220 vs 240 volt

[Bill Dufour]

In school they taught us 1 + 1 is 2. So 110 +110 is 220 so much for that lesson

True, but 120 +120 =240. That process is called generalizing what you learned in class into real world examples.
110 changed into 120 about when biplanes got replaced by monoplanes. 120 was firmly established before the Army air corp became the air force.
Bill D

[Bill Dufour]

Edison invented the electric chair to demonstrate how dangerous AC was. This was after he electrocuted an Elephant in a public spectacle to show how you should only use nice safe dc. Which he just happend to have the patents on. While he had no rights to ac power.
Bil lD

[Bruce King]

Edison invented the electric chair to demonstrate how dangerous AC was. This was after he electrocuted an Elephant in a public spectacle to show how you should only use nice safe dc. Which he just happend to have the patents on. While he had no rights to ac power.
Bil lD

It didn’t go well when the attempt to kill the elephant failed. The electrical shock wounded him and he was suffering so they shot him.

[Stan Coryell]

I don't think anyone who competed with Edison had an easy life. To confuse things more, some old machines, particularly from the Northeast have old two phase motors or 200v three phase. A vfd handles the 200v but nothing really handles the two phase very well. Dave

I've never run across 200 vac. Plenty of 208 vac 3 phase.

2 phase was common in Philadelphia, Camden, and Niagara Falls (so I've heard). I can only speak of Philadelphia. There is still quite a bit of it. Most of the industry is gone, but it's still common for elevators. Which are costly to convert for a number of reasons.

PECO's 2 phase distribution is 4Kv. It's fed from a 3 phase 13.2 Kv transformer that steps it down to 2 phase 4 Kv. I've read somewhere that the idea of 2 phase, was to parallel 2 single phase circuits. In the teens an 20's when DC and single phase generation bit the dust, there must have been quite a bit of two phase already in use. For what ever reason PECO has resisted changing it over until recently. Mostly because of 100 year old 4 Kv distribution cable failures. Also, a lot of the 4Kv equipment was installed just after WW2. Maintenance and replacement parts are now astronomical.

I worked for a shop, starting in 2000. One of the newer presses was about 1965. It was factory wired from Cincinnati 220 VAC 2 phase, 4 wire (two neutrals). That's the newest machine I've run across wired for 2 phase.

To the OP:
A lot of people use 220 and 240 interchangeably. Myself included. However, in the global economy, 220 vac single phase is sometimes associated with 50 hz (220 phase to ground). We have a few presses that have 240 v to 220 v transformers in them. AC motors run at lower speed on 50hz. Here in the US we use 60 Hz. This lower speed may cause overheating issues. Unlikely with a table saw.

[Bill Dufour]

Yes Tesla invented ac motors and had the patent against Edison's dc motor. AC has transformers allowing higher distribution voltage which are much smaller and more efficient use of metal.
Tesla Motors has no relation to N. Tesla and family except they stole his name. I believe the company donated some money to the Tesla museum. There is a small powerhouse on the truckee river in the Sierra Nevada named Tesla.
Bil lD

[Rob Damon]

Typical Residential is 120/240v single phase because you are taking the 240v off the pole/ground mounted transformer and tying to the middle of the 240v winding to getting two legs of 120v to gnd. You will see a single transformer on the pole or ground. This 120 volt generally range from 108v to 132v and the 240v can range from 216v to 250v. But they generally try to hit the median voltage.

Small businesses that are generally located in rural residential areas that does not have 3 phase overhead lines will have sometimes 120/240v 3 phase. You use A/B/C legs to get 3 phase and use A or C legs to get 120v but you can't use the center (high) leg to get 120v (you generally get a 277v you can't use.) You will see two transformers on the pole, as small one and a larger one.

Small and Large business in commercial areas with 3 high voltage overhead lines will give you either 120/208v, 3 phase or 277/480v, 3 phase. You will see 3 transformers of equal size on a pole or a larger ground mounted transformer.

For motor loads you will see nameplates that may say 200v which will run on 208v system. Sometimes the name plate will say 200v - 230v which means it will run on 208v or 240v. Most will be on 208v even though the measure range from the transformer can be 10% above/below that voltage.

Most service transformers have taps so if the measured voltage is really low or really high, the utility can connect to a higher or lower tap on the transformer to adjust the voltage to feed your building or if the service conductors run to a transformer that is far from the building to account for voltage drop.

This also holds true for the 480v system, you will see some motors listed as 440V, 460V and maybe 480v but most are labeled 460V. This is for just the US.

I have seen an odd machine that may have an odd voltage and that's when special transformer come into play to adjust the voltage to suite the machine.

[Jason Roehl]

The building in which I am the maintenance tech uses pneumatic HVAC controls, so I have a large two-motor/two-pump compressor to supply the pneumatic system. At one point we had to replace both motors because a contactor failed, causing the motors to run continuously—one was still running, on fire, when my co-worker discovered it. Anyway, the replacements we found for it were 200V—made for continuous duty on a 208V 3-phase system (what my building is). Upon a further online search, 200V is a standard nameplate rating for a 3-phase motor intended to run on a 208V system.

