AC Voltage & Phase Question - Page 1
Forum section: electronics / general

author: otpowell
author_rank: Regular Contributor
timestamp: August 21, 2017, 10:30:29 pm
content: I have a quick question, when someone talks about 2 and three phase power, this is all in relation to running to sine waves out of phase to get a more stable power source, correct? In these instances, is there a way to double your Voltage? Say running two 120v AC lines 180 degrees out of phase could you get 240v?Additionally, can one run AC lines in a parallel arrangement and series, same as batteries to get double the amperage or double the voltage? My initial thoughts are that AC doesn't work like DC because each hot and neutral are pulling the sine wave in different directions so really one would end up with the same Voltage and amperage. Amperage and Voltage is dependent on the transformer and breaker on controlling the lines? One of my coworkers seems to think that one can run two 120v lines in parallel and get 240v but I don't think that's the way it works.Additionally, from my experience most AC circuits, have a ground, a neutral, and a hot. Are there applications where instead of one neural, there would be two hot wires and one ground? This is what my coworker seems to think because we are getting very low resistance between the supposed hot and neutral terminals. His reasoning is that maybe we don't have a neutral but two hot wires (each 120v which would total 240v). My response was, without a neutral, there wouldn't be a proper sine wave without noise and the power would have nowhere to return to? He said perhaps ground and neutral share. I don't think that's possible? Any suggestions or info would be greatly appreciated. FYI, I'm taking power systems this semester and probably will fill a lot of these AC gaps of knowledge that I have. Thanks all in advance!Sent from my iPhone using Tapatalk

author: Zero999
author_rank: Super Contributor
timestamp: August 21, 2017, 10:40:04 pm
content: You don't use a two phase system in the US. It might be what it's commonly known as but, if it is, it's a misnomer. Two AC sources connected in series, with the neutral in centre point, is split phase, not two phase, which is a 90ophase shift.AC sources can be connected in parallel, but they must be exactly the same voltage and be in phase with one another, otherwise a large current will flow, causing overheating, possible fire and smoke.Electrically speaking, neutral unusually is the same as ground, because it's typically connected to earth i.e. the soil but only the neutral conductor can be used for carrying current. The ground conductor is just there for safety purposes.

author: Cubdriver
author_rank: Unknown
timestamp: August 22, 2017, 01:48:43 am
content: In US residential systems, center grounded 120/240 service is the norm. In this arrangement, the distribution transformer on the utility pole drops the typically 12-18kV powerline voltage to 240V using a center tapped secondary. This is then fed to the home's breaker or fuse panel as Line 1, Line 2 and Neutral. Neutral is tied to earth ground at the service entrance (typically the panel, though it may be at a breaker outside at the meter box if the panel is some distance away). The voltage is 240V when measured from L1 to L2, and 120V from N to L1 and from N to L2. N to L1 and N to L2 appear to be 180* out of phase with one another, but are not really two phase because they come from the same transformer secondary.Industrial settings are usually fed with three phase power; a common 'low voltage' (light industrial) arrangement is 120/208V 3 phase. (I say 'low voltage' because US industrial power may be 3 phase 120/208, 277/480 and I think in some instances 346/600) This is a 'wye' connection, with three 120V transformer secondaries, each with one end tied together (forming the 'neutral'). The primaries are each fed from one leg of the incoming three phase line, and the resultant output waves are 120* out of phase with one another. (L1, L2 and L3) Because they are 120* apart, they will measure 120V from line to ground, but only 208V from line to line (the phase differences mean that they do not add directly).Single phase 120V AC circuits will usually have a hot (the line), a neutral and the ground (a protective earth). In normal operation, there should be no current flow in the ground; it is there only for safety. All current should flow through the line and the neutral wires only; they form the circuit. This may be either L1orL2 and neutral in a single phase installation, or L1orL2orL3 to neutral in a 120/208 3 phase system.In residential circuits where 240VAC is needed (for instance to power an oven or range, electric clothes dryer, or a large air conditioner), circuits may be two hots (L1 and L2) and a protective earth only. In 3 phase 208V settings, 208V may be derived from any pair of lines (L1 and L2, L2 and L3, or L1 and L3), with a safety connection to the protective earth line. (Again, in either the 240 or 208V scenarios, the earth ground should carry no current in normal operation - it is there to prevent the chassis of whatever it is connected to from going above ground potential in the case of a failure resulting in a short or leakage.)AC distribution lines are not typically paralleled in my experience; if more current is needed then heavier conductors are run from the panel and a larger breaker is used.How are you measuring resistance between hot and neutral - I'm assuming that you're doing this on the device to be powered, rather than the wires from the panel. Knowing what the device is would help to answer this question - things like incandescent lamps have very low 'cold' resistances and will appear to be near shorts if measured with an ohmmeter; once they are powered up their resistance increases with increasing temperature until they stabilize at their normal operating voltage and current.Line to line will certainly work and provide a good sine wave without a neutral reference, but in that case both lines will be hot relative to ground. Ground and neutral in a properly wired circuit never share, though they are connected at the point where power enters the facility. From there on, they must be kept separate for safety purposes, even though they ultimately are connected together at that entry point.Your upcoming power systems class should make all of this MUCH clearer - it is difficult to describe in type, but seeing schematic diagrams should help immensely in your understanding of it.-Pat

