Avionics power

Avionics is the term for 'aviation electronics'. These electornics devices are for example used for:
Airplane control
Status monitoring (temperature, pressure, valve positions and so on)
Communication (radiosystems)
Navigation (artificial horizon, compass and so on)
Weather (like a storm radar)
anti-collision systems

 Avionics voltages
The majority of avionic devices are powered using these electrical power:
28 VDC for instruments functinality
5 VAC for dial illumination

But more sophisticated aircrafts like airliners and military aircrafts use more complex power systems:
115 VAC 400 Hz (single phase)
115 VAC 400 Hz (three phase)
28 VAC 400 Hz (single phase)

AC vs DC
Alternating Current (AC) is used for functionality. Position indicators of for example gyroscopes use synchro's and/or resolvers. Synchro's and resolvers need a reference AC signal to be able to work. The good thing about synchro's and resolvers is that positions can tramsitted and received with high (analog) accuracy. There are no wearable parts in synchro's and/or resolvers like moving contacts and they can withstand a high amount of vibration. Therefore synchro's and/or resolvers are expensive and very reliable for avionic and military use. The gyroscope position can be transitted to the cockpit indicator using synchro's and/or resolvers which use the same AC refernce signal.

400 Hz frequency
400 Hz AC systems are 'the standard' in avionics nowadays. This seemds rather odd since the 'rest of the word' used 50 or 60 Hz af a default powersupply frequency. The main reason to use 400 Hz instead of 50/60 Hz is that transformers much smaller and therefore weigh less than 50/60 Hz transformers. Even electric motors have approximately (400 / 50 = ) eight times more power in the same size than the 50 Hz equivalent. Due to standardisation for crosscompatibility 400 Hz is 'the standard' to use.

 dynamotor inverter
A rather 'old school' system of generating three phase 115 VAC 400 Hz signals is with a dynamotor inverter. A dynamotor is a device that has a electric motor connected to a generator. The word dynamotor is a composition of dynamo (generator) and motor. A (DC) motor is mechanically connected to a generator that converts the mechanical energy in electrical energy. So no semiconductors or other electronics is needed. Dynamotors were used in old army transceivers to obtain high voltages and (low) filament voltages for the vacuum tubes in the radio. For avionics these dynamotors were used to obtain for example three phase voltages out of a single phase direct current source. The positive side is that these dynamotors are usually very stable and have a ling lifespan. The downside is that dynamotors are very noisy (mechanically) and the neutral is missing likely.

To speed up the process of reviving the Panavia Tornadi TV TAB display unit, I bought a dynamotor. This was the easiest step to create 115 VAC 400 Hz three phase power. The creation of the variable frequency drive (VFD) power supply will probably take more time. This dynamotor is a nice intermediate step.

I bought a 'Class A Inverter' from the manufacturer 'Eicor Inc.' The device is marked as built by drawing number '1-100A115D-7'. The input voltage is 27,5 VDC according to the type plate and the current draw is 9,2 Amps thus consuming 253 Watts of energy. The output power is three phase 115 VAC 400 Hz power. According to the type plate the dutycycle is continous, the power factor is 0,95 and the available power is 100 VA.

115 VAC is the usual voltage for avionics. 115 VAC is measured between line and neutral. To obtain the line-line voltage, the line-neutral voltage (115 VAC) has to be multiplied by the square root of three. The square root of three is also the same as three to the power a half (3^0,5). Therefore 115 VAC * 3^0,5 = 199 VAC. Thus the line-line voltage is approximately 200 VAC between the lines.

To calculate the maximum current per phase, the following formula is needed: I = P / (√;;;3 pf V)
Where I is the current, P is the power in VA, pf is the powerfactor and V is the (line-neutral) voltage. Filling the formula, I got this result: I = 100 VA / (√;;;3 0,95 115 VAC). This means I = 528 mA per phase. I guess '100 Watts' of power thus 528 mA per phase should be sufficient for now. And the solid state VFD power supply will replace this dynamotor eventually...

 400 Hz PSU: Static inverter
The most ideal way to get 115 VAC 400 Hz three phase power is by using a so called static inverter. Usually the static inverter needs 28 VDC to generate the three phase power. Since static inverters are relatively expensive for hobby use. Avionics equipment is professional high quality stuff and therefore usually rather expensive unless broken of outdated. An example of a static inverter is shown below. These inverter are sold on eBay for approximately 500 each. Remind that there are also one phase versions, 1 kHz versions and low current versions. If you want to buy an off the self static inverter, beware to chose the one with the desires specifications!

