240V from a 3 phase main ?

I don't know about everywhere else, but I rented a shop that had 208-3ph. It was just as Doug described. Phase to phase = 208 volt, any phase to neutral was 120 volt. Greg

It was probably set up as a warehouse originally. This was often done because all that was needed was power for lights. The 208 wye service has the added advantage that your load is more balanced since you have three 120 circuits to use not just two. Plus you can run just about any 3-phase power machinery.

The only drawback to the 208 wye service is that any 240 2-phase stuff that you use won't get the power for which it was designed. But then, for just about anything that you find in a shop, that's not a problem anyhow. For instance, if you've got a 240 2-phase air compressor it will run happily on 2 legs of the 208 service. The power output might be a bit less, but you'll never notice it - plus the manufacturer overstated it in the first place anyhow.

Oh, and one more thing: Where Doug was saying "Wrong" and "NO.." up there. You now know that the correct answers are "Right" and "YES.." don't you?

George.

What newsreader are you using? Maybe I need to change readers. They were scrambled on this end.

Bob

If you're using a MS-Windows based reader, make _sure_ that you've selected a *fixed*pitch* font ("fixedsys" is one, so is anything with 'courier' in the name).

MT-NewsWatcher on a Macintosh.

I suspect the real problem is that your reader was trying to display the text as HTML or in a proportional-width font. If you can somehow force your reader to display the article as plain text in a fixed width font, you should be fine.

And anyone in Southern California that wants to see a working 2400 VAC 60Hz 6-phase to 600 VDC Rotary Converter station only has to go to the Orange Empire Railway Museum on an operating weekend.

Looks like something Rube Goldberg designed, complete with a ball-and-worm armature shaft wiggler to keep the brushes wearing evenly, and big live-front contactors with open arc chutes...

They now have a solid-state converter for everyday use, but they can still fire up the rotary - for demonstrations, if they're working on the other power plant, or if they're going to be running a lot of rolling stock at once.

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We have 440v and 220v 3 phase in our building. When we need 220v single phase we just take any two lines from the 220v 3 phase. When using plugs, we use the 3 phase plugs, but only use two of the taps going to the load. Works on motors, welders, everything, everytime. I never knew there was a problem until I read this thread. (this ain't theory, we been doing this for years). Oh yes, when we need

This much shouldn't be a problem for either of the 3 phase setups that have been discussed here - the voltage across any two legs will be the same. Of course if the equipment needs a neutral as well and you have delta power then it will matter which legs you use.

For 240V delta power, two of the legs will give you 120V compared to neutral. For Y 208V power, any of the legs will be 120V.

Since you haven't had any problems you probably have a 208V Y configuration (or you just happened to use the right legs). For what it's worth, before this thread I had no idea that anyone provided delta power. I knew what it was (although not the name) because that's what you get from a phase converter where you're using two legs as they are and generating the third. But every industrial building I've worked in around here (near San Jose, CA) had 208V Y power. Sometimes they'll have some higher voltage as well, but since that tends to just go to the HVAC systems I've never paid any attention to its configuration.

allan

For what

That's an interesting observation. I've had three phase power in three different structures, all of them in residential areas, and it's been no problem at all to get three phase delta. The one problem, however, is getting them to run it in underground. I have been told by more than one EE that there are fire problems associated with service of that nature. Not sure I understand it, but I've learned to live with the three transformers overhead.

Harold

It isn't that the motor won't run, but that a) the efficiency and life of the motor will likely be reduced because b) the current draw will go up at the lower voltage.

The speed of induction and synchronous motors (which most 240V tool motors will be, single or three phase) is determined primarily by the line frequency. The power needed by the motor is determined by the load-the motor doesn't care how much current it draws... it draws what it need to to meet the power demand of the load at the run speed. At the lower voltage (about 12%) the current will be higher (again, about

Motors are generally not conservatively rated-it isn't economical to overrate. You get the 10% or so maximum supply variation built in, and that's it. Go below that, and you need to begin derating the motor rapidly, go above that, and the likelyhood of insulation failure goes way up.

I can't speak to what direction you are referring to about customer cable. Around here, it varies so much. We have some customer owned primary and secondary. But when I'm talking about the grounds not being a return to the sub, say you have a 3-phase primary (just 3 wires, no neutral) going out from the sub a couple of miles and now you install an pad-mount transformer (or string of transformers) being fed from a riser off of this 3-phase primary. Your grounds on all these transformers are tied together, like you say. But there is no metallic return to the sub on them.

Yes

I can't say if it's practical for other than utilities, you'd have to weigh the costs. And yes, they have to redo the potheads, and elbows. Around here we don't use that many potheads, we use 3M termination kits.

Don

OK.

I'm referring to the legacy potheads in the underground transformer vaults which the utility desperately wants to phase out in favor of pad mounts.

Fire nature is based on the IR drop. You draw the current through the smaller than needed current and drop some voltage. I^2R or E^2/R that is power. The heated wire doesn't dissapage heat except down the wire. Some place gets to hot.

There is times when water leaks in and there is more IR drop - mostly to water.

Such is life.

Martin

I agree that power = I * V * cos(pf), so the current will be greater for a lower voltage at a constant power draw. I also agree that a fixed load device such as an air compressor could have trouble with a lower supply voltage, but variable load devices will simply have less power available -- if your table saw is a little weaker due to the voltage being somewhat low, just don't push the work through so fast.

Although motors are sized to meet their (presumed) load, motors are built in discrete sizes -- 1/2, 3/4, 1, 2, 5,...HP (or the equivalent in watts). If a load is 7/16 HP, the manufacturer is going to use a 1/2 HP motor which is then going to have >10% reserve. Motors are also designed to work in a particular voltage /class/, so they'll produce their rated power output at the minimum supply voltage without damage (or at least without catching fire). If the voltage is higher than the minimum, a motor can produce more than its rated output without damage. It would be a violation of various standards (UL, etc.) to manufacture a device that relies on a more than minimum supply voltage to safely produce the necessary power.

Industrial installations are another matter, but this discussion started as question about a "home" woodworking shop.

I think that would be hard for the user to judge. You would need a motor starter with properly sized heaters to ensure you're not drawing too much current.

The thing to remember here is that with induction motors (as opposed to smaller permanent magnet rotor motors) the torque is proportional to the cross product of the stator and rotor currents. To a first approximation, if the supply voltage is 86.6% (208/240) then the torque will be 75% of that at rated voltage.

RB

Everett M. Greene wrote:

Your math escapes me. If the supply voltage is lower, the current will be higher for the same power output, thus the torque would be higher, not lower, by your statement.

It doesn't work that way. The impedance of the motor's winding doesn't change just because you have a lower voltage. To get the same torque you need a higher current but you won't get it.

If as the voltage is lowered the current rises then when the voltage is zero (as in a short across the input to the motor) the current will be how much?

If the voltage is 86% the torque developed will be 0.86 x 0.86 or 75% of that at rated voltage (240 volts.)

RB

Everett M. Greene wrote:

torque != power

We're talking about two different things