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| Subject - How can a 240v device work off no neutral/ ground? | |
| seagull369 |
Hi. I was wondering if anyone could offer an understandible explaination as to how I can get a measurement of 240 volt across two 120 "hots" without there being a need for a nuetral or ground in the equation. I have even plugged in a 240 volt compressor I have across these hots, omitting the ground altogther (I know, not the safest setup), and the thing ran. When you take a voltage measurement for one 120v line, you need either a nuetral or ground to complete the circuit to get a measurement, right? Why isnt a nuetral/ ground necessary in a 240v setup for a device to run? Any explanation (or link) that can help solve this mystery would be appreciated. Thanks. |
| JimmyDee | http://www.electricalknowledge.com/forum/topic.asp?TOPIC_ID=301 |
| lctrc789 | Look at the picture that Jimmy hypyerlinked for you real careful. You can get 240 volts acroos the the two legs of the transformer and across your equipment for it to work, the ground is for safety and has nothing to do with the fact the load will not work at all. Just like a 120 volts needs to complete the circuit for the load to work so does the load work with it completeing the circuit for 240 volts across the transformer thus your load will work as well. |
| seagull369 | ok, so if I understand this correctly, the center of the transformer winding (which is tapped to earth ground) is 0V. The two hot legs of coming off the transformer are 180 degrees out of phase from each other, making there instantaneous voltages opposite of each other. If I am right there, it's interesting to see that a +120V and a -120V at a given point in time aid each other to give 240V instead of cancel each other out and equal 0V. I guess by looking at their vectors we see the magnitudes aid each other: +120V at 0 degrees and -120V at 180 degrees is calculated: +120V - (-120V) = 240V. Is this correct? Thanks for the great stuff offered thus far. |
| JimmyDee | I think you have tried to make it too hard. Think of the transformer's secondary coils as 2 individual transformer coils that are capable of being series or paralled. Each coil produces 120 volts, RMS. In the configuration we use feeding a house, we series the 2 coils together. If we added lables to the 2 leads feeding the coils, one coil would be labled lead X1 and X2 and the other coild would be marked X3 and X4. by splicing X2 & X3 together we have series the 2 coils, thus producing a series circuit of 2, 120 volt coils which add up to 240 volts. Now if we ground the X1-X2 junction and also add an additional white wire at that junction, you will see 120 volts between X1 and the X2-X3 junction and 120 vlots between the X2-X3 junction and X4. Also 240 volts between X1 and X4. -----^^^^^^^^^^------^^^^^^^^^----- x1..........................x2..x3....................... x4 Jim |
| Energreen | seagull369 asked: " +120V - (-120V) = 240V. Is this correct? " Exactly right. The resultant voltage is the difference in potential between the two nodes (X1 and X4). The potential difference in this case is 240V. However, the numbers you're using are Vrms values, which don't technically work for an example of INSTANTANEOUS voltages such as you posed. It's ok to use Vrms for a basic discussion like this. Your question shows that you basically understand what's actually happening... the relationship between the two hot lines in a 240Vac circuit. But be aware that the 120Vrms sinewave is actually closer to 170V peak to peak. So in reality, if you choose the instantaneous peak value it's not 120V, it's 169.7V (120V/.707). And if you choose the negative peak instantaneous value, it's -169.7 volts. So, rather than 240Vrms, you're actually going to have 338.5 Volts at the instant described in your example. (A Vrms meter would read 240Vrms. An oscilloscope would show 338.5Vpp.) FYI - In AC theory we refer to Phasors, which represent magnitude and phase angle. (Vectors represent magnitude and direction, which is different.) More FYI - Vrms is Volts Root Mean Squared. It's a mathematically derived value that represents the "heating value" of the AC voltage. If you apply 120Vac to a 1000W resistor (or any resistance value of your choosing), it will produce the same amount of heat as if you applied 120VDC to the same resistor... even though the 120Vac is actually 169.7 Volts peak to peak.  |
| lctrc789 | I say real good job here guys, it may not be easy for those who do not truly undertsand but peak voltages and mean voltages and nominal voltages and let's add 180 degrees out of sync and wow how does that work.... |
| blackrd | I really hope this isnt a dumb question. How do you, (or what does it look like schematically) paralell the secondarys? And if you do, what do you end up with on the output side, using the transformer used for the prior examples. |
| JimmyDee | quote: Tie X1 and X3 together for one lead and X2 and X4 together for the other. It will produce 120 volts with double the ampacity of the 240 hookup. Jim |