Compressor motor, 1phase or 3 phase?

[dkmc]

Question for the electrical guru's on this site.

I have a 3 horsepower air compressor (Quincy 308 with the oil pump) that I'm building up, and I have the opportunity to use either a Single phase or Three phase motor on it.
If I use a 3 phase motor, theoretically it will use less power and be cheaper
to run. But if I go with a Single phase motor, some day it will probably
have more resale value and a larger market appeal.

My question is, how much more electric power will the Single Phase motor
use compared to the Three Phase motor?
My electric is billed at about .20 cents per KWH.

Thanks for any insight!

 

[exranger06]

Do you have 3 phase service in your shop? If not, you can't use a 3 phase motor unless you get a phase converter.

 

[rockwithjason]

you will never notice the savings on a power bill, it's too small. The cost of the equipment to run the 3ph on a 1ph service will take 50 years to recoup.

 

[manwithtools]

You can run a three phase on single phase without a phase converter by using a VFD. It's actually a better solution given today's VFD'S. Still, not much opportunity for energy savings unless you can run the motor slower and a compressor is not a good candidate for this.

Sent from my SM-G920V using Tapatalk

 

[dkmc]

I thought it would be obvious, but YES I do have 3 phase 240 service
at my shop.
So.....I have a choice. I can put a 3 phase motor on it with a mag starter, and run it
on 3 phase
OR I can put a 1 phase motor on it and run it on single phase of 2 legs of the 3 phase
service. I have both motors here, no need to purchase either.

Generally, no one would ever want a 3 phase 3HP compressor. But if it had a 1 phase motor on it, that would open it up to a much larger market.
Hope this clarifies my position.
Not that I will care when it's time to sell it.....probably be an ash.....pile.

 

[Bib Overalls]

I don't think you save much if anything with three phase when you make it from single phase. If you have true (metered) three phase in your shop that is the way I would go. Otherwise, I would go single phase and avoid the expense of a conversion setup and the inherent complexity.

 

[manwithtools]

If you have three phase than there is no question, three phase is the way to go. You already are paying the minimums. Go for it. Replacement motors are cheaper, wire sizes are smaller, etc...

Sent from my SM-G920V using Tapatalk

 

[dkmc]

If you have three phase than there is no question, three phase is the way to go. You already are paying the minimums. Go for it. Replacement motors are cheaper, wire sizes are smaller, etc...

Sent from my SM-G920V using Tapatalk

My question, mostly out of curiosity, is HOW MUCH would the cost
difference be to operate it on 1ph VS 3ph?
Like, if the power costs .20 per KWH, and I run it for 100 hours, single
or 3 phase, at the end of 100 hours, how much higher will the single phase bill be VS if it was run 100 hours on 3 phase.
Hope this is clear?

That's why I wrote "Question for electrical GURU'S on this site".
I'm looking for the numbers not general opinion.

 

[theoldwizard1]

3HP is pretty small so, likely, you are not using it a lot. I concur with the others. It will take a long time to pay back.

You can't get 240V (without a transformer) out of 3 Phase Wye. The best you can do is 208V.

You can get 240V out of 3 Phase Delta if one of the winding has a center tap for neutral.

Most modern motors are rated for 208-240V, but double check.

 

[dkmc]

3HP is pretty small so, likely, you are not using it a lot. I concur with the others. It will take a long time to pay back.

You can't get 240V (without a transformer) out of 3 Phase Wye. The best you can do is 208V.

You can get 240V out of 3 Phase Delta if one of the winding has a center tap for neutral.

Most modern motors are rated for 208-240V, but double check.

Holy cow!....Lots of speculation...
Did I say I have Wye?
I have Delta service with a 'wild leg' here.

 

[larry_g]

Question for the electrical guru's on this site.

I have a 3 horsepower air compressor (Quincy 308 with the oil pump) that I'm building up, and I have the opportunity to use either a Single phase or Three phase motor on it.
If I use a 3 phase motor, theoretically it will use less power and be cheaper
to run. But if I go with a Single phase motor, some day it will probably
have more resale value and a larger market appeal.

