MEP-803s and UPS loads

[Demoh]

Well all things aside I am used to splitting a hair then splitting it again. I build a trailer to a 1/32 tolerance and I pulled the running gear off a M105 to find the true center of gravity and built massive spreadsheets for all my different options to load up with tongue wait and fuel calculations for a rental generator I use as a spare. Ive never had to pay for a wheel alignment because the machine always comes back 0.0 for toe in/out.

So if the waveform is slightly deformed and has a couple % for those types of loads, what would be the derate be on the double delta? I asked the wrong question because I know these things have no problem above 10kw. What I meant was would there be a measurable efficiency difference between zig zag and double delta in regards to this particular head?

 

[DieselAddict]

These generators in reality have several different ratings. The ultimate short term power rating is all about engine power. That is why they will go above their rating. Because the engine has enough power output to keep swinging the load. The long term power limit is more about thermal saturation of the alternator.

If you go and look up the power output for the LPW4 engine you'll see its rated at around 18kw at 1800 rpm. That is a generous amount over the generators 10kw rating. Sure you have to subtract all the loads such as fan, alternator, friction , bla bla bla. Lets say that leaves you a net of around 15kw. That is how much (in a perfect world) rotational power you can translate into electrical power. I'm keeping this simple so don't jump in here and tell me all the other places where power goes with kvar versus kw. Yes, I'm fully aware of that but just humor me a bit. This is more about concepts than absolutes.

Circulating current is still current. It still consumes energy from the engine to make it flow even if it does zero work to move your loads outside the generator. The impedance of the generator coils is quite low (Impedance is a reactive form of resistance and in this case I would like to use the correct term since we are dealing with inductors - aka generator windings). This means any slight difference in voltage potential between where the two sets of coils join in parallel will translate to quite a lot of current. Current is heat. Current is what burns up coils (excessive voltage in comparison blows out insulation). You may have many amps of current circulating through these parallel coils (we could do the math but I don't want to send us down the rabbit hole). This could easily equal a couple thousand watts of power doing nothing but generating heat. Add to that the current of your external load and you can see if you tried to draw your normal safe level 12kw or so out of the generator the windings themselves are producing more like 14-15kw internally. You are at the limit of the engine output AND may be at the long term thermal limit of the alternator. Obviously in Alaska the thermal limit of the alternator is reached at a different total load than it is in the desert. Just concepts here. Remember, this circulating current is not measured by your power meter or your overload system since it never leaves the alternator except as heat.

With the gross assumptions above you would need to derate 15-20% to keep a decent safety margin on the thermal envelope of the alternator. Many people say 30% but I think that is more than is generally necessary.

Whew. I need a beer.

 

[Demoh]

I'm keeping this simple so don't jump in here and tell me all the other places where power goes with kvar versus kw. Yes, I'm fully aware of that but just humor me a bit. This is more about concepts than absolutes.

Yea, we dont need to get into all that and muddy the waters even further. Concepts come before real world when learning anyways. Full humor coming your way (thats what you asked for right?... oh, that wasnt literal.)

This means any slight difference in voltage potential between where the two sets of coils join in parallel will translate to quite a lot of current.

So basically to make sure that I am following, the potential of coil 1 (120deg) plus coil 2 (240deg) is not equal to coil 3 which it is paralleled to (0deg) correct? So when connected either the pair of coils in series or the single coil is going to be "producing" more power which is flowing to the other set that is "drawing" the power, therefore that is the definition of circulating current?

Theoretically zig zag could have circulating current due to manufacturing differences in the 2 same angle coils that are paralleled correct?

If that being the case, I knew it. (without formal training) I always knew there was bound to be something like this because of how my mind works however I always leaned towards double delta due to symmetry. I could never explain it because I dont have any formal training but growing up I have always been "in" this field (my dad is an electronic tech and mom electrician, I was installing outlets by 7 or 8 ). Ive actually spent many hours on different forums and google researching alternators, double delta vs zig zag, etc but not a single thread touched on this. I left after my days of research finding that the field was split on which method and it was all based on preference, aka I wasnt able to gather any concrete info.

Like thats why I HATE connecting batteries in parallel, there are differences, one is going to work harder. If one gets a bad cell it sucks the life out of the other battery to keep dumping current into the bad cell. (This is why I have an 8D in my truck instead of a pair of 65s)

Really 20%? I was thinking it was going to be less than that, like a few %.
Now my turn. Go enjoy your beer.

Theoretically still, so this circulating current that is current that never leaves the alternator. but really it does because the 12wire connection is outside the alternator right? So a current sensor/tap measures the sum of the currents going through the coil. Assuming L1 and N and 10A on L1-N. if you run L1 through the CT it will read 10A. If you run L1 and N through the CT it would read 0A. Could you run different wires through 1 or multiple CTs to see what either the total (circulating + to the load) or just the circulating current would be? I dont know how CTs react when you throw phase angle into the mix but its worth pondering. Im the type of person who will (if possible) install a gauge that will allow me to see circulating current or total current, or find some spots for thermal sensors in/on the alternator but are these alternators already rated taking into account 10kw production to the load plus circulating current when in single phase mode?

I sure am tiptoeing around the rabbit hole. Im going to start doing some outside research now that I may be able to know what I am looking for where I wasnt previously.

Edit, sidenote, does anybody know the insulation class on these alternators?

 

[DieselAddict]

Either method is a compromise that reduces the total energy the alternator can deliver to the load. In my opinion I prefer the zig-zag due to all the paralleled windings being in phase with each other. With the double delta you are summing two windings and paralleling it with another. If all the windings are perfect the circulating current is minimal. The reason we can still get rated output of these generators is due to the alternators being well over-engineered. Commercial grade gear tends to be that way where home store gear is not. If you were to take a light duty 3-phase generator and wire it for single phase you'd need to be a lot more conservative with load. More like 25-30% derating to stay within the thermal envelope.

You are correct that the circulating current will be present on the alternator leads and if the CTs are on those leads then circulating current would be accounted for.

If I were to guess I would expect the wiring to be class F.