VFD Motor Starter with Diesel Generator 3

[txsafr]

Can you gents that are electrical gurus help me with a motor starter / generator question I have.... I'm a mobile equipment fleet manager and though I will have this in the fleet I'm not qualified to know the answer without guidance.....

We have to put a small (10 electrical HP) rotary screw compressor onto a trailer to power some diaphragm pumps (pretty continuous air requirement, hence the screw compressor). The operators of the controls on this machine will be close to the compressor, so it'll be in an accoustic enclosure. As we need a minor amount of electrical power on the unit to run a computer and some lights, we want to use a single industrial enclosed gen set to power everything. My question - DOL starting on the motor is going to require a sizeable generator for inrush power (28 amps running voltage at 230 x 3 phase). I think we can put a VFD on the compressor which will reduce our starting power enough to allow using a 15KW generator, which we have available at site. Is this a viable way to go on a machine which will see a fair amount of bouncing around as it's on a trailer? The VFD would be mounted inside the compressor enclosure, so it'll be out of the weather.

Couple of other bits of info on this..... we looked at using a small diesel engine powered compressor with a side diesel generator, but because of the sound the operator would be exposed to (95 db vs. about 75 db), we'd like to go the generator route. Gasoline engines are not allowed at the site this will be used at, so we're stuck with diesel. The additional cost of the VFD is worth it if we can stay with the smaller gen set vs. going up to a larger sized one.

Thanks for any input.

 

[waross]

Put an over sized battery charging alternator on the diesel engine. Consider two batteries and a charge isolator. One battery for starting and one battery for lights and computer.
Use an inverter to develop 120 VAC from the battery to power the lights and the computer. Depending on the capacity of the auxiliary battery you may use the lights and computer for a time without running the engine. You may run the computer alone for a long time.

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

 

[txsafr]

Thanks Bill for the quick reply.

I'd thought about doing just this, but the small diesel compressor can't be sound enclosed due to cooling considerations (I checked into this), and the little bugger is loud, about 100 db. at 3m. The overall amount of noise generated by the package is one of the driving factors on this whole equation, and the quietest way I can figure to do it, given the space and equipment constraints to work with, was the gen set / electrical compressor. Both of those items are industrial goods also, which will have a much longer lifespan than the 17 HP Kohler motor on the diesel compressor. The operations group put together the first iteration of this unit, and put a small recip compressor on with a single phase motor which they plugged into a light plant to run, and promptly burned up. I got wind of this and am working to get them something that will be as operator-proof as possible and totally self-contained so they don't have to use anything external to run the unit. It's oilpatch work, so you know how an anvil can be broken without much effort if given the chance..... Overall, if a gen set with electrical compressor would be doable, I'm sure it would run for a long time; I had a 20 HP electric motor screw compressor which had 20.000 hr on it at a previous operation and it was still going strong with nothing ever having been done to it except routine compressor maintenance.

 

[jraef]

A VFD is a viable solution to lowering the genset size. But you are right to be concerned about the vibration / road bouncing. 10HP at 230V will be a sizeable VFD with a lot of capacitors, their mass and orientation will lead them to an early demise in my opinion. I know of OEMs that do this all the time, but they typically only really care if the VFD outlasts the one year warranty. If you want to try it, look for one that uses a DC choke because although the choke is heavy, it is very robust and the use of the choke will reduce the amount of capacitors being used, which will reduce the failure potential. I would also get out the RTV gun and goop the heck out of everything you can safely goop.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

You've stated that the gen set is breaking your sound budget. You really should reconsider a diesel powered compressor. That gets rid of three large or heavy devices, the generator AND the motor, AND the VFD. Each of which - you have to pay for. Also the the losses incurred by the generator, the motor, and the VFD which you will continually be paying for.

Lots of applications need mobile direct-drive air. Find something that works. Get one with a remote cooling package so it can be silenced. Don't get hung up on screw compressors. Recips last a looong time too. It could drop your costs and broaden your alternatives.

For the lights and computer follow waross's suggestion but go further. Install the correct number of batteries and a battery isolator for long duration comp and lights. Charge them all from the vehicle engine. Switch to LED for lights so you can run with 12V and very high efficiency. Make sure the computer is a laptop so it's efficient and has its own UPS(internal battery).

One alternative, if you figure out the comp and lighting to be reasonable, would be to run a single deep cycle battery and use the air compressor's diesel alternator, for it's own starting battery, to supply 12 or 24V to the comp/lights battery.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

"One alternative, if you figure out the comp and lighting to be reasonable, would be to run a single deep cycle battery and use the air compressor's diesel alternator, for it's own starting battery, to supply 12 or 24V to the comp/lights battery."
I agree with the comment;
"It's oilpatch work, so you know how an anvil can be broken without much effort"
These guys regularly blow up their cell phones (banned on site by the way) by plugging them into the cigarette lighters on 24 Volt systems on front end loaders and other equipment. If they kill the battery by forgetting to turn the lights off there is to high a possibility that booster connections will blow the alternator.
When I find a trailer mounted gen set with a full to the brim fuel tank, dead battery and blown alternator I assume that first the fuel truck missed this unit and it ran out of fuel. Then either auto-start or a roughneck killed the battery trying to start it.
When the fuel truck arrived the jumper cables were hooked up wrong polarity and the alternator smoked.

Back to the issue at hand. I would spend some effort looking for a solution to the sound issue. Then use itsmoked's or my suggestions.

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

 

[txsafr]

Keith, Bill

Thank you for the suggestions. This is one of those situations where there is more actually involved with it than a short thread here can say. In my career I have designed and built mobile equipment which is being used in about 30 different countries over the last 25 years, so a lot of the hard-learned experiences there have come into play with this project design. Given the budget we have, the space to work with on the in-use trailer we have to mount everything to, what they're actually going to do with it, and what the benefits may be having power from a gen set vs. just the diesel compressor (we'll also need to mount a hydraulic power pack to test a well blow out preventer's rams on it that will need to be powered, hence the benefit of power), and the availability of gen set we have and good price on a compressor manufactured locally that can have the VFD factory-installed, I've plotted out this scenario. Dollar-wise I'm a couple of grand more to go with the gen set and VFD than going to a straight diesel compressor (without sound enclosure - new cost, under 10 grand for this), so the money is not a huge make-or-break on it.

I agree with waross on the inverter and batteries for the laptop and/or lights, I've done that on many units and am looking at that on this one again. Rotary screw on this one because of the duty cycle - will be at 100% of air useage for a lot longer than recips are designed for due to compressor air end cooling. We have the compressor selected we want, it'll run with the electric or diesel motor. On this note, the compressor is a small one- basically the size of what you see on tire service trucks. It is about half the size of the small tow-behind units that you may be thinking of as a small diesel compressor, we've considered that scenario but cost gets way up there compared to what we're able to do on the small compressor, especially if we use our gen set.

Thanks again for your quick input. First time I've used this web site, it will be a great resource and I'm sure I'll be able to contribute also in the realm I have expertise in.

 

[waross]

Thanks for the come back and information. That's worth a star.
One more option that may work. Can you direct drive the compressor from the gen-set diesel? Possibly belt drive, possibly a jack shaft from the front of the crank? Probably not but it doesn't hurt to explore all options.

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

 

[txsafr]

Bill, I'm actually looking at that option right now too. I can get a canned gen set and compressor, but it may be better if I just put together a small diesel engine drive hydraulic pack, enclose it all, and still get a canned hydraulic drive compressor. I'll let you know where this all ends up...... Ah, the fun of lots of options.