My question is, how much more electric power will the Single Phase motor
use compared to the Three Phase motor?
My electric is billed at about .20 cents per KWH.

Thanks for any insight!

The question you need to be asking is a three phase motor more efficient than a single phase? Motor efficiency changes with a few different factors so you will have to look up the efficiency of the particular motors you have in hand.

lg
no neat sig line

 

[dkmc]

The question you need to be asking is a three phase motor more efficient than a single phase? Motor efficiency changes with a few different factors so you will have to look up the efficiency of the particular motors you have in hand.

lg
no neat sig line

Both are 80%

 

[Junkman]

I have a 3 phase 3 H.P. Baldor motor that I removed from a Quincy compressor that I will sell you. It has very little use on it, since the company went belly up a few months after they opened their doors. I got a good deal on the compressor because it was a 3 phase motor, and I have since purchased a single phase motor for the compressor. I am in CT on the MA border (Interstate 84/Ma Pike interchange), so you could pick it up, if you are relatively near.

 

[rlitman]

Both are 80%

Then both will cost the same to run.

80% is not that great BTW, but you pay dearly for higher efficiency motors (and unless you're running continuously, there's not much call for it).

3 phase will have a little more starting torque. Though with a working unloader, it won't matter.

 

[larry_g]

Both are 80%
So.....I have a choice. I can put a 3 phase motor on it with a mag starter, and run it
on 3 phase
OR I can put a 1 phase motor on it and run it on single phase of 2 legs of the 3 phase
service. I have both motors here, no need to purchase either.

Without brands and motor part numbers so I can confirm this I'm calling ********. Post up pictures of the said motors name plates.



lg
no neat sig line

 

[theoldwizard1]

Holy cow!....Lots of speculation...

Nope, just stating the fact.

 

[theoldwizard1]

I know, in small commercial retail stores (think strip mall store) they run the furnace blower motors on 3 phase because it is cheaper. Those blower motors are about 1-3 hp but they are running many hours a day.

 

[dkmc]

I have a 3 phase 3 H.P. Baldor motor that I removed from a Quincy compressor that I will sell you. It has very little use on it, since the company went belly up a few months after they opened their doors. I got a good deal on the compressor because it was a 3 phase motor, and I have since purchased a single phase motor for the compressor. I am in CT on the MA border (Interstate 84/Ma Pike interchange), so you could pick it up, if you are relatively near.

I am in upstate NY I have an open frame motor I can use on it.
If it's a TEFC motor with reasonable efficiency that you have, I'd be interested except for the shipping. Usually 3 phase motors are somewhat plentiful in the scrap yards around here anyway.

Then both will cost the same to run.

I don't believe you are correct.

Without brands and motor part numbers so I can confirm this I'm calling ********. Post up pictures of the said motors name plates.

it's a question for a person that's well versed in electrical service, not a contest to
prove I'm telling the truth. Theoretically for the discussion and to compare I am saying
BOTH motors are 80% efficient. I won't waste time taking and posting pictures.
But if the right guy that understands my question posts here, he will understand the question, and if/when he answers, you may learn something.

 

[dkmc]

I know, in small commercial retail stores (think strip mall store) they run the furnace blower motors on 3 phase because it is cheaper. Those blower motors are about 1-3 hp but they are running many hours a day.

Finally someone has posted some pertinent and useful information.
That is very interesting, and now please tell us how you 'know' it's
cheaper? How did you hear that?
How many hours a day they run is completely irrelevant however.
Cheaper is cheaper if it's 1 hour a day or 24 hours a day.

 

[theoldwizard1]

Finally someone has posted some pertinent and useful information.
That is very interesting, and now please tell us how you 'know' it's
cheaper? How did you hear that?

Dad ran a store. Whne the blower died, that was the first time I ever heard of 3 phase. The guy replacing the dead blower motor said it was cheaper to run.

How many hours a day they run is completely irrelevant however.
Cheaper is cheaper if it's 1 hour a day or 24 hours a day.

