kbsparky

I have a job to connect an irrigation system in a rural area. It requires 480 Volt 3 phase power.

The only available utility power here is 120/240 Volt single phase.

I am trying to determine whether I should recommend a rotary phase converter for this setup, or should we simply move on to the 21st century, and specify using a VFD?

The irrigation pump is 25 HP, and the rest of the system has (4)½ hp drive motors, and a 2½ booster pump. All of them operate at 480 Volts, 3 phase. Even the control box requires 480 Volts for its operation.

Would it be cost effective to get a VFD unit(s) that have a 240/1 Volt input, with a 480/3 output?

Edit to add: I understand that most rotary phase converters only add a phase, and I would also need a transformer to step up the output to 480 Volts as well.

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-KB

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HARRY304E

I would say that you are better off using a VFD.

With a Rotory phase converter i believe you would also need a transformer right.

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micromind

I don't know of any VFDs that have 240 1ø input and 480 out. It might be hard (and expensive) to find one that could handle this much load. Some 3ø drives can accept single phase input, but you have to oversize them.

A rotary phase converter and a transformer would likely be less money and more readily available.

Is the pump submersible? If so, it will be more sensitive to voltage imbalance, a VFD will produce better balance than a phase converter. a 2-1/2HP motor sounds IEC to me, these motors are way sensitive to voltage imbalance.

If you could get a transformer that is 240∆ to 480∆ it'll help with the voltage balance. I'd suggest a 45KVA, a 30 will be pushing it a bit. Check into drive isolation transformers as well as basic power transformers.

kbsparky

Yes, the pump is submerged in the well.

The 2½ booster pump is located at the far end of the irrigation array. It boosts the flow for the outer nozzle, so it can get passing cars wet ....

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user4818

I'm curious as to why this is the case. No 3 phase primary voltage available?

retiredsparktech

Why was this equipment installed in the first place, if there is no power source. 25 HP is a little large for a single phase VFD. If one is available is the service large enough to handle it.

kbsparky

Nope. This is in a rural area, a corn field out in the middle of nowhere.

This system has been operational for many years using a tractor-mounted generator. He uses the same gen set for 3 different fields, irrigating one, then moving to the next, and so on. There are cord sets on the pump control box, and also the irrigation control that plug into the gen set.

The farmer is getting tired of hauling the generator around, buying fuel, engine maintenance, etc. I figure that would get O-L-D after a few years ....

The power company has single phase service out there, supplying a few houses across the road. They won't run 3 phase without costing a fortune, however.

So we are left to deal with using single phase, and trying to make it work.

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JRaef

You need to do some serious consideration of how you are planning to get power to this. It might be worth considering the cost of getting 3 phase dropped from your utility.

VFD issues:

1. You cannot run multiple motors from one VFD independently, meaning they need to ALL turn on and off together by using the VFD, and they will all run at the same speed should you choose to take advantage of that. So a VFD does not make for a good SYSTEM phase converter, it's best used as a controller / phase converter for one motor at a time. That means buying VFDs for each motor. (although I have no idea what your entire system is doing here).
2. VFDs do not as a rule step up the voltage either. So you would have to get a transformer to step up your 240V to 480V single phase. Then you would need to use 480V VFDs that are capable of 1 phase input. Technically all are, but some do not allow it, because they have built-in phase loss protection that cannot be defeated. It's just something that you need to look out for if you go that route.
3. To use a VFD as a phase converter for a single motor, you must DOUBLE the size of the VFD to the motor size. Therefore for your 25HP irrigation pump, you must buy a 50HP 480V VFD. Then the NEC rules for supplying and powering a VFD call for a MINIMUM circuit size of 125% of the VFDs maximum rated input power, NOT the motor connected to it. A 50HP VFD is roughly 72A (maybe more), so that means the MINIMUM circuit size you can feed it with is 90A on the 480 side, which means 180A on the 240V side. That is JUST the irrigation pump so far. So as it relates to #1 above, this means you need a 50HP + a 5HP + (4) 1HP 480V VFDs.

RPC issues:

1. RPCs are a better solution to running multiple motors that will turn on and off independently of each other. But the drawback is energy efficiency, you are running an "idler" motor to make that single phase into 3 phase and that motor must run all of the time when anything else is running. So to power this system, you have approx. 30HP connected and if they ever all have to run at the same time, you will need probably a 50HP rated RPC which will likely have about a 10HP idler motor. That then means if you have to run only one of the 1/2HP motors, you will also be running a 10HP idler. It will be basically unloaded and not consuming MUCH power, but it will be consuming more than a VFD would.
2. You still will need the transformer, but with an RPC I would do the transforming on the 3 phase side because it would be a smaller cheaper transformer.
3. You have to make sure that the control panel power is taken from the "real" legs, not the manufactured leg. Not big deal usually, just something to be sure of.
4. You will need to buy essentially a 30HP 1 phase combination starter for the 240V side of the RPC, that means again, the circuit size will be around 150A. Make sure you have that much available in your 240V 1 phase service.

In either case, also make sure you account for voltage drop, it's a big deal at that kind of power level.

kbsparky

Not going to happen. Got 100 grand lying around? Didn't think so.

We were considering using a 50 hp rated unit. So far so good.

Not likely. Those motors control the wheels on the irrigation array, and have to all operate together. Otherwise, the array would get out of whack/sync. And THEY don't run unless you have the water pumping thru as well. They all have their own VFD's in the main control box, as each motor has to operate at a different rpm in order to keep the array aligned straight.

