"Half Voltage Scheme" Inverter?

[Glider]

I have been doing some reading that has raised some questions for me regarding "half voltage" inverters.

Apparently some inexpensive inverters send 60v of power out each wire: 60v on the black side, and 60v on the white side. The voltage combines at the load to produce the full 120 vac. In this case, the white side is not bonded to ground, for obvious reasons.

While it is, apparently, not an issue to power a load in this manner, it would be an issue to power a service panel, as the white wire would send 60v to ground. Not good.

I have a 2000w Zamp pure site wave inverter sitting in my living room. I just checked the output side with my ohm meter, and found no connectivity between either white and ground, or between black and ground, but full connectivity between white and black.

My conclusion: the Zamp inverters appear to be of the half voltage ilk, and are not suitable for connecting to a service panel.

Does anyone know anything about this?

Thanks.

[boywonder]

Does the Zamp have a GFCI receptacle built in?

[Wowbagger]

I am an electrical engineer, and I have never heard of this sort of scheme. Now, I have seen some crappy inverters that simply didn't have any connection between the 120V side of the inverter and case ground, so if you were to measure the resistance between the neutral and the ground on the receptacle you would get an open circuit. However, if you are using such a device to feed a panel, and you bond ground and neutral at the panel like you are supposed to, then nothing bad would happen, and it would behave "correctly".

I would suspect that any device like that would me a modified-sine wave inverter. That may have been good enough ten years ago, but with modern MOSFET devices and control circuits there is no reason not do to a proper full sine wave, and I would trust such low cost devices approximately as far as I could throw them
into the wind
On Jupiter.

[ShuttlePilot]

Originally Posted by Wowbagger

as far as I could throw them
into the wind
On Jupiter.

That's funny. Jupiter has the highest wind speeds known in the solar system. OK. Never mind.

[arctictraveller]

Originally Posted by Wowbagger

I would suspect that any device like that would me a modified-sine wave inverter. That may have been good enough ten years ago, but with modern MOSFET devices and control circuits there is no reason not do to a proper full sine wave, and I would trust such low cost devices approximately as far as I could throw them into the wind
On Jupiter.

I suppose square wave inverters have a place somewhere. I've had a 2500W one in use for over 10 years with no bad effects other than burning up a couple electric blanket controlls. Since then I've installed a second true sine wave to power critical loads. I agree that the price on true sine wave's have come way down, but in some applications a dirt cheap square wave may be adequate. Not that I'd go back though, your point is well taken.

[Glider]

Thanks for the ideas. Especially about the inverter toss on Jupiter. I'm available next weekend--we could make it an annual gathering.

I've learned a few more bits since last night, but the one thing I am certain about is that I don't know everything I want to know just yet.

1. Boywonder, the Zamp does not have a GFCI receptacle.

2. Many inverters--including the Zamp--do NOT have a UL listing. This blew my mind.

3. 120v electrical systems in RVs are not supposed to have the neutral bus bar bonded to the vehicle chassis, as they would be in a building (in the case of a building, it would not be the vehicle chassis--it would be the ground). The reason for this is that if, in a vehicle, the neutral were bonded to the chassis at the panel, and the neutral and hot wires were mistakenly reversed, the chassis of the vehicle would carry a 120v potential with respect to the earth ground--which would be a rude awakening for the person grabbing a door handle from outside the rig.

4. There are two UL listings that apply to the subset of inverters that bother to carry UL listings: UL1741 and UL458.

5. The NEC requires inverters used in residential applications to be listed to UL1741. A UL1741 inverter must allow for the neutral to ground bond to occur only at the main AC service panel--so in the UL1741 inverter, neutral is floating with respect to ground. Wowbagger, it would seem that the open circuit between neutral and ground is actually required in these applications--which is not to say that there are not also some un-listed, poor quality inverters out there that might also happen to have this condition.

