Genset

Hi all,
I was hoping someone here could help. here's the problem I'm facing. My welding generator puts out 800 amps at 48 volts on two circuits (2x400 amp). The engine is a vw rabbit diesel converted to natural gas. I purposely chose a smaller engine in order to maximize the efficiency ratio. I plan on using four 12 volt golfcart batteries as surge protection for the welding shop and house hold current at night. Here is the problem ;The ac generator part is only 1500 watts at 120 volts. I need to devise a surge protection device using the batteries combined 48 volts inverted to 120 volts ac for household and shop power. Any ideas?

Can you provise for us a simple circuit of what you propose?

In other words, a circuit showing how you intend to wire up the welder's 48Vac/120VAC OUTPUTS and THE batteries.

Perhaps a solar panel DC to AC inverter designed to interface and synchronise with the mains power would do what you want.

Thank you cowboybob
I'm sorry but I do not know how to synchronize sine waves. I was hoping you could provide that information.
Thanks again
Henri

Thanks crutschow
Yes that seem reasonable. I will check that out. I was hoping for something specifically designed for surge protection. Maybe I'm over thinking the problem.
Thanks
Henri

We need far more information on the whole system and purpose of the system.

Do you not have electrical power where you are at or is it incredibly expensive or what?

Is the welders output AC or DC constant current or constant voltage and are the outputs each driven off of independent windings or one big 800 amp one that's split into the two 400 amp outputs?

Also does it use any form of feedback circuit to regulate and control its output currents or voltage?

Thank you for your post. The welder is a dc generator Hobart Pipeliner with an ac 1500 watt exciter. It puts out two (2) 400 amp circuits at 48 volts. I am going to use four large 12 volt batteries as a cushion for the welders and inverting to 120 ac household power. All of that is not the issue. The devise I need is a sine wave synchronizer for the ac exciter and the inverter configuration. The exciter is only 1500 watt and the batteries will have about 100 amps of inverted ac power, I want to use those batteries as a cushion/ surge protection for the 120 ac circuit (household ) .
The reason for doing this is triple fold
1. the welding shop running 10 hrs will cost ~90.00$ of grid power @.24 per kw
and the equivalent ng per gj is like 5.00$
2. 80% is free heat
3. Equipment on hand. Learning curve. Independence. Flexibility.

Sorry-... $0.24 per kwh

If you are looking at doing the most basic form of co gen then you are over complicating the whole process with the welder generator, battery, then grid tie inverter concept.

All you need is a common AC induction motor tie dot the grid and your engine to spin it faster than its stock RPM.

For example if you had a stock 40 Hp 3 ph motor that ran at 1740 RPM fully loaded you could essentially turn it into a (40 x 746) ~30 KW co gen unit just by using your engine to spin it at around 1860 RPM or slightly higher.

To synchronize it all you need to do is spin it up to 1800 RPM then flip the switch. Natural inductive slip of the rotor will do the rest as you increase the drive speed from there.

I have done it many times with many different sizes of induction motors and it works surprisingly well despite the bare minimum of components and no actual synchronizing systems.

The only trick to using it on single phase is to make sure you don't drive its output current past what was its maximum rated input current level was for extended periods on the two lines you are using for the single phase power output.

Yes that definatly is an option if I wanted to tie into the grid as a producer. I would consider it later on. It is not as far as I'm concerned right now to be a financial alternative to the welding shop. Cash flow wise. I was considering a 48 volt dc motor / 120 volt ac motor ( used as a generator) combination as you detailed but was thinking the efficiency ratio would suffer on low amp (10) load and could never economically use the full amperage (100) available from the batteries.

I am not following what exactly you are after then.

The idea is based on motor capacitors. The capacitors simply supplys the extra voltage needed on start up. A dc surge protection for computers is based on the same principal. If the line voltage fluxuates the dc battery (that is inverted to ac )makes up the differance.
Economically and efficiency speaking it's wiser to run the vw engine on the generator at a higher rpm maxed out for hp and geared for proper rpm.( for generator) And use battery banks to cushion load startup. I need sine wave synchronization for the ac exciter and inverted ac voltage off of the batteries to compensate for line fluctuations because of different ac motors starting up. The idea I'm getting is to just use the inverter and forget the ac exciter altogether.