I figured as much. Wouldn't this also solve the problem of using the control power from any of the output legs as well?

Difficult to do once the 480 transformer is installed, no? All the controls require 480 Volts here.

We were planning on installing a separate 200 Amp service for this project.

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micromind

Given the above new info, I would lean more toward a rotary phase converter. Feeding a 3ø VFD from the output of another VFD will almost certainly be trouble.

If you use a 240 volt phase converter and a 240∆ to 480∆ transformer, it won't matter which two legs the controls are on. It will also balance the voltage better.

A 200 amp 1ø service will do it, but just barely.

Se if the POCO will connect a 480 1ø service. Around here they will. If so, see if the VFDs on the wheel drive motors will accept 1ø. They might, probably not though.

The wheel drive VFDs will be the critical factor here. I don't think they'll accept 3ø power from another VFD. Possibly if there are both line and load reactors involved, but I'd need to see hard data from the manufacturer. They might however, accept DC power from the main VFD. You'd need to look into whether the main VFD will be able to supply DC and whether the smaller VFDs will accept it. Just because it has DC terminals doesn't mean it'll work though.

I have a sneaky suspicion you'll be using a phase converter and a transformer.

JRaef

Well that's new info. Most likely they may be 1HP drives on those 1/2HP motors, because on a lot of 480V VFDs, 1HP is the smallest they make. So that may allow you to consider just buying two VFDs, a 5HP for the booster and a 50HP for the pump. But in fact if the booster and the pump are always going to run together and you don't need to vary the speed of either on,you may be able to get away with a single 50HP VFD to run both. It's probably a toss up by way of cost.

You really have 480V controls? No control power transformer? Haven't seen that in years. But anyway, having the 3phase transformer won't really "cure" the ills of the manufactured leg that comes off of an RPC, the issues just pass right through. But the problems are worst for electronics and controls that use a Neutral, i.e. 120V off of a CPT, so if you don't have that. it might be fine.

If it were me, I'd go with the RPC. I love VFDs but sometimes they can create new problems that you didn't already have, and farmers are not big fans of electrical problems in my experience. They like things simple and clean.

Good luck.

kbsparky

Yeah, I won't be needing to vary the speed of anything here. My idea to use a VFD came from a recent elevator job we did. The elevator had a 3 phase motor on it, but only required a single phase circuit to run the thing. The VFD had its own inverters built-in, and "made" the 3 phase from the input which was rectified into DC first, then back into the 3 phase AC. A battery back-up was also built in, allowing the elevator to return to the ground floor level in the event of a power failure. Such a system should be able to start with just about any voltage, and make its own output at whatever level it chooses.

It was a small VFD, but I figured they make `em in all sizes so why not explore the possibility here?

I will still inquire at my local supply house if they have anything that will do the job, and compare the hardware costs, etc.

I have an inquiry into the POCO about them providing single phase 480 Volt service, but I am not holding my breath. They would have to set another transformer, in order to provide that voltage. Don't know if they are willing to have 2 transformers sitting out there, when one would normally do.

The closest 3 phase is miles away, so that cost is prohibitive, otherwise this whole discussion would be moot.

The farmer believes that even single phase power with the extra hardware will be preferable to his current routine, and at lower costs.

I figure we will have to install some sort of time-delay circuit to allow for the rotary phase converter to get up to speed before the pump and system kicks in. Probably will use a control transformer and small relay to do the job.

Finally, yes, all the controls require 480 Volts. I had to do a double-take on that as well. No neutral, no 120 Volts, etc.

I'll try to get some pix of that control box next time I'm out there ....

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-KB

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Jlarson

It pretty much be a no brainier for me. Farmers want no frills, cheap, and bulletproof. Defiantly a RPC w/ xformer job. A simple circuit to start the RPC first and shut it down last and all is well.


On a side note, why the disbelief with 480 controls, it's like a weekly thing for me.

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hardworkingstiff

I'm sure you know this, but I'll state it for those that are reading and might not know it. The 3rd leg created by the RPC must not feed the control circuit. It's only to be used for the motor.

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alpha3236

The farmer is definately correct that this will save money. The cost of diesel being what it is. I do this type of work all the time and have found that roto phases take a lot of expensive replacement do to surges & lightning. I replace 5 or 6 a year. For the last couple of years I have been using a static/electronic unit from Phase Perfect and love them. They only come in a 240V configuration but they will supply the transformer for the boost to 480V & give you a great warranty. They also have intelligent people to talk to over the phone. The equipment's not cheap but gives a great install. As any of my customers that have replaced a roto phase can tell you, it's worth every penny.
This is the only static type converter I know of that will handle variable loads, gauranteed. And the POCOs like them because starting a roto phase is a real line draw issue. Let me know how you make out, or if I can be of assistance.
www.phasetecnnologies.com

kbsparky

And how do you tell which is which, once it has been transformed up to 480 Volts?

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Big John

More to the point, once it's been transformed, why would it matter? Seems like it would simply appear as a very slight load on the derived phase, and I can't imagine that would be a problem.

-John

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kbsparky

Exactly my point. An insignificant load if there was any at all....

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kbsparky

Bad link.

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MDShunk

Since the drives already exist in the control panel, and the control panel already exists, the only reasonable option (in my mind) is the rotary phase converter. If this was being designed from scratch (nothing existing but the motors), my opinion might be different. As it stands, get a rotary phase converter and call it a day.

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