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Petrol Gen for 7 1/4 locomotive

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Andy Cameron04/09/2019 15:02:08
9 forum posts


Not being an electrical engineer but wanting to build an petrol/electric locomotive I was wondering if people think its feasible to use a small 4 stroke petrol generator possibly with components separated to make it fit to provide power to an electric bogie. I know the generator high capacity output would be AC but I think as most are listed as high power outputs i.e. 1-2KW would the following seem possible:-

Generator-----AC 13Amp--->rectifier 12/24v-----ESC 4Qd--------DC motors

Not sure of the maths i.e. AC 13Amp to DC Amp but I assume as P=VxI the above would result in a high potential current output on the DC side ?

I know its possible to use a car alternator but the above doesn't require any drive belt setup etc.

JohnF14/09/2019 23:46:22
915 forum posts
114 photos


Clive Hartland15/09/2019 08:35:45
2498 forum posts
40 photos

I have always found that what a gen. is rated at is not what you get. I then triple the Gen. rating to get what I want and this of course means a jump in size. It all sounds feasable for your project and I would at first set it all out and prove it before comitting to hardware on the track. Generating leccie is about 4 x the cost of household leccie.

Andy Cameron15/09/2019 10:57:08
9 forum posts

Thanks Clive

I didn't think about the ambitious ratings some cheaper devices quote, Plan was to purchase an Electric loco with good dimentions then I could use it with either battery or generator after making some dual fittings etc. I will try to stick to major manufacturers if I give it a try.

Clive India15/09/2019 10:59:04
204 forum posts

Andy, the answer is yes. I can see the attraction - many of the generators available are compact and fairly quiet.

Work back from the power at the output to get the power you need to generate at 240 a.c. - it will not be 100% efficient but that will give you a starting point to work from. There are some quite small, relatively inexpensive pulse bench power supplies available, taking mains voltage and producing 12 or 24v, with current limiting, at reasonably high currents. These might be worth a try?

Like Clive says, a bench mock-up would be good. Be careful though, it might be just a small generator but it is mains voltage from a low impedance source, so it can be dangerous.

Robert Atkinson 215/09/2019 11:49:00
492 forum posts
21 photos

What voltage is the Loco motor? If it's 12 or 24V an engine driving a car / lorry alternator would be a better idea than a mains generator and transformer arrangment. A major limitation with mains generators is the need to keep the engine revs constant tto keep the correct frequency, this is not efficent. If using a vehicle alternator the engine can run at it's optimum revs as he AC frequence does not matter as it's turned to DC. This i what the modern "inverter" generators do, they generate "frequency wild" AC, convert to DC and then chop it up into AC at 50 (60) Hz.

If you use a vehicle alternator you could remove the regulator, connect the motor to the alternator output and control power and speed by varying the field of the alternator. Depending on the motor and if it has a governor you might have to provide some throttle control to that too.

Separate enge and alternator gives layout flexibility too.

Robert G8RPI.

Robert Atkinson 215/09/2019 11:50:26
492 forum posts
21 photos

Where are you located Andy?

Andy Cameron15/09/2019 12:45:08
9 forum posts

Thanks all definitely some food for thought the loco would ideally be 24V to keep current low, Robert I am located in the forest of dean Lydney

Howard Lewis15/09/2019 14:30:25
2691 forum posts
2 photos

Coming in late, as usual.

If you are talking of a generator capable of delivering 13 Amps at 240 volts AC, ( 3 Kw in round figures ) that will need a transformer to reduce the voltage to, say, 24 volts.

This would need to be capable of delivering 130 Amps, IN THEORY; probably nearer 110 Amps (assuming 85% efficiency ) in practice. The rectifier (A Full wave bridge, presumably ) would need to have a current capacity of 120 Amps, to be safe.

I would imagine that this set up would be capable of powering four motors, each drawing 25 Amps (ie two bogies of B-B or A-1-A configuration ).




Edited By Howard Lewis on 15/09/2019 14:31:18

Howard Lewis15/09/2019 14:32:54
2691 forum posts
2 photos

A bit far to go to Hereford S M E at Broomy Hill, (At least you'd be approaching from the Ross side of town ) for help or advice, and to run?


