Why the 120/240 split issue?

[Metalflames]

Hello all, glad to be a member! I just purchased a MEP-016D From GL the other day and in my research stumbled amongst the site. I keep reading that there needs to be modifications made to run 120/240v. Most of the post refer to well pumps. Couldn't you just hook the 240v right to the panel via a transfer switch or interlock and be done. Why all the jumping and load balancing? Is it because of the well pump, I have city water so I may be missing something. If I recall doesn't the panel get feed with straight 240v.

 

[storeman]

I agree. Too much theory and little practical documentation. My feed is 220 to both panels of a 400 amp panel setup. have had no problems. I'm only sending in my 002a';s meager 26 amps but that jewel runs all my 110, including well pump. 4,000 sf house which is managed conservatively in power outages but maintains gas heat and gas hot water.

Jerry

 

[Isaac-1]

I think you are not understanding some things:

Common household power is 120/240 split single phase, this means you get 240V Line to Line and 120V line to neutral with a center tapped neutral, so in a diagram if would look something like L--120V--N--120V--L where the 120+120 L to L =240V

As originally wired with MEP-016 family (at least the a,c,d versions) allowed you to select 120V single phase only mode which is 120V line to line , or 240V line to line, with no middle neutral like 120/240V split phase has. The modification (if needed on your specific unit) effective adds that center tap to turn 240V line to line mode into 120/240 split single phase just like household power is.


Connecting a stock unmodified MEP-016 family generator in 240V only mode to L1 and L2 of a conventional breaker panel would only provide power to the 240V L-L connections and would provide no 120V power since nothing would be connected to the neutral.

Ike

 

[Metalflames]

Ok but if back feeding 240v into the panel via interlock wouldn't that in turn run through the center tapped neutral and do the same thing without having to watch load balancing?

 

[NDT]

Ok but if back feeding 240v into the panel via interlock wouldn't that in turn run through the center tapped neutral and do the same thing without having to watch load balancing?

Why don't you try it and let us know how it turns out. I have personally never seen what happens when you put a TV set in series with a refrigerator.

 

[simpleserf]

There is no neutral in stock form... therefore, you can't get 120 from the neutral. You have two wires from the genny, when you need three. The Mods that you are reading about allow you to get the third connection that you need to get the neutral. The neutral in your box is the neutral for the incoming grid feed.

Without the mod, the smoke comes out of your appliances. It's hard (and expensive) to put it back in.

If you don't fully understand what is going on in this circuit, have an electrician set the unit up. These generators put out more than enough power to kill you.

 

[Keith_J]

Why don't you try it and let us know how it turns out. I have personally never seen what happens when you put a TV set in series with a refrigerator.

Funny but please don't DO IT!!! EVER!!! Something will get damaged because it will set up a voltage divider circuit, the weaker one will get more voltage, possibly 240.

 

[rosco]

That might provide a cool picture!

 

[Metalflames]

Why don't you try it and let us know how it turns out. I have personally never seen what happens when you put a TV set in series with a refrigerator.

I don't try things. I spend the time to learn and understand what I am doing before I do it. I was hoping some of the more knowledgeable people here would be willing to share some of that knowledge. But thank you for your in put anyway.

 

[Metalflames]

I am more than comfortable wiring the generator. I am just not familiar with some of the information I have read. Let me ask this a different way. Why is this different than a commercial genset? When that is back feed it provides 120/240v without having to load balance. What component allows that?

 

[Keith_J]

Load balance is something else, don't get corn fused here, OK?

Commercial generators provide 240 through at least 3 wires, two legs and a neutral. Most have 4 wires, two legs, a neutral and a ground.

The MEP 016 series is wired to only have two legs in a series delta connection. There is no neutral line in stock configuration.

What happens if you hook it to your house, using 2 legs is all the 240 stuff runs fine. If you plug in a 120 volt light to an outlet, the current flows (for lack of better term as this is AC) from the leg to neutral. But neutral isn't connected to the generator so there is no flow so no light. Now if an identical light is plugged into another outlet in the house fed from the SECOND leg, current WILL flow from the second leg, to the light and to the neutral. It then flows from the neutral through the first light bulb through the first leg and back to the generator. Both bulbs will turn on as long as they are EXACTLY the SAME TYPE and POWER.

Now, if one bulb is a 25 watt and the other a 100, the 240 volts will be divided unequally with the 25 watt bulb getting far more voltage, it will burn out because light bulb's resistance is proportional to temperature, the 100 watt bulb will never get hot enough to shine. It would be over 200 volts to the 25 watt bulb.

Now THAT is load balance. If both legs are perfectly balanced, no current flows through neutral. It is perfectly normal for current to flow through the neutral on utility power. But in generators, it is best to keep legs balanced within 25% of total current.

 

[Metalflames]

Keith thank you that clears up a bit of the confusion. Now I am not familiar with the delta connection which I will be learning about next. I am assuming the added neutral will allow the lights to run as they should.

