single phase vs. 3 phase

[George]

Count the wires up to your pole.

[Rico]

That's what I did on my 3 hp 3 phase milling machine, but it wasn't nearly free :) So far I have not noticed any drawbacks to the simple cheap way. If I was running the machine at full capacity in a commercial environment I probably would.

On my 3 hp 3 phase lathe, I got a VFD because I wanted variable speed control. Then I found out how nice they are for controlled rate stopping and starting and a bunch of other things. I would even use one on a metalworking lathe if I had three phase power. I probably wouldn't bother with a VFD on wood working tools, although it would be tempting if I got a ww lathe for turning large diameter items.

Rico

[Jon]

In commercial/industrial even single phase equipment can require the same. It's not just because it's 3-phase.

Just for the record..... Jon Veeneman

[Bruce Rowen]

You state that the reason it the need for "big" equipment that has the advantage of being cheaper up front (purchase price) compared to (lesser quality) single phase equipment. I'm not sure of your intended use (production vs. sporadic home type use), but in general the most cost effective way to take advantage of the "cheaper" three phase equipment is with phase convertors. As stated before you do lose all the efficiency gains of true three phase but if the equipment is only actually running for a tiny fraction of the day these savings are invisible. Several neighbors have 5-7hp lathes and mills in their garages that they power with rotary convertors (made from a spare three phase motor) that work just fine. These things are only used for short times and the price of a surplus three phase lathe is far cheaper than any equivalent 1 phase tool.

If I was planning on what you are intending, I'd just get the 3-phase equipment and live with a phase convertor. Should the need arise later for true three phase, you probably can get it installed if the service is near by for several thousand dollars.

-Bruce

[JMartin957]

Correct on everything but the starter. Overload protection is a good idea for any motor, but you don't need it for either single phase or three phase.

John Martin

[Mike D2]

look at the number of lines coming from your transformer into your shop. 3 lines = 3 phase, 2 lines = 1 phase.

[Gfretwell]

As long as the equipment is capable of being wired for 240v 3p (most do) it won't know about the center tap. The center tap is only used for the 120v loads. This arrangement also gives you the 240v single phase the other equipment in your house expects. It was what the utility advised but they weren't anxious to do it. It came down to money in the end. I could have got an upgrade to 400a 1p for free but the 3p was just called "expensive" by the engineer at PEPCO and neither of us pursued it farther. I didn't actually go to the sales office for how expensive it would be. Of course you also have to add the price of the service disconnect, main panel rewiring and the extra 3p panel. You might actually be able to get by with the capacitor trick (I think it is

4mfd per HP if memory serves) although a commercial phase converter is the better solution. I did get a 3p 3/4hp (3330 disk drive) motor spinning with a 3mfd capacitor but I never really used it for anything.

[JMartin957]

Three phase is more economical to run, and the motors are cheaper. It also allows you to instantly reverse the motor (without allowing it to coast to a stop or to slow way down, as you do with a single phase motor).

But, three phase power is generally very expensive to bring into a residence.

Except for a few special metalworking machines where the instant reversing is a plus - such as for some tapping and lathe operations - you would always choose a single phase machine in a home or small shop.

If you get a used three phase machine, you can run it on single phase current by using a phase converter or a VFD (variable frequency drive).

John Martin

[Roy Smith]

Maybe. I've got 3 wires coming to my house. It's 240V single phase. There's two hot legs plus a neutral. On the overhead drop from the pole on the street, the neutral is bare, but once it goes into a conduit down the side of the house to the meter and panel box, all three are insulated.

This is dangerous stuff to be guessing about. If you're not sure, the best thing to do is call an electrician. He'll be able to tell you what you've got in about 30 seconds by looking at it. Might cost you a few bucks for his time, but it beats burning your house down.

[Gfretwell]

Did they just repeal part III of article 430 and not tell me. I hate it when that happens. ;-) It's true that a high percentage of motors, particularly those sold retail, have internal overload protection (check the labels on the motor) but it still needs to be there somewhere. The branch circuit breaker is sized to the starting current, not the full load current and can typically be 250% of what the motor will safely run at. (see table 430-52) That can also be more than twice the regular ampacity of the wire feeding the motor. (stuff like 40a breakers on 14ga wire). It is the overload protection that keeps this safe.

[Gfretwell]

The number of INSULATED lins ... There is also a neutral that is typically uninsulated. Total is 4 wires for 3p, 3 wires for 1p (in the US).

[Rico]

Neutral is typically (always?) insulated in a single phase system. Single phase = 3 insulated wires plus uninsulated ground.

Rico

[Lazarus Long]

the neutral at my address is uninsulated. So it isn't always insulated.

[Wade Lippman]

An office I used to work in had 2 phases of 3 phase supplied to them, so they had 208v. It still isn't 3 phase, since they did not get the 3rd phase.

A factory I worked at had real 3 phase, so they used mainly 208v motors, though some of the big ones were true 3 phase.

[Bay Area Dave]

that I don't know, since I was only concerned about the voltage in the shop I leased

dave

Rico wrote:

[Scott Lurndal]

While the grounded (aka neutral) conductor in premises wiring is indeed typically insulated, the original poster was refering to the service drop, where in most cases the grounded conductor (neutral) is uninsulated.

scott

[Scott Lurndal]

Santa Clara doesn't have _a_ voltage. Power is brought into Santa Clara via high-tension transmission lines where the voltage is somewhere around

115,000 volts (115kv). It hits a substation where it is transformed to lower voltages for distribution (22kv is common in south san jose, for instance, although newer systems use higher voltages). The 22kv is then distributed to home, commercial and industrial users. The 22kv will be transformed to 240v (nominal) single-phase for home users, and typically 480v three phase for commercial (office parks et. al.) and higher voltages for industrial users. Commercial and industrial users have on-site transformers to produce various voltages for various purposes - 240/277/208 volt systems are quite common in commerical and industrial facilities - 277v is very efficient for flourescent lights while 240/208 is used for industrial purposes. 240 can be centertapped to ground to provide the common 120 (as is done by the grounded conductor at your home service entrance, for example). In all cases, the 22kv distribution will be three phase and where the commercial or industrial entity has its own transformer, the three phase will be distributed to the business. To save wiring costs, three phase isn't distributed typically within residential neighborhoods (past the 22kv stepdown transformer).

scott

[Eddie Munster]

Hope your shop is in commercial space and not your home.

John

Mike B wrote:

[Andy Dingley]

Stop it ! You're scaring the Europeans 8-)

[Andy Dingley]

I thought some of you guys had (domestic) heating that ran off it ?

Here in the UK it depends how your power is delivered. If it's underground (nearly everyone) then it's a serious spend to get 3 phase. If it's an overhead feed (just the rustics) then it's a lot cheaper as there's normally line-of-sight to a suitable transformer.

-- Die Gotterspammerung - Junkmail of the Gods