[modeleng] Re: Topsy - to charge and how to charge, that is the question!

You should have a voltage regulator which controls the field current so that 
when the battery reaches its proper charged output voltage the alternator is 
reduced to just a trickle charge. Some alternators have an internal 
regulator but if not it is easy to fit an external regulator. For 28volts I 
suppose  you will neeed to source the regulator from an aircraft supplier.
To prevent any external voltage source damaging the alternator such as 
regenerative braking you need a heavy current diode in the main alternator 
output connected so that its normal output current is conducted but any 
reverse current is inhibited. Just connect the diode +ve terminal to the 
alternator and the negative terminal to the load. It must be mounted on a 
good heat sink. Make sure that the voltage regulator input terminal is 
connected after the diode as you will lose about 0.7volts through the diode.
Not sure of the load but it is easy and cheap to buy diodes that will carry 
say 70 amps.
This would be a standard sort of setup and will solve any problems of 
overcharging and reverse current through the alternator. The charge will 
also automatically increase to take up the load of the motor.

John Rippengal.

From: "Peter Sheppard" <peter.sheppard@xxxxxxxxxx>


> All,
>
> Next phase.
>
> Last night I finally got the electrics in state that I could try it.
>
> Apart from the petrol engine not running properly (it ticks over OK but 
> seems
> to suffer from fuel starvation at higher revs - it runs for a while then 
> dies!)
>
> With the alternator field "off" the voltage was around 26.2 volts (one 
> battery
> was in use on an electric locomotive over the weekend, so the volts may be 
> down
> slightly).  With the engine running and the field on (apart from slowing 
> the
> petrol engine quite markedly - and this was with no load on the battery!) 
> the
> battery volts rose to around 28v.
>
> So, it looks like the alternator works.
>
> So my question remains, what is better, weaken the field (which is 
> intrusive to
> the alternator) and leave the engine running all the time, or leave the 
> field
> as is and switch the charge to a dummy load whenever power is taken by the
> motor?  I am still concerned by regenerative braking taking the volts up 
> to 36
> and damaging the alternator.
>
> I suspect it will depend on how much time the locomotive is running (i.e.
> hauling, not charging), versus sitting there idling (and thus charging).
>
> Cheers
>
> Peter

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