If you have not read our troubleshooting tips for alternators and generators, it is recommended that you do so as an aid to your understanding of this section. Click here to review our troubleshooting section on alternators and generators.
When attempting to get a twin engine aircraft charging system to parallel correctly the previously described test will come in very handy. One of the most common causes for aircraft charging systems to not share the load is an imbalance in the performance of the alternators or generators. The full field test allows the service technician to determine if these units will output relatively close to the same under the same conditions. It is necessary to full field each unit and compare their output at the same rpm. Running the engines at 2000 rpm will provide enough speed for full output and give the technician a very good idea if they are close to equal.
If the aircraft in question is equipped with amp meters these can be used to monitor the output during the full field phase of this test. If cockpit amp meters are not available the technician should use an inductance (clamp around) type amp meter. This should be connected around the main output wire at each alternator during the engine run-up. At 2000 rpm the alternator outputs should be within 10 % of each other.
After determining that the alternators are capable of providing very close to the same amount of amperage, it is time to adjust the paralleling feature using the voltage regulators. First start one engine. Under a no load condition adjust the voltage to whatever voltage is called for in the aircrafts maintenance manual. Now shut down this engine and start the other. As before, without load, adjust this side to exactly the same voltage. Next start both engines and bring them to 2000 rpm. Turn on the avionics, landing light and pitot heat. Monitor the amp meters. They should now be very close to sharing the load. If not, make final tweak changes at the lower of the two regulators.
Most paralleling problems are caused by one of the following:
1) An unbalance in the alternator amp output at the same rpm. The "full field" test is used on each alternator to determine amp output at approximately 2000 rpm. The two alternators should be within 10% of each other. Be advised this can also be caused by a slipping drive gear.
2) The regulator paralleling circuits communicate with the alternators by way of the alternator field resistance. A general rule of thumb is that the field resistance should be between 4 and 8 ohms on 12 volt systems and between 8 and 16 ohms on 24 volt systems. In order to parallel properly the two alternators should have field resistance within 10% of each other. On single field alternators connect your ohmmeter between the field post and ground. For alternators with two field terminals connect your ohmmeter between the two post.
If you have any technical questions regarding your starting or charging systems feel free to call me toll free at 800-634-0190.
Thanks, John Evans