Neil Wyatt15/09/2019 14:43:27
17044 forum posts
690 photos
76 articles

Have you seen recent issues of ME? There's a series looking at exactly this in detail for a 7 1/4" loco.


Andy Cameron15/09/2019 16:22:10
9 forum posts

Looks like its a project begging to be done, thanks all excellent input. Neil do you know the issue numbers or dates of the ME articles so I can back order if available. Howard yes I am a member of Hereford SME havnt been for a while but have seen some good petrol hydraulic and electric only models in action and excellent tracks.

Bazyle15/09/2019 19:34:31
4885 forum posts
195 photos

Looks like you need to do some research to find out from people running similar battery locos and manufacturers if you are going to buy one what the current draw and power actually used is over various operating conditions, passenger number, hills, pulling away and cruising.
Batteries will produce very high currents for just the few seconds it takes to pull away from the station and then cruise at a much lower power. You might consider a battery loco but a separate generator car that tops it up on the run. This would allow some periods of nice quiet running, getting useful energy from the generator when sitting at stations, and providing a convenient portable power unit for track maintenance.

not done it yet15/09/2019 20:57:57
3897 forum posts
15 photos

As above, do stick to a 24 volt alternator, if that is the voltage you are choosing. Nobody has been electrocuted by a 24V DC supply - as far as I know!smiley

Every voltage conversion loses energy. Many modern generators are a) rated for maximum output at 0.8 power factor (so kVA, not kW) and b) not many have a continuous output at the advertised maximum.

There are PMGs (Permanent Magnet Generators) out there that could be direct coupled to an engine, but a couple of belts and a pair of pulleys should not present too much hardship to engineer? Electronics controlling the engine speed should not be too difficult to arrange.

At the very least, go for a 110V generator if you must...

Andy Cameron15/09/2019 21:24:30
9 forum posts

Im not to worried at this point about efficiency i.e. can I get it to work then build from I have had a couple cans beer I was also thinking about single phase AC motors ? i.e. generator direct to AC motor that just to simple to be feasible obviously with some sort of ESC.

John Olsen16/09/2019 02:39:07
1001 forum posts
86 photos
1 articles

If you want to be a bit different you could have a three phase motor, which would be smaller than a single phase of the same power. You would supply it with a VFD (AC motor control) which would be supplied from the Generator. The downside of this sort of arrangement is that everything is high voltage. The nice side is that everything is available off the shelf.


Andy Cameron16/09/2019 13:20:39
9 forum posts

Yeah I suppose an AC setup raises all sorts of Health and Safety concerns especially if public running.

Neil Wyatt16/09/2019 20:22:32
17044 forum posts
690 photos
76 articles
Posted by Andy Cameron on 15/09/2019 16:22:10:

Looks like its a project begging to be done, thanks all excellent input. Neil do you know the issue numbers or dates of the ME articles so I can back order if available. Howard yes I am a member of Hereford SME havnt been for a while but have seen some good petrol hydraulic and electric only models in action and excellent tracks.

Part 8 is in issue 4621, but I think most of the stuff you are asking about is in parts 6 & 7 in 4618 and 4619. The parts seem to be consecutive.


martin perman16/09/2019 21:07:15
1727 forum posts
70 photos

As a slight aside I use a Mobility scooter if I'm at a steam rally, not allowed to walk for longer than twenty minutes at a time, and over the last couple of years I've noticed the odd scooter, particularly at the GDSF, running around with a small generator attached and running charging the scooter batteries, this has got me thinking. Is there any reason why a scooter drive train consisting of a 24vdc motor, gearbox and diff could not be used to power a 71/4 diesel locomotive using the drive shafts to drive the two bogey wheel sets.

Martin P

Clive Hartland16/09/2019 21:55:35
2498 forum posts
40 photos

One post mentions Hydraulic drive motors. A petrol engine driving a pump to drive the wheels, foolproof. No electricity, no danger wow.

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