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#37063 04/21/04 09:08 AM
Joined: Jan 2003
Posts: 4,391
I
Moderator
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.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#37064 04/21/04 09:09 AM
Joined: Oct 2001
Posts: 597
E
Member
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).


Al Hildenbrand
#37065 04/21/04 09:14 AM
Joined: Jan 2004
Posts: 105
C
Member
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.

#37066 04/21/04 09:15 AM
Joined: May 2001
Posts: 717
G
Member
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.

#37067 04/21/04 09:56 AM
Joined: Jan 2003
Posts: 4,391
I
Moderator
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


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#37068 04/21/04 11:38 AM
Joined: Mar 2002
Posts: 360
T
Member
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

#37069 04/21/04 01:26 PM
Joined: Apr 2002
Posts: 2,527
B
Moderator
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.

#37070 04/21/04 01:31 PM
Joined: Dec 2003
Posts: 886
H
Member
...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

#37071 04/21/04 02:21 PM
Joined: Sep 2003
Posts: 650
W
Member
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

#37072 04/22/04 09:20 AM
Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and
Member
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


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
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