author: Connoiseur
author_rank: Regular Contributor
timestamp: August 22, 2017, 03:16:48 am
content: Quote from: otpowell on August 21, 2017, 10:30:29 pm*Yes the angle refers to the difference in phase between the sine waves.*Indeed you get more stable power with more phases, as the peaks become closer together in time, reducing the power ripple.*Theoretically, yes you can double the voltage, but it is almost never done in reality. I have never seen a generator used that way; possibly because keeping two prime movers in sync would be nearly impossible this way and you can always use a step up transformer without the negative consequences; however a niche application of such an arrangement is cascade transformer.Quote from: otpowell on August 21, 2017, 10:30:29 pm*Any generator or inverter tied to the grid is actually connected in parallel with numerous other generators in the grid. It is easier to connect AC sources in parallel than in series. For paralleling ac sources the a) Voltage, b) Frequency and c) Phase Sequence should be the same. Once synchronised, the generators effectively lock with each other share the load based on excitation and per unit impedance.*AC conductors (I mean from PCC or MCC panel to the respective load) can be connected in parallel to increase current carrying capacity and is fairly common practice in industry. High current loads like compressors by design have two glands for cable entry. In the past I've used 2 runs and even 3 runs of 240sqmm 3.5 core cables on certain occasions. This is done since a single thick cable may not be flexible enough to bend properly in the space available. We had a DRUPS that used 32 parallel 3 core cables to supply 1670kVA at 400V to the UPS PCC panel.*No. You cannot get higher voltages by paralleling.Quote from: otpowell on August 21, 2017, 10:30:29 pm*How did you measure the resistance between hot and neutral? I hope you didn't probe it with your multimeter. You can irreparably damage your multimeter if you poke a live circuit in the ohms range. It is a good thing that there is low resistance, how else do you expect the source to supply a lot of amps to your load otherwise?

author: otpowell
author_rank: Regular Contributor
timestamp: August 22, 2017, 11:10:03 am
content: Quote from: Cubdriver on August 22, 2017, 01:48:43 amThank you Pat for your freakin awesomely detailed answer! YES, you are correct, we measure the resistance on the device to be powered to make sure there's no shorts or anything. It's called a safety power up where I work. Well, we usually run this test on a UPS (uninterruptible power supply) which powers a big ass sever rack and usually the resistance between neutral and hot (or two hot perhaps since this is a 208V industrial setup) and the resistance is usually 100k or higher. On this particular UPS, which is different from others that we've tested, we get about 3 ohms. We aren't sure if the cord is bad or if this is normal for this UPS.Sent from my iPhone using Tapatalk

author: otpowell
author_rank: Regular Contributor
timestamp: August 22, 2017, 04:41:49 pm
content: Quote from: Connoiseur on August 22, 2017, 03:16:48 amThank you very much! No I wasn't proving the live line for resistance. Just checking things before powering up.. thanks for such great info!Sent from my iPhone using Tapatalk

author: Cubdriver
author_rank: Unknown
timestamp: August 22, 2017, 05:36:49 pm
content: Quote from: otpowell on August 22, 2017, 11:10:03 amYou're most welcome - hope it made things clearer. 3 ohms seems awfully low to me for the line side of a power supply, but I'm not familiar enough with UPSs to be certain. The other thing to remember with measuring resistance and AC circuits is that your ohmmeter is using DC to make the measurement, while the AC will see impedance rather than pure resistance and a low DC resistance *may* not indicate a problem (mind you in this case I would be very suspicious of the 3 ohm reading; that strikes me as shorted input territory especially considering that you say that you typically see much higher input resistance).I'd be suspicious of that particular UPS. I don't suppose that there are any more of that model that you can get at to make a comparative measurement? (Probably not, or I'd have guessed that you would already have done so, but I'm throwing that out there just in case...)-Pat