Generator wiring options

OffGridiot · September 2021

Hey all. I’m in the market for a new generator to replace the Honda 3000 watt inverter generator I’ve been using to charge my batteries for the last decade and a half. It’s done a somewhat adequate job for the most part but there have been times when it has come up short, and it’s getting pretty long in the tooth at this point. I’m thinking a diesel replacement in the 5 to 7 kW range is in the cards. I want it to either be the variable speed inverter style, or steady 1800 rpm type, and my budget will most likely mean shopping for a used unit. My question(s) is (are) mainly about circuitry to my MagnaSine 4024 inverter/charger. I understand that there is a capacity for ‘pass through’ juice beyond the 29 VAC charging current that the charger half of my inverter will process, enabling me to power other household loads while charging the batteries.

So, it seems that most generators I’ve been considering have breakers rated for around 30 amps. Being rated as 6 or 7 kW, I assume (from reading online data sheets) that this either means there are two 120 volt circuits around that capacity, or one 240 volt circuit at that amp rating to arrive at a total wattage output rating of the machine. My MagnaSine won’t accept 240 volts, and using only one 120 volt circuit @ 30 amps takes care of my battery charging needs but leaves virtually nothing for ‘pass through’ household loads.

Do the series/parallel rules used to make battery banks of different voltages apply to AC power as well? Could I (or a qualified electrician) combine two 30 amp, 120 VAC circuits in a parallel manner to (safely) create one 60 amp circuit? Or would it make sense to run one 30 amp circuit to each of the two AC input terminals in the inverter (I’m not certain if these separate inputs can be used simultaneously, or if they are only to be used one at a time, for convenient source-switching purposes)?
I’ve got my eye on an Onan Cummins 7.5 kW unit that’s almost brand new, and about half the price I’d have to pay retail from a dealer. And there are plenty of 6 kW light plants available on Kijiji at relatively reasonable prices. Opinions on these choices are also welcome.

Thank you.

Bruce

mike95490 · September 2021

If you only need 120VAC, look for a generator with what's commonly called "Full Power" switch.
This changes from 240V 30A to 120V 60A ( or whatever size the genset is )

I don't know of any variable speed, diesel, inverter gensets. Diesel is pretty much going to be a
a conventional alternator . Double check you can get Full Power at 120v from it

RCinFLA · September 2021

Some units have a switch to allow full load power on 120vac only.

Most of the time you can rewire the two 120vac output windings in parallel.

You have to get the phase polarity correct. To do this from all four wires disconnected, ohm out to find out which two sets of wires go to each separate winding.

You have to be careful about field winding pickoff power. Sometimes they mix it up with output winding connections near socket panel. Most generators have their own separate stator windings for field rotor power so should not be an issue.

Pick two wires, one from each separate winding and connect them together. Run gen and check voltage across two open end wires. If you get near zero AC voltage you got them in parallel for 120v. If you get near 240vac you have them connected in series and need to disconnect your first guess and swap with other end of same winding. Repeat test to check you get near zero AC on two open wires and you should now be able to connect the two open wires together.

Largest AC socket for 120vac is 30 amps. If generator is greater then 4kw you should change to higher amperage socket. You could also use two of the sockets keeping to their current limit for each or use half a 240vac 50 amp sockets. Don't just parallel the two original L1,L2 240vac socket outputs.

The Magnasine or any other hybrid inverter is limited on pass through current to what their connect relay is rated for.

OffGridiot · September 2021

Thanks for the reply Mike!

Onan Cummins makes the QD line of diesel generators that are variable speed. I inquired about a new one, and the sales rep said that IF he could even get one to me in the next 6 months, because I had told him that I’d be using it for off grid, the warranty would be void. I’ve seen other options out there that turn at speeds of between 2200-2800 rpm (but constant steady speeds, I believe) and sales reps have told me that there’s an electronic gizmo (sorry for the technical jargon) that keeps it at 60hz. I guess they’re inverter units too? All pretty moneyish for my Scottish sensibilities.

I guess I’m hoping to find a crazy good deal on something with which I can safely install my own version of a ‘full power’ switch…. if that can be done. Hard wire it into the system instead of using the twist lock receptacle that my current system has in place.

Cheers!

mike95490 · September 2021

If you can't get the power you need at 120V, you can use a step-down transformer, 240V - 120V
If you want long life, look for 1800RPM generators
https://www.electricgeneratorsdirect.com/power/1800-rpm-generators.html

m151 · September 2021

Honda eu7000 inverter has a 120/240 full power switch, it's not diesel, but with fuel injection may be as efficient.

RCinFLA · September 2021

QD line does have diesel engine permanent magnet alterators (makes HV DC) with sinewave inverter output. Better quality sinewave and freq control. With diesel rpm-torque profile you won't get much ECO mode variable rpm control. Most of the time you cannot use ECO mode when driving a hybrid inverter anyway.

Only thing I don't like about them is diesel engines that run at 3600 rpm. I don't believe they have the long term reliability of a normal 1800 rpm diesel.

wellbuilt · September 2021

There are some nice Surplus units around
I think they come in 3000watt 5000watt and 10000watt
they seam to be selling for 3 to 5k for low hours unit .

I looked at for sale and I mite of bought it for 3k if I could hear it run . It did not start ? ☹️

I use a little honda2800 watt unit for my place .

It’s only a few years old and it gets 120 or so hours a year on it .

It seams pointless to go diesel
I use the Honda , it sips fuel any way and cost 1000 bucks .

I would like a 7000watt unit but it burns more fuel cost 5 times the money . And I don’t have a need now

Generator wiring options

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