@tcmtech
38.4 kwh is worth about ~ $400.00 in welding income.
38.4 kwh is worth about $9.00 sold back to the grid.
Both cost about .50 to produce using natural gas.
These are just very rough figuring just to give you an outline of the cash flow and R.O. I .

Hey merry Christmas to all and thanks all for your help. I guess I needed the feedback to figure the obvious. Lol
H.

Sorry-... $0.24 per kwh

Click to expand...

38.4 kwh is worth about ~ $400.00 in welding income.

Click to expand...

Huh?

38.4 x .24 = 9.216 not 400

You appear to be missing a lot of info and understanding of how co gen systems plus generators and induction motors in general work here.

Can we start from the beginning by you describing exactly what it is you are hoping to accomplish ere and why you think as such?

Sorry but u seem to be under the impression I want to sell the power to the grid. I merely pointed out rather than selling the power to the grid for ~ 9.00 it is worth to me ~400.00 in shop time as previously explained. So the set up I described has a better roi with the value added and efficiency ratios as per stated and set up. All I needed and was looking for,was a device to sync. the sine wave off of the exciter with the inverted batteries. The one fellow suggested the solar setup, which was perfect but cost more than it was worth. I was hoping for a cheaper way and there is;- with two light bulbs but needs an experienced hand because the consequences can be costly. Like I said its not worth it for 1500 w when I have the 100 amps of inverted battery power.
H.

I follow that you are not trying to sell power back to the utility but to use it yourself from that however I am lost in your explanation of where, why, what for, and how much peak and continuous power you need.

I am getting the impression that you want to use the stock auxiliary power output as a reference point to synchronize a grid tie type inverter setup to it in order to use the higher capacity welding power output as the primary power source for your shop and home instead of paying for electricity from the utility company but again I am not sure.

As far as the rational of your cost estimates of NG powered generator operation VS $0.24 KWh utility power I only vaguely follow how you may be coming up with your numbers.

In your PM you sent to me you mention a $80,000 annual electrical cost.
Where and how are you getting that number? Does your welding shop use that much power a year? If so that works out to you having a base load of around 38 KWH per hour which is where I am guessing your one number may have came from.

However how it relates to $400 in welding income does not. At $.24 per KWh a welding job that burns 38 KWh would cost about $9.12 in electricity and you are suggesting that to use NG to power a generator system it would cost you around $.5 to do the job. So are you saying that you can only afford $.50 in electrical outlay costs to do a $400 job?

Now from that I am getting the impression you want to produce your own electrical power for your welding shop which I can see looks good on the up front numbers however what I am not clear on is why you are trying to run a discombobulated gen set system to do it opposed to using an outright normal power generator that puts out the correct voltage and frequency.

Going back to relating to my comments on using a large induction motor as a co gen unit if its connected to your side of your main power meter you don't necessarily sell anything back but rather you are just using it to keep your utility meter from running up a higher bill. Every KWh you co gen on your side of the meter is a KWh you don't pay utility rate for. That's all.

As far as the batteries and what not I have no clue as to how they fit in to all of this as of yet.

