Neutral wiring of an Chinese ST generator

[Hank]

I have a 7.5kw ST generator set up for auxiliary power. It is wired in parallel therefore I have 120 volts coming out of 2 wires. Well this was all well and good running a few extension cords.

I want to hard wire it into a service panel with breakers, etc., we are totally off grid.
The question I have is, if it has 2 wires feeding 120 volts which wire will be the neutral wire? Or does it matter?
I could wire the gen for 240 volts then I would have 3 wires with the center being neutral but I don't currently have use for 240 volts and don't want to be balancing my loads.

I've asked several electricians and they are clueless. As a side note I ran the ground of the receptacle to the generator frame (which is not grounded) and if I take a voltage (unloaded) reading between any of the terminals (hot or neutral) and this "ground" I get 1/2 of the voltage. ie, if the output voltage is 120 I get 60 volts between any of the terminals and "ground' (generator frame).

I want to be somewhat code compliant and certainly I want to be safe even though we barely ever use this generator.

Any help or insights will be greatly appreciated.

Hank

[Frank S]

This should help you out
 st_manual.pdf (127.22 kB - downloaded 928 times.)

[Frank S]

maybe a little easier for everyone to see this one



Connect output wiring to the output terminals U1 and U2 inside the terminal box of the
generator. Pay particular attention to making sure that the wires connected to the
generator output are of a heavy enough wire gauge to carry the output current rated
output current.
Open the outer covering plate of the rear cover and inspect the brushes to ensure that the
brushes are making good contact with the slip rings. If this is the first operation of the
generator, the slip rings should be cleaned with fine emery cloth abrasive sheet.
Ensure that the wiring to the load is correctly installed, secure, and that switches and
fuses or circuit breakers rated for carrying and interrupting the rated power output are in
place.

[Hank]

Thanks Frank for the diagrams, but I already have these.
My set up is for 60Hz 115 volts as per diagram, therefore my take off power lines will be U1 and U2 for 115 volts. I still don't know which one is the neutral, or if it even matters.

Can I just pick one and call it neutral, wire it accordingly in my panel and ground it? Or is there another way. I guess it may not be neutral per say but a grounded phase conductor. Will this serve the same purpose as a neutral for safety/fault purposes when wired with bonding grounds?

Hank

[bob g]

i think you can simply pick one and call it neutral, however
my concern is the fact you report some 60vac from either lead to the generator housing.

i would find out why this is happening first!  with the thing shut down
check the resistance of each lead to the generator housing, you may need to separate each set of windings. you may have a short in one of the coil sets in or about the stator.

if this is the case it is not only dangerous but likely will cause a burnout sometime later, so find it and fix it.

it also might be associated with the harmonic winding (referred to as the "Z" winding) the st heads in this size run around 60volts excitation. it might be the rectifier is either wired so that it shows up on your meter, (not sure how, but maybe?)

all i know is my st heads have no voltage from the output leads to the case of the generator.

something doesn't seem right, so either check it out or have someone qualified take a peek at it.

fwiw
bob g

[Frank S]

Thanks for jumping in Bob. I was just about to ask him to do an ohm test while it was shut down as well.
 I'm not really much on single phase. in fact I have never lived in a home where I didn't have 3 phase, worse case was wild leg 120/120/200
 My concerns are from what I remember I think you should not have more than 5% voltage from neutral to ground.but again I am not positive on that
 I would worry about having a false neutral to ground.

[Hank]

Thanks Bob and Frank,
I'll check the resistance from the leads to housing more carefully when I get back out to my site. From what I recall the resistance was in the Meg ohm scale when I checked. Not sure what it means however. I was going to put a resistive load between one of the leads and housing and recheck the voltage but never got around to it.

Hank

[dnix71]

From the looks of that diagram, there is no true neutral. US regulations require any genset over 5kwh to have a proper neutral/ground/hot output like a house is wired for the express purpose of allowing a safe connection between the genset and house wiring. Since this is Chinese and may not have been certified for export to the US, it may have only a floating neutral, which means you cannot hook it up to your house.

According to this forum post http://members.rennlist.org/warren/generator.html , the ST series has a floating neutral. This makes it illegal to connect to a house in the US in spite of the instructions shown in the post. Neutral and ground are supposed to be bonded in US house wiring.

http://members.rennlist.org/warren/gt5000c.html The same person also does not recommend rewiring Chinese ST head gensets to neutral-bonded ground.

https://www.youtube.com/watch?v=Cjjn9rMxYZU This video shows how to install a main breaker interlock that uses the main panel breakers to backfeed your house from a genset, but it requires a genset that is wired for neutral bonded ground.

