Battery voltage and starting the 737N's APU

[allendc]

Hello
I have a question regarding starting the APU on the 737 NG aircraft. Here are the conditions (aircraft is on the ground):
No ground power available, no engines running, fully charged batteries (28v).

1. If one sets the DC meter to read "BAT", what would the voltage read before an APU start.

2. What voltage would the DC meter position "BAT" read during an APU start.

3. Is there a noticeable dimming of any lights that are illuminated during the start?

What I am trying to understand is the voltage sag during APU start.

Thank's again to everyone for their help with prior questions.

David
www.737FlightSim.com

[B737CL/NG_Mech]

1. Depends on the condition of the BAT -> min voltage for APU start is 22 VDC With the meter selector switch in "BAT" pos you´ll read the actual voltage (fully charged 28VDC)

2. Voltage drops down to ´bout 19 Volts

3. Yes. You´ll rather see the indication ligts (warning ligths) because they go dim.

The B737NG has a nice feature to keep the draw from the BAT "low". The start power unit changes 115v ac or 28v dc electrical power to 270v dc power. Transfer bus 1 or the battery supplies power to the start power unit (SPU). And the start converter unit (SCU) changes the 270v dc power to AC and sends it to the starter-generator.


hope this helps a bit. For more Information fell free to send me a private message.

Yours,

[Trash Hauler]

The battery voltage should read around 24 VDC before the start not 28 VDC.

The DC supply system will read nominally 28VDC when powered by the TRUs.

Cheers

Trash Hauler

[allendc]

Thanks to B737CL/NG_Mech & Trash Hauler for their answers, so a follow up question.

If the battery voltage is at 24V and the batteries are in good condition, what will the battery voltage drop to during APU start?

Thanks Again

David

[Trash Hauler]

On initial activation of the APU starter the battery voltage will drop to about 21VDC.

It will then commence to climb from 21 to 22.5 VDC as the APU accelerates to 20% RPM. It will then remain constant until APU starter cutout (50% RPM?) at which time it will return to approximately 23.8-24VDC.

When the APU generator comes on line the voltage will then go to 28DVC as the TRUs power up.

Hope this helps

[allendc]

Thanks Trash Hauler for the follow up answer. Do you know when the Bat Discharge lamp extinguishes? (%RPM or Time)

David

[Trash Hauler]

I am not sure on the discharge lamp however it should be out when the TRUs are powered (DC volts at 28VDC).