electricity cost of single vs three phase

[petroremedy]

I am designing a groundwater remediation system in a town that does not have three phase power. The closest three phase line is 0.5 miles away. Because the system would not be there for more than about 5 years, the power company wants about $15k to install three phase to the site. I have always used three phase in the past because the motors have to be XP (Class I Div I area) and there are usually a couple motors above 5 hp. The total horsepower of the system will probably be about 25 horsepower. If it was a three phase system, there would be one 15 hp blower motor, one 7.5 hp blower motor, and a couple 1 to 2 hp transfer pump motors.

I'm trying to determine whether to use phase converter/VFD, limit motor size to 5 hp (requiring multiple blower units), or pay to get three phase. Some people have told me to get the three phase because the motors will last longer, especially with frequent start/stops. I am also trying to evaluate the electrical cost of operation of the system in single phase versus three phase. Is it more expensive to operate the same sized equipment single phase?

Thanks in advance.

 

[BJC]

Is there natural gas available? If so I would consider a natural gas generator. I might even consider a diesel powered genset.
The genset has a salvage value, you know the installation cost of three phase system( more than a generator of 75-100 KVA), you know the energy cost. Sound like a textbook engineering econ problem.

 

[itsmoked]

petroremedy,

I have to agree with BJC. You should probably seriously consider a diesel generator.

1) They can be so quite it's amazing. I have stood in front of generators that I thought were surprizingly quite even for idle, (not audible at 50 ft) only to be shocked when I looked through the glass window at the gauges.. It was putting out 90KW!!! 208 3ph. They are now used all the time in construction in residential areas. They drop them on a skid and drop a skid mounted fuel tank next to them and leave them running for months while they keep trenches dewatered, etc. WhisperGen is one brand.

2) With the current price of electricity a lot of companies are finding on site generation (diesel) cheaper then buying it from an already installed utility.

3) I suspect your project is temporary in nature so that salvage value makes a lot of sense.

4) You can arrange with a fuel company to come by on a schedual.

The one key though is if you go this way make sure the gen can't grow legs.

As to the single phase, even if you can jury rig it up[note bias ] The utility may be unhappy about three phase sized motor loads on their 1Ph line.

 

[UKpete]

petroremedy, a couple of points:
1. Three-phase motors are more efficient and more reliable than single-phase motors (which may have centrifugal switches, and capacitors)
2. If you are running the three-phase motors from a single phase supply, using VFDs (mains feed single phase, motor feed three phase) then there is not much difference in system efficiency or reliability compared to running three-phase motors DOL on a utility three phase supply. This is because VFDs have an efficiency of 95-98% (depending on rating) i.e. very high compared to the motor (typ 85-90%).

When I looked at all the major VFD suppliers (AB, ABB, WEG, Siemens ...) I didn't discover a 3-phase motor drive running from a single phase supply above 3kW.
However, provided the utility company are happy for you to draw the 25hp from a single phase feed, it may be possible to run larger motors from modified VFDs. A VFD has at it's front end a rectifier to produce a dc link which is then fed to the inverter stage, so you could use say a 25hp VFD and replace the 3-phase rectifier on the input with a single phase rectifier of the same power rating - I am not necessarily recommending the latter option as it is non-standard therefore extra costs apply, merely pointing out that it is technically possible. You would have to speak to a drives expert.

Regarding the diesel generator I make the running cost about US$0.20/kW-hr for a Kohler 20EOR/Z (20kW at 1.67 gall/hr, diesel at $2.30/gall - is price this realistic?).

http://www.kohlerpowersystems.com/pdfs/g3026.pdf

Maybe it would be cost effective.

 

[jraef]

Assuming you have already considered site generation and ruled it out (environmental restraints are usually the big issue), there really is no direct difference in the electricity costs of 3 phase vs 1 phase, a kwh is a kwh. There is some slight difference in efficiency in the motors as the HP goes up, but probably not enough to offset the cost of running 3 phase. Because the current draw will be higher at 1 phase, the IR losses in the system will be slightly higher as well, but that is most likely a utility problem since your equipment will be relatively near their metering point.

That said, there is some truth to the issue of 3 phase motors lasting longer if you have a high duty cycle. 1 phase motors have starting switches and/or capacitors that shorten the overall life compared to 3 phase. That is also why they are difficult to find in ex-proof versions. Using VFDs to convert your 1 phase to 3 phase for the motors may be a good solution, as long as the control equipment does not also need to be Class I div. I. That can be very expensive for VFDs because of needing to dissipate heat.

If you can do it with VFDs, it is best if you put one VFD on each motor. It is OK to put multiple motors on one VFD, as long as it is acceptable to shut down ALL of them if one motor should fail or overload. The thing you cannot do is switch motors on and off behind the VFD. For instance, if you wanted to use one VFD to power the entire 25HP load, you would need to set up your controls so that if even only the 7.5 HP motor overloaded and tripped, you would shut down the VFD, which means shutting down the entire machine.

That single VFD would need to be rated for 50HP by the way, because the inceased 1 phase current drawn by the rectifier section (front side) will be higher than the 3 phase motor current by a factor of 1.73, which would make it equivalent to 43+ HP, and the next VFD size up from that is 50HP. Use the same forumla for individual VFDs as well, except on 3HP and under. Most VFDs in that size range are already rated for 1 phase input capacity.

"Venditori de oleum-vipera non vigere excordis populi"

 

[UKpete]

petroremedy, if I read jraef's last paragraph correctly (not the one in latin) then you can actually buy a large enough VFD designed to operate from a 3-phase supply but only connect up your single phase, provided you get one rated at least 1.73 times your motor power. So you don't need a separate rectifier as I suggested.