I forgot, you already have 1 of each !

Some big commercial operations pay for "phase correction" (I hope that is the correct term). This is caused when one or two legs are pulling more power than the other. Big single phase motors will cause that (not that 3 hp is big).

 

[dkmc]

Just remembered I know a guy that will be able to answer this question with
authority. I will ask him over the weekend and report back here with his answer.

 

[larry_g]

it's a question for a person that's well versed in electrical service, not a contest to
prove I'm telling the truth. Theoretically for the discussion and to compare I am saying
BOTH motors are 80% efficient. I won't waste time taking and posting pictures.
But if the right guy that understands my question posts here, he will understand the question, and if/when he answers, you may learn something.

After 30 years in the business I can recognize BS when I see it. If you want a discussion on theory then state so. If you have a need for facts on what you have in hand say so. If your just trolling that to will become obvious.

lg

no neat sig line

 

[dkmc]

Which "business" would that be.... might I ask?

You seem to feel the need to turn this into an argument for whatever reason which
seems puzzling. If the simple question I'm asking makes you that frustrated, skip
this thread by all means. I don't need your input, really.

So far I'd say your living up to your SIG kine quotes pretty well.

 

[matt_i]

Good post over at Practical Machinist on this subject from a couple years back.

http://www.practicalmachinist.com/v...d/3-phase-vs-1-phase-motor-efficiency-171972/

I like the idea of looking at the FLA on the motor nameplates, you have to use the SQRT(3) factor on the 3 phase motor when comparing.

I have never compared in person but I have to believe that the inrush current is a lot lower on a 3phase motor.

Just pulled up some examples from the web. (Baldor 3ph ,1hp, 1750rpm 4 pole motor, uses 3A per leg on 240vac)
https://www.zoro.com/baldor-electri...gclid=CPaY-fiXpssCFQUIaQodGhoLMA&gclsrc=aw.ds

Hopeful equivalent, 1hp 1ph, looks like 6.5A Full load @ 230vac
https://www.zoro.com/baldor-electri...gclid=CKWU9bmYpssCFYSAaQodjfoLfQ&gclsrc=aw.ds

So 3* SQRT(3) = 3 * 1.73 = 5.19 A total draw for 3-phase motor

Lets assume running at full load as most compressors do. In 1 hour of motor run time, the Power * time = Watts * time = V * I = 240vac * 6.5A = 1560 W-hr or 1.5kWHr

3phase 240vac * 5.2A = 1248 W-hr or 1.25kWHr

a page back I think you said you paid 0.20 per kW-hr, so you will pay 30cents/hr on single phase and 25 cents per hour on 3 phase. Not a ton of savings, but keep that nickel machine running

Hope all is well

 

[dkmc]

THANK YOU Matt!


.05 savings per hour x 10 hours per week x 40 weeks per year x 10 years = $200
savings in 10 years at that usage level. Consider the bit higher price a single phase
user might pay VS a user willing to live with the 3 phase motor...
STILL a wash.
But now I know, and my curiosity satisfied as to weather the savings would be significant.

Your info is MOST helpful!

 

[matt_i]

You know...thinking of this more...we should do a 3hp comparison. I did a 1 hp comparison...

 

[TheEquineFencer]

If you're building this compressor and plan to keep it, I'd go 3PH, if you plan to sell it in the near future for a profit, I'd go 1PH.

I wish I could afford 3Ph to be run in my shop. The equipment is cheaper to buy used. Welders and machines are a lot cheaper. My useage, or lack of makes it cheaper for me to stay 1PH. They charge for 3PH meters and useage even if you do not use it much, there's a monthly min charge.

 

[dkmc]

If you're building this compressor and plan to keep it, I'd go 3PH, if you plan to sell it in the near future for a profit, I'd go 1PH.

I wish I could afford 3Ph to be run in my shop. The equipment is cheaper to buy used. Welders and machines are a lot cheaper. My useage, or lack of makes it cheaper for me to stay 1PH. They charge for 3PH meters and useage even if you do not use it much, there's a monthly min charge.

