Rotax 503 Charging System
By: Jim Hayward & Doc Green
The charging system of the Rotax 503 is a 12 pole, flywheel generating system. The FAA refers to it as a lighting coil instead of an engine driven alternator. It's located on the magneto or starter end of the engine and consists of 12 integrated permanent magnets built like a flat "cup" and mounted on the crankshaft.
The other four coils are used for the ignition system, two for each system on a dual ignition setup. This gives the system it's independency for each ignition system. They are called the "charging coils" even though they have nothing to do with charging the battery. The output of these coils consist of two pairs of wires. In each pair, one is white and the other is green. The white wires connect to each other and to ground inside the unit. The gray wire is an output used with a tachometer. The brown wire is simply a connection to the metal frame of the assembly, a "ground" wire.
As you can see, some way to keep the voltage at a nominal 13.8 VDC is needed. Enter our voltage regulator which takes the DC voltage from the rectifier and regulates it to approximately 13.8 VDC. An automotive alternator uses a field winding to generate the magnetic field which causes the stator to produce the electricity. This field winding is fed from the voltage regulator and creates a magnetic field to induce voltage in it's stator windings. The regulator senses the battery's voltage and, by varying the voltage to the field winding, varies the magnetic field which in turn, varies the output voltage of the alternator. Since we have permanent magnets generating a constant magnetic field for the Rotax stator windings, an automotive type of regulator cannot be used.
What Rotax uses is typical of many motorcycle regulators. It's called a shunt regulator since it momentarily shorts (or shunts) the output of the stator to ground whenever the voltage rises above a particular factory-set voltage, thereby keeping the system voltage constant. As the load on the lighting coil increases, such as turning on lights, radios, auxillary fuel pumps, or charging a battery, the regulator has to shunt less voltage to ground because the various loads use the excess power. Rotax calls these loads "consumers".
In lightly loaded situations such as a fully charged battery and no electrical equipment turned on, the power draw can be as little as 15 watts or just over an amp of current. In this case, the regulator is shunting a lot of excess power to ground which keeps the voltage at a nominal output of approximately 13.8 VDC. It's a very power-wasteful regulating system but is simple and usually works rather troublefree if set up correctly.
TROUBLESHOOTING THE CHARGING SYSTEM
(You may wish to refer to the wiring diagrams at the end of this article to get an overview of the complete electrical system.)
Occasionally, there may be a problem develop in which the battery won't charge at all or, the charging voltage is either too high or too low. If the battery is known to be good, it's time to check the charging system. The wire connections should be checked first to make sure they are not only tight, but free of corrosion. Corroded or loose connections will prevent a proper charging voltage from reaching the battery.
Another check to be made is the correct battery voltage. Without the engine running, a healthy battery should show about 12.6 to 12.7 VDC. With it running, that should be up around 13.5 VDC to as high as 14.5 VDC which indicates the lighting coil/rectifier/regulator system is working. A voltage of 13.8 is ideal as it gives a good charge rate without causing the battery to gas. Lead-acid batteries begin to gas about 13.9 VDC.
If the voltmeter reads higher than about 14.5 VDC, replacement of the regulator is recommended since higher voltages will boil the battery and cause water to be needed much more often, not to mention the possible damage to electronics from too high an operating voltage.
If the meter only indicates the battery voltage with the engine running, connect the voltmeter to the yellow output wires and set the meter to read AC. Run the engine and see if the voltage varies with engine speed and according to the values previously given (which are not absolute). If it does, the lighting coil is probably okay. Since the rectifier/regulator is a combined unit and is getting it's required input from the lighting coil, it should be replaced and the voltage check repeated for the battery. This check should now be good and the problem solved.
If the lighting coil (or stator) output isn't what it should be, then it should be checked for shorted windings or other problems and will probably have to be replaced.
Wiring Diagrams from Rotax
This section consists of six wiring diagrams taken from the Rotax manual that show how a given voltage regulator is wired into a given electrical system.
Circuits for two different regulators are shown: # 264 870 and # 866 080.
Note that the 866 080 regulator requires a minimum current load of one amp in order to prevent excessive AC voltages from being applied to the regulator input. The 264 870 regulator does not require a minimum current load.
For each of these regulators, wiring diagrams are given for:
A word of explanation: Because the same alternator is used to provide electrical power to each of the circuits, and because the wiring diagram of the alternator assembly is shown above, that part of the wiring diagrams has been removed. This permits the remainder of the diagram to be enlarged to provide greater clarity.
In all the following diagrams, the yellow and yellow/black wires of the "lighting coils" connect to the yellow, AC input wires of the regulator.
Regulator # 264 870 No battery.
Regulator # 866 080 No battery.
Regulator # 264 870 With battery and electric start.
At first glance, this circuit appears rather complicated but it really is not all that much more so than the others. The only thing added is the starter circuit.
First of all, the terminals of the solenoid, B and M, are the posts to which the big battery cables connect. These are shown as extra bold in the diagram. You can trace them from the positive terminal of the battery to the starter. The solenoid serves as the "switch" for this circuit.
All consumer devices may be switched in this circuit. There is no minimum current requirement for this regulator.
These photos are three regulators commonly used with our planes.
Jim Hayward & Doc Green