Contrails ! Low voltage notification

This page describes the basic electrical architecture of our MCR 4S project, and the troubleshooting of a Tyco battery contactor noise issue.

Background

The airplane's power unit is a Rotax 914 turbo four cylinder engine. Its 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 was 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 its 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 its 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 switching capacity to light the corresponding panel annunciator.

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How the system works

When the Master switch is switched to 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 its 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 low voltage annunciators 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 battery 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 failure, the MAIN BAT and AUX BAT lights notify 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.

Further tests

My investigation zeroed in on a noise issue on the part of the Tyco EV-200 contactor.
Following experts advice, I tried hooking up a variety of capacitors, but to no avail.

Workaround

My intention was to bring a digital oscilloscope, and to hook it up to see what was going on regarding any bus noise.

But as the opportunity to borrow such a unit was postponed, I ended with replacing Bob's LVWM module. As the unit from Perihelion's seemed impervious to the Tyco EV-200 contactor perturbation, I installed one, and experienced no more problems.

In my opinion, there must be some pertubations caused by the presence of the Tyco EV-200, that disturbs Bob's unit and not that from Perihelion's.

Not being an expert, I leave it to the big boys to delve deeper into the problem. I'd be much interested in listening more on this Tyco EV-200 noise problem.

 

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