Center Tap Neutral for 016B??

[ETN550]

Hope Someone can help here.

I have decided to attach a 4 wire cord to my output lugs and take it to a custom made box with a variety of outlets on it which I can use for different devices.

The manuals have a large number of errors in them so I am wary of trusting the manuals as there are contradictions. I'll post up pictures of the manuals as the discussion unfolds if needed.

I will be only using 120 and 240 single phase. No three phase.

For the 120 single phase function I will be powering my RV and the usual tool and misc. light that plugs into the standard household 120 outlet. The manual says to ground the L2 and I am fine with that and the RV does not bond the neutral and ground as is proper. The RV uses the special slanted 120 VAC pronged plug with the round ground prong. It is the 30AMP 120VAC plug not the 50AMP 240VAC.

So no questions about the 120 1ph switch setting, wiring, and grounding.

Now the 240 setting.

I read that the L2 lug is to be grounded when in 240 mode.
I also read that the courtesy outlet on the front is directly taken from the T1 / T4 leads on the generator. (bypassing any reconnection switch settings)
I also read that all the generator stator output leads are floating and none are referenced to ground. Stator lead grounding only occurs when a selected output load terminal is externally jumped to ground.
When running without any jumper to ground in the 240 mode both L1 and L2 indicate 120VAC when referenced to LO with a meter.

So my questions: When in the 240 mode can I draw 120 from both L1 and L2 if the neutral leg is LO? (Regardless of which output is grounded)

Can I Ground LO in the 240 mode when using L1 or L2 for 120? If yes, then LO and Ground would be considered bonded, correct?

If not acceptable to ground LO in the 240 mode then is there any issue grounding L2 as stated in the manual in the 240 mode and still tapping L1 and L2 against LO for 120VAC as long as LO is never bonded to ground anywhere in the system?

If I ground LO then I can see my way towards backfeeding my house at the main. If I ground L2 then I can only see applications in the field for 240 / 120 and not the house. (which is okay because I can backfeed the house 120 on the 120 single phase setting and skip the 240 for the house) BTW this generator runs my MIG welder with no issues so a typical field application would be 240 to the welder and 120 to the hand grinding tools, etc.

I realize the 120 take off in the 240 mode is probably limited to 10 AMPS or less. I'm guessing each winding is good for about 10AMPS. (The F1 F2 fuses are 7.5AMP thus protecting the T1 / T4 winding they are directly attached to. )When in the 240 mode my main use will be 240 and the 120 is just for small tools, etc. In fact for my output box design I am planning on just one 120VAC duplex outlet with one plug on L1 and the other on L2. I might entertain a 7.5 AMP fuse on each half of the duplex outlet so as not to overload the generator windings.

One reason I bought this generator is to have full output in 120VAC single phase mode for the RV, 30 amps 120VAC input.

Thanks in advance for any advice on grounding and 120 outputs in the 240 single phase mode. I'll post up some of the diagrams if this gets involved.

Doug

 

[ETN550]

Help, Anyone??

After looking further into things I believe I can safely get 120 from L1 and L0 in the 240 single phase mode although with limited amp draw. (7-1/2 - 10?)

What puzzles me is the wire diagrams for 240 with the zig zag layout would seem to indicate that LO is at the T4 connection which means none of the other load terminals could be paired with it for 120VAC.

Yet the hand meter registers 120VAC between L2 and L0 when in the single phase 240 mode. I do not understand how this can be unless the diagrams are all wrong or the L2 L0 is backfeeding from something and really cannot support a load.

Can someone comment on getting 120 out of the machine when in the 240 single phase mode?

Thanks,
Doug

 

[Isaac-1]

There are lots of mistakes in the MEP-016b manuals, the connection diagrams are just one of them. As to grounding I will not say too much, other than the L2 grounding in 240V mode is not to be confused with 120/240V split phase with a grounded L0 neutral, and for most residential applications is probably not the best way to do things.

Ike

 

[ETN550]

There are lots of mistakes in the MEP-016b manuals, the connection diagrams are just one of them. As to grounding I will not say too much, other than the L2 grounding in 240V mode is not to be confused with 120/240V split phase with a grounded L0 neutral, and for most residential applications is probably not the best way to do things.

Ike

Ike,

thanks for reply. I was hoping you were out there reading this.

Yes it is the errors in the literature that make things more confusing for sure.

I'm trying to confirm if the 240 grounding to L2 is required or can I ground L0? I suspect because L2 is grounded in 120 that it is a matter of convenience to keep the ground at L2 for 240 but I'm not sure. Further, I think it is far more risky to ground one leg of 240 leaving the other leg at 240VAC to ground. Whereas grounding L0 might only give 120VAC to ground for L1 and L2, much safer IMO. They do state that the generator stator outputs are fully floating, which I would like to believe, but I now question about everything in the manual.

My big question is when in 240 single phase mode how can I be seeing 120VAC from both L1 and L2 to L0 with the reconnection wired per the manual? Can I actually get about 7-10 AMPs per leg out of L1 and L2 both referenced to L0? Can L0 be a common neutral? Can L0 be grounded?