[Hank]

Dnix,
Thanks for the response and references. Actually I have read these but I don't think we are talking about the same type of generator. This is not your portable type of generator, unfortunately it's a single phase generator that if I had used in a 240 volt configuration I'd have a neutral, but wired for 120 volts this option does not seem to be available. Now this is an off grid application, there is no house wiring. What I have are two RV's set up at my site that are basically powered by solar and wind, batteries and inverters. Occasionally I use the Gen set to charge my batteries and rarely to provide RV power. I just want to set it up as a more permanent set up (rather then extension cords) and be safe about it. A service panel with several circuits on it.

Thanks for your input,
Hank

[ChrisOlson]

That generator has a floating neutral, which is no real problem.  In fact, the floating neutral configuration is preferred for installations where a person may come in contact with a hot conductor and ground at the same time.  Most RV/marine, or otherwise mobile inverters, use floating neutral.  And some small generators like Champion Power Equipment also use floating neutral.

For connection to house wiring you will have to wire it for 240 split phase.  Since you do not have split phase service, then you'll have to use either just one leg of the generator, or install an autotransformer to leg balance the generator.
--
Chris

[Hank]

Thanks Chris,
I can either run it as 240 and just use one leg or I can rewire it, as per diagram up above for just 120 volts and use the full power rating of the gen set (if I need it).

I have no service, so any service that I get will be from this generator. I can wire it for split service(240 volt) if I want, then there is no problem (I think). However if I wire it for 120 then I lose my neutral.
Probably the best analogy is 3 phase. In the USA we use "Y" 3 phase and center tap for neutral to provide consumer power (240 or 120 volts). In Europe, I believe, they use "Delta" 3 phase configuration to provide 240 volts. In that case there is no neutral. I'm sure they ground there circuits somehow.

[ChrisOlson]

Yes, you need the neutral for residential, and you need a neutral ground bond.  Usually the bond is made at the service entrance.  But if the generator is the service then a neutral ground bond has to be made at the gen.  You can only have one neutral ground bond in the system - otherwise the ground and neutral has to be separate.

So, this gen being a floating neutral unit, only has a non-floating neutral when wired for split phase.  You cannot bond a floating neutral to ground.  So you have no choice for residential except to use it in split phase configuration.
--
Chris

[dnix71]

I agree about using floating neutral for portable installs. As long as the RV wiring is isolated from any metal frame/shell part of the RV then it shouldn't matter which wire is which. The RV wiring ground at the panel is connected to the genset ground and either the genset has a GFCI to protect itself and users or you have to be very careful to earth the ground with a proper rod.

[ChrisOlson]

The beauty of the floating neutral for portable applications is that neutral is not bond to earth ground.  So, say you are using a small drill and standing in a puddle of water.  You can accidentally come into contact with the hot conductor and you won't get a shock from it.

With a bonded neutral, if you come into contact with the hot conductor you have a completed circuit thru the water to the earth ground and neutral.

The floating neutral in a RV, marine or other portable application isolates the chassis ground from the neutral conductor to allow for the conditions that may be encountered in portable applications.

The bonded neutral in residential wiring provides for ground fault protection and is required by code in every jurisdiction, including off-grid wiring.
--
Chris

[Hank]

Gets more confusing to me as I go on. Okay if I float the neutral (not bonded to ground or grounded) and bond all my boxes and ground wires to earth thru an appropriate ground rod will I be safe?

[ChrisOlson]

I will explain this, but if you don't understand it get an electrician out there to do it.  Electricians are cheaper than dead people.

No matter if you have a generator, inverter or grid power, you should have a Main service panel in your house that the power source feeds.  There will be a ground rod driven at this, what is called the Service Entrance.  This is your earth ground.  The Service Entrance is the term given to the point where all the power is distributed in circuits protected by a main breaker for the power source, and circuit breakers for each circuit.

Inside that service panel is a provision to install a green bonding screw that bonds the system neutral to the earth ground.  That green screw should be installed in there and this is your one and only neutral ground bond allowed in the entire system.

