply this technology to the missile and ground control equipment and later as head of new product development I developed and patented high-speed alternators directly connected to gas turbines. It was desirable to produce normal 60 or 400 cycle power at high efficiencies from variable speeds drives and high frequencies with small high speed, lightweight generators. High efficiency electrical switching power conversion was developed for military equipment and has provided the background for all almost all power conversion and switching power supplies to the present day. The highest power solid state devices at the time were the new silicon controlled rectifiers (SCRs) which had multiple kilowatt capability and low power loss. I switched pulses of the high-frequency power(at least 10 times of power output frequency desired) and the cycles or parts of cycles were added until the integrated (filtered) power were of the desired power waveform and frequency. This design concept permits a very high-efficiency circuit in that switches are all on or off thus dissipate very little power and the high frequency allows small, light, efficient components. Presently almost all power conversion uses a high-frequency transformation and control to the desired voltages which means a small transformers and small chokes and capacitors for filtering. Switching a DC voltage on and off with pulse width modulation or by choosing the number of on pulses to get a sine wave is usually used to efficiently generate an efficient power sine wave today.
High efficiency electrical power transmission systems use DC, especially in undersea cables where the distributed inductance and capacitance would cause extreme losses for alternating current. They also use switching power conversion systems to get back to AC from the DC. The DC is converted to a high-frequency and high-voltage AC and pulse width or pulse selection convert the power back to 60 cycle power line frequencies. Electric car power converters use semiconductors to convert battery voltage to variable frequency AC feeding an induction motor drive system using the same conversion system. This allows a very high-efficiency, high power density converter system.
I regret it is that I did not follow-up on the power switching, pulse width and cycle selection advanced technology power conversion systems that I developed and predominates today. It would be many years before this advanced technology was applied in industry. It is now the standard design from milliwatt to megawatt power supplies and inverters. Electrical power grid DC transmission systems convert dc back to ac with high efficiency by this technique.
Class A power amplifiers use a plus and minus bus and let proportional valves carry proportional current off the bus to the output resulting in about 50% efficiency. The concept of integration or filtering the output of electrical power circuits is good engineering and it allows full on-off high-efficiency switching to achieve a sine wave output or regulated DC from DC or high frequency AC power. It is also used in analog computers which essentially consist of integrators and differentiators. We also developed the last powerful analog computers for designing and optimizing tank suspension and gun stabilization systems for military tanks. Most system functions can be simulated by analog differentiators, integrators, addition and subtraction.
It was really wonderful to be part of the new technology world opened up by electronics, the transistor and solid state physics opened up by quantum mechanics, by integrated circuits, computers, data communications, servo mechanisms, automation and the systems concept. It was an exciting challenge and interesting world. I now look forward to being a part of the new energy world called Quantum Energy, Atomic Transmutation Energy (ATE) or LENR.
The new switching power amplifiers and power supplies have efficiencies of 98% and involve class D and switching with IGBTs and SCRs and usually involve high frequencies to keep transformers and filters small, lightweight, low cost and efficient. As far as I know this idea was first used in equipment covered by my patent 3,173,076 filed in1960 and granted in 1965 and patents 3,187,188 and 3,157,793. They converted a 10 to 60 Khz power frequency to 50 to 400 Hz output power or dc using selected one half cycles or parts of cycles using efficient, small, light weigh, low cost components. Silicon controlled rectifiers are shown in the drawings but IGBTs could also be used. The equipment was extremely small, lightweight and efficient and delivered up to several megawatts. One of the uses was to clear ocean magnetic mines which recognized and were triggered by the magnetic signatures of large ships passing over the mine and not small ships. Clearing required high power and a waveform at a fraction of a cps. The power was generated by gas turbines driving high frequency alternators aboard minesweepers and the circuitry could generate any desired waveform down to direct current. The power fed a towed magnetic field device fed by the conversion circuitry. By using the high frequency output of the high speed flux switch generators ran at turbine speed the systems were small, light and efficient. These flux switch generators ran at speeds up to 100,000 rpm, generated up to 15 Kw per pound, generated frequencies up to 60khz and were ideal power for switching power converters. The generation and electrical power converters weight, size, efficiency and cost are directly proportional to the frequency which can be several megahertz. Later many more uses were found for the power conversion system and it is ideal for variable speed drive systems such as vehicle drives.