 

[weh3]

Had a similar difficulty with a remote state park site, which required a couple of 20+ hp motors. Supply was single phase, with three-phase several miles away. Between keeping single phase, running three phase, and installing phase converters, we decided to spring for the capital cost of the three-phase line. Here were some considerations:

1. It was a permanent installation.
2. The motor size is difficult to find in a single phase.
3. The current load for single phase motors of that size is very large.
4. A phase converter requires essentially the same large, single-phase motor that you are trying to eliminate.
5. The application required running for several hours a day.

You have small short-term loads that would be well served by single-phase service. However, $15k might also be a drop in the bucket compared to the price of the rest of the project or to the payback, and payback is typically only a secondary consideration in an environmental project. Most utilities have a minimum revenue ratio requirement. If you can show them enough projected kWh, they might spring for it.

William

 

[aolalde]

The new 3-phase transformer installed by the utility company is your best economical and safe option, for the reasons Weh3 exposes.

 

[mc5w]

An alternative is that Mirus International makes a combination static phase converter and unversal harmonic filter for running 3-phase input VFDs off of single phase power. The model series is 1Q3 and it will make your harmonic current distortion about equivalent to an 18 pulse 3-phase rectifier. This would reduce power quality problems due to harmonics especially on a single phase line.

DO NOT try to run a VFD off of a conventional phase converter because the 5th and 7th harmonics will melt the rotor of the rotary phase converter. Phase converters have to be built specifically for the purpose when powering VFDs.

 

[jraef]

If he bought a rotary phase converter, why would he need the VFDs? He never mentioned needing variable speed, that was just a way to get phase conversion.

 

[petroremedy]

jraef is right, I don't need VFDs, I've just been told that you can get VFDs that will do the phase conversion. Can I have multiple motors on a single phase converter with those motors turning on and off and not have problems.

Talking about VFDs, we have a different site with a three phase VFD on a 15 HP motor and we are having alot of trouble with interfering with AM radio in the area of the site. The VFD supplier added a filter of some type that has reduced the interference, but it is still there. That is something I don't want to have to go through again if I put VFDs on each motor at this site.

 

[peebee]

Regarding your AM interference: I have no direct experience with this problem, but there's a lot of literature out there on this. There was a link to an ABB paper some time ago in a thread on this site that covered that issue in addition to VFD reflected waves and other similar VFD issues.

I did find this similar paper on a Google search:

http://www.pge.com/docs/pdfs/biz/power_quality/power_quality_notes/vfd_emi.pdf

It specifically relates to output-side VFD EMI issues, and suggests various solutions such as: metallic conduit, triplexing the cable, including a grounding cable, shielded cable, chokes, etc.

Hope this helps.

 

[jraef]

I would be willing to bet that the output conductors of your VFD are portable cord, like SO cable right? If so, they are like a very powerfull antenna.

In answer to your other question, yes you can have multiple motors turning on and off independently with a rotary phase converter. It will just cost you in additional losses.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Farkel]

U.S. Drives makes drives which will accept the single phase input and thur producing 3ø on the output. Also the least expensive would be a roto phase converter, and if the system is designed correctly with line reactors one should not have a problem with harmonics.

 

[jraef]

Farkel,
Virtually ALL modern PWM VFDs will do this. It is an inherent design issue. PWM VFDs use incoming AC power simply as a "raw material", converting it to DC for use by the transistors in recreating a psuedo-sine wave output. In the process of rectifying the incoming AC to DC, the rectifier section is typically "unaware" of whether there is power on all 3 legs. The issue on over sizing them is because the diode or SCR devices used in the rectifier section must be rated to handle 1.73 times the current that the transistor section will be putting out because the source is 1 phase, not 3 phase. There are a few VFDs that have phase loss protection built in so that a unit sized for 3 phase input is not damaged by a blown fuse, but even then, most of them can be defeated if the VFD is oversized properly.


"Venditori de oleum-vipera non vigere excordis populi"

 

[weh3]

I would still try to get 3-phase just to keep down the capital cost of your service equipment. You may be looking at a 400 A service at 120/240.

In most of these remote locations, to get three phase, the utility only needs to run two phases. Our local utility (central Virginia) loves the open delta, which requires only two transformers, and one can be smaller. You end up with a 3-phase, 240 V, with 120/240 V on one of the phases.

10 hp, 3-ph, 240 V: 28 FLA
10 hp, 1-ph, 240 V: 50 FLA

Regards,
William

 

[ScottyUK]

Hi jraef,

The other thing to consider when using single phase input to a drive designed for 3-phase input is that the reservoir capacitors are typically much too small when used with a single phase supply instead of 3-phase, leading to high ripple on the DC bus. Easy to correct with an external cap bank, if not exactly elegant.



----------------------------------

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

 

[EEJaime]

One source of information you can try. I've used these guys for years. You can get static or rotary phase converters, they have an applications guide and can provide cost information, buyer's guide, etc.... Try Ronk Electrical Industries, Inc. at

http://www.ronkelectrical.com

 

[aolalde]

petroremedy:

Consider possible problems with standard motors when VFDs are applied. Insulation failures and bearing currents could destroy your motors if not constructed for VFD duty.

 

[jraef]

ScottyUK,
Good point worth mentioning about the caps, I often forget to mention that. Of course, upsizing the VFD to take care of the bridge rectifier also takes care of the capacitor issue as well, but it is good to keep people well informed as why.

"Venditori de oleum-vipera non vigere excordis populi"