3ph for 1ph motor substitution--sizing

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A diveshop owner (hp & lp compressor loads, open delta service) in Fla was told by an "electrician" that he could replace his 8hp single phase motor with a 5hp three phase motor to save utility costs. Likewise replacing his 5hp lp compresser single phase motor with a 3hp three phase motor. What is the rational for this and does it hold water? Would it if he had a true three phase service rather than open delta (ie., single phase with a wild leg. Is this the same as a monocyclic configuration)?

Larry
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Larry, HP is HP. A 3ph motror will have less line current than the same HP 1ph motor, mainly because there are three lines, but that doesn't mean a compressor can run on a smaller 3ph than 1ph motor.

The service configuration has no bearing on this, presuming it's of adequate capacity for the load to begin with. An open-Delta supply is "real" three phase power, by the way.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Larry's right, HP is HP. His "electrician" is probably not really, and even if he is, he's wrong. It can happen... ;)

It's a common misconception unfortunately. People, especially those not in the trade (but not exclusively) will often make bonehead assumptions like that based on some misinterpretation they partially over heard from some guy in a coffee shop or a bar. I've heard that one several times and once when I challenged an older guy on a job site who claimed to be "a 'lectrician", he got very hostile because he had been telling people this for years, fully believing it. I was accused of calling him a liar, I pointed out that he had done most of the work to that end on his own.

The other variation I hear a lot is that 3 phase motors are more efficient because single phase motors are only getting power twice per cycle rather than 3 times, or minor variations to that theme. That's not true either, in fact motor efficiency has everything to do with motor design, not the phases, and there are some sizes and designs in which 1 phase motors are actually MORE efficient than 3 phase.

HP is a shorthand expression of an amount of torque at a particular speed. There is nothing in the HP formula that refers to voltage, current, phases, weather or the price of tea in China.

The only real thing you can say is that the power delivery of a 3 phase motor is very slightly more smooth than a single phase of the same HP, because of the 3 to 2 issue. The only time anyone notices that is when you are doing some forms of high precision machining where the tiny little fluctuations in torque make tool bits cut differently or polishing equipment show striations.
 
Larry's right, HP is HP. His "electrician" is probably not really, and even if he is, he's wrong. It can happen... ;)

It's a common misconception unfortunately. People, especially those not in the trade (but not exclusively) will often make bonehead assumptions like that based on some misinterpretation they partially over heard from some guy in a coffee shop or a bar. I've heard that one several times and once when I challenged an older guy on a job site who claimed to be "a 'lectrician", he got very hostile because he had been telling people this for years, fully believing it. I was accused of calling him a liar, I pointed out that he had done most of the work to that end on his own.

The other variation I hear a lot is that 3 phase motors are more efficient because single phase motors are only getting power twice per cycle rather than 3 times, or minor variations to that theme. That's not true either, in fact motor efficiency has everything to do with motor design, not the phases, and there are some sizes and designs in which 1 phase motors are actually MORE efficient than 3 phase.

HP is a shorthand expression of an amount of torque at a particular speed. There is nothing in the HP formula that refers to voltage, current, phases, weather or the price of tea in China.

The only real thing you can say is that the power delivery of a 3 phase motor is very slightly more smooth than a single phase of the same HP, because of the 3 to 2 issue. The only time anyone notices that is when you are doing some forms of high precision machining where the tiny little fluctuations in torque make tool bits cut differently or polishing equipment show striations.



Couple of points here.
  1. It seems to me that the original conversation is directed toward a non-technical person, by an 'electrician' and I don't know what to attribute to the quotation marks.
  2. Compressor types mechanical efficieny varies, so it is possible that one type uses siginficantly less energy than another.
  3. Dependent on the sizing of the unit one may cycle more than the other to deliver the same maintainable pressure.
  4. The size of the storage tank could be also a reason why a smaller compressor may be able to perform the same duty. It would just cycle more frequently.
  5. Unless you are paying demand charges - highly unlikely in the case of a small user like this, but in the EU there are some even at these levels - the issue and benefits are emaningless in practical terms. (Somewhat similar to the home-powerfactor correction 'benefits'.)
  6. You pay for kWhr, not kW. So even with a smaller unit that is less efficient than the larger unit you coupld be paying more.
 
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3 ph/1 ph - "true or open Delta" etc.

3 ph/1 ph - "true or open Delta" etc.

Thanks for all of your remarks, they confirm what I remember from way back when I studied energy conversion. The open delta service does have "demand charges". As I understand it from earlier posts in this forum, the open delta suffers from unballanced currents and is not recommended for individual 3ph motors greater than 10 hp, but is fine with mutipule motors of that size. The diving center is ramping up to a stable of: (I'll need to inject some common sense into this next month!!!!!!)

3ph 15hp 230v/37.4a
3ph 5hp 230v/12.6a *
3ph 3hp 230v/
1ph 7hp 220v/36a *
1ph 7.5hp 230v/36a
3ph 10hp 230v/23.2a *
1ph 5hp 230v/21.5a *
those starred currently installed, based on what he was told, the owner is/was planning on replacing some 1ph with 3ph from the list.

Of course, not all running at the same time and certainlt not starting at the same time.



Laird
 

BJ Conner

Senior Member
Location
97006
Did the Electrican measure the current to the single phase motor. If it was some fraction of the rated HP the require HP could be less.
Motors only do the work they have to. The big single phase could be loafing.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
There are plenty of good reasons to use 3 phase instead of single phase for motors if you have it available, better load balance being the best one. But that does nothing with regards to changing the amount of HP that a load requires, nor does it change the amount of HP that a motor delivers. HP is HP is HP. If you take out a 7.5HP 1 phase motor, you will need a 7.5HP 3 phase motor to do the same work. That's what is counter to the premise from the user's "electrician" mentioned in your original post.

As to whether or not a full delta would be better than an open delta, it isn't a cut and dry situation. Certainly in the current state of affairs where there are some significant 1 phase loads, the voltage imbalance is probably high and that is going to make motors run hot and shorten their life. So just switching to all motors being 3 phase will help that situation regardless of open or closed delta. Sometimes though, a utility will pull a rabbit out of their hat on an open delta service if they know what the loading is going to be and use dissimilar transformer sizes to help mitigate the voltage drops caused by the single phase loads on one transformer. If you then close that delta and leave those two existing dissimilar transformers in place, you end up with a new set of problems. The people you need to talk to about this are the utility engineers.
 
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