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Low Voltage Continious Duty 7.5 hp Motor

Low Voltage Continious Duty 7.5 hp Motor

Low Voltage Continious Duty 7.5 hp Motor

(OP)
I'm working on a Solar/Wind-Turbine powered Oil & Gas pumping application.  It is about 3,700 Watt requirement, but factoring in the hydraulic losses I decided to bump the motor size up 50% to 7.5 hp (5,655 W).  The first design called for a 240V ac motor and a $10k (Canadian) inverter.  The specs on the inverter said that at full-load (which I never expect to see) it provides 80% of the input power to the output and I don't want to pay the $10k or take the inversion efficiency hit.

My question is since I'm planning to have 32 batteries to handle the calm/dark times and I can wire them up any way I want, should I be looking at 48V dc (i.e., 4 groups of 8 batteries in series, the groups wired in parallel) or 96V dc (i.e., 8 groups of 4 series batteries, the groups wired in parallel)?  The 48V dc batteries that I've found have been designed for golf carts and electric vehicles and are not recommended for continuous service.  I've found a bunch of 90V dc motors in the size I'm looking for that are rated for continuous service.

Anyone have any recommendations on which voltage I should be looking for?

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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RE: Low Voltage Continious Duty 7.5 hp Motor

Generally the conversion losses are lower as the voltage goes up, certainly in the voltage range you are looking at. Paralling batteries is generally not a great idea: find a larger cell or bloc. Blocs usually come in 6V or 12V and up to about 200AH or so for the 6V types After that you're looking at individual 2V cells just to make them more manageable in size. The duty sounds similar to that for a UPS - long periods on float charge followed by heavy discharge. Might be worth considering the types optimised for that kind of service - cyclic types such as vehicle batteries use different construction.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Low Voltage Continious Duty 7.5 hp Motor

Why not hook the batteries up in series and feed the DC bus of a VFD?

They are usually way more efficient than your quoted inverter efficiency.  Depending on the pump you use, the variable speed aspect could be used to optimize your energy use verse supply availability.  It could also assist with reducing the large inrush of a start to a limited source system like you'll have.

DC motors have brushes that will need to be cared for and DC motors are something like 80% efficient which is a lot less  than a three phase AC motor.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Low Voltage Continious Duty 7.5 hp Motor

(OP)
I knew I was going to get bitten by the old definition of "easy" (i.e., "someone else has to do it").  So you guys are saying that the AC option (with tweaks) will give me better power usage than any of the straight DC options?

That sure seems backward to me, but I'll accept it (especially it is in the same direction as the packager's recommendation).

Thanks for looking at it.

David

RE: Low Voltage Continious Duty 7.5 hp Motor

All things battery/wind/solar hinge on very small details.  Many large details are missing from your description, so we are only talking in broad strokes.

As and example your battery lifetime can be one of the most expensive issues.  Over drawing them murders them rapidly. So your supervisory system needs to take many things into account to optimize battery life. Meanwhile the pumping may have some sort of payback that might make sense to kill batteries over.   If the wind isn't blowing at certain times you may get little utilization from it.

There are so many little details that go into systems using ONLY renewable energy sources you have to work all the numbers.  You can't just buy from column A,B,and C like you often can with utility fed jobs.

Good luck, sounds like a fun project really.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Low Voltage Continious Duty 7.5 hp Motor

(OP)
I've done 2-3 of these renewable projects over the years (this is the first one with a significant motor load) and the details do make or break the project.  The reason that I brought it up here is that the quote from the fabricator didn't "feel" right.  It actually felt like they were doing their engineering from the surplus list (e.g., the max load is 3.7 kW so they sized a 13 kW wind turbine since they had one in stock).  I was trying to get a reality check.

David

RE: Low Voltage Continious Duty 7.5 hp Motor

Just to be clear, $10k, even Canadian, is an exorbitant price for a 7-1/2HP 240V inverter, and 80% throughput is abysmal. I would venture to guess that you did NOT contact your friendly neighborhood VFD supplier. I would have sold you a VFD that size and rating for around $850 US, which even with a bad exchange rate is a far cry from $10k! A standard VFD would be at about 95% throughput efficiency worst case, especially with a DC input. This is not a solicitation, I'm just pointing out that something is seriously wrong with your source my friend.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: Low Voltage Continious Duty 7.5 hp Motor

(OP)
That's how it looked to me.  If I can hijack my own thread, can you bring dc into a VFD and get ac out?  So a VFD is a soft start, a speed control, and an inverter with 95% efficiency?  These things seem like magic.  Actually the inverter in the bid was 15 hp (from the surplus list I'm assuming).

