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Advantage (s) of 3-phase electric over standard

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Forum topic by Joeshop posted 01-29-2011 04:45 AM 2359 views 0 times favorited 55 replies Add to Favorites Watch
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Joeshop

49 posts in 2580 days


01-29-2011 04:45 AM

Topic tags/keywords: electric phases question

I realize that as you go up in HP the mfgs. at some point go over to 3-phase. I’m wondering what are advantages or disadvantage.

Thanks for any info !

Joe

-- ~You cannot do a kindness too soon, for you never know how soon it will be too late.~ - Joe


55 replies so far

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Loren

8302 posts in 3111 days


#1 posted 01-29-2011 05:09 AM

I think it’s cheaper, both in terms of the price of the motors and
the cost of running them. That’s why it’s used in industrial settings.
Higher HP motors (over 7 or so) probably require 3 phase.

With a phase converter I don’t think you save much on electricity
but 3-phase motors are still cheaper. Used ones are abundant and
cheap anyway.

These are just generalizations from what I’ve read. I could be wrong
and I’m certainly no authority. If you can get real 3-phase at your
location, it does open up some cool possibilities if you’re into old ‘arn.

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PurpLev

8523 posts in 3111 days


#2 posted 01-29-2011 05:13 AM

actually it’s a common misconception – 3 phase motors are not about the higher HP as most machines that are 3 phase are running motors in the 1/2hp-2hp. the motors are cheaper to make, and are simpler in design, ironically they are also more efficient and faster to start as they do not require a separate motor (capacitor) to start them.

the main purpose of 3 phase motors is for industrial usage where 3 phase is available, and the motors are more efficient. for home use single phase is easier to get to work though.

the higher HP do dictate the use of 220v as opposed to 110v in certain countries (like the US) as the amperage would be too high to run on 110 which will heat the cables too much so 220 is a must above 2hp motors

-- ㊍ When in doubt - There is no doubt - Go the safer route.

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canadianchips

2349 posts in 2460 days


#3 posted 01-29-2011 07:41 AM

3 phase is more efficient. (Using a phase converter looses this efficiency !) A 3 phase motor will start under a bigger load (conveyors, augers) 3 phase motors are easy to reverse. (change any 2 of the 3 lead wires).
BAD PART IS: It is expensive to bring it into a residential home.

-- "My mission in life - make everyone smile !"

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TopamaxSurvivor

17669 posts in 3139 days


#4 posted 01-29-2011 08:34 AM

It is not really practical to use single phase motors over about 5 hp even though they are made. A 5 hp draws 28 amps on 240 v and a 10 hp will draw 50. The biggest problem is starting them. They almost knock the earth off it’s axis where a 3 phase will just turn on.

Lots of guys on here are always using the old 746 watts = 1 hp to calculate their motor circuits. On single phase, it is more like 1500 watts = 1 hp. 3 phase motors get a lot closer to the ”magic number” but they are probalby in the range of 850-900 watts = 1 hp. These are not calculated numbers, just practical educated guesses numbers to illustrat the point. They won’t be far off, though.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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canadianchips

2349 posts in 2460 days


#5 posted 01-29-2011 03:44 PM

Topomax….That explains it, all these extreme weather patterns throughout the world have nothing to do with global warming, it because all of us 1 phasers are turning our table saws on at the same time !!!! LOL

-- "My mission in life - make everyone smile !"

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richgreer

4541 posts in 2537 days


#6 posted 01-29-2011 04:30 PM

Many years ago, on our farm, we purchased a silo unloader. They recommended that we get 3 phase service and use a 7.5 hp motor. The other option was to stay with 1 phase and get a 5 hp motor. We opted for 3 phase. I always wondered why a 5 hp 1 phase was considered equivalent to a 7.5 hp 3 phase. Now, after many years, I know. They are not equivalent. It is just that 5 hp was the practical limit on 1 phase. Note, this was in the early 60s.

There were probably some farmers that could not get 3 phase and they were forced to use an underpowered motor. I don’t know this but they probably “geared down” the machinery with the 1 phase and delivered the silage a little slower.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

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HorizontalMike

7144 posts in 2377 days


#7 posted 01-29-2011 04:42 PM

This all begs the question:

Does 3-phase service cost any more than single phase service on a monthly basis? Or is it ONLY the setup that varies the costs?

-- HorizontalMike -- "Woodpeckers understand..."

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TopamaxSurvivor

17669 posts in 3139 days


#8 posted 01-29-2011 06:37 PM

Power is power, unless there are different rates; however, 3 phase services can have what is know as a demand fatco. They charge a higher rate for peak usage periods. Depending on the power company, it can be more expensive if you are using power during peak demand periods. Lots of commercial and industrial users will have peak demand limits in their equipment to do non-critical operations in off peak hours.

