Author Topic: Generators  (Read 3090 times)

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Offline Peaky 556

Generators
« on: Feb 20 2013 08:17 »
Hoping you folks don't think I'm being a control freak but I thought it best to have different threads for each major topic, in the interest of consolidation and avoiding too much chaf to wade through.  The first heading of "Diesel-Electric?" seems to mainly focus on cooling, not an easy issue!  I kicked off one about engine types.... now I want to do the same with that lump of electromechanical ironmongery that turns mechanical input power to electrical Watts.  It may also have some other demands upon it...

My take is that the generator (I shall call it that as a broad descriptor that would cover AC and DC outputs) should have these minimum properties:
  • high efficiency of energy conversion;
    ability to become a motor to start the engine when required.

    Please add to the list of essentials and desirables!
    I am currently playing with a disabled scooter motor that is pretty good on both accounts.  More anon when I have checked out the efficiency by testing.

    Regards, Tim (sorry I can't get these darned bullets right!)
Thank you,
Tim
Facebook.com/AppleTreeRailway/

Offline cabbage

Re: Generators
« Reply #1 on: Feb 20 2013 09:27 »
If you are going to use a DC motor to turn over your engine then it has to be a huge 3 pole monster. I would imagine that you are going to use external batteries for this (as I do). Several people have experimented with the RE-850 as combined dynamo/starter.

http://www.mfacomodrills.com/motors/850.html

The other option is to use an outside runner 3 phase motor as your combined alternator/starter.

http://www.giantshark.co.uk/boat-motors-c-25_165.html

I would recommend that you have an independent DC battery supply for on board control systems. As I have found this to be far easier!!!!

Advantages of DC generation:

No Rectification losses.
Output is always DC -smoothing capacitors in a "ripple kill configuration" are best.
Simplest.



Advantages of AC generation:

Far more efficient than DC.
3 Phase generation gives more constant load to engine removes "cogging effect".

Despite the efficiency of AC generation the problems with using a 3 phase motor as a starter do (I feel) preclude its use. My recommendation would be to stick to a huge DC motor!

regards

ralph


Offline Peaky 556

Re: Generators
« Reply #2 on: Feb 20 2013 12:43 »
If you are going to use a DC motor to turn over your engine then it has to be a huge 3 pole monster. I would imagine that you are going to use external batteries for this (as I do). Several people have experimented with the RE-850 as combined dynamo/starter.

http://www.mfacomodrills.com/motors/850.html


Despite the efficiency of AC generation the problems with using a 3 phase motor as a starter do (I feel) preclude its use. My recommendation would be to stick to a huge DC motor!

regards

ralph

On the first point above I have to disagree.  The motor am using is huge yes, but has 16 poles so very little cogging effect.  It turns over the glow-engine very happily with an on-board 12V, 4.2 Ah lead-acid battery.  No I do not want to remove the roof at a public event and fit big crocodile clips to the inards!!!  These beasts need to be self-starting.  No compromises!!!!!

On the second point by Ralph, I fully agree that a big DC motor is the best solution to act as 'Motor-Generator'.

Details of my solution will be made available as I fully explore it.  So far I know the torque required to turn over the glow-engine, and the stall torque of the 'motor-generator'.  It is actually rated at 24V, but even at 12V it gives about 400% of the torque required.

Regards, Tim
Thank you,
Tim
Facebook.com/AppleTreeRailway/

Offline Peaky 556

Re: Generators
« Reply #3 on: Feb 20 2013 12:48 »
I forgot to say the RE-850 has a claimed stall torque adequate to turn over my .40 glow engine.  I have not tried it, but my feelings are the gearing would need to be 1:1 between the two to allow starting, and at the 'sensible' revs I want to use the electrical output wouldn't pull the skin off a rice pudding ;D.  I'll have a re-think on the latter, now I have revised my tractive power requirement considerably downwards to 65W.
Regards, Tim
Thank you,
Tim
Facebook.com/AppleTreeRailway/

Offline Peaky 556

Re: Generators
« Reply #4 on: Jan 27 2014 22:42 »
I'm dismayed to see it's nearly a year since the last post, but some minor progress has been made.  I have spun the generator up to about 4600 rpm and measured output voltages and currents for a few loads, along with the torque to turn it.  I'd like to spin it faster but my poor old pre-war lathe complains too much! 
At this speed it generates about 20 V, so I may settle for a nominal 18 V system rather than the 24 V I had hoped.
I am waiting for a high power rheostat from HK to take some higher power measurements, but at around 30 W output I'm getting an electrical x mechanical efficiency of at least 45% when acting as a generator, with a trend for better efficiency as the power levels increase, up nearer to 60%.  I'm pretty pleased with these figures. 
At worst I will need a mechanical power input of 65/0.45 = 145 W.  Equating to 0.2 HP, this should be readily available from the engines through the gearbox.  I can keep you posted as and when I get more data.
Regards, Tim
Thank you,
Tim
Facebook.com/AppleTreeRailway/

Offline Peaky 556

Re: Generators
« Reply #5 on: Feb 02 2014 21:11 »
 Chinese rheostat has arrived, and despite it getting so hot that one of the solder joints melted, the geny has been spun up to 4600 rpm with increasing loads and we now have a volts/speed curve for various currents up to 5.4 A. Modest extrapolation tells me with reasonable confidence that spinning up to 5200 rpm will give the required output voltage of 21.7 V whilst delivering about 5 A.
This should be enough to power a loco pulling  rake of heavy BR Mk1s, even Kingscales!
Why 21.7 V? An 18 V SLA battery will need around 21 V to keep it charged, and there will be around 0.7 V added to get past the charging diodes, taking advice from our learned Tech Advisor, thanks Ralph.
Efficiency of mechanical to electrical power conversion is also good at 60%.
I can now design and make the gearbox!
Regards, Tim
Thank you,
Tim
Facebook.com/AppleTreeRailway/

Offline 454

Re: Generators
« Reply #6 on: Feb 02 2014 21:30 »
This should be enough to power a loco pulling  rake of heavy BR Mk1s, even Kingscales!
Regards, Tim

How many "heavy" BR Mk1's had you got in mind at 6Kg each then Tim?
 8)

Cheers
Dave
454

Offline Peaky 556

Re: Generators
« Reply #7 on: Feb 02 2014 23:30 »
Dave, if you buy one per year then there'll be ten to lend me by the time I've finished... ;D, Tim
Thank you,
Tim
Facebook.com/AppleTreeRailway/