Single Phase to 3 Phase efficiency 3

[ptmoss]

I have a client interested in constructing a septage treatment facility that would include dewatering presses and pumping resultant liquid component about 180 feet (vertical head) up a hill to some leaching fields.

He has a great property for it except 3-phase does not come to his property, and is way too far away to realistically have the power company extend it.

He's been told the best way to go is by getting two diesel 3-phase generators. Two so that he'll have back-up when the other is being serviced. Seems an expensive option to me.

I was curious how the single-phase to 3-phase converter's operated from an efficiency and cost standpoint. Does anyone have experience with these? Are they really that inefficient and expensive to run?

 

[dpc]

How big are the motors?

The power company will be willing to do quite a bit if you are willing to pay for it. Are you saying it is unrealistic due to cost?

If the motors are not too large, VFDs can be used to provide three-phase output from a single phase input.



David Castor

http://www.cvoes.com

 

[ptmoss]

Seven figures to extend it and it would be my client footing the bill. It's a long ways off.

The sewage pump isn't huge, 3-4 HP maybe, but he said their efficiency will be at about 40%. I think the same pump with 3-phase would be no better than 70%.

I'm waiting to hear sizes for the dewatering unit. And there's a few other pumps involved as well.

What are VFDs?

 

[waross]

Two generators for 3 or 5 HP may be massive overkill. The same pump will have close to the same efficiency whether on single phase or three phase. At that power level use single phase pumps or VFD driven 3 phase pumps. (Powered from the existing single phase.)
Note:
At that head and power you may not be able to find a suitable pump. Back in the day before VFDs were available we had an application with about a 180 foot head. We had to belt up 15 HP motors to get the head out of the pumps.
Now with cheap VFDs available we would have just used a direct coupled motor and used the VFD to get the speed that we needed.
Sizing a small pump to run overspeed to deliver a high head is not trivial for a layman. You should talk to someone who has direct experience in this area.

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

 

[rbulsara]

In this case it may be much simpler to look at an electro- mechanical phase converter like this one

http://www.phase-a-matic.com/RotaryDescription.htm

Rafiq Bulsara

http://www.srengineersct.com

 

[jraef]

ptmoss,

A VFD is a Variable Frequency Drive. They are a device that allows the speed control of 3 phase AC motors by changing the voltage and frequency together, a necessary step to changing speed without losing torque. To accomplish this, they convert the incoming AC power to DC, then re-convert, or "invert" the power back to AC again, but in a way that allows them to independently change the voltage and frequency.

A side benefit to this process is that they can also act as a sort of "Phase Converter" that allows you to run 3 phase motors from a single phase source. Because the first stage just converts AC to DC, it doesn't matter if the AC is 1 phase or 3 phase. So when it inverts the DC back into AC there is no direct relationship to the incoming power and the outgoing power phase relationship anyway; the incoming power is "raw material". When being used this way, it's unimportant that you don't need to vary the speed on the output, that's up to you. But that said, there are other benefits to matching flow rates to the requirements. That however is a longer discussion.

So here are some caveats:
1) At the sizes you are looking at, you may have to buy a VFD that is 2X the size of the motor. There are many VFDs that allow single phase input without de-rating up to 3HP, but very few that extend beyond that. It's a matter of the sizing of the components on the AC-DC conversion section and the capacitors used inside. 1 phase current draw will be higher by the sq. rt. of 3, so 1.732X the 3 phase current rating of the motor. The electronic devices (diodes) on the conversion side need to be sized for that higher current and you also need extra capacitance inside the VFD to smooth out the extra DC ripple that results from AC-DC conversion from 1 phase. So the safest way to make sure you get that is to go 2X if it is over 3HP.

2) Remember when sizing your 1 phase service that the current will be 1.732X the 3 phase values. So if for example you have 4 x 3HP motors and the FLA is 4.2A each, the 1 phase current supply will be 4.2 x 4 x 1.732 for sizing your 1 phase service and protection devices.

3) If your 1 phase is 230V, as it most likely is, use 230V motors and VFDs even though it appears as though higher voltage VFDs are cheaper. Otherwise you need to buy a transformer to step up the 1 phase to the higher level, and then not all higher voltage VFDs can accept a 1 phase input. It's just a quirk of the industry.

4) This works great for 3 phase motors, but don't power up other systems from the output of the VFDs. Use separate VFDs for each motor (assuming multiples) and allow everything else to be powered directly from the 1 phase source. Keep it all separate. By the way, the VFDs will serve as the motor starters, so no need to duplicate that.

