ECN Forum
Posted By: Attic Rat Phase Converters - 04/21/04 04:51 AM
... Hi gang, I have a prospective customer who needs 3 phase in a single phase restaurant...Why you may ask ?? ... His central A/C system that is slated to be installed needs 3 phases.. as it will be alot cheaper for him to run... So he's been asking me..everyday now, what can I do...I told him I "think" that a converter can be used... Am I correct ?? Can it run a compressor and the air handler, or are they used only for motor loads...
... He suggested a buck/boost xfrmer,..but I don't think that'd work...Help me PLEEEZE and tell me something I can tell this pest...(he's actually an old friend o' mine)
Russ
Posted By: Lostazhell Re: Phase Converters - 04/21/04 04:56 AM
Russ...
http://www.phase-a-matic.com/

Tell me if this helps [Linked Image]

-Randy

PS... They state Air Conditioning equiptment as an application [Linked Image]

[This message has been edited by Lostazhell (edited 04-21-2004).]
Posted By: chi spark Re: Phase Converters - 04/21/04 04:59 AM
Howdy Attic Rat- this place is pretty good at applications and there's some good info on their website: http://www.kayind.com/index.htm

Be sure to mention that you will be powering a compressor and the converter is sized for starting that. Your customer may reconsider when he gets the price. Good luck
Posted By: Attic Rat Re: Phase Converters - 04/21/04 05:04 AM
...Thanx sooo much guys!! [Linked Image] [Linked Image] [Linked Image] [Linked Image]
Posted By: caselec Re: Phase Converters - 04/21/04 05:43 AM
I would like to see the numbers on this. How can it be cheaper to run 3-phase HVAC equipment off single phase using a phase converter instead of using 1-phase equipment? Phase converters are not 100% efficient.

I think your customer/friend saw the circuit sizes for 3-phase and single phase equipment and figured the 3-phase equipment must be cheaper to run since the circuit size is smaller.

Curt

[This message has been edited by caselec (edited 04-21-2004).]
Posted By: nesparky Re: Phase Converters - 04/21/04 05:53 AM
It's not cheaper to run the phase converter.
It is usually 3 phase power is not availble from the POCO. The installed cost of the phase converter is usually less expensive than a new 3 phase service and the POCO charges. Sometimes 3 phase equipment is all that is availble for the size of load the equipment serves.
Posted By: caselec Re: Phase Converters - 04/21/04 05:59 AM
Nesparky,

I agree with you. It was the comment as it will be alot cheaper for him to run that I’m questioning.

Curt


[This message has been edited by caselec (edited 04-21-2004).]
Posted By: winnie Re: Phase Converters - 04/21/04 11:35 AM
Three phase motors are generally regarded as more efficient than single phase motors, but you have the inefficiency of the phase converter to consider, which depending upon type is either a mess of power electronics (static phase converter) or essentially a single phase motor/three phase generator rolled into a single rotating machine (rotary phase converter). You can even use a plain ordinary three phase induction motor as a rotary phase converter; you power it from your single phase supply through two of the leads, do something mechanical to start it spinning (since a single phased 3 phase motor won't _start_), and once it is spinning you can draw the third leg from the motor terminal.

For air handling equipment, rather than using a phase converter, it is worth while considering using a variable speed drives. Air handling is the classic case where variable speed can save energy. For a given duct system, the power required to move air scales roughly as the cube of air speed, but the volume of air moved scales linearly with the air speed. Rather then running your motors at full speed, and then throttling the airflow or turning the system on and off, operating at low speed continuously with minimum resistance to air flow can save lots of energy.

-Jon
Posted By: Attic Rat Re: Phase Converters - 04/21/04 11:37 AM
...Caselec,thats what he thinks,someone must of told him that it'd be cheaper for him in kilowatt/hours to run the A/C at 3 phase rather than at single... Down the line,wouldn't the usage come out the same?? I mean a 60,000 btu unit is gonna consume the same amount of power,whether it's 3 phase or not, it's just divided by 3...or something like that...no..??
Russ
Posted By: iwire Re: Phase Converters - 04/21/04 12:52 PM
Russ by using a phase converter down the line it will just cost more money in electricity.

