how should load be connected directly with 3-Phase AC high voltage generator that has no neutral wire? Should three loads be connected; something like L1 - L2 , L2 -L3 , L3 -L1 !
Moreover , there is voltage between all three phases. if the voltage is measured only between L1-L2 i.e 100v , is that the actual voltage output of the generator? . Should the voltage is to be read as L1-L2 +L2-L3 =>100V+100V= 200V?
I am not an expert.But with the description of the problem it seems to me,if some delta type of connections of windings is arranged at output terminals from where load is to be connected,this problem may be eliminated.
Theoritically it seems to be o.k,but practical implementation results will tell about actual success.
If your load is balanced, you can connect isnstar. Balanced load has zero neutral current and hence needs no neutral.
If your load is unbalanced, you have to connect it in delta only. However the loads should have sufficient voltage rating.
What you measure between the lines,( L1-L2 or L2-L3 or L3-L1) are all line voltages.
Supply via delta/star or star/star transformer, will get neutral, normal(balanced), H.V transmission is done with 3 phase 3 wire line,without neutral......
If you have single phase loads usually, because of their nominal voltage, you cannot supply them through a system which has no neutral wire. However, if their nominal voltage allows, you can supply them using two phases of the three phase system. Another method is to use a delta/star transformer and connect its neutral point to the ground. Then, using the wire connected to the neutral of the transformer and the one of the phases at the star side, you can supply the single phase loads .
Regarding your question about the terminal voltage of generators, you should consider that when it is said that VL1-L2 is 100v, it is the RMS value of the voltage. As the power system voltage is sinusoidal, the waveform of voltage can be represented by sqrt(2)*Vrms*sin(wt+phi1). While the waveform of VL2-L3 is sqrt(2)*Vrms*sin(wt+phi2). Hence, when you add VL1-L2 to VL2-L3, you cannot add their amplitude or RMS value and say the sum is 200v! A simple method to calculate VL1-L2+VL2-L3 is to use the phasor concept.
The voltages between L1-L2 = L2-L3 = L3-L1 are all measurements of line-to-line (or simply 'line') voltages, which may be 100V as you have stated. That is the voltage rating of the generator (alternator to be precise). Since there is no neutral, your supply system is a 3-phase 3-wire system. If your load is also rated at 100V each, then 3 such loads can be connected in "delta", ie one load from each set of one line to another.
However, if your load voltage is 100/(sqrt3) = 57.7V, then only you can connect three identical loads in "star" connection ie one end of each load to one line and the other end of all loads connected together (to form the 'star' point) without connection to any other terminal. In this mode, while the line-to-line voltage is still at 100V, the line-to-neutral (or 'phase') voltage of the load side is 57.7V. Note that there is no corresponding neutral on the alternator (supply) side and hence your supply system is still 3-phase 3-wire type.
Indeed, no need for a neutral in that case, just conneting single phase loads between lines. For small grids or generators it is better without a neutral (better use of copper).
But you should get 110-120V, rather than 100V? Generators without electronic control do give a voltage proportional to speed. If you adjust the speed governor of the engine a bit higher, it could give the right voltage. Sometimes it is a compromise between the right voltage and the right frequency. I normally prefer the correct voltage. Good luck.
In Europe we have 230V and 400V grids. The 230V grids (15% of grids) have no neutral connection. If necessary and not done yet by the system one can earth one of the lines in 230V grids..
Dear Shah,
There is a high-tech solution to your problem as well:
Add a converter that has a DC-link inside and generates a proper AC-Supply from this DC-link.
You would just need a device that has a compatible input voltage range.
This will also get you rid of fluctuation in the generator's output voltage.
Dear Pankaj,
This is completely wrong: if no neutal connection is given a star load has a completely wong balance and also the voltage is too low to use, some 60-70V, even if it is balanced.
However one can connet a delta star transformer if people want really a neutral, but it is not needed and in increases cost and losses.
