Alternator matching regulators ?

[catmandoo]

Have 80 amp and 70 amp alternators discharging to common post on relay prior to feeding battery banks . If one regulator set at 14 .2 volts and other at 14.6 volts would I get negative flow through 70 amp one and not other presumably weaker one would increase current and voltage drop to achievecommon voltage at common post what ever voltage that would be ?

Funny before things failed at voltages 15 +I measured negative current through 70 amp one Also open circuit voltage of 70 amp one was 20 v after stopping engine disconnecting cable and starting up again- so not bust
Funny?
So second question is do both regulators have to have same set points
These systems of control are simple proportional control feed back loops measuring deviation fromset point and adjusting current out put accordingly ! but negative???
Perhaps something failed but what
To many variables these days
I despair

 

[William_H]

I presume you have 2 alternators running (2 engines) into a common point. Most standard alternators sense battery (charge and output) voltage at the output terminal of the alternator. Inside the alternator you have the cathodes of 3 didoes feeding this terninal. So if an alternator is trying to feed (charge) a battery whose voltage is above that of the regulated point of the alternator (from the other charging source) the regulator will shut down field current so that the alternator will not be generating. The higher voltage coming into the output terminal of the alternator should go no where because it is up against the 3 (blocking) diodes. If you have or are measuring negative charge then there is something odd going on most likely in the other wiring to the amp meter circuit. If you genuinely have negative charge back into the alternator by fitting an amp meter at the alternator output terminal then you have a real problem.
Whilst you have 2 alternators of uneven regulated voltage you will always get the alternator with the higher voltage doing most of the charging. The actual ouput of that alternator will depend on the load ie battery charge state andf engine revs. The alternator will be limited by its own capabilies to provide the current. So the regulator may find voltage low so opens up full field current but if the engine revs are too low or alternator is simply overloaded then voltage will not reach regulated voltage. If this actual output voltage is lower than the other alternator regulated voltage then it will start to turn on the field current and it will also do its best to bring the common output voltage up at least it's own regulated voltage.
So the whole sytem should in practice work ok. On small twin engined planes they have balancing regulator which forces both alternators to contribute equal currenty. You may like to research this option. Although I am not really sure why they are so concerned about equal load except it means equal horsepower from both engines. I am not sure you should worry. The working hard alternator may get hotter and have a limited brush life etc but it is most likely it is still running mostly way below ratings so should last a long time. If it dies or that engine is stopped your other alternator should pick up the load albeit with a lower voltage. good luck olewill

 

[catmandoo]

Thanks for that . I had ben following the same reasoning but wanted some back up . I had not considered the alternator diodes though in the overall system . I use an portable "megger" with a split loop to measure current so the negative amps although small - 3 to - 4 was a bit of a shock . I wonder if the diodes are faulty ?

 

[lw395]

It's possible you've damaged one of the regulators.
Some do not like running with no load, and running with a load voltage higher than the regulated voltage might work out very much like no load.

It depends on the exact circuit of the regulators, you have to remember that they are not constant DC voltages, but analogue circuits with response times and so forth. So a peak voltage from one could cause the other to mis-regulate.
I think it's better to have split circuits, one alternator per bank when charging, unless the regulator is fully specified to work with paralleled alternators.
It's not something a normal car alternator is expected to do, so it wouldn't be a requirement of the circuit design.