I guess I can figure it out by metering all of the generator leads and outputs while it is running. It appears that the front recepticle is directly connected to T1 and T4 when in 240 and T4 has no corresponding load terminal in the diagram but sure seems like it is L0 when metered out.

As a side bar it is interesting that the main breaker has a lower amp trip for T3 than T1 and T2. Is the T3 winding different or do they just let more through T1 and T2 for keeping rated load with single phase? Maybe the only way they limit 3 phase current is by having T3 be the trip point. Odd.

Let's face it to run a small house on 3Kw is a challenge. I need the 240 to run the blower on the furnace and a hot water heater. I'm considering disabling one of the heater elements and switching the other from 5000 to 3000 watts. Then running the 3000 watt element only when hot water is needed. And maybe 1 or 2 light bulbs if it is dark outside. When the water is hot I will remove the hot water heater, reconnect, and run the gen in 120 single phase and take as much as I can up to 3+ Kw for the house. based on my electric bill for the house I draw only 650watts overall average. I should be able to manage with 3+Kw.

 

[Isaac-1]

Let me start by saying I AM NOT a professional electrician so everything to follow should be taken with a BIG grain of salt.

Ok, here goes a couple of thoughts, as you say running a house on 3KW is a challenge even with gas heat. I have my MEP-701a/016b configured for 120V only single phase mode so I get all 3KW or so on a single "leg" this has one side of the leg grounded (neutral) and one hot, the military 240V only connection uses the same theory, with one side of the 240V grounded and the other hot, as to if this is safer than grounding a center L0 leg as seen in common household 120/240 split phase is a matter of debate, since split phase leaves either leg still hot with 120V with a break, and 240V with corner ground gives a 50/50 chance of an open circuit.

Next point, the reason I choose 120V only single phase mode is to get the full 3KW on one leg, if wired for 120/240 split phase (which is not officially supported) then you would get 2 legs of 1.5KW at 120V making load balancing that much more difficult. Time for an example:

Assume the output rating per the data plate as 31.2 amps at 120V, for ease of math lets round that up to 32 amps, hey they always under rate anyway

In 120/240V split phase you get 2 legs of 16 amps at 120V

Now lets assume you have 3 10 amp loads: (can't be balanced in split phase without overloading one side)

How about 2 10 amps loads and an 8 amp, again can't be balanced, 1.5KW per leg is just too limiting in the real world without loosing a large fraction of the potential power output, loads are simply too course most of the time, or too trivial with CF light bulbs, cell phone chargers, etc drawing a fraction of an amp.

Now your problem is you need to make hot water and power your 240V fan motor (I am lucky my furnace fans are 120V),

lets tackle each one separately:

Assuming you have a powerful water heater with a true 5000 watt element some basic electrical math (ohms law) tells us at 240V a 5000 watt resistive element has 11.5 ohms of resistance, more math tells us if we feed 120V into an 11.5 ohm resistor (wire up water heater for 120V) it will dissipate 1252 watts which should still be plenty to heat water slowly and leave some power to run other important household items. (you could also wire up both heating elements to run in parallel on 120V to get 2504 watts, I am not sure how the water heater thermostat would work, but suspect it would work ok at the lower voltage, again consider safety of such action as this is just theory)

For the 240V heater fan the simple solution here may simple be use a 120/240V transformer (I don't know the amp rating of your motor but assume it is less (probably a lot less) than the 15.6 amps that this generator is rated at in 240V only mode), one thing to remember about transformers is they are bi-directional, meaning a 240V to 120V step down transformer can be ran backward to get 240V from 120V, any number of transformers may be found on the used/surplus market that would be suitable for this task at prices of well under $100 (search ebay for 120/240 KVA transformer, and fill in the KVA demand you need for your blower with a safety margin, it seems 1KVA start around $50 used). You may even be able to get one for free if you know of an industrial site with 3 phase power upgrading their electrical systems.

Ike

p.s. on the 3 pole breaker question with the lower trip rating on the 3rd leg, I assume this is a trick they use to allow the same breaker to be used for both single phase and 3 phase operation, this would of course assume that when used in 3 phase that there would be no significant single phase load connected at the same time and therefore the 3 phase draw would be equal across all 3 legs which is a somewhat reasonable guess for such a small generator.

 

[ETN550]

Yes, I understand what you are saying. The hot water heater has an electronic control system so I don't know about putting 120VAC across it. I could bring a pigtail off of one of the elements and just plug into it and monitor the temp. It would take a while to build if I put 120VAC across it, 2,400 watts / 50 gal.

Another thought I had is to install a dummy breaker for 220, turn off the main, short the two outputs of the dummy breaker thus connection both L1 and L2 in the breaker box. I could then drive all 120VAC loads in the house with the Gen set for 120VAC L1 to the panel, L2 to the house neutral ground. All 240VAC breakers would be opened.

In your reply you say the gen can produce 16A X 2 , 120VAC in the 240 mode. Is this with grounding the L0?

Doug

 

[Isaac-1]

In theory, yes this would be grounding L0, however before doing so in practice it would be best to check all wiring diagrams as well as checking for conductivity/resistance on the set as wired.