[Alan Lightstone]

And let's not forget JP Morgan funding Edison in the fight against Westinghouse, and Tesla.

Now why are 3 phase systems 208V? Is it because the amplitude of the voltage difference between two phases is sqrt3 times the amplitude of the voltage of the individual phases (120x1.73)? Or some other reason a standard was chosen.

Looking back, it was amazing that I wasn't taught a thing about electrical distribution in lower level electricity and magnetism courses at MIT. But I still can recite Maxwell's Equations. I guess they saved that for the true EEs.

And how do you tell looking at a utility pole what service is being provided, or available?

[John K Jordan]

...
Now why are 3 phase systems 208V? ...

I once drew out the 3-phase waveforms and subtracted one wave from another other to see why.

The power company here, out in our rural area with no industry around, recently replaced all the poles on the nearest country road and ran wires on 3-phase hangers. But they only ran two wires in addition to ground instead of three. I assume they did that to allow balancing the load between different areas, either now or in the future, and let them easily change to 3-phase if needed. Maybe someday when I don't have anything else to do I'll drive around and see if some side streets are fed by different phases.

The poles, BTW, are much taller and set back further from the road than the original. They cut down a LOT of trees. I could have picked up 200 logs just along the short road our lane connects too.

[Rod Sheridan]

And DC is likely to replace AC for the large high voltage distribution lines. Less loses with DC & modern low cost electronics needed to change the voltage is removing the high infrastructure cost barrier.

Agreed, however probably only for insulated cables, such as submarine applications....Rod

[Steve Demuth]

In school they taught us 1 + 1 is 2. So 110 +110 is 220 so much for that lesson

The electrical voltage standard is reducing a very complex energy delivery problem to a single number. Some details necessarily get fuzzy when you do that.

The standard in North America is that the RMS voltage at the service be between 114 and 126 volts, service to ground. That's 120 volts plus or minus 5%. The voltage drop across your motor is supposed to be between about 108 volts and 124 volts.

That's an "average" voltage in the sense that the actual voltage is cycling as a sine curve 60 times per second between a positive maximum with respect to ground, to a negative minimum with respect to ground, and back to the positive maximum. The 120V nominal is the root mean square (RMS) average of that cycle. The actual voltage you'd measure with an oscilloscope, where you could see the wave-form, would be varying between +172V and -172V 60 times per second. The RMS voltage is used because for a sinusoidal AC voltage, it represents the DC voltage that would provide the same power as the measured AC voltage. That is, a standard North American 120V AC circuit at some defined current flow, delivers the same power as a DC 120V system at that same current flow.

You get 240 V in North America by taking the voltage across two 120V "legs" that are in precisely opposite phase - so one is at their maximum positive with respect to ground when the other is at their maximum negative. That doubles the voltage difference at every point in the curve, and yields the 120V RMS +120V RMS = 240V.

But if you take two "legs" that aren't in exact phase (from a 3 phase system), the voltages don't double, because the second leg is not at it's negative minimum when the first is at it's positive maximum, but somewhere between ground and its negative minimum. When you do the math, that makes 120V RMS + 120V RMS add up to 208V RMS.

It works this way for standard low voltage delivery systems pretty much everywhere in Canada, the United States, and Mexico. Outside that region, it can be quite different. Japan has two different frequency standards in the same country, requiring essentially separate grids in the East of the country and the West, and runs at 100V RMS, instead of our 120V RMS e.g.

[Mike Henderson]

Agreed, however probably only for insulated cables, such as submarine applications....Rod

I don't think that's correct Rod. There are a number of terrestrial high voltage DC transmission lines. One advantage of DC is lack of skin effect. Also good for transmission between grids that are not synchronized to each other.

Mike

Wikipedia has a good article here.

[Frank Pratt]

Agreed, however probably only for insulated cables, such as submarine applications....Rod

Overhead too. There's an 800 kM, 500 kV, 1500 mW DC transmission line in India, China has an 1100 kV line that's 3300 kM. There's an intersting read on the subject in Wikipedia: https://en.wikipedia.org/wiki/High-v...direct_current

Oops, Mike beat me to the link.

[Wes Grass]

Unless it”$ three phase, then 120 + 120 = 208


Regards, Rod

Or in my shop in CA: 120+120+220 = 240+-, 240+-, 240+-.

Two of the CNC machines were on transformers. One *needed* 380 (European), the other wanted 208 (Japanese). Service Tech strongly suggested stepping it down in case of any warranty issues.

[Steve Demuth]

I don't think that's correct Rod. There are a number of terrestrial high voltage DC transmission lines. One advantage of DC is lack of skin effect. Also good for transmission between grids that are not synchronized to each other.

Mike

Wikipedia has a good article here.

There have been High Voltage DC transmission lines in the US for over 30 years. I believe the first was built between Washington State and LA in the 70s. There have been quite a few built since. They are quite common in Europe,China and Japan. The big advantage is significantly lower power loss in long distance transmission due to the complete lack of reactive loss.