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

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

From: Peter Sheppard <peter.sheppard@xxxxxxxxxx>
To: "'modeleng@xxxxxxxxxxxxx'" <modeleng@xxxxxxxxxxxxx>
Date: Wed, 30 Mar 2005 13:54:31 +0100
Firstly, apologies for the big snips - the mailing list only allows 20 lines of 
text carried forward!

OK, I was trying to make this a KISS project (keep it sweet and simple!)

All your points are valid and noted.  Some are quite simple to resolve.

Forward / Reverse -  there will need to be an instruction to the driver(s) that 
forward/reverse direction can only be changed when the locomotive is 
stationary.  Not perfect, but without fitting a ZVR as we do to the real trains 
(for door release) there's not much else I can do.  (ZVR = Zero Velocity Relay, 
in reality it operates at < 5kph)

I will fit freewheel diodes and earthing resistors as suggested.

Now, with regard to the alternator......

This was such a simple idea at the time and I think I can keep it this way!!

Imagine this as a conventional battery electric locomotive.  I have two 12v 
batteries connected in series driving a shunt wound DC motor controlled by a 
4QD Uni electronic controller.  Forward/reverse on the hoof is an issue, but 
I'll manage that through "procedures" (and beatings of errant drivers if 
following the procedures doesn't work!)

Topsy is not a huge locomotive (well it didn't start off that way) so I was 
limited to two batteries.  The problem with stored charge is when it uses it 
all, it has to get some more!  So, I had a small (0.75 - 1.0hp) brand new BSA 
single cylinder petrol engine lying around and I thought that if I paired it up 
with a 24v alternator I could put some electrons back in the battery and be 
able to play for a bit longer.  This was never intended as a "petrol electric" 
as I knew that the petrol engine did not have the power to drive a large enough 
generator.

So, we then came to the two questions most recently posed by yours truly.  In 
view off the comments received (thanks Rich) I think I will design the system 
so that the batteries are only charging when the locomotive is stationary and 
the controller is not drawing any current.

So, my plan is as follows:

There are two separate contactors for direction.  If I use a double pole double 
throw switch with a central off position I can arrange it that a relay is only 
energised when the switch is in the "off" position which totally disconnects 
the motor from the 4QD.  In view of your comment, it may also be an idea to use 
the same switch to disconnect the 4QD from the battery at the same time. 
 Opposite contacts of the same relay can be used to connect the alternator from 
a dummy load to the battery.  I could have a big red light that says "NOT 
CHARGING" to remind people to put the switch in Neutral.

The only area I will need to investigate is how hard the petrol engine has to 
work under differing battery conditions.  I suspect that I should be able to 
find a throttle position that will do for most circumstances.  There is a 1:1 
belt drive between the motor and alternator so as long as I get around 800 rpm 
the alternator should be charging.  There will be a throttle control in the cab 
so that it can be adjusted.  The alternator (being from a diesel lorry) has a 
tacho output, so I can always use that (as I did on Stumpy) to drive a LED 
bargraph and get the drivers to make sure the petrol engine is in the 
green/yellow and adjust it if not.

Does that seem reasonable (and simple!)

Cheers

Peter







>
> I would bear in mind issues like - damage to the controller when you
> switch from one direction to the other:-
>
> 1) if the motor is running, due to the resulting voltage spikes
> 2) if the motor is stationary, due to the static charge that the insulated
> motor windings can carry relative to the systems earth.
>
> ...I'm sure 4QD have advice leaflets on it, but I would consider fitting:-
>
> 1) a freewheels diode BEFORE the direction changing switch
> 2) some static dissipating 1MOhm resistances between the motor and
> controller sides of the switch.
<snip>
> Some kind of governer, to adjust the throttle to keep the revs level when
> charging the battery, but that when the battery voltage reaches a certain
> level, the revs are backed off.
<snip>
> > I haven't looked closely at the alternator, I got it from a lorry scrapyard
> > and
> > was only too pleased to find that it worked!
> >
> > If I get a chance I'll have a look, but I don't remember anything obvious.
>
> Have a look, but if not, there are ways and means of determining ratings -
> let me know what you find, and if you need a few pointers.
<snip>


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

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