Even the highest quality (expensive) phase converter will not be 100% efficient.

Any heat coming off the phase converter is wasted energy.

The best option from a energy consumption standpoint is a true 3 phase service if available.

The next would be having single phase HVAC equipment if it is available in the BTUs needed.

The least electrically efficient method is the phase converter.

In the end watts are watts no mater how many phases or what voltage, it will take a certain amount of watts to remove a given amount of heat.

Bob
Posted By: iwire Re: Phase Converters - 04/21/04 01:08 PM
Check out these links, the first one has some info on the math.
https://www.electrical-contractor.net/ubb/Forum1/HTML/001802.html

This next one is just plain fun to see what can be done by someone with a creative mind.

This "Hillbilly" is no idiot. [Linked Image]
https://www.electrical-contractor.net/ubb/Forum7/HTML/000234.html

It is interesting that the collection of motors running together gives him the power he needs to start the big one.
Posted By: ElectricAL Re: Phase Converters - 04/21/04 01:09 PM
I've run into clients, in the past, who have paid attention to the wrong (or incomplete) information. They hear someone say:
  • 3 Ø motors are more efficient
  • 3 Ø motors are quieter
  • 3 Ø motors are simpler, less moving parts, therefore are more reliable
  • yada yada
The client fixates on the "little truthes" without realizing that there is more that the 3 Ø motor involved.

The client has to be educated about added complexity of the 3 Ø converter itself, the added equipment and installation cost.

In my experience, the client doesn't want the education. . .in fact will push for the bottom line in an eternal quest to save money. The client's confusion results in the HVAC guys having to quote several prices for different systems, and the EC also having to quote several prices. Then, with all these different quotes, the client trys to find the cheapest combination.

Russ, if the resturant cooling load is 60,000 BTU (can I assume that?) then tell the guy to stop the order for the 3 Ø equipment and rethink the project.

In my opinion, equipment for converting 1 Ø to 3Ø is of value only when one is stuck having to use 3 Ø equipment (because nothing else is available) on 1 Ø power (because nothing else is available).
Posted By: chi spark Re: Phase Converters - 04/21/04 01:14 PM
As was said before, this application of phase converter is uncommon because the HVAC is normally a design item-it wouldn't be designed to be 3 Ø in a 1 Ø area. The only ones I have sold are used for a specific piece of equipment that was obtained(mill, stamper, autoclave, etc.) and the shop was only 1 Ø. You can get a price pretty fast, and I think that, along with conversion power loss, may change his mind. Good luck.
Posted By: George Corron Re: Phase Converters - 04/21/04 01:15 PM
Remember one of the first rules of Electricity:
You can NEVER do better than your original source.
Otherwise, all of us in this trade would have a 480 volt transformer in our houses.
Posted By: iwire Re: Phase Converters - 04/21/04 01:56 PM
ElectricAL I think you hit the nail on the head.

Quote
The client's confusion results in the HVAC guys having to quote several prices for different systems, and the EC also having to quote several prices. Then, with all these different quotes, the client trys to find the cheapest combination.

You may have a semi electrical savvy HVAC tech telling the customer that 3 phase draws less amps and therefore uses less power.

The client fixates on that and know looks for you to come up with 3 phase.


George

Quote
Remember one of the first rules of Electricity:
You can NEVER do better than your original source


Also well stated. [Linked Image]

JMO, Bob
Posted By: Trainwire Re: Phase Converters - 04/21/04 03:38 PM
In the circles I run in, there are home machine shops. Phase converters all over theplace because most machine tools are built around 3 phase motors. Because they are smaller for the given power, and prob'ly the real reason, they are easy to reverse.

This business owner might be in for a unit big enough that single phase power is a special order. but I doubt it. This is going to cost more in engineering and upkeep than it's worth.

:two cents: now I'm too broke to pay attention [Linked Image]

TW
Posted By: Bjarney Re: Phase Converters - 04/21/04 05:26 PM
Be extremely cautious of the claimed capability of “Phase Converters.”

