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CHP. 180v 800w DC/DC Buck... Help!

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dan-graham

New Member
Hello all, my first post. I am looking for help with my project, building a CHP generator. Most of the mechanics are finished and I am now starting on wiring etc.

Background info, I have a single cylinder diesel engine, connected to a 5KW AC alternator. There is also an 180V PM motor via belt to flywheel of engine, used to start engine and provide 180VDC when engine is running. There are also 2x 3KVA inverters (UPS's APC type 48V) I also have 16x 100AH VRLA batteries. I also have smaller SLA 12V 7AH type etc etc...

What I want to achieve? Well my plan is to use 8x 12V 7AH batteries in series to crank the engine via the PM motor, (I need to build a soft start system to do this). I have tested the motor and around 80V is enough to start the engine a small 96V bank will be more than enough to crank it. When testing the motor with a relay we were seeing around 18A @ 80V so soft start would be good.

Once the engine is running and warmed up (AC @ 50HZ) the PM motor puts out 173V, as the AC is loaded and unloaded this will fluctuate max 190V. I need to step this down to 48V to feed my battery bank. Multiple outputs would be good if I could have 14.5V & 5V for other applications.
The idea is to use 1 inverter overnight, direct from battery with engine off (control required) then in the morning the engine will start up and charge the battery bank for inverter 1, while inverter 2 will provide mains from alternator and fall over to battery if there is a problem. Inverter 2 will charge its bank from the AC inverter 1 will operate in the cold start mode i.e. no AC supply. I will also pull AC direct from alternator in workshop for welder, plasma etc.
The engine is controlled by a 1940 design governor and diesel pump. I would like to remove this and attach a servo to the diesel pump control and control the engine based on the AC HZ from synchronous alternator. The old governor is not very good. At present if I connect a 3KW load to the AC it dips to 46HZ and is slow to recover, once 3KW load is removed it then over revs to 55HZ and again is slow to recover. Servo with control from AC feedback should be much more stable.

I am looking at developing a microcontroller to run the whole system and safety features. Not sure what controller offers the best features etc. I am new to microcontrollers so will probably buy a kit and work from there. There is also a heat exchanger to provide hot water, this will go through some diversion valves etc and need to be controlled. I have been looking at SMPS power supply for the DC/DC converter but not sure on components that will take the power or if I can parallel certain fet’s etc????
I am sure you will all have lots of input and can help me in the right direction before I waste money on components that are not suitable.

I would like to start with an MC that will offer most of the features I need and build from that, I have been looking at the PIC32 and various AVR’s but not sure where to start because there are a lot of systems to control. I am currently working my way through some Ibrahim books but I learn by doing rather than reading!

Thanks in advance Dan.
 

Mr RB

Well-Known Member
Sounds like a cool project.

Re the stepping down 170-190v to 48vdc, most switchmode power supplies will take that input voltage, especially the ones that accept input from 230vac to 110vac. These could be a cheap way to charge your 48v battery bank, ie a float charge using regulated DC voltage via a shunt resistor. So not much to build there.

As for building the servo controller to control the diesel speed, that shouldn't be too hard, but it might be easier if you start with a simpler micro than the PIC32 series(!) as most of the easy code examples you will find on the net are for the more common and simpler 16F series pics. Maybe try a 16F877.

I assume you want to auto-start the engine with no human supervision? That might need some other sensors...
 

smanches

New Member
I doubt you'll be able to actuate the fuel pump electronically. I don't think any diesel engines have electronic fuel pumps due to the pressure they need. You could still manage the governor electronically though.
 

dan-graham

New Member
Thanks Mr RB,

The reason I need advice is because I would like to bring all of the control systems together in the same controller, if that is possible? Or separate controllers that can all be monitored together, these items being.

Engine start - 180V Brushed DC PM Motor, Soft start from 96V bank. H Bridge not required just low side driver that can handle the load. The problem is once the engine is started I then want to draw power from the motor not drive it? My old fashioned brain is telling me to use a large relay to switch it over to another circuit once the start up is complete but I am sure there is a way to integrate this?

I also need to switch on fuel servo, set the ramp rate on the motor, crank time until compression lever is closed and a feedback to say motor is running, this could be voltage increase above the 96V but that may damage the circuit? Or tach on the fly wheel or use HZ from AC alternator.

DC/DC step down Once engine is started PM motor output is stepped down to 48V (need to handle 850W peak) I would then also like to have multi stage battery charge, temp adjustment etc. I also need to keep the 96V bank charged and monitored, and 14.5V would be nice for the workshop!

Engine Control Once start-up is completed I need to run the motor as close to 50Hz as possible with fast recovery for large AC loads, this will be the closed loop control of the fuel pump servo using AC HZ feedback? I also need to have safety controls, oil pressure, engine temp, Heat recovery and thermostat controls etc. Auto shut down when batteries charged, water tank heated with a manual override if I need to keep using AC.

