Architecture and low voltage notification

 

This page describes the basic electrical architecture of our MCR 4S project.

Background

The airplane's power unit is a Rotax 914 turbo four cylinder engine. It's main feature is the absence of mechanically driven fuel pump. The main and boost pumps are electrical. The engine is therefore electrically dependant.

In order to ensure a proper fuel supply redundancy, it has been decided to power the two fuel pumps from truly separate sources.
The architecture diagram can be seen here.

Brief description

The basic architecture is as per Bob Nuckolls' Aeroelectric figure Z16 , with the addition of an auxiliary battery, following figure Z 30. There are two separate batteries, each with it's own always hot battery bus. Each battery feeds one of the fuel pumps.

The battery contactors are Tyco Kilovac EV200 AAANA with a low currrent draw (140 mA)
The main fuel pump is powered direct from the Main battery bus (MAIN BAT BUS) whenever the accessory contact of the keyswitch is ON.
The boost pump is powered direct from the Auxiliary battery bus (AUX BAT BUS) through it's own normal BOOST PUMP switch on the panel. Each battery is monitored by a low-voltage warning module with a corresponding annunciator on the panel.
The main battery low-voltage module has an Auxiliary Battery Management Module (ABMM) feature. This particular module is from Eric Jones'. The auxiliary battery is monitored by a Low Voltage Warning Module (LVWM) AEC 9005-201 from the Aeroelectric Connection with a switching capacity to light the corresponding annunciator.

How the system works

When the Master switch is switched in the ON position, the main battery contactor is energized and the main battery (MAIN BAT) comes online. After engine start, when the the alternator comes to life and the main bus voltage rises above 13 volts, the Auxiliary Battery Management Module (ABMM) turns off the MAIN BAT annunciator and energizes the AUX BAT contactor. Thus the AUX BAT is connected to the ship's circuit only when the alternator voltage is sufficient. Whenever the ship's voltage falls below 13 volts, the ABMM disconnects the AUX BAT, enabling it to keep it's full charge regardless of what happens to the rest of the circuit.
In order to avoid running down the auxiliary battery, the AEC 9005-201 is connected to the battery through a small relay. It monitors the auxiliary battery only when the E-bus is energized.

Viewed from the cockpit

When the pilot flips the the MASTER SWITCH, the MAIN BAT and AUX BAT annunciator low voltage lights flash. After engine start, when the alternator supplies sufficient voltage to the main bus, the MAIN BAT light goes off, followed by the AUX BAT light, as soon as the ABMM connects the auxiliary battey to the ship's circuit.
In case of failure of the ABMM, the AUX BAT light should give the pilot an active notification of the problem. In case of total alternator/regulator/battery contactor the MAIN BAT and AUX BAT light notifies the pilot within seconds.

Ground and flight tests

The system works as expected except the AUX BAT light : it has not been possible to have the two low-voltage lights to flash simultaneously with the master switch on.

Only the main battery low voltage light flashes

The main battery low-voltage warning light flashing when the Master switch is ON and the engine is not running. The AUX BAT light to the right should be flashing too.
The aux battery low voltage warning light flashes only when the Kilovac contactor is not energized

When the E-bus is energized direct with the E-Bus switch only, or with a bench power supply or an ordinary Cutler-Hammer battery contactor, it flashes. The wiring seems to be correct.
Different relays tested to pinpoint the LVWM problem

Three of the miniature relays tried to connect the LVWM to it's battery. At the top the regular Cutler-Hammer contactor.
With this contactor jumpered in place the problem vanishes. Therefore the presence of the Kilovac contactors in the circuit seems to disturb the LVWM.

Transparent relay used to verify relay operation

Test with a fourth type of relay.
The transparent case reveals what's going on inside. A close examination of the picture should show it is energized and the relevant contacts are closed. Additional tests with shunted contacts show the same behaviour.

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