Now, no matter if you install a generator, hook up grid power, or an inverter, you only run hot to hot, and neutral to neutral, and ground to ground.  You DO NOT make another bond between neutral and ground at the generator frame or anywhere else.

The generator you have, when wired for 120 volt, will supply 60 volts from neutral to ground, 60 volts from hot to ground, and 120 volts from hot to neutral.  This is because the sine waves are technically split phase 120 volt power being you wired the generator windings in parallel.  It has a floating neutral.  This type of generator can NOT be wired in to a system with a bonded neutral.  Period.

If you rewire the generator for split phase, now you have a good unit with a generator neutral.  You have 120 volts from either leg to neutral, and 240 volts from leg to leg.

With the generator wired for 240 split phase you can wire it to your service going hot to hot, neutral to neutral, and ground to ground.  However, you will have one hot leg from the generator that is not being used.  This means you cut the gen output in half and you will only use half the generator windings.  If you want to use the full output of the generator, then you will have to install a transformer that converts the split phase output of the 240 gen down to 120 volt.
--
Chris

[Hank]

Thanks Chris, I understand this fully now. I also understand that there can only be one point for grounding neutral. What I didn't understand is about the neutral in this gen set up for 120 volts. I have heard everything from grounding either one of the terminals and calling it neutral to it can't be done.

Just one question for you. You said I will get 60 volts from either terminal to ground. I understand this, similar to inverters, but where is my ground in this case as I have only 2 output wires? When I measured voltage from either terminal to the generator housing I got 60 volts. Would that be my ground? If so then I can't bond this to earth. Is that correct?

[ChrisOlson]

Yes, the generator frame IS ground.  If you want, you can drive a ground rod for the generator as long as it's at least (I believe - check the code book) 9 feet away from the service entrance ground rod.  You just can't bond the generator neutral to that ground rod at the generator.

And no, with a floating neutral you cannot ground it and call it ground.  That defeats the purpose of a floating neutral generator and will cause power to flow in the ground circuit all the time.  Power always flows in the neutral under normal conditions.  But in residential wiring power only flows in the ground when there is a ground fault condition.  You cannot ground one leg of a split phase power source, and that's basically what a floating neutral is, only that the amplitude of the sine wave is at 120 volts instead of 240 on this generator when it's wired in parallel.

That generator was built to be a residential unit with the 240 volt, and to be a portable unit when wired for 120.  Champion, Honda, Yanmar - a whole bunch of manufacturers do this.  It is very common.  Just that residential service at 120 volt only is not that common.  Residential service is 240 split phase normally.  Being you only have a 120 volt service, the best option is to pick up a transformer so you can use the full output of the generator.
--
Chris

[Hank]

So if I understand you correctly I would run two ground rods, 1 for the generator frame and the other for the service panel and no where will the neutral be grounded or bonded, seperation of ground rods per code.

 It was my understanding that if I wired the generator for 120 volts only (parallel) I would be able to get the full output. If I wired it for 240 then I would get 1/2 the output per 120 volt leg and full output across 240.

[Frank S]

T  Being you only have a 120 volt service, the best option is to pick up a transformer so you can use the full output of the generator.
--
Chris

If it were mine and I had to have 120v I would use the transformer

[ChrisOlson]

So if I understand you correctly I would run two ground rods, 1 for the generator frame and the other for the service panel and no where will the neutral be grounded or bonded, seperation of ground rods per code.

No, you should have one neutral/ground bond at the service entrance.  That is the only neutral/ground bond allowed.  At the power source (grid transformer, generator, inverter, what have you) neutral and ground is kept separate.  The neutral is the return path to the power source, and if you bond that to ground in more than one place you provide multiple paths for return current to the source, and one of those paths happens to be ground.

It was my understanding that if I wired the generator for 120 volts only (parallel) I would be able to get the full output. If I wired it for 240 then I would get 1/2 the output per 120 volt leg and full output across 240.

Your understanding is totally correct.  It's the application that determines whether or not you can use the 120 volt configuration, and for residential power you can't.  So the only option you have, to meet NEC, is to use the 240 volt configuration.  If you want to use the full output capability of the generator then you will have to purchase an autotransformer that takes both legs of the 240 and outputs a single 120 volt sine wave to power your 120 volt stuff.  The Outback FWX-240, as an example, is rated at 4 kVA without cooling, and 6 kVA with a cooling fan blowing on it:
http://www.affordable-solar.com/store/outback-autotransformers/outback-fwx-240-auto-transformer

This is the fan cooled version of it - the PSX-240:
http://www.affordable-solar.com/store/outback-autotransformers/outback-psx240

Typically, with a genset on prime power duty you're going to run at 50% load, and this transformer would handle the full surge load of the generator for a few seconds.  And few gensets put out their rated power unless you're at sea level on a nice cool day.  So I think the fan cooled one would work fine for your application.
--
Chris

[Hank]

Thanks all, I think I finally understand it now.
Just have to dig up some cash for the transformer and go from there.