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There are also homebrew alternatives which maybe are also interesting. See the ideas in tha paragraphs below.

 400 Hz PSU: Motor drive solution
A ready to use 115 VAC 400 Hz three phase power supply is called a static inverter. Usually the static inverter needs 28 VDC to generate the three phase power. Since static inverters are very expensive I was looking for more affordable alternative. First I thougt of a three phase audio amplifier setup. But to get a large amount of power three rather big amplifiers are needed. And I expect a voltage drop when current draw rises... Then I found another idea!

variable frequency drive solution
By using a industrial motor drive, it's possible to generate a 250VAC three phase signal with a rather large amount of current. I had two motor drives in the 'junk box' and one of them can generate signals up to 400 Hz! (The other one can generate 50 Hz at maximum.) I connected a three phase transformer to the secondary side of the frequency drive to 'create' a neutral since a frequency drive has no neutral of it's own. I also use the three phase transformer as a step down tranformer to get 115 VAC from 250 VAC. The frequency drive can also lower the voltage, but the step down option seems a better choice. The transformer is designed for 50 Hz, so the load has to be reduced using 400 Hz. But that's no problem since the transformer is rated for 5 Amps and the motor drive can only deliver 100 Watts. The three phase transformer was a gift which saves a lot of money since a 400 Hz three phase transfomer is a custom made product and therefore rather expensive. The used transformer is a YNa0 type transformer which means that the input is a 'star' configuration (Y) with a neutral (N) without phase shift (0). The interesting part that this transformer is an autotransformer (a) and has therefore no galvanic separattion. For safety reasons a galavnic separation is needed. Therefore I used an additional 250:250VAC safety transformer at the input side of the frequency drive. A simple safety transformer is much cheaper than a custom made three phase transformer. In an ideal world I would use a three phase 400 Hz transformer with 115 VAC and 28 VDC taps and galvanic separation between the primary and secondary side. Since the budget is limited my second best setup is fine I guess. ;-)

The first test setup is shown below. The safety transformer is shoen at the left. In the middle is the variable frequency drive visible. This is a Yaskawa VS-606V7. It's 'just' 100 Watts version, but this one was directly available. A 500 Watts one would be even better. The three phase transformer is shown right. The 400 VAC connection is fed with 250 VAC. Therefore the 350 VAC taps are 120 VAC in reality. Due to the lower input voltage and higher frequency the voltage indication is not representing the real world voltages. Probably I'll do some settings tweaking to get 115 VAC instead of 120 VAC.

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As expected the output signal looks terrible. The rectified mains voltage of the frequency drive is 'chopped' in 16 kHz samples to recreate a sinewave. The raw 400 Hz sinewave shape is clearly visible. Also the suqare wave portions are visible. These square waves generate a lot of high frequency harmonics. These harmonics has to be reduced to prevent damage to the avionics equipment. Normally a transformer will damp these high frequency components, but since an autotransformer is used the interference is directly fed to the output. An additonal sinewave filter is needed to smooth the raw singnal to an acceptable wave shape.

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I looked also to the three output signals. A plot is shown below. The 120 degrees phase shift is visible. Therefore a desired 120 VAC 400 Hz three phase signal is created. The next step is to smooth the signals using a sinewave filter.

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 28 VDC power supply
A power supply for 28 VDC devices is rather simple. 'Just a simple' powersupply will do fine. A switch mode power supply is the easiest and safest choice. A transformer, rectifier and linear regulator will also work but the efficiency isn't that good. And when the linear regulator fails, there is a possibility that the unregulated voltage is fed to the avionic devices... My choice would be a (Meanwell) 24 VDC SMPS (Switch Mode Powersupply) where the output voltage is set a little higher to 28 VDC.

I use a Meanwell DRP-240-24 switchmode power supply (SMPS) als shown below. This is rather small 240 Watts 24 VDC power supply. There is a voltage adjustment potentometer to set the 24 VDC to 28 VDC. 28 VDC is well within range of the powersupply. At 24 VDC is the maximum current 10 Amps. Due to the higher voltage of 28 VDC, it's possible that the maximum current is 8,6 Amps but I haven't fully loaded the powersupply yet. I made also a simple quality analysis of the output power with a Siglent SDM3045X digital multimeter. The voltage stability seems to be very good. The standard deviation of the test is just 0,0002! I guess this powersupply is perfect for avionics use.

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