Not wanting to hijack the belt sanding thread I thought I’d start another to discuss the options open for 3Ø power.
I happen to be in favor of 3Ø upgrades to existing service. Others have suggested rotary or solid state converters.
My reason for prefering the upgrade is that for the cost of about a 30A rotary, I was able to get 200A 3Ø 240V service (more than 6x the power). Solid state converters are even more expensive.
I can understand the utility of buyng or building a rotary if you were to come upon a used, bargain 3Ø machine, or if you inherited one. Then, you would size the rotary to the machine.
The disadvantage though, is that single phase power, converted, is still single phase power at the meter, and you have the added efficiency losses of the converter to the cost of operating the machine. You also still have to breaker, and supply, the single phase power to the rotary at the level required by the woodworking machine, so you’ve lost the installation advantage as well.
So,
No istallation advantage (breakers and conductors are still large),
No operating advantage (still paying for single phase amperage to converter) and.
No initail cost savings (you have to add the cost of the converter to the base cost).
I’m open minded. If someone has a different perspective, I’m listening.
Tom
Replies
If you have three phase tools, or want to run three phase tools, obviously having a three phase service installed would the best way to go for ease of operation. However, it isn't necessarily easy.
Most people probably don't have three phase power available at the power pole out in front of their house. If your shop is in an industrial or commercial area, or if you live along a main road or street, there's a better chance it's out there but if you're in a residential area it's not very common.
If there isn't three phase power running by your house, you may be able to convince the local utility to bring it in if they have it somewhere nearby. However, it'll probably cost you...I just finished up the design of a new wellhouse for a local town (I'm an electrical engineer), and it will be built in a residential neighborhood about three blocks from the closest three phase. The local electric utility is going to charge the city about $30,000 in installation costs to extend the three phase into the neighborhood.
The utility may or may not be interested in hooking you up with three phase. They will look at the amount of revenue they'll get in return (i.e. how much electricity are you going to use every month), and if it's not enough to make it worth their while they may refuse to do it. Also, some utilities just won't serve residential property with three phase, only commerical. I can't speak for all electric companies, but that's how it works around here.
Finally, the monthly base charge for a three phase service can be quite a bit higher than it is for single phase.
It seems like every electric utility has totally unique rules and fee structures, so your mileage may vary depending on where you live.
I guess it was presumptive of me to expect that we all have 3Ø power in the street. My power company had to install a couple more transformers on the nearby pole. That, and the connection to my mast cost me $700. well worth the cost.Tom
I'm just impressed that you got that "phase" symbol to show. How?
"Everything should be made as simple as possible, but no simpler." A. Einstein
http://www.albionworks.net
Well now, I don't suppose you know the secret handshake either, do you stranger?Ø is <shift-option-O>
ø is <option-o>Feel free to impress your friends with it, at parties and social gatherings.TomBTW- "that phase symbol" is called a "theta"; it's the eighth letter of the Greek alphabet. (I am very popular at parties, ...I think)Edited 1/28/2005 11:23 am ET by tms
Edited 1/28/2005 11:23 am ET by tms
That is a phi. This is a theta: q.
You're right, of course. The font was a bit foreign to me.Tom
You know what? I wasn't thinking very hard, I read the three phase in your post and didn't pay attention to the fact it was a 240V three phase system.
I'm guessing that your local power company hooked you up with a 240V three phase open delta system, which can be done off a single phase line by adding a second transformer. You end up with a voltage of 240V between any two of the three phases, but from the three phases to ground you get 120V, 120V and 208V. The 208V leg is typically called the 'wild leg', and as a result every third slot in your circuit breaker panel can't be used for a regular 120V circuit. I looked around online but I can't find a decent illustration for this setup, if I come across one later I'll post it.
Around here, the power companies don't support this kind of electric service any more; there are still some older buildings that have it, but they won't install any new open delta services. That's why I didn't even think of it as a possibility, but you're right - if it is available, it's a reasonable way to get three phase service. It sounds like you got a pretty good deal.
You're right I do have a Delta system.
"Wild Leg", I've not heard that before. Around here we call it the high leg, but wild leg is probably more appropriate. I have measured voltalges over 400 VAC against ground, and yet against the A or C leg, it's a consistant 240V. I'm in Seattle, and I'm pretty sure that there are many more Delta systems here than there are Wye systems. I think that the Wye systems are limited to the industrial sections of town. On the other hand, I think that a Delta sysem is within reach of most folks. In this town at least.Tom
I'm guessing that your local power company hooked you up with a 240V three phase open delta system, which can be done off a single phase line by adding a second transformer.
Are you sure that's right? I don't know how you can generate three phases, 120 degrees apart from each other, off a single phase line.
I thought the "wild leg" deltas were three transformers on a 3-phase distribution network, but the ground is picked off of the center tap of one of the transformers. So the voltage between this ground, and the twp phases the center-tapped transformer is connected to will be 120V, but it will be higher between the other "wild" phase and the ground.