Hank

[ChrisOlson]

Hank, just note that you can get everything wired up and get it working, just using one generator leg.  Then add the transformer on to improve the gen performance when you get the cash for it.  That gives you some time to shop around a bit - there's many sources for autotransformers.

I can't vouch for the quality of the Chinese ones for off-grid duty but I know you can get them from Eaton and several other companies other than Outback or Xantrex.  Most big generator dealers will handle various ones, but the two Gold Standards for off-grid use are either the Outback X-240 or the Xantrex/Trace T-240.  We have a T-240 on our system for leg balancing our dual inverters, and although quite expensive it performs flawlessly day after day and is quite efficient.
--
Chris

[Hank]

Chris, thanks for the info. I guess where I got all confused is when I read a post from http://www.utterpower.com/installa.htm where George talks about converting this Generator head to straight 120 volts, then goes on to say to pick any one of the two output leads and call it neutral and no mention of grounding it.
My power requirements are small, the 7.5kw generator is an overkill so I may just rewire it for 240 volts center tap to neutral and wire my panel accordingly. I'll probably never need 240 and since my requirements will probably not exceed 1 kw/leg, I'm sure one leg will be able to handle it without a problem. And when I say service panel all I mean is to have a central panel in a shed equipped with breakers to provide 4 circuits. Mostly by some cheap inverter and occasionally via the generator using a dpdt switch for isolation purposes. In reality I could  accomplish all of this using extension cords but that's messy.  All this provides power to our camp of two RV's that require 120 volts.

Thanks again,
Hank

[equiluxe]

Before you go and bond a live to earth (ground) make sure that the center tap which is your present neutral is not already bonded to earth and or the frame of the gen set. As long as there is not already a bond on the center tap it does not mater which of the 240 wires is used as neutral as a neutral wire is only a live that is connected to earth and therefore at earth potential.Your 60 volt is most likely down to filter capacitors I bet there is no current.

[bob g]

let me throw a wrench into the works!

should you decide to draw most of your loads from one leg of an ST head be forewarned that it "may" cause it to howl or growl a bit.

the reason being each leg is "not" displaced 180 electrical degree's around the stator as one might expect.

the original design of these heads was to make 230 volts 50hz for the part of the world they were to be used in...

when the market opened for 60 hz 120 volt units they simply opened the stator mid way and brought out leads, changed the turn count for 60 hz operation speeds to get the voltage right and away they went.

so what you end up with if you pull most if not all your power off of one leg is magnetic loading on one side of the stator and none on the other. while technically/electrically it works just fine this lopsided magnetic field causes some of the heads to make wierd moaning/growling and sometimes even howling noises.

while i have never head of any failures related to this phenomenon it is something that needs to be known about up front, if you have such a head that howls like a banshee it will drive you and your neighbors out of their minds.

mine sounds like an air raid siren, and makes it so that i cannot be around it without ear protection. it doesn't produce this sound all the time, but enough so that i don't run it without a balanced load across the stator outputs.

many folks that reported this problem, report back after installing a transformer the noise doesn't come back.

fwiw
bob g

[Hank]

Thanks Bob, I'm aware of the consequences of running unbalanced loads and that was the prime reason for trying to go with the 120 volt only configuration, although I have never done it on this gen set. The closest neighbor is about a mile away so no worries there, damaging the generator however is a possibility.
I still have some work cut out for me before I continue, the fact that I'm getting 1/2 of my line voltage when measuring from either line to the housing is of some concern to me. When I had it wired for 240 I don't believe that was the case bit it's been a long time ago. When I get out there again I will be changing the dog house and rectifier.
I'm still looking up NEC codes for guidelines for separately derived 2 wire systems systems to basically feed several circuit breaker protected outlets. NEC 250.26.................?

Thanks for the help, should have paid more attention when I was studying electric circuits, but that was a life time ago and much has changed especially NEC codes.