6. UL458 is the standard for vehicle applications. UL458 inverters have internal neutral-to-ground switching relays. When the vehicle has an external source of AC power, the relay is open, and the inverter emulates a UL1741 setup. When the vehicle is utilizing inverter power, the relay closes, and neutral is bonded to ground at the inverter.

7. For vehicle applications, it would appear that the selected inverter should be capable of providing a neutral-to-ground connection when the vehicle is off-grid and the inverter is in operation--but should provide for an open circuit between neutral and ground at the inverter when the vehicle is utilizing AC power from any other source.

Resources and references:
The Samlex web site has a great collection of white papers:
White Papers & Circuit Diagrams | Samlex America


My bottom line at the moment: the Zamp is going back to Lowe's. I am going to keep reading, and I'll see what I come up with.

[Scalf77]

What are you looking for in a inverter? The new cheap replacement for the Modified Sine Wave inverter is the high frequency Pure Sine Wave inverter, they are smaller and lighter, but.

There would be a difference in meeting UL 1741 and neutral floating with respect to ground and having 60 volts on neutral such as the Zamp.

You might want to post your findings back on the cheap Zamp at Lowe's thread.

-greg

[86Scotty]

I like Greg's answer best. Mostly what I want to know is if I plug a microwave into it to heat my coffee, will it get hot? Will it do this on camping trips for a few years?

If so, I'm good with it.

[Scalf77]

Originally Posted by 86Scotty

I like Greg's answer best. Mostly what I want to know is if I plug a microwave into it to heat my coffee, will it get hot? Will it do this on camping trips for a few years?

If so, I'm good with it.

Yes, and no. First, I am not a fan of Zamp period. But in general not meeting UL requirements, also makes me wonder what other kind of testing they do, and by they I mean whoever makes it for Zamp. The high frequency inverters actually use a lot more parts than the low frequency, also have tighter design constraints. For the money they were charging, you won't be out much, if it breaks early.

Here is an explanation of the difference from magnum-dimensions Inversion Methods Explained: High Frequency vs Low Frequency | Magnum Dimensions

They also show that they make high frequency models(a MSW & PSW), all of which are UL 458 compliant as do others. Just a MSW inverters had there place, so do High Frequency Inverters, it is just the low price point is where you generally see the most crap come out.

-greg



-greg

[Glider]

Originally Posted by Scalf77

What are you looking for in a inverter?-greg

Originally Posted by 86Scotty

Mostly what I want to know is if I plug a microwave into it to heat my coffee, will it get hot? Will it do this on camping trips for a few years? If so, I'm good with it.

I'm not finished with research on this yet. I still have some open questions, but:

1. I just got off the phone with a technical guy over at Samlex. According to him, the half voltage scenario is very common in lower-cost inverters. For example, the Samlex SAM series uses this approach.

2. What I'm looking for: Greg, my ideal scenario is an inverter that I can tie into my existing 120v system. If I were just using an inverter as a stand-alone, and plugging the coffee pot directly into the inverter, none of this would really matter. I could have either a half-voltage or a full-voltage unit, and my electrical appliances would never know the difference. Eric, I'm guessing that is how you are doing it.

However, if I am going to tie the inverter into my van's panel, then I need to be sure that I am assembling the proper components in the correct manner. Based on what I know so far, I need to:
a) be sure that my inverter is not a half-voltage unit;
b) be sure that my inverter meets the appropriate UL spec--in this case, UL458;
c) determine where the neutral-to-ground bond is supposed to be located--and where it is NOT supposed to be located;
d) determine where I should and should not install GFCI outlets--and why;
d) etc.

The neutral-to-ground bond arrangement is the most interesting question in my mind at the moment. Does anyone have any good information on how neutral and ground should be bonded--and not bonded--in a vehicle application--including how this is handled inside the inverter?

Clearly, an error in this bit of the wiring arrangement could lead to a shocking surprise, which I'd prefer to avoid.

Thanks.