Hermetic-compressor manufacturers state that over 2% voltage unbalance is unacceptable, to the point of disqualifying warrantee replacement.
Posted By: hbiss Re: Phase Converters - 04/21/04 05:31 PM
...or essentially a single phase motor/three phase generator rolled into a single rotating machine (rotary phase converter). You can even use a plain ordinary three phase induction motor as a rotary phase converter; you power it from your single phase supply through two of the leads, do something mechanical to start it spinning (since a single phased 3 phase motor won't _start_), and once it is spinning you can draw the third leg from the motor terminal.

Something that has always bothered me about this- how do you get true three phase which is 120-120-120 degrees out of single phase which is 180-180 degrees unless you use a true motor generator? (Single phase motor coupled to a three phase generator.)

All rotary converters I have ever seen use the single phase hot legs and create a third phase from them. This works if a three phase motor loses a phase- it will generate the third. But the "input" is already two phases at 120-120 degrees.

There was recently a question from someone who was using a rotary converter that supplied 240 volts. He was using a buck transformer arrangement to buck it down to 208. He couldn't figure out why the voltage was not consistant on all three phases. I suggested it was because the phase difference was not 120-120-120 degrees as the transformers expected to see.

-Hal
Posted By: winnie Re: Phase Converters - 04/21/04 06:21 PM
Quote

Something that has always bothered me about this- how do you get true three phase which is 120-120-120 degrees out of single phase which is 180-180 degrees unless you use a true motor generator? (Single phase motor coupled to a three phase generator.)

All rotary converters I have ever seen use the single phase hot legs and create a third phase from them. This works if a three phase motor loses a phase- it will generate the third. But the "input" is already two phases at 120-120 degrees.

There are three questions hidden here.

1) How do you get _any_ phase angle difference from the single phase machine?
The answer is that you have stored energy in the mechanically moving rotor, just as you would in a true motor-generator. The single phase supply attached to one of the phase bands causes the rotor to move. The moving magnetized rotor interacts with the other phase bands in turn, and since the other phase bands are mechanically displaced, the voltage on those phase bands is displaced in time.

2) How do you get _exaxtly_ 120-120-120 with balanced voltage?
Alas, you don't. You have three separate circuits which are _not_ the same. Current flows (and thus voltage drops) are different. The system will come close. If you use an ordinary three phase induction machine as your phase converter, the machine is designed with balanced coils, and _must_ produce unbalanced output. If you design the machine with unbalanced coils, then at a particular load condition it could produce balanced output.

3) If you have a three phase 120-120-120 supply, and one phase legs goes dead, what is the phase relation on the remaining two legs?
The phase to _neutral_ phase angle remains 120 degrees. However for a motor without neutral connection, losing _one_ of the phase legs means that you have lost _two_ of your three phase supply circuits. What is left is a _single_ phase circuit, with 180 degree phase relation between its two supply terminals. For _any_ load connected between _two_ supply terminals, be they two hots or a hot and a neutral, or the hot and a tap on an autotransformer, the current flowing in from one supply terminal must be balanced by the current flowing out on the other supply terminal.

-Jon
Posted By: Scott35 Re: Phase Converters - 04/22/04 01:20 PM
Tell the Client to use 2 or 3 separate 1Ø HVAC Packages, and forget the whole Phase Corn-Verter idea + 3Ø HVAC Package!

Just venting my 2¢ [Linked Image]

Scott35
Posted By: nesparky Re: Phase Converters - 04/26/04 03:04 AM
Caselec
I believe the answer was well covered by the rest of the posts. It will not be cheaper to run the 3 Phase equipment vs properly sized 1 phase equipment. Watts are Watts. Add the unavoidable inefficiency of a phase converter and the running cost will increase.
Like ElectricAl I have had clients who wanted 3 phase in normally 1 phase areas (Apt complex). They cannot understand why the POCO bills are always slightly higher than there 1 phase buildings. After all a slick salesman had sold them on CHEAPER power and they cannot admit the got bamboozaled.
© ECN Electrical Forums