Hank

[bob g]

the only reason i mention the unbalanced loading issue is because of the design of the st head as compared to just about every other generator head available here in this part of the world.

90% of heads made the only concern with unbalanced loads is overloading one side electrically, stay under the single coil limit and there really is no problem, you just get half the available power thats all.

with the ST you not only have the aforementioned issues related to half power and not overloading, but also the issues relating to magnetic unbalance forces from the stator winding layout, and the noise that can occur.

the first time you hear it, you will think that you have ruined your genhead or something horrible is about to happen... it doesn't happen to everyone but those that it does scares the living crap out of the first time it happens.

fwiw, to you, maybe you already were aware of the issue?  if so disregard and hopefully it might be of use to someone else down the road.

bob g

[ChrisOlson]

where George talks about converting this Generator head to straight 120 volts, then goes on to say to pick any one of the two output leads and call it neutral and no mention of grounding it.

Hank, it can tend to get very confusing because the load does not care which is hot and which is neutral.  To the load they're both "hot" and you can interchange them and the load don't care.

Where it matters is in residential wiring and making that bond of the neutral to ground.

At the loads you mention, you should not even need a transformer because you're not even close to overloading one leg with the gen wired for 240.  So this all may be a moot point.  If the gen can handle all the loads on one leg, wire it up and be happy.

Like Bob said, this can cause a vibration in some generators, depending on how they're wound.  If it does, then get the transformer.  But it's not going to hurt the gen.
--
Chris

[Hank]

Thanks Chris,
Well I don't have a residence there unless one considers a travel trailer a residence. As stated earlier all I want to do is to set up a  panel equipped with circuit breakers, in a shed, from which I could run a few outlets.

I'm not sure if that makes a difference?

[dnix71]

Maybe if you are that far off the grid, you should consider your loads and go "12v" as much as possible. I can run my washing machine from a fairly inexpensive sine inverter bought from Home Depot. I have a 120v rice cooker that runs just as well from the same inverter. My fridge is an Engel 12vdc. A much smaller 120v common genset would suffice if you focus on load management. My fan is a 12v battery fan bought at Walgreens. There are plenty of LED and CFL 12v lights available from auto supply stores now.

My DSL box runs straight from 12vdc and my laptop is kept charged from a 180 watt sine inverter hooked to batteries.

If you need killowatts of power, get a Lister with an axial alternator and boost your batteries that way when you need to run heavy loads on an inverter.

https://www.youtube.com/watch?v=VyJYdHzwsy4 This on is based on Dan's design.

[Hank]

The nearest grid power is over a mile away. What I have there at our camp site are two travel trailers. For power I have about a 1000 ah battery bank being charged by a PV system as well as a wind mill. The 12 volt battery power is run thru some cheapo MSW inverters which in turn supply 120 volt ac to our trailers and exterior lighting. The 7.5Kw gen set is driven by a 195 Changfa diesel which is also equipped with a separate automotive alternator as back up battery charger. At lower engine rpm the alternator works just fine in charging the batteries and the gen won't even excite (produce power).

Now one will ask "why the overkill with 7.5 Kw", well the price was so good I couldn't pass it up. It also gives room for future expansion and power increase needs.

I have used the gen set for powering welders, water pumps, compressors, etc. as needed.
Of course this was all from one extension cord coming from the generator. All I want to do is to set up a more permanent service in the shed (the only permanent structure) so I can run several pieces of equipment at the same time or for that matter even the trailers (which are not hard wired). Getting 12 volt appliances have no merit for me as the trailers have all the appliances I need (12volt, 120  volt ac, and gas), I basically use 120 ac volts and gas.

[Hank]

It's been some time but have been quite busy.
Here's the scoop:
Measured voltage from one of my hot leads to frame and got 1/2 the full voltage (60 volts). The same for the other lead to generator frame. Connected a load from one hot lead to frame, voltage went to 0. Used a different multimeter to measure the voltage from hot to frame, no load, and I got 0-1 volts. Repeated this several times to confirm  and it was the same. One multimeter measures 1/2 of the voltage the other measures virtually 0. Go figure!
At any rate, based on all your suggestions and advice, I decided to go with an autotransformer.
The Outback transformer is quite pricy and I've seen transformers on e-Bay that were cheaper.
Any suggestions as to what transformer to pick or keep away from?

All suggestions appreciated,
Hank