Need advice on remote site - generator plus VFD

miro · Jul 31, 2014

We've got a small shop far away from the main panel for the utility service.

And we've got a a couple of machines there that only run infrequently - one or twice a year - and they have 3 phase, 220v motors- 5 HP.

Rather that spending lots - and I mean thousands of $$ - to bring 220 V to this remote shop for such infrequent use, I thought a better way might be to find a 7 or 10 kW generator ( used) which makes 220V single phase, then get a 5 HP VFD to convert the generator output to make the motors work.
It certainly won't set us back nearly as much as the utility wants to charge.

I already have some experience with VFDs, and I like the idea of the soft start.

Thanks,

miro

SteveBausch · Jul 31, 2014

From what I've read, you will need to oversize the genset by a factor of 3 or 5.

5 HP is roughly 5 KVA, so.... 15 to 25 KVA ?

Might be better off getting a surplus largish UPS iwith a genset capable of charging the batteries....75 KVA UPS units are out there, and I doubt those things hold their value.

wheels17 · Jul 31, 2014

Watch out for the cheap generators. They tend to have very poor waveforms and the VFD may not like the quality of the power. Here's an example of the problem, in this case with a standby generator.

http://support.fluke.com/find-sales/Download/Asset/3497420_6112_ENG_A_W.PDF

I knew a guy that had a native 3-phase unit that was originally built for the military that he picked up at auction for an excellent price. It was a really nice unit. I'd assume that they had some pretty tight specifications on power quailty.

steve_p · Aug 1, 2014

If you go the surplus mil gen set, make sure it is a 60hz unit, not a 400hz unit designed for avionics.

ahall · Aug 1, 2014

I have to agree with the others, if you need three phase power, look for a second hand 3 phase gen set.
I have never been in the market for one, by I suspect the demand is lower than for single phase gen sets.
They may be less expensive per KW.

Several years ago I lived in a rural area that was hit by an ice storm. Knocked out power in the area for weeks.
Lots of folks bought welders and generators to drive things around the house.
Several folks lost the printed circuit boards in their gas heaters because the signals produced by the generators were not "clean".

DaveKamp · Aug 1, 2014

I'll echo the same- a 25kw three-phase liquid-cooled three-phase generating plant of the 'industrial' grade is not difficult to find, and in most cases, inexpensive.

Reason being... that there's quite a few around in areas where small 'process-sensitive' industry areas are either closing or growing. Closing, the machines are pulled out and auctioned or scrapped. Where growing, the old machines are replaced with larger, higher capacity plants with integrated remote monitoring, emmissions compliance, and a bunch of whiz-bang features. This makes the smaller ones easy to get... just have cash, and a trailer and equipment to move it... because the best deal is found as-is, where-is.

miro · Aug 1, 2014

I appreciate the views and inputs.

But we are operating motors, no electronics.

The most electronic -y thing will be light bulbs - incandescent type, and besides in July we have very long day light hours
and we can open the shop ( actually more like a shed) because it is warm.

I understand wave form distortions and higher frequency harmonics but so long as they are not outrageously bad, I think the motor should be OK.

It would be nice if the power utility were a bit more reasonable, though.

I also understand that lightly loaded alternators are quire inefficient. The starting current should be moderated by the soft start of the VFD.

So a 10 kW - 12 kW - 15 kW genset is probably what we should look for, or even a PTO powered alternator ( we have tractors)

But I'd like to hear more views on amount of power needed. Aside from starting current for the motor which will be moderated by the VFD soft start
is there any compelling technical reason to get a bigger machine?

Thanks,
miro

mike_kilroy · Aug 1, 2014

I'll play devil's side....

harbor freight 7kw gas 120/240 gen, $ 400.00.
= install time: 15 min

7.5hp non-china drives, $ 700.00
= install time: 30 min - use all default settings to run quick

$1800.00 total. only concern: I have no experience running vfd on generator re: if noise is possible issue

Or 25kw 3ph w/o vfd softstart. $ ? never saw that even surplus (running) for even double this price.

Tony Quiring · Aug 1, 2014

With cell carriers upgrading to equipment that requires less power they may be selling off portable generators.

Likeky already done but they ate in the market.

Also rental companies change out fleets as they age.

Smog limits also causes tjem to be sold off as well...those would be in ca and cheap.

Diesel and propane and many are "wisper watt" built to be very quiet.

25 kw to 50 kw

jakefreese · Aug 1, 2014

A challenge with generators and pure motor loads is in many cases the pure inductive load can turn your power factor on its head in a blink of an eye. Adding a resistive load to the out put will help balance the inductive load helping the generator start the load. Actually harmonics can destroy motors just as much as electronics.

DaveKamp · Aug 2, 2014

Here's what I've come across in the last few years:

Ex Civil Defense Kurtz & Root 20kw 3phase 208/125Y (EM or Westinghouse alternator) powered by Hercules 4 cyl, gasoline driven 6v starting battery, good running condition for $650.

Ex industrial 18kw 3ph 480/277Y or 208/125Y EM alternator driven by Waukesha OHV 4 cyl gasoline/natural gas/propane, 6v starting battery, excellent condition, with Startix system, $400 each (bought two, should have bought all seven!)