 

[Isaac-1]

Ok lets try this another way:


Commercial generators may have a switch on to select 120V only mode or 240V mode, but in reality what they mean is 120V only mode or 120/240V split phase mode (smaller commercial generators usually those under about 5KW are often 120V only) When in 120V only mode they deliver 120 volt ac over 2 wires, when in "240V" (120/240V split phase) mode they deliver 240V line to line between 2 wires with a 3rd neutral wire that is at 120V relative to either line.

The stock MEP-016 delivers 120V only mode over 2 lines on 2 wires, or 240V only over 2 lines on 2 wires with no neutral.

If you simply connect the 2 wires with 240V from the stock MEP-016 to the L1 and L2 on your panel you will have no center tapped neutral that evenly splits the 240V into equal 120V halves, instead the laws of electrical resistance will split it, and if resistance is not balanced across the circuit one side or the other will have higher voltages, this is the flip side of needing to balance the loads on each half of the generator. As to your comment about not needing to balance the loads on commercial generators this is NOT True, in fact one of the leading causes of failures on contractor generators is running too many 120V tools off one of the 120V legs when in 120/240V split phase mode. (note running the generator in 120V only mode would eliminate this issue of load balancing)


Here is another way to think about it:

Your MEP-016 will produce a total of 3KW of electricity, this can be all as one 3KW lump sum in pure 120V only mode or pure 240V only mode (like when needing to run a 240V only motor) with the wiring modification though, you can produce 120/240 split phase (household power) mode where each of the two 120V lines has a maximum output of 1.5KW (1.5+1.5 = 3), or this could let you use 1KW worth at 240V across both halves for something like a well pump (.5 +.5 =1), and at the same time leave 1 KW of 120V available on each half for other loads for a total of 3KW.

Ike

 

[Keith_J]

Yes, the neutral is ESSENTIAL for split phase twin leg 120/240 to provide 120 power.

Delta and wye are the two connection schemes common to military gen sets. Most all 3 phase sets are termed 12 lead because there are 6 independent windings in the stator, the part where the electricity comes from.

Each individual winding produces 120 volts in these generators. Connecting all 6 windings (12 leads) can be done in three fashions. The output voltage varies with connection.

Delta is a triangle, wye is a star or Y. Since each line segment represents a winding, either a Y or a triangle has three line segments so there are TWO Ys or TWO triangles. They can be stacked which in Y gives 208 from one end point the other or 120 from one end point to the center. Stacked in triangle gives 120 from corner to corner.

So, on single phase, most are connected in triangle or delta. For 120, they are stacked. For 240, the triangles are side by side with one corner joining the other. This connection between the triangles is neutral. So what about the high points? Not used, the sum of the currents through these legs contributes to the bottom leg.

Now, other connection schemes for 12 lead 3 phase gensets include dogleg and some other freaky stuff. You can also get double voltage by series connection of the windings. So 408 volt 3-phase wye, 240 volt delta .

I'm a Mechanical Engineer, power fascinates me. It bores the EEs to death.

 

[quickfarms]

If you do not understand electricity you should hire an electrician. Electricity can kill you or burn down your house.

 

[Keith_J]

But you don't LEARN. If you surrender at any task fully documented and proven, you cannot reach for the stars.

Safety should never be ignored. Fortunately, correct connection of a 12 lead genset is easily verified by regular 120 volt light bulbs. If they illuminate correctly, then you have the right connections. Plus they can be used to synchronize two generators .

Light bulbs are cheap enough if you burn a few out in testing.

 

[Metalflames]

If you do not understand electricity you should hire an electrician. Electricity can kill you or burn down your house.

I would like to say I have homeowners so I guess I'll die trying. But seriously I am not going to put myself in a situation for which I am not comfortable.

 

[Isaac-1]

I think the thing here that worries some of us, is that this may be true, but that a blind man can be comfortable standing in the middle of a freeway at rush hour.

Ike

p.s. there was just a long message thread over on the smokstak board regarding a professional generator technician that was killed because he assumed a circuit that logically should be off was indeed off without testing it (a site electrician had done some very creative wiring that allowed power to be backfed to the load side terminals of an open breaker), so as we say electricity can kill you

I would like to say I have homeowners so I guess I'll die trying. But seriously I am not going to put myself in a situation for which I am not comfortable.

 

[s205designs]

Not sure if it helps you out but it helped me to see the standard NEMA plugs when comparing a MEP to a commercial generator. Wikipedia has a good diagram here:

http://upload.wikimedia.org/wikipedia/commons/0/0d/NEMA_simplified_pins.svg

You will notice there are three "categories" 120, 120/240, and 240. From my understanding many of the smaller MEP generators will only put out 120, or 240. I have a 016D so I'm in this boat.
As you can see all of the 120/240V plugs contain a neutral wire, which is not brought out from the generator head in many MEP generators.

Not sure if this helps but it helped me to see the difference that the board members here are discussing. If it doesn't help, than just ignore me!

J

 

[LuckyDog]

Here is a post with pictures of different ways to wire up generator heads.

http://www.steelsoldiers.com/auxili...ewiring-mep004-single-phase-5.html#post673191

I was looking at the manual (TM 5-6115-271-14). The schematic for the the MEP-016 A/C/D on page 136 shows the generator head on them is a 6-lead. SO, that would be the same as the "parallel" 12 lead connections.