Interesting perspective.
Again, some speculation, and perhaps some places there is a minimum charge. Here in upstate NY, there is NO minimum charge, but they manage to have a heap of other charges that ad up.....before they ad in the actual
electricity used. A commercial 1 phase service is the same, SAME basic service fees and 'wire maintenance' fees, etc. I think I pay close to $40 per month in fees, before the cost of the electric. So the both services are near equal in the respect of fixed costs.

 

[sberry]

The big advantage for a 3 hp would be it could run on the same service as other equipment in the facility. As a practical matter the cost savings in electric isn't worth calculating.

The guy replacing the dead blower motor said it was cheaper to run.

A maintenance guy one remembers from childhood should always be considered an expert.

 

[dkmc]

The big advantage for a 3 hp would be it could run on the same service as other equipment in the facility. As a practical matter the cost savings in electric isn't worth calculating.

A maintenance guy one remembers from childhood should always be considered an expert.

It is worth calculating.......to determine if it's not worth being concerned about....


I think you're just being sarcastic with your maintenance guy comment....

 

[rlitman]

I don't believe you are correct.

You may choose what you believe, but if you believe that I am incorrect, then you do not understand the definition of the word "efficiency".

I know, in small commercial retail stores (think strip mall store) they run the furnace blower motors on 3 phase because it is cheaper. Those blower motors are about 1-3 hp but they are running many hours a day.

They use 3-phase, because they have it. They use 480V, again, because they have it, and the WIRING costs are lower.

Once a single phase motor has reached speed and the centrifugal switch cuts out, it runs in the same manner as a 3-phase motor. It is just heavier.

Good post over at Practical Machinist on this subject from a couple years back.

http://www.practicalmachinist.com/v...d/3-phase-vs-1-phase-motor-efficiency-171972/

I like the idea of looking at the FLA on the motor nameplates, you have to use the SQRT(3) factor on the 3 phase motor when comparing.

I have never compared in person but I have to believe that the inrush current is a lot lower on a 3phase motor.

Just pulled up some examples from the web. (Baldor 3ph ,1hp, 1750rpm 4 pole motor, uses 3A per leg on 240vac)
https://www.zoro.com/baldor-electri...gclid=CPaY-fiXpssCFQUIaQodGhoLMA&gclsrc=aw.ds

Hopeful equivalent, 1hp 1ph, looks like 6.5A Full load @ 230vac
https://www.zoro.com/baldor-electri...gclid=CKWU9bmYpssCFYSAaQodjfoLfQ&gclsrc=aw.ds

So 3* SQRT(3) = 3 * 1.73 = 5.19 A total draw for 3-phase motor

Lets assume running at full load as most compressors do. In 1 hour of motor run time, the Power * time = Watts * time = V * I = 240vac * 6.5A = 1560 W-hr or 1.5kWHr

3phase 240vac * 5.2A = 1248 W-hr or 1.25kWHr

a page back I think you said you paid 0.20 per kW-hr, so you will pay 30cents/hr on single phase and 25 cents per hour on 3 phase. Not a ton of savings, but keep that nickel machine running

Hope all is well

Your math is completely wrong! Volts times amps gives you a number in the units of VA, not watts on an AC system.

 

[theoldwizard1]

Lets assume running at full load as most compressors do. In 1 hour of motor run time, the Power * time = Watts * time = V * I = 240vac * 6.5A = 1560 W-hr or 1.5kWHr

Unfortunately V * I = Watts does not apply to AC power. Close, but not exactly. It has to do with "imaginary numbers" (yes, they are real, and I forgot all about then >35 years ago).

 

[dkmc]

You may choose what you believe, but if you believe that I am incorrect, then you do not understand the definition of the word "efficiency".

They use 3-phase, because they have it. They use 480V, again, because they have it, and the WIRING costs are lower.

Once a single phase motor has reached speed and the centrifugal switch cuts out, it runs in the same manner as a 3-phase motor. It is just heavier.