Overnight power monitor similar to this Übersetzte Version von http://www.dg7xo.de/selbstbau/nt-power-modul.html

So I can monitor how much power we are pulling from the batteries overnight etc.

Rather than buying lots of separate useless bits, if we bash it out I am sure I will be able to find a controller and relevant peripherals to run the lot. Ideally a remote monitoring and control of the system from my PC would be good.
Again advice is really appreciated, when I speak to sales reps for advice they only seem to be interested in parts, like motor control or engine control, I need to bring all of these bits together and work out the best MC to build on.

Thanks,
 

dan-graham

New Member
Just to clarify the fuel pump is high pressure driven by the cam. The control is via a rack and pinion style, as the motor reaches 1500 rpm the governor opens via centrifugal force and pushes the rack. This rack then reduces the fuel supply. If the governor is removed the rack can easily be opened and closed with your finger so a servo will operate this well, only 3cm of travel. The pumping of fuel will not change.

I have lots of PC power supplies but as far as I can tell they will not take the 800W and the intermediate DC voltage is around the 380V. But I do have lots of components to play with! I think the main thing is to find an MC that will cover the majority of these systems to keep my external components low.

Thanks for your input.
 

Sceadwian

Banned
No, I guess new users get their post moderated briefly, it'll stop after a short period of time.
 
surplus telecoms equipment

Hi Dan,

I think you are in luck with regards to the 180Vac to 48Vdc, since 48Vdc is a standard telecoms supply voltage. The supply’s that I’ve used in the past are very tolerant of input voltage and frequency variations, being rated at 85Vac to 275Vac and 45Hz to 66Hz. These units being rated at 1500W would be ideal as they are designed to charge at 54Vdc for your 48Vdc battery bank, they are also designed to work individually or load share if multiple units are used.

You should be able to find something suitable from any Telecoms surplus store, they are often just called ‘Rectifiers’.

Hope this helps.

Leftfield95.
 

Mr RB

Well-Known Member
Hi again! Wow that is an impressive wish list of features. I would use a large cabinet for your "controller" and continue to add and improve features as you go along. It sounds a little much to learn to use the microcontroller, and implement a heap of real time closed loop features all in one hit!!

It's also a good idea to do the bits you know you can do with technology you know will work. Save your headache time for the few things you MUST learn. So I would use 2 large disconnect relays for the DC motor, one for start motor function, one for generator. You can do the soft start with a big resistor (like a jug element) for the moment and add a nice microcontroller soft start later.

Controlling the governor from the microcontroller or other circuit should not be too hard if you keep that as your only task, again get that part working ok THEN work on the autostart feature for instance.

Like Leftfield and I said, you can get the 180v to 48v converters pre-built and pretty cheap. Most of these switchmode 48v supplies have voltage regulation and current limiting, so they make a nice "float" style battery charger. It only needs to top the battery back up so it is ok for the next start cycle.
 

tcmtech

Banned
Most Helpful Member
nice project!
I have tinkered with that myself before. I would recommend a basic PLR for the control system myself. There are no big up front cost outlays when getting one. All you need is a link cable to connect it from your computer to the PLR unit when you want to change programming.

B&B Electronics carries them and they are reasonable to work with and a completer system with link cable will run under $200.
The software is free on line and rather easy to work with. If you used a combination of relay and analog input and outputs you could control everything from one small box.
Check them out at bb-elec.com you may be surprised at what you find and what you can do with basic industrial micro controllers!
 

dan-graham

New Member
For the DC/DC buck this is similar to what I need.

DC-DC Converters, DC-DC Bulk Power Supply, Custom Power

Only problem is the input is not wide enough to take the 180v and I don’t need boost. I have basic circuits for low power dc/dc buck converters but not sure on how to scale them up to handle the power? All I need to do is change 180v to around 55v but high current. This is my most immediate problem because I need to get them on charge.


The bb-elec.com has nice stuff but a bit out of my price range. Buy the time I add up all of the parts I will need!
 
You said you have some PC PSUs lying around, as long as they are all the same rating, with wide range (85-265vac) input, you could connect 4 up, all the mains sides together (parallel) and the 12v in series. they should all share the load as the current is equal everywhere in a series circuit. If you need 53v or so use another PC PSU's 5v or 3v3. Just make sure that the cases of the PSUs are isolated from eachother. Most PC PSUs have hiccup overload prtection so if you try to draw too much current nothing gets destroyed. I think that if you use dissimilar PSUs (different current ratings on the 12v, the one with the lowest will start hiccupping first.
 

Mr RB

Well-Known Member
Don't look for dc-dc converters, try "switchmode supply" (brands like Soanar and Powertech) these are normally 120v-240v ac input and 48v dc output. Should be under $60. Most have a trimpot to give some adjustment of the output voltage, if not you usually only have to change one resistor near the optocoupler to change the DC output voltage.
 
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