Using a generator with step-up transformer

[WA1LGQ]

Gentlemen, I have a 120 volt Champion generator, 3500W, 4k peak supposedly. Also have a hefty 3kva or so 120/240 transformer. Would this setup be ok to run into the power panel? Any potential problems? Leg balance issue or what? I am not sure if the transformer has center taps yet.
Thanks....Larry

[W7TFO]

I've done that several times over the years, to no ill effects.

I even used a beefy filament transformer to raise the 28-Volts from a little military generator to 120/240 without any hassle.

A neutral, or 'ground' is necessary for any loads other than pure 240-V.

73DG

[KA2DZT]

Better to just jump both legs together and run everything off the 120 volt generator.  Of course trip off the main while doing this.  It will run everything except 240 volt loads, and you don't have to worry about balancing the loads.

Fred

[steve_qix]

I have done both -  used an auto-transformer (could also be a standard 1:2 step up), and also have jumpered both sides of 240 together and have run everything that way on 120V.

The obvious advantage of stepping up the voltage is that 240 volt loads will function when the generator is active.

The auto-transformer allows you to create a "real" neutral / ground which is carried through from the generator to the main service entrance panel.  Neutral and ground may come together only at the service entrance.  All other panels (sub-panels) and of course loads require the ground and neutral to be separated.

If you use a standard transformer, you will need to carry the neutral / ground connection from the secondary (and service panel) back to the generator for safety.  It is vitally important to ensure the integrity of the grounding and neutral (you probably know all about this anyway) back to the generator itself.

[Opcom]

If the transformer's oversized to the load being drawn, you should not have balance issues. If the load's difficult, the winding which is alone could give a little lower voltage. 8-) At those low power levels, a boost via a filament transformer could be helpful but probably won't be necessary at all.

It may be best to remove as many reactive or nonlinear loads as you can. Bad power factors and high peak currents will diminish the performance and efficiency of the generator, and make starting an a/c unit harder. It will also affect the way the transformer works. A little RCA 9" TV set even. Been through this with the military radio truck project. -A current transformer showed me the phase of the load current there, including to an HVAC blower, and the huge ugly 7 Amp peaks every half cycle. The TV I could do nothing about, the motor's lagging current was dealt with by adding 600V oil capacitors across it.

Your project ought to be just fine. With a little extra measuring you can see how to improve the efficiency too.

[WA1LGQ]

Thanks for the replies. Fred, I have thought of jumping the legs together, and I might just go with that idea since the only thing in this house that needs 240 is the clothes dryer. Its simpler and it might even be more efficient than the transformer method if a large enough conductor is used for the connection, though I like the idea of having the isolation of a transformer in between. Maybe the thing is able to wired 1:1. Nope, just checked it, 1:2 only. Steve I was wondering about the autotransformer method also. I have some decent sized ones kicking around, but don't think I have time to mess with that.
Thanks...........Larry

[W1ITT]

Larry...
I suspect that electric clothes dryer is closer to 5 kw to run, so your 3.5 kw won't be man enough to run it in any case.  Run a rope out to a tree and dry them the old way.
The worst thing about running a generator is convincing the less technical members of the household that if they run a hair dryer, and a toaster, and a few lights, they have pretty much used up the capacity of the generator.  Then if the refrigerator kicks in all is lost.  I finally gave up and got a 13 kw diesel  rig. 
73 de Norm

[W2VW]

I had a few days here with nothing much to do after I got back home a couple days after Sandy.

Tried all kinds of stuff with the generator feeding the house. I have several really nice 5KVA transformers with multiple split taps. I was able to make the toosmall generator run the Pulverizer in the shack but couldn't come up with enough joules to get the lift motor on the tower to work. The Pulverizer's plate iron made an evil sound while working with the less than FB waveform.

My too smart for it's own good gas fired water heater did not like the 30 plus year old Honda's juice. I hoped the transformer would help that situation. No joy. The cure was to turn on the 300 watts of front porch lighting.