Ex-USAF 35kw 3ph 12-wire, 480/277Y or 208/125Y Kato with Hercules 6 cyl, gasoline/NG/LP, 24v start

Now, you may not have such an easy time finding machines like this on your side of the border, but I'm betting you could snag one down here, and tote it up there without taking much of a beating at the customs office... and not a single one of 'em is at-risk of being damaged by a high start surge... most have 120% overload rating of 12 hours. They'll start a pretty heavy motor without even much of a grunt.

mike_kilroy · Aug 2, 2014

jakefreese said:
A challenge with generators and pure motor loads is in many cases the pure inductive load can turn your power factor on its head in a blink of an eye.

Besides the 600% starting current if no VFD, this is the other reason everyone says get a 20-30kw gen set for his 3-7kw load - so you don't have to worry about that.

Here's what I've come across in the last few years:

Dave, If I EVER came across those kind of deals, I would buy one too! In 10 years of watching, I have NEVER seen such deals. Why I finally broke down and got a slightly undersized Yamaha 6.3kw inverter model and paid a rediculous $ 3500 for it.

You have to be in the right place at the right time. For OP unfortunately, I doubt that is not going to help him get a good deal for his immediate need.

Wanna sell your extra 18kw unit? It probably has depreciated from $ 400 to what, $ 200 by now? I will even pay shipping!

jim rozen · Aug 3, 2014

Don't forget that the application in question can use the VFD to soft-start the load motor(s).

That will reduce the inrush current a LOT.

My initial guess on the OP's question, was maybe so, it might work. 10KW generator
might start a 5 hp motor via a VFD.

Possibly the best test would be to mock it up with a drive, a generator, and a machine tool.
Downside on the test is, you smoke the drive.

mike_kilroy · Aug 3, 2014

jim rozen said:
My initial guess on the OP's question, was maybe so, it might work. 10KW generator
might start a 5 hp motor via a VFD.

My guess is a 5hp generator will start a 5hp vfd driven motor just fine. I don't see any reason at all to oversize it. Remember that as it accels up to speed it is a linear increase in "HP" from 0 to 5; so even with accel ramp it does not require more than motor rating (3.7kw). It doesn't even draw the magnetizing current from the generator. So say starting the motor with a 5 sec ramp, if one uses full nameplate rated current, requires power from 0kw to about 3.7kw when it reaches speed - and assuming it begins doing full 5hp worth of work immediately, which is highly unlikely - no inrush, no 600% etc. If one wants to test it, heck, use a $ 200 3kw el cheapo generator - it should work fine. a 3kw prob is plenty big for that 5hp motor with what they will use it for too since they probably won't load it anywhere near 5hp if they are typical. I thought they had TWO pc 5hp motors tho; and if both need to work at same time, then get a 6-8kw one.

jakefreese · Aug 3, 2014

Generator sizing to motor output is challenging. At best the generator head is going to be 80% efficient, you are going to have the same efficiency issues on the motor. Plus reactive load losses. Problems with vfd on a generator is the load is non linear which typically means your generator has the be sized for twice the rating of your drive. For pulse width modulated drives you can typically go 1.4x the drive rating. To really nail down the size you have to have starting and running power factor on the motor, name plate HP, run kW, run kva, locked rotor kva and HP. The last thing is whether the load is a high or low inertia load. I consider many mills low, the majority of lathes are high.

Modelman · Aug 3, 2014

The OP's location is Ontario, last time i checked that's in Canada, so all the mil surplus stuff is likely unavailable to him. I'm sure the Canadian military sells surplus items from time to time, but I suspect the volumn is much less than the US.

One possibility is a used generator from a railroad refrigerator car. These are typically diesel driven, three phase, either 230V or 440V, likely changeable. Single car generators range from 12.5kW to 20kW. Single refrigerated cars seem to be losing favor to refrigerated containers, which often use a separate skid mounted power unit that rides on the end of the flatcar and can power up to five containers. These are typically 50kW, and could also be a possibility.

Dennis

mike_kilroy · Aug 4, 2014

I don't think I said a word about ENGINE size when I said "My guess is a 5hp generator will start a 5hp vfd driven motor just fine." I did not read into OP comments that he wanted to design his own generator from the ground up, so size of the engine has nothing to do with this discussion. 5hp of generator is still 3.75kw output no matter how you slice it up.

OP needs to also keep the two concepts discussed here separate: if NO VFD, yes, all those starting currents, reactive currents, and PF things matter. If VFD used, then they are meaningless terms that just will cause buying an excessively large generator.

miro · Aug 5, 2014

Thank you to all who have contributed to the discussion - it certainly has helped a lot .

I think we'll go with a VFD and a rented generator to see if it works out. I plan to use my clamp -on AC ammeter to see what the current
from the generator is . I plan to use a 10 second start-up ramp on the VFD.

Thanks again to all.
miro

jim rozen · Aug 12, 2014

Do please post up the results.

It's an interesting experiment and I for one want to find out
what happens!

Tnx - Jim

Need advice on remote site - generator plus VFD

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