The single phase motor "runs in the same manner" as in.....the shaft turns
just like the shaft on the 3 phase motor turns.
Electrically they are much different, and the 3 phase motor is more efficient.

Believe it or not....no one's trying to convince you.
You think of it how ever you like.
Meanwhile I'm using (some) of the info posted here to make a decision
for my needs....which won't affect you in any way.

 

[theoldwizard1]

Volts times amps gives you a number in the units of VA, not watts on an AC system.

True, but it is close, especially for single phase systems.

They use 3-phase, because they have it. They use 480V, again, because they have it, and the WIRING costs are lower.

Yes and no. Sure 480 allows thinner wire (THHN is good for 600V). That does not mean that 480 is available in all commercial locations.

Once a single phase motor has reached speed and the centrifugal switch cuts out, it runs in the same manner as a 3-phase motor. It is just heavier.

NOT TRUE ! On a single phase motor, the armature is "coasting" much of the time. On a 3 phase, the second and third phase "fill in"/overlap that coasting.

 

[larry_g]

grab a calculator here . Pay attention this calc does not include efficiency of the motor.

Grab from baldor a motor 3 phase

Grab from baldor another motor single phase

Run the numbers through the calculator and see for yourself the power difference is nominal.

Lets hope that question creep does not continue. If you have actual motors sitting on the floor then use the data from those motors and plug it into the calculator.

lg
no neat sig line

 

[rlitman]

NOT TRUE ! On a single phase motor, the armature is "coasting" much of the time. On a 3 phase, the second and third phase "fill in"/overlap that coasting.

And? That has nothing to do with efficiency.
That's like saying a 6 cylinder engine is more efficient than a 4 cylinder engine. Ok, so a 4 cylinder engine spends more time coasting between impulses than a 6.

It's just a question of whether I give you one bigger push for each turn, or three smaller ones. "Coasting" is irrelevant.

As I said, a single phase motor will be heavier, for the same horsepower and efficiency. There's no getting around that.

If more phases meant more efficiency, then we'd see poly-phase motors all over the place.

The single phase motor "runs in the same manner" as in.....the shaft turns
just like the shaft on the 3 phase motor turns.
Electrically they are much different, and the 3 phase motor is more efficient.

Again, clearly you do not know what the word "efficiency" means.
If two motors have the same efficiency (as you have claimed, by pulling numbers from your sphincter; which as others have pointed out clearly means that you're line of questioning is trolling), then that means they have the same efficiency. Period.

 

[dkmc]

ef·fi·cien·cy
əˈfiSHənsē/
noun
noun: efficiency

the state or quality of being efficient.
"greater energy efficiency"
synonyms: organization, order, orderliness, regulation, coherence; More
productivity, effectiveness
"we need to make changes to improve efficiency"
competence, capability, ability, proficiency, adeptness, expertise, professionalism, skill, effectiveness
"I compliment you on your efficiency"
an action designed to achieve this.
plural noun: efficiencies
"to increase efficiencies and improve earnings"

technical
the ratio of the useful work performed by a machine or in a process to the total energy expended or heat taken in.


Read the RED sentence.
3 Phase motors do MORE work for the given ammount of energy they take in compared to single phase motors.
Stick that in your sphincter.

 

[rlitman]

...technical
the ratio of the useful work performed by a machine or in a process to the total energy expended or heat taken in.


Read the RED sentence.
3 Phase motors do MORE work for the given ammount of energy they take in compared to single phase motors.
Stick that in your sphincter.

Ok, so you passed reading, but failed high-school physics (and spelling, and debate).

If two motors have the same efficiency, AS YOU YOURSELF HAVE STIPULATED EARLIER, then by definition, they perform the same amount of work, for the same amount of energy taken in.

 

[dkmc]

Happy for you that you're right...in your mind

 

[matt_i]

You're right about the VA <> Watts, but a close enough assumption for apples to apples comparison without actually doing live, clamp meter testing, or digging up the power factor data.