Devlopment of 270 VDC

[otto452]

In working with military aircraft power systems, I've come across 270 VDC systems before. Where does the decision to go with 270 VDC come from?

I had heard that it was derived from using 3 phase 400 hz 115/200 VAC power but I'm not sure how do the conversion to get 270 VDC.

 

[IRstuff]

My recollection is that A/C power is 400 Vac, and the 3-phase, full-wave rectified DC is 270 Vdc. 120 Vac is derived from conversion of the 400 Vac.

TTFN

FAQ731-376

 

[FieldTeam]

There was a really good post in this forum that has already talked on this.
"+\- 270V (540V) supply as opposed to a 0-270V supply".
thread2-168599

 

[otto452]

Thanks for the information IRstuff and FieldTeam, but I'm still struggling to see how the three phase rectified voltage from 115/400 VAC results in 270 VDC.

The RMS value of 115 VAC is 115*.707= ~81 VDC. How do three phases of 81VDC lead to 270VDC? I know I'm missing something, but the previous entries to not adequately explain the conversion.

 

[FieldTeam]

I never worked this type of power set-up.
With a steady voltage input, a transformer/rectifier could do this. We use TR's to convert 115VAC to 28VDC so why not 270VDC. It probably wouldn't be efficient.

Most likely, the generator system is designed to output 270VDC. Generator with a power converter. Less weight because the gen does not need a CSD and associated controls. The power converter outputs a constant 270VDC from the variable generator input.

 

[otto452]

Thanks for the information everyone.

I was also wondering where the decision to use 270VDC came from. Sure the current goes down as you increase voltage, but why not use the rectified 3-phase AC voltage directly? Then you wouldn't need a transformer/rectifier (you could just use a rectifier) and could still have the higher VDC to keep the current down.

 

[Compositepro]

The examples I have seen of 270 vdc have been motor/generator sets that are part of WWII electronics. They converted 28 vdc to 270 vdc. I have always assumed that this was the standard for vacuum tube plate voltage supply.

 

[IRstuff]

The 270 Vdc does come from the rectified 3-phase AC. However, the available power is usually limited, so each box generates its own 270 Vdc from the 3 phase.

Your calculation is only for single phase. You need to calculate for 3 phase.

TTFN

FAQ731-376

 

[PHovnanian]

3 phase 115 Vac line to neutral has a line to line voltage of 200 Vac. Both of these are RMS values, the peak voltage being sqrt(2) times that. Given a three phase system and the appropriate configuration of rectifiers, the DC voltage will equal the line-to-line voltage peak, or 1.4 * 200 = 280V. Deduct 10V for ripple and losses an you get 270 Vdc.

 

[IRstuff]

It's generally not allowed to run aircraft equipment across phases.

TTFN

FAQ731-376

 

[PHovnanian]

Define 'running aircraft equipment across phases' in your context. Most 3-phase loads like motors, TRUs and the like are 3 phase, 3 wire loads and, as such run across phases. This includes 270 Vdc supplies.

While unbalanced line to line loads are discouraged, when I was working electrical power systems at Boeing, the window heat controller manufacturers plead for (and received) an exception to this rule. Being some of the largest electrical loads on the airplane, they messed up our attempts at maintaining load balance across phases.