Lookin' up at the pole, from the ground, that's what it looks like they did. That would also explain why the 'B' leg is so freaky.Tom
Here's a link to a page that describes open and closed delta services: http://cipco.apogee.net/foe/ftdttd.asp. From that page, it says, "Two single-phase distribution transformers can be used to supply an open delta service where one phase of the delta is "missing" or "open". This arrangement provides three-phase service for small three-phase loads within rural, residential or light commercial areas."
In this setup, you have two 240V center tapped transformers connected at one end, like this picture. You get your 120V single phase power from either phase A to neutral or phase B to neutral. If you tried to connect between phase C and neutral, the voltage would be a lot higher. So, in the panelboard every third slot is not usable for a 120V circuit.
View Image
Edited 1/28/2005 5:46 pm ET by Stuart
OK you only need two transformers, but you still need access to a 3-phase distribution network; e.g., on my street, we only have single-phase, meaning there's only two wires on the pole: one grounded wire, and one "medium voltage" ~3 KV distribution feeder. There's no way one can hook a couple of transformers up to those 2 wires and get 3-phase service.
"OK you only need two transformers, but you still need access to a 3-phase distribution network; e.g., on my street, we only have single-phase, meaning there's only two wires on the pole: one grounded wire, and one "medium voltage" ~3 KV distribution feeder. There's no way one can hook a couple of transformers up to those 2 wires and get 3-phase service."
You're right, I had my story a little garbled up. For an open delta setup, the electric utility will add one additional feeder wire to the overhead poles, so essentially they will have two phases on their side of the transformer bank. One of the feeders is connected to the primary side of one transformer, the other feeder is connected to the primary side of the other transformer, and the grounded wire is connected to the center point where the two transformers are connected to each other. On the secondary (or customer) side of the transformers, because of how the transformers are connected, you get three phase power as shown in the diagram in my earlier post. You can sometimes get weird voltage imbalances with this setup, which is one reason they don't do it very often (as I said earlier, around here the electric utilities will continue to support existing open delta services but they won't install any new ones.)
A conventional three phase setup has three feeder wires and the one grounded wire. Since the utility can do the open delta setup with only two feeders, it costs less money. There's one less wire, of course, and there's less work required to install it - they can usually install the extra feeder wire on the existing poles, but if they ran two extra feeders they'd have to add crossbars to the poles to keep the feeders spaced apart from each other. That's a bunch of extra work, and with crossbars the poles need more clear space around them so they'd have to trim back any trees that are in the way, so it all adds up.
The bottom line is that yes, three phase power is preferable for running large machinery, but three phase power isn't necessarily going to be available out at the pole in front of your house.
Anybody look into getting a 3 phase generator? Get one used from a construction company?. Gotta be cheaper than paying union linemen.
"Anybody look into getting a 3 phase generator?"
Run that 3 phase generator with an electric motor rather than a fueled engine and we have a rotary converter. Good ones reportedly have about a 97% efficiency, and drive the 3 phase motors at full power (unlike most static converters)
________________________Charlie Plesums Austin, Texashttp://www.plesums.com/wood
Okay, now we're back to my question in the original post.Why would anyone purchase, install, and operate a converter, when a 3Ø service upgrade is available?Tom
If three phase power is available, it is far better for motors - go for it.
You cannot make three phase power from single phase (all that is in most residential areas) without running a generator or doing some very expensive electronic tricks. You have to bring it from a generator (or borrow some from an industrial area where the three phase power exists to run the big motors).
No amount of money will get three phase power to me in my neighborhood - all of the underground utilities for the entire neighborhood would have to be replaced. And as some others have suggested, it may cost tens of thousands of dollars to bring the three phase power the last few blocks via poles. I just bought a large combo machine with three 5 hp motors, and chose to go single phase. Another brand used only three phase motors, but had an internal electronic converter if you wanted single phase. If I had chosen that brand, I would have spent the many hundreds of dollars on an external rotary converter and bought the three phase machine)
Many people use static converters. Over simplified, it gives you about 70% of the power from your 3 phase motors, with an inexpensive converter. (I bet my new saw would run pretty well with only 3 1/2 hp available instead of 5 hp). The more expensive rotary converter will give your 3 phase machines 100% of the power, not just 70%________________________Charlie Plesums Austin, Texashttp://www.plesums.com/wood
Hi Charlie,I've seen your posts on the MiniMax group. I also have a CU300Smart, that I bought single phase because three phase was not available without a special order from Italy.Are you certain that three phase is not in your neighborhood? Even if you live in a developement with underground utilities, most power distribution is 3Ø, with the transformers in vaults at major intersections. likely, all you would need is a fourth conductor pulled through your chase.Tom
My MiniMax 410 works great on single phase, even though three phase motors have far better starting torque and may last longer because of the smoother application of torque. Felder was the second choice, which is the brand that only uses three phase motors, and has a built in converter if you want single phase (more electronics than I want built into a heavy duty machine).
I was told that we did not have three phase in the area, but even if we had, I would have considered single phase because of the uncertainty around the power distribution in the next location I might move to, etc. Total cost of wiring for the combo was about $70, most of that for the locking plug and receptacle, 5 feet of #10 wire to the breaker box, breaker, and power cord. I never could have done it that cheaply for three phase, even if it were next door. ________________________Charlie Plesums Austin, Texashttp://www.plesums.com/wood
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