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P O L Y T E C H . N U

Static Inverter 1A2501B - 115+28V 400Hz

This article is about the 1A2501B static inverter. This device generates 115 VAC and 26 VAC 400 Hz for avionics purposes from 28 VDC. This device is used in (small) airplanes since some instruments require 400 Hz power and small airplanes have only a 28 VDC supply.



image description: static inverter
brand: Avionic Instruments Inc.
model: 1A250-1B
weight: 9,3 Lbs. = 4,2 kg
connector: MS3102A-18-9P (plug: MS3106A18-9S)
mounting: base mount – orientation not critical

input voltage: 24...32 VDC
input current: 1...13A
full load current: 13 A @ 28 VDC input voltage

output voltage: 115 VAC and 26 VAC
output frequency: 400 Hz
output phase: 1
power factor: 0,9 leading ... 0,8 lagging
output power: 250 VA
output waveform: sine

device description
The static inverter is a device that converts 24...32VDC (usually 28VDC) to 115VAC 400 Hz and 26VAC 400Hz. Some avionics needs 400 Hz power and this device created the needed sine wave signal. Resolvers and synchros are 'sensors' that need this AC signal to operate. Therefore 400Hz signals are rather common for airplanes. Sometimes three phase 400Hz power signals are needed, but this device creates only a one phase signal. (Three phase is common for larger/military systems.) This device is rated for 250VA. That's approximately 2 Amps for the 115VAC output.
The output voltage is stabilised so if the input voltage changes, the output voltage stays the same. The exact design philosophy is unknown, but there's an educated guess of the design: There's a rather loud 400Hz tone generated that's likely due to a 400Hz square wave. By switching a 28 VDC voltage on and off 400 times per second and feed this to the primary side of a transformer, two 400Hz signals are generated at the transformers' secondary side. It's likely the square wave is converted to a sine wave by applying a 'LC' low pass filter. It's likely that the DC input voltage before switching is regulated to influence the output voltage. When the current draw is larger at the output, the input voltage is probably increased to 'send' more energy to the transformer core. The duty cycle of the square wave must stay 50%, so only the energy to the transformer can be regulated by varying the input voltage.


imageAs usual technical documentation of avionics is hard to find, so I had to figure out the connections.

Based on some educated guesses the pinout is known now. My 'reverse engineering' steps were:
The left connector is capped and therefore likely a diagnose connector. This is therefore not further investigated.
The other (right) connector is therefore likely the main connector.
The top panel is removed and pins A, B, C, D and G are connected. Pins E and F are note connected.
Since the input voltage is lower than the output voltage, the input current is higher. That makes is logical that the thick pins A and D are used for the power input.
Pin D has a black wire attached and pin A has a red wire attached. Black is usually ground/return and red is usually the positive power. Therefore my educated guess is that pin A is +28VDC input and pin D is 28VDC return.
There are three pins left. The output voltages are +115 VAC and +26 VAC (400 Hz) according to the type plate. And there's likely a pin for the power return ('ground').
Pin C has a black wire attached and therefore likely is the return/negative/ground.
Pin B has a white wire attached and pin C has a yellow wire attached to it. There's no logic found in the wire colours except that one is likely +26 VAC and the other is +115VAC.
Based on the analysis shown above an input voltage is increased and the expected output pins are measured with a Voltmeter.
The inverter started as expected and the expected output voltages are present. Yey!

The connections of the inverter are therefore as shown below.

pinusenotepin location
A+24...32VDC input1...13Abottom right
D24...32VDC input return1...13Abottom left
B+115VAC 400Hz output-middle right
C+26VAC 400Hz output-top right
G+115VAC/+26VAC 400Hz output return-middle


inside view
The inverter is very well built. High quality components are used and everything is mechanically fixed well. Even the printed circuit boards are coated to prevent corrosion.



Static inverter in use
I made a video using the static inverter. See the video below to see the device in action. On the video is some testing with a Honeywell VG311 vertical gyroscope and a Smiths H6 artificial horizon visible. Both devices are powered by the static inverter with 115VAC 400Hz.

Help needed
I don't have the plug that fits on the static inverter. If you have the corresponding plug (for sale) that fits on the receptacle, please let me know!