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Thread: Behavior of Multiple Alternators

  1. #1

    Join Date
    Aug 2013

    Behavior of Multiple Alternators

    I am somewhat unfamiliar with the intricacies of the alternator-regulator, and I am putting together the final touches on the following configuration:

    • Engine is a ULPower 520is with an integral 50A alternator and regulator, of which 15A is used for engine related systems and support (net about 35A, these systems are powered from the main plane bus, battery a bus, and/or battery b bus, depending on the load involved). The ULPower alternator is self contained - no external field wire is available for control (hence, its always on - period).

    • Installing a B&C 40A alternator with a Primary LR3C-14 regulator (I have a VP-X as well in the plane). Hence... both alternators are continuously enabled.

    • There are two batteries, on their own contactors, which are brought to a parallel bus (main plane bus), which then powers the starter and most non-engine plane systems. Both batteries are 16Ah EarthX units.

    • The B&C will be attached to the VP-X, so it will get field power and voltage sense from there (as if its a primary alternator).

    Key questions:

    1. What is the behavior of both alternators in this configuration? I assume that the B&C need to be set to the exact same output voltage as the ULPower alternator (~14.8VDC).
    2. Do the two alternators just balance the bus load, or are there nuances on this? For instance, if the total load is 60A, that load has to be shared across the two alternators as the load is not (and practically cannot) be easily segmented to only one of the alternators. In reading some online articles, it seems that both alternators will constantly "seek" and output will vary slightly back and forth, but this should be a non-issue.
    3. I have the ULPower regulator output on the main bus, as opposed to one of the two batteries directly. Is this preferable, or should the regulator go onto one of the two batteries directly (before the contactor)? If, yes, then I assume the B&C regulator should go against the second battery?


  2. #2
    CharlieN's Avatar
    Join Date
    Sep 2017
    Vermont, USA
    I would probably run the two batteries and alternators on separate buses with separate loads and only parallel them when needed.
    President Chapter 968
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  3. #3

    Join Date
    Aug 2013
    Unfortunately, fully isolated buses are not possible in this configuration. It would also increase complexity by adding another contactor.

  4. #4

    Join Date
    Oct 2011
    Not positive, but since the output is directly related to the voltage, one of the alternators will not supply any output if the voltage is higher than it's regulated setting. You can set the voltages the same, but one will always output more than the other one. This is not a good situation.

    VP-X has provisions for two alternators. What you can do is place a relay (or contactor) between the ULPower alternator and the bus. Connect the relay to VP-X and make it the first (J12-11) or second alternator. Doesn't matter which. Use VP-X as the field source for the second alternator also (I think you can use any pin for that). VP-X will only turn on one alternator at a time. See page 29-30 of the VP-X manual.

    The battery is normally isolated from the bus through the contactor. Turning on the master supplies power from the battery to the bus. The only purpose of the battery is to start the engine. After that the alternator supplies an unlimited amount of power as opposed to the limited supply from the battery. With this in mind I don't understand the reasoning for the second battery. Yeah, if all the alternators fail and the first battery goes dead maybe a second battery would be nice. But when was the last time you lost one or both alternators and the battery went dead?

  5. #5

    Join Date
    Jul 2011
    Tustin, CA
    I suggest you put the B&C alternator in the standby position and control it with their SBK-14 voltage regulator. That way you don't have to worry about the two alternators playing nice with each other.

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