David

RE: Low Voltage Continious Duty 7.5 hp Motor

A VFD COnverts the AC utility power to DC, then INverts the DC back to variable AC. So putting in DC just bypasses the first stage. The DC must be at a fairly high level, about 320VDC for a 240VAC drive, so keep that in mind fr your battery configuration.

Not every drive is capable of DC input because of where some derive the control power to run their own electronics, but probably 95% will be.

And yes, the technology involved has become very efficient over the years.


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RE: Low Voltage Continious Duty 7.5 hp Motor

I didn't see any inverters there sreid?

zdas04; You could also use an inverter/charger.  This may be what you were considering in the first place.

They have them that take in wind/solar/generator and output  120VAC while keeping a large battery bank charged.

You would want a VFD to prevent large hits to the system when starting.

http://www.xantrex.com/web/id/2/type.asp

Keith Cress
kcress - http://www.flaminsystems.com

RE: Low Voltage Continious Duty 7.5 hp Motor

Re the sizing of the wind turbine. Average power runs about 15% to 20% of peak power, so the 13 kW turbine doesn't look as bad as you may think. They have average at about 28%, other suppliers may have spec'ed the turbine at 18.5 kW or even 27 kW to supply 3.7 kW average.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Low Voltage Continious Duty 7.5 hp Motor

(OP)
Bill,
This application has 10 kW of solar panels, 13 kW of wind turbines, and a 15 kW gas powered generator.  With maximum pump capacity, the pump needs 3.7 kW.  With the well's estmated liquid capacity (estimated by offsets), the pump needs less than 2 kW.  What the packager did was take the max hp (that no well in the field has ever needed), then designed each of the 3 systems to supply 300% of the requirements (since it is dark at night, the solar panels need to provide a day's power in 8 hours, some days are calm, etc).  I would probably design it for a combined 500% of nominal instead of 900% of maximum.  I'm still trying to get my arms around how much safety factor is warranted and how much is wasted capacity that will never be utilized.

David

RE: Low Voltage Continious Duty 7.5 hp Motor

In addition to voltage and capacity of the batteries, I assume you would want to know something about the voltage drop of the batteries under load.

=====================================
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RE: Low Voltage Continious Duty 7.5 hp Motor

(OP)
Good point, the line item on the quote for batteries is $420 (Canadian) per battery, so I don't think they're Wal Mart stuff, but I haven't seen the specs.

David

RE: Low Voltage Continious Duty 7.5 hp Motor

I'm guessing the $10k inverter will be a renewable energy specific inverter. It likely has some controls for battery charge/discharge limiting in it and might take a lower voltage input to get the higher voltage AC output.

A standard industrial drive could be used but you'd have to plan for a minimum 360VDC battery bank to run at 208VAC 3-phase. Basically, the VFD would run the motor from the batteries all the time. The solar, wind and genset would then all have to charge the batteries in parallel. You will need some kind of charge/discharge controller for each system and finding 360VDC parts in this power range can be tough. You would also need a battery monitor to turn-off the VFD if all else fails to ensure you do not over discharge the batteries. It sounds easy to use an cheap industrial VFD until you try to do it.

I would investigate using 48VDC with each system charging the batteries. Then, use a 48VDC brushless motor and controller.

I agree with Bill. The wind turbine won't produce anything close to 13kW most of the time. Still, you will need a resistor bank or a shutdown controller because you must use the power the turbine generates or dump it off in heat or furl the blades to limit the power produced.

You won't get full output from the solar panels in the morning or evening so they're not just oversized to account for it being dark at night.

The gas genset changes things. You will need to decide how often you want it to run before sizing everything else. The integrator might have picked 13kW because that was the smallest 3-phase unit he had available. The desired duty cycle of this genset can lower the size and cost of the wind, solar and battery components.
 

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