In response to Mr. Pankaj Anand's answer, I think the neutral of the machine also should be grounded. In case of unbalanced circuit, the neutral current requires a return path to the source, that is the generator in this case. Even if the load is balanced, I go with Mr. Alex Van's reply. The voltage will be too low.
If the output of 3-phase AC generator is 380 V between L1-L2, L1-L3 and L2-L3, we use an AC/AC transformer 380 V / 220 V for a load voltage 220V. So an output of 220V biphase.
Dear Smail Semaoui,
Since about 2003 the standard in Europe ichnaged to 230V and 400V and not 220 and 380 any more. http://en.wikipedia.org/wiki/IEC_60038
Still about 15% uses 230V line to line three-phase. It may have a star point of and 133V but the star pont is not given to the customer. However, it is also permitted to earth one of the phases in that case. So one can have two voltages of 230V on each point of the wall plug. Strange but permitted.In some countries (Suriname) they use the 127V.
The USA is used to single phase supply of 115V and an 180° opposite phase, so they get also 230V if they want for cooking and airco, but 60Hz. Nothing is the same across the world...
The type of connection which may be suitable for such kind is 3Q, THREE WIRE, STAR connected. In this way one of the three wire will act as neutral wire In Order to balance the load.
Caution: there are inaccurate answers above. If the generator has a DELTA output and the LOAD has a WYE input, the Line-to-Line voltage is still the same. In other words, you can connect L1(Gen) to L1(Load), L2(Gen) to L2(Load) and L3(Gen) to L3(Load). Of course, it doesn't matter which line of the generator connects to which line on the load, it's all arbitrary so long as each and every Generator line connects to a separate Load line. Keep the Load Neutral open unless you have a 3-phase delta to wye transformer. But this really shouldn't be needed. Make sure you understand your load and whether or not there might be some reason you do need the load neutral connected. If it's absolutely required by the load and the GEN has no Neutral, then the DELTA to WYE AC Transformer is needed. SImply connecting the Load Neutral to Generator chassis ground will not help as the GEN chassis will almost certainly be isolated from the GEN 3-phase line outputs. ALso, one cannot simply add the Line output voltages without the COS(theta) of the angle between them, which is 120 degrees in all cases herein. Finally, make sure that your GENERATOR output voltage matches (within reason-+/- 10 to 15%) your LOAD voltage requirement. If it does not, again, you need a 3-phase transformer to match them, which can also have the DELTA primary to WYE secondary built-in, allowing a neutral connection.
Djamila, that all depends on the exact application the system is used on. It's not good to assume things like this. THe original post never mentioned either the voltage or the frequency and only gave an example for calculation purposes of 100v & 200V.
Shahzad, a single phase load doesn't require a neutral. I don't know why you would say that as it is untrue.
Here's context from your post above: "noting here of course that a single phase load obviously requires the neutral line, but which is absent as pointed out." OK, I'll make it simple. You take a resistor (resistive load) and connect it across L1 to L2 and it will be powered with no neutral needed. Granted, there are reasons one may or may not want to connect a single-phase load this way, but it doesn't hurt the generator or the load.
Well, your statement is your opinion, and I disagree with it in that you're trying to generalize all generator hookups (3-ph and 1-ph), delta and wye. And I never heard of a recommendation either way ("this practice is NOT ENCOURAGED OR FOLLOWED"). I guess that's you that's encouraging or recommending. It all depends on the application. It is not discouraged to connect a single phase load to a 3-phase generator. That's an engineering decision and depends on a lot of factors, including cost. Who are you (where do you derive your authority) to throw out blanket statements in capital letters about practices being NOT RECOMMENDED in connecting electrical generators?
Gentle men,
The discussion is getting more and more personal. Let the spirit of exchange of knowledge be upraised.
Regarding the question, there should be no hard and fast rule that a three phase generator should not be used for a single phase load.
Regarding the uneven heating of the stator, as mentioned by Dr. Shazad Abraham, the insulation and other things provided in the machine, are meant for continuous usage within the rated power range. Even if the generator is run on a single phase load continuously, the heat produced should not damage the machine, as it is designed for it.
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