Since you didn't know what a VFD was, I strongly suggest hiring some expertise though, this is great stuff but not for the novice.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[ptmoss]

Oh don't you all worry, there's good reason why I'm a civil engineer and have nothing to do with electrical!!

He has been talking to an electrical guy who I believe works for a company doing the electrical engineering for all their plants (concrete and some related industrial). (There's a close friendship between my client and the owner of these plants, which is why he has been able to run things by his engineers)

Your mention of the inverter - that was one of the options he spoke to my client about. But it sounds like you need one for every piece of equipment and/or pump that should use 3-phase. Which is why he was suggesting the diesel generators - which to me would seem to be more than a tad bit expensive of an option.

My question was more towards the phase converters you see advertised (correction..."I" see advertised...) - would you also need one of these for each piece of equipment? I believe that is similar to what rbulsara suggested.

I also should have gone back and read my notes before my original post - this project has been on the backburner for a while and my memory wasn't too good!! - we had two pumps spec'd by the pump company he uses....one 5-HP at the proposed facility and then a second halfway up the hill to push the effluent to it's final disposal area. I didn't like this setup for a variety of reasons but we aren't at the point where I'm designing it yet so all we're going by is my clients discussion with his pump guy where he gave him the vertical separation between facility and disposal field.

jraef and waross - thank-you for your comments!

I know I've oversimplified things. As usual when I think I'm looking for a simple answer it never is! I'm trying to get my client some alternate thoughts as I really don't like the diesel generator suggestion. That to me is a short term solution if you were waiting for 3-phase to be extended to the property. Which as I mentioned is cost-prohibitive.

 

[VEBill]

How about a diesel-powered pump (skipping the electric)? Use holding tanks so that the pump only needs to run once in a while.

Obviously you'd want a backup pump. If they're cheap, maybe three in total. The backup pump(s) could be much smaller, they'd just run longer.

 

[fsmyth]

How many three-phase loads?
If just one or two, the VFD options are simpler.
If the loads are variable or greater than 3 or 4,
then a rotary phase converter makes more sense.
Size it as large as the panel rating if the loads
are unknown. The one nice thing about converters
is that the more 3-phase loads are applied, the
better it works. Static converters are also an
option, but not recommended.
<als>

 

[jraef]

RPCs (Rotary Phase Converters) are great for things like machine shops where you need to power multiple machines at once sometimes, but they are horribly inefficient if the system has to be on "stand-by" such as a pump station application.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

I mentioned a high head application in my first post. This setup originally had two 15 HP pumps on an alternator. The system worked well for years. Then changes were made "up the hill" and the head was increased. This extra head pushed the existing pumps so far up their curves that with one pump running the flow was just a trickle and with both pumps running the flow through each pump was so little that the effluent was actually boiling in the pumps and burned the paint off.
A new pump was purchased and installed. It was belt driven to increase the pump speed to get the head needed.
That left us with two 15 HP pumps that couldn't quite do the job.
Others were going to replace both of them with one more belt drive pump to provide back up. I had the pumps re-plumbed in series so that the discharge of one fed the suction of the other. It worked extremely well.
The point is:
There is no need whatsoever to put one pump halfway up the hill.
The second pump will need a housing capable of withstanding the total head but finding such a pump should be fairly easy. The money saved by putting both pumps at the same location makes it well worth the time to locate a suitable pump.


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

 

[waross]

Something to consider is the cost of running the diesel generators.
You will get about 13 KWHrs from a gallon of fuel with a loaded genset. If the main load on the set is a motor, the set must be oversized and so will not be operating at the most efficient loading.
The result? You will probably get less than 13 KWHrs per gallon.
Now take the fuel cost and double it to allow for oil, filters and repairs. Your estimate will still be low.
A real world example;
I spent quite a bit of time on a small island. The local utility used diesel generators to power their grid. The price of electric power was about twice the price of fuel, but that is not a fair comparison as their sets were loaded and working at almost 14 KWHrs per gallon most of the time.
What is a fair comparison is the large numbers of people with standby diesel generators who had found out over the years that it was cheaper in the long run to buy their power at twice the cost of fuel to generate it themselves.
So, calculate the cost of fuel and double it and you will still be low.

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

 

[MikeWryley]

Modern VFDs are the way to go for this app,
go to Ebay and do a search for 5 hp VFD, if it has a single phase in, buy two, one for each motor. Comparatively speaking, they give the most bang for your buck, a diesel will just be a pain in the butt.

you could probably get a 4 hp to work with a 7.5 hp VFD three phase in/three phase out, only using two of the three line inputs.

I use these in my home shop for three phase lathes and mills. Also in a large building for a ground water heat pump system. They also have the advantage of a ramped start up, saving wear and tear on your motor.