737 NG APU; SCU,SPU and IGV questions..

[bwheelz05]

When we received the new -800's for C check recently we went thru a Gen Fam for Classic to NG differences.

The APUs starting cycle is a little bit different I know. The APU ECU,SPU and SCU I need a little clarification on:

On start, (can be either AC thru transfer bus 1 or DC from the battery) the APU ECU tells the SCU that its ready to start, the SCU tells the SPU which (Have AC on the aircraft, Normal start) receives 115 AC to convert it to 270VDC. The SCU takes the 270VDC and converts it to 3 phase AC correct?

What exactly is the output of AC then from the SCU to the starter-generator after the conversion? 115VAC with three phases??

Also the IGVs control the pneumatic air input to the APU load compressor and works in conjunction with the surge control valve at times to relieve the bleed air load on the load compressor when bleed air is not in use correct? Much like the IGVs on the engine .... correct comparison? The IGVs are controlled via the Fuel control unit- I guess its actually the lube module that controls the actuator fuel pressure regulator. (fuel actuates the IGVs, much like the HMU, which meters fuel to the engine VBVs, VSVs ect....)


Appreciate the clarification/ Thanks!

[rmm]

The APU FCU contains an 'actuator pressure regulator' that maintains 250 psi to the IGV actuator.

Much like the IGVs on the engine .... correct comparison?
Yes

As for the output voltage from the SCU, not sure it's 115 or 200 VAC. I'd have to check the CMM or wiring diagrams.

[ampclamp]

I am uncertain and it is not in the AMM but do recall from the course i did it being a variable a.c. output maybe 115 to 175 vac?
Don't take that as gospel as i am really unsure.

[rmm]

A quick check of the CMM (if I'm looking at the right table) appears to have it as variable AC, 0 - 115.

[ampclamp]

Thanks RMM. I had it the wrong way about starting at 115 and building.