@tcmtech
38.4 kwh is worth about ~ $400.00 in welding income.
38.4 kwh is worth about $9.00 sold back to the grid.
Both cost about .50 to produce using natural gas.
Ok let me reexplain this post
38.4 KWH ( PER HR) from my welding generator running on ng make 400.00$ PER HR income. This what the welding generator produces per hr : 38.4 Kwh :
38.4 KWH (per hr ) @ .24 of grid power is about 9.00$ PER HR. I do not want to use the grid. I do not want to sell it back to the grid,this power my generator can produce. I will therefore use that power to make 400.00 $ per hr.
The natural gas that the system consumes , the motor welding generator, cost about .50 per hr in natural gas. To produce 38.4 KWH
The system designed as I laid it out is running at max efficiency. This produces four welding stations at 100.00$ PER HR. Thats 200 amps per welder. Now if all 4 welders and the compressor and a drill press and various other shop machines all started up exactly at the same time, the welder/ generator would stall. Now if there are 4 cat batteries in parallel totalling 1000 amps as a surge protection that (stall)would not happen. The shop power and house all get their power from inverters hooked up to the batteries.
All of this has been gone over by the two electricians I have on staff. It's a perfect system. This like I've said several times , is not the problem. All this translates to 80,000.00 $ per MONTH..INCOME. PLUS 200,000 btu of hot water per hr.
this is rough draft but the number are close enough.
Now my orginal quest. As stated "problem" and no longer is a problem bc the cost is prohibitive I might add, was to tie the exciter to the batteries for the purpose to use the batteries as a surge proctector for that ac line. 12.5 amps tied to the whole system with a 5,000.00$ sine wave sycronizer is not worth it. Problem solved.


The exciter , 1500 w 12.5 amps is an ac alternator device that produces ac power and is rectified to dc to excite the windings in the main dc generator.

@tcmtech
Once I get back to the shop I will finalize the plan with the coefficient of performance ratio and all and submit it to the engineer. I can let you know the outcome if your interested.
I would like to generate 3 phase power and i do have and i know about three phase motor converted to a generator. i can belt it off of the system for 3 x 3hp pumps for the heating system. ( and maybe making my own ng) but do not want to compromise the vw hp ...so I hear what you were saying. That can come later when I switch to a 1.9l Jetta tdi.
Thanks for your input and look forward to further posts
Sincerely
Henri.

So are you actually already making $80,000 a month doing welding work with four people welding as is? If so what is your monthly electrical bill as is and what do you expect your NG bill to be with this system?


The next question I have is what quality of welds will they produce with wildly fluctuating power sources supporting them and the whole shop?

As a well experienced welder/fabricator and former welding shop service tech myself plus being a well experienced industrial tech on top of all of that and a rather knowledgeable person on grid tie inverter design and construction I can you wont get very far with a system of discombobulated devices such as this.

Second as an industrial tech I can assure you your peak load power levels are way past what a set of 48 volt 1000 amp cat batteries and any type of synchronous inverter you could possibly ever afford let alone synchronized to a mere 1500 watt base reference point will handle or work with.

For one the peak AC load demands being supplemented by a set of batteries and an inverter concept you are way off on your numbers in so many ways it bothers me.

To be honest if it was me trying this I would set up one dedicated welding station ran solely off of a stock NG fueled engine driven welder and see what the realistic operating costs plus inconveniences add up to for a month.

If it even remotely pans out then consider having each welding station running the same plus having a main NG powered generator producing your necessary shop power for everything else but in the end the only practical and realistic way I can see this full NG based power system will work is that you need to run the whole shop off off a large industrial gen set built to handle all of your peak loads and be done with it.

If a large gen set built out of dedicated parts designed to generate full three phase power at the energy levels you need for the whole shop does not pan out on the numbers your discombobulated system will be far far worse off yet.
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Lastly related to the over spinning induction motor co gen system you are not actually selling any power back to the utility. What you are doing is just generating the average power you use while staying connected to the grid and just using the grid as a giant AC battery of sorts to support the higher than average peak loads.
When you are not using the full output of the co gen system the meter does run back wards but when you use more power than your co gen system produces the meter runs forward taking that power back you put on it earlier just like charging and discharging a battery but its being done with your full three phase AC power system without all the complicated batteries, inverters, control systems and what not costing you more operating money and loosing overall system efficiency.

This works over the long term if you are balancing the total averages of power produced by you with the power used by you in that same time period which gives you the end result of the net meter readings staying at zero. To me that is the only way I can rationally see a home built co gen system working on a NG fuel source while still allowing your shop to run at 100% normal function.

But thats my opinion as someone who has studied co gen in detail and attemped to use it myself. Sorry but I can assure you there is far more to this that what you are seeing/dreaming of up front.