Bottom line is as stated in previous posts. If the only 240 volt load is something too large for the genset there's no point in making 240. Tie the legs together and feed both sides of the 240 for a pair of 120 breaker buses.

If 51W Fred said that he really means it. Fred loves transformers.

[KA2DZT]

Larry,

No need for any isolation, better to skip the xfmr.  That Champion generator is probably better than a 3KW 240 generator.  You can't load either side much over 13 amps.  With a single pole generator you don't have that problem.

I have a 3.2KW Power Mate 240 volt generator that works great.  With average loads it will run 22 hours on 5 gals of gas.

I don't connect it into the mains.  My house has a completely isolated set of circuits that only run from the generator feed.  There are a number of sets of four outlets in important locations in the house.  It requires moving plugs from the grid outlets to the generator outlets.  It takes a few minutes to move the plugs but it works FB.  The furnace has a set of twist lock outlets.  Also the sump pump, the TV system equipment and lights in the basement.

Using the isolated system I can better control what is being powered from the generator.

Fred

[WA1LGQ]

Yeah, I am going to skip the transformer. I bought this generator at Cabela's in East Hartford a couple-3 years ago and I have no complaints, it has worked out very well. I made up a heavy duty break-out box for it and have loaded it up with extension cords to run to 2 fridges, a freezer and a sump pump, add to that lights, big screen tv etc. Its biggest test was during the Halloween Hellstorm, it ran for a week straight except when I stopped it once to change the oil. Just kept it fueled up, which wasn't easy since all the gas stations around here were without power. I was able to get help from the guys on the local 2M repeaters to find open stations (with mile long lines of cars). Tensions were high, saw a couple slight altercations. Anyway, it has a 4 gallon tank and it went through about 28 gallons. The extension cords are a pia so it got me thinking, but I guess I'm better off without the transformer. I will have to find another use for it. Its label says that it can be used for 240/480 also. Hmmm

[W2PFY]

That transformer may be a good 120 volt source for a class E rig?

[RolandSWL]

Hi All, On a Whim, I bought a Duro-Star 4kw generator at a too good to be true price. I thought it would sit in the garage until the second coming. Boy was I wrong. Hurricane Sandy pressed it into service A.S.A.P.

The only downside was that the gen only has 120volt outputs, making a rats nest of extension cords all over the house. I think the generator was so cheap because it measured out at 50hz instead of 60hz. Anything I connected to it didn't complain save for a slight high pitched buzzyness from the washing machine.

Anyway, I rescued an arm stretcher multi tap auto transformer from my job, which, if cobbled into an un-used 220volt dryer socket, would eliminate the extension cord problem.

Could one of you fine folks draw me a picture how I could wire the auto-trans? The main breaker would be turned off, of course. I'm trying to grasp how to keep the ground/neutral correctly oriented.

Thanks, Roland

[ka4koe]

OK, time to pull the EE out of the box.
 
Some rules from NFPA 70:

1. This is essentially a separately derived service on the transformer secondary.

2. Bond neutral ground together at the secondary OCPD, and ONLY at the seconday. A zero reference must be provided. If running 240V winding with a center tap for 2 - 120V legs, then any imbalance in the load on either leg will case the voltage across the full secondary winding to divide up, eg Phase A may be 150V and Phase B 90V...not a good situation. Things tend to go POOF at this point.

3. Carry a ground wire throughout.

4. Provide a secondary breaker no further from 10' from the secondary.  Ground the case of the breaker.

5. If you are running 240V loads with no neutral, you still must provide a neutral-ground bond at the secondary breaker

6. Rule of thumb for secondary voltage transformer sizing: In general, don't load the transformer beyond 80% of its rated secondary KVA rating.

[RolandSWL]

Thanks for the info. Would it make any difference if I used a step up/down isolation transformer instead of the auto transformer? Also, are generators that can supply 240vac single phase or two phase. I don't recall seeing that info in the specs.

Thanks again, Roland