I was asked to do a peak demand limiting job on an electrical boiler at a hospital in the mid 80s. It was a 13,800 volt boiler. The job required shutting down the hospital’s service to install some of the equipment. I felt a one man startup shop had no business shutting down a facility with life safety issues involved. I advised them to use a company that could bring in 40 men to repair any potential problem in a few hours if necessary. They didn’t take my advice on that issue, a fiasco was the result.;-)) But, they eventually got the job done and it attracted nation attention for the cost savings.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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TopamaxSurvivor

17669 posts in 3139 days


#9 posted 01-29-2011 06:53 PM

Rich, The motors are definitely not equal!! One of my uncles had a refrigeration motor changed to a larger one to cool milk. All he had was single phase service. The new motor kept tripping out. None of the local boys seems to be able to figure out what was causing the problem. The new motor out at the barn a long ways from the transformer had so much voltage drop it could not reliably start. I suspect that is the issue the silo unloader company ran into. When the tried selling single phase motors bigger than 5 hp they opened up cans of worms they wish they had left alone ;-)) In some cases, I doubt if it was practical or possibly impossible to use big enough conductors to start a 7.5 or 10 hp motor under load out at the silo.

My uncle ran into the same situation on his manure pit agitator and loading pump. He ended up using a tractor rather than an electric motor. 3 phase was too expensive to bring a few miles.

One thing I hate about working around manure pits is the potential “forgets” that most of us do from time to time. Unc forgot to open the lid on the manure wagon one day when he started to load it. He got a shower he never forgot!! ;-))

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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HorizontalMike

7144 posts in 2377 days


#10 posted 01-29-2011 07:04 PM

The reason I asked about any monthly electrical rate changes associated with 3-phase VS Single-phase electrical service is:

1. I don’t know any better as I have ONLY had Single phase service.

2. Water Service (another utility) changes rates according to the SIZE PIPE hook-up you have PLUS usage. A 5/8” water hook-up costs less per month than a 1” hook-up costs per month, etc.

So does anyone know how most utility providers handle this?

-- HorizontalMike -- "Woodpeckers understand..."

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patcollins

1420 posts in 2328 days


#11 posted 01-29-2011 07:09 PM

The reason for three phase is because power transmission lines can transmit three times the amount of power for the same induced magnetic field. As voltage and amperage raise the magnetic field around a wire are greater, and the lower the dielectric breakdown (chance to get zapped though air from it) .

Going back to trig a single phase is a sine wave, and the three phase are all offset by 120 degrees so the power at any instant is fairly constant compared to a single phase, that is why a three phase motor is more powerful at a given rating and thus more efficient.

Sorry for the engineer answer :)

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TopamaxSurvivor

17669 posts in 3139 days


#12 posted 01-29-2011 07:13 PM

HMike, My experience in business is everything has a residential and commercial rate. Since three phase is typically a business service, I’m 99.9% sure they get stuck too :-))

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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HorizontalMike

7144 posts in 2377 days


#13 posted 01-29-2011 07:53 PM

Topa,
That’s what my common sense tells me (though there are those that don’t think I have ANY sense, much less the common kind) so I guess any perceived savings is just good fodder for LJs discussions ;-).

-- HorizontalMike -- "Woodpeckers understand..."

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rcs47

182 posts in 2592 days


#14 posted 01-29-2011 10:12 PM

A single phase motor gets a “kick” with each cycle as it turns. A three phase motor gets a “kick” from each power leg, each one 120 degrees out of phase, giving almost a continues push.

The utility point of view… Three phase service is typically only available in commercial, industrial and Ag areas. In our area, typically two wires on insulators on the pole is 12kV single phase, three wires is 12 kV three phase, four wires is 21 kV three phase (the fourth wire is the neutral). We work to balance the load across all three legs.

Single phase services (read residential, small commercial, small Ag) are balanced on the three legs as they are added. When we design residential developments, we only run single phase. Three phase service is not available. We have factored in the load of the houses in the overall design of our system, splitting our legs, balancing the load.

What if you want three phase? You will have to pay to have the additional cables installed on the poles, or pulled in the underground system. If it’s underground, it was probably not designed for three phase (conduit too small), or the conductor is direct buried. The equipment installed (transformers, switches, boxes, fuses, etc.) may need to be replaced because there isn’t anyplace to terminate the cable(s) at the end of each pull… You will need to install an additional panel for a meter and get new commercial rate because there is not a residential three phase rate. Can you get a permit for a commercial service in a residential zoned area?

People have talked somewhat about rates. Our three phase customers had demand meters, that measured their peak kW draw each month, plus the normal kWh. In many cases, they had a minimum demand charge because we installed the three phase transformers. As demand increases, customers are required to change to time of use rates, paying more for the power they use between noon and 6:00 PM.

-- Doug - As my Dad taught me, you're not a cabinet maker until you can hide your mistakes.

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Dennisgrosen

10850 posts in 2578 days


#15 posted 01-29-2011 11:16 PM

now I´m glad we have standard 3fase 380-400Volt and 1fase 220Volt in nearly all houses in Denmark
and newer house´s has it allways easy to wire up a new wire from the box and if you realy pull Amp
you just say it to the comp. and they change the main fuse infront of the house to a bigger one
and no exstra charge :-)

take care
Dennis

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