Ike

 

[ETN550]

Yes I will be metering all the leads to figure out the wiring. Since it does meter 240 VAC , L1 to L2 in the 240 mode and 120VAC L1 to L0 and L2 to L0 at 120VAC I remail hopeful. Can't wait to get home to begin working on it again!

 

[ETN550]

Well, the diagrams are incorrect enough to make it virtually impossible to figure out how the output reconnection is actually wired. On the diagram it looked like if I jumped #5 and #6 on the output reconnect switch I would have 120 VAC between either L1 or L2 and L0. Well, as it turns out My reconnnect switch for both units has no leads attached to #5 or #6 on the reconnect switch!

Further, when running in the 240 single phase mode the unit produces:

L1 - L2, 240 VAC as designed.
L1 - L0, 120VAC, which is a direct take off of the T1 - T4 winding, (same as used b the convenience outlet on the front)
L2 - L0, 120VAC
L1 - L3, 120VAC
L2 - L3, 120VAC

The diagram clearly shows L3 as a dead end when in the 240 single phase mode. So how I'm getting 120VAC using it I have no idea.

I made an outlet box and will start another thread but I have load tested using the L1 - L0 and L2 - L0 leads to get 120VAC in the 240 single phase mode with no issues. Each handles around 1250 watts at about 90% on the load meter for it's respective lead.

I'll start a new thread showing the outlet box.

 

[Buzz254]

240 vac explained...

OK. Here is the scoop.
Look at page 4-3 in your tech manuals (Army TM 5-6115-615-12)
1. 120 volt 3 phase (upper left): T-6 SHOULD be T5
2. 240 volt 1 phase (lower right): T2 SHOULD be T6

3. I am going to refer to windings in the stator of the generator end as (in order of phasing, T1/T4=W1, T2/T5=W2, and T3/T6=W3)
4. The 240 Volt single phase connection is a BUCK-BOOST, in which when the rotor spins in the stator, and excites the widings, NOTICE that W1 is FORWARD +, then connected in SERIES with W3 IN REVERSE (AND 120 DEGREES LATE), and then W2 is connected in series (again, 120 degrees later).

This is a Buck-Boost type of wiring arrangement. We get 60 volts NET from EACH of the windings that are on the OPPOSITE phase at the instant in time that the single full phase is giving us 120 volts in the other direction...thus 120 + 60 + 60 = 240 Volts SINGLE Phase.

The only limitation is that you can only pull a current draw that is the equivalent of any single phase by itself.

So, there you have it.

Please mark your manuals accordingly.
NEXT...I am going to test and see if I can close a contact (which will give us a NEUTRAL between these phases...HA HA! If it works, the neutral will be L3!

Stand By!

 

[Buzz254]

Questions...??

Look at this url. I have posted it in a location that you all can see it. It is a .pdf of what I am talkiing about.

https://dl.dropbox.com/u/78370251/THREE%20PHASE%20UNDERSTANDING.pdf

 

[Buzz254]

Why L3 shows 120 VAC in 240 VAC single phase mode

L3 shows 120 VAC due to the A2M1 voltmeter leakage path that backfeeds L3. There should be no current to it. I am going to jumper contact 1/2 and run in 240 VAC mode, and see what L3 can handle with a load.

Stand by.

 

[Buzz254]

Ground Problem Solved

Here is what I have found.

Refer to your TM. Figure 4-1, on page 4-3, (and taking into account the corrected terminal numbers on this page stated prior in this thread).

240 VAC Single Phase:
1. T-4 & T-6 are "grounded" to L0 by default (T-4 is ALWAYS connected to L0)
2. T-1 to T-4...120 VAC (you could also state L1-L0...120 VAC)
3. T-2 to T-6...120 VAC (you could also state L2-L0...120 VAC)

Here is gets tricky, but the truth comes out...

4. T-2 to T-5...120 VAC (expected, as this is a single isolated winding)
5. T-3 to T-6...120 VAC (expected, as this is a single isolated winding)

SURPRISE...look at the TOTAL of T-2 to T-6 in item 3 above...ALSO 120 VAC.
So we can deduce that W2 and W3 are both contributing 60 VAC EACH...within the same phase relationship as when W1 (T1-T4) is pushing its OWN 120 VAC.

So the push-pull is with W2 and W3.

So...YES....L0 SHOULD be a good NEUTRAL...and just happens to be bonded to the frame as well (just like a good ground and neutral are bonded in your home (sub-panels are the exception, as they need to be locally bonded to ground per the NEC).

So there you have it...and all of that WITHOUT an oscilloscope!!

Cheers!

 

[Isaac-1]

thanks for taking the time to look into this.

Ike

 

[Buzz254]

One last comment concerning load factors

This Gen-Set is conservatively rated at 8 HP...
Which translates to a total 5968 total watts can be deliverd by the engine at the flywheel.
Taking into account how conservative the engine rating is (add), and then subtracting the in-efficiencies (subtract), this thing should EASILY deliver 25 amps/leg at 120 VAC..OR a hard 25 amps over a 240 volt load...REGARDLESS of what the TM says...

So, we REALLY have a 6 KW (RAW) gen-set...