DC-AC Power Inverters DANGER

[fs]

I have been looking for a replacement of my older DC-AC inverter to go
camping and hook up inside my truck and wired up as code with the natural at
ground and case ground potential. Going over several DC-AC Sinewave Power
inverters there seems like there is only a few that DO a good job without
noise riding on the output, and provide the kind of output that I need. The
requirement I was looking for was that the NATURAL of the AC out must be at
case ground. I looked at some of the lower cost ( WAGAN sinewave) and others
inverters, and their so called natural was floating above ground as part of
the sinewave. The reason is that when I hooked these inverters up to my
camping trailer that normally plugs into the camping site AC power the AC
line going to the Camper is hooked up as it should be, natural and ground is
hooked to camper ground as required by code. I used this to power the small
AC lights and other devices plugged into the inside AC sockets running from
the Trucks DC battery supply. Now here is the problem that is very
DANGEROUS, and should be looked into. From the cheaper modified sine wave
or puresinewave inverters from many of manufactures, the NATURAL side of the
AC out is HOT ( part of the waveform to save a small cost in building) , the
problem is that if someone touches the truck, and the camper next to it
guess what BAD SHOCK. Keep this in mind if you are hooking the inverter to
something wired external. Some of the Xantrex Modified Sine Wave Inverters
,Prowatt,and Portawattz and their Prosine series,have this with grounded
natural output, and EXELTECK seem to also has a good grounded AC natural
with a clean sinewave output. I do not know about Samlex sinewave series but
I am looking for more input.

 

[Robert Morein]

I'd like to hear more input on this.
It would seem from your description that some of these inverters are
providing what is known as "balanced AC", which is actually desirable in
some situations.

As long as the design of the inverter is transformer isolated, and I think
they all are, what you are referring to is a floating AC waveform with no
ground reference. This is not in itself a shock hazard, and you could make
the neutral-ground bond yourself.

I'm not trying to be authoritative, these are just some informal thoughts,
and I'm curious what other people have to say.

 

[fs]

Robert MOST of the inverters are not transformer isolated from the DC
input, as I have only one 1000 Watt that is, if they were all isolated I
would hook the out put AC jack with the netural to ground. Its in the cost
to make the inverters as cheap as possible and actually it is not that much
more to provide the correct grounding AC output. Most builders drive the AC
socket from a H bridge switching supply witch makes both sides of the AC
output line inverters HOT compaired to the incomming DC ground.
I might mention that I am talking about the small 50-
some 1000 Watt inverters, not the big ones.
Some manifactures do development where the AC Hot side does swing above and
below ground pontential as in a normal AC jack in the home.

 

[fs]

Big spelling mistake.


ITS NEUTRAL not natural.

 

[ptaylor]

Every inverter I've ever seen has had a transformer in it.It *has* to to
transform 12V (24,48,etc) to 120Vac.
12Vdc comes in,goes (pulsed at high freq.) through a set of
(HEX)FETS,stepped up to ~150-200V by a small ferrite
transformer,rectified(now back to DC),filtered with a large cap, and
gets chopped up by an H-bridge at 50/60hz (back to AC).
Thats the usual setup for a cheap square/"modified sinewave" inverter.
I'd imagine a sinewave inverter would be similar,with the exception of
the H-bridge,and the drive signal(s) to it.

Now,wether or not it's designed with a floating output or grounded
neutral,etc. is another story.. Probably varies alot by manufacturer.

 

[fs]

Every inverter I've ever seen has had a transformer in it.It *has* to to
transform 12V (24,48,etc) to 120Vac.
12Vdc comes in,goes (pulsed at high freq.) through a set of
(HEX)FETS,stepped up to ~150-200V by a small ferrite
transformer,rectified(now back to DC),filtered with a large cap, and
gets chopped up by an H-bridge at 50/60hz (back to AC).

Thats right but the HV CAP in some inverters are grounded to the input DC
supply to have a referenct to adjust the High Voltage Regulation to.
If they used optico isolators for their HI Voltage feedback with it woulb be
possible for isolation. Now the problem is an isolated supply for the
switching transistors . Rember the High Voltage FETS drive must come from
somewhere, and if it is powered from ICs or transistor drivers they must get
their power from somewhere. Most use the incomming DC and regulate to power
them. Now we are not isolated.


I did not say there was no transformers used but some inverters are NOT
isolated. I am stating that there is a possibilty for a shock hazord on some
inverters. Here you are going off like you know so damn much and trying to
dispute my findings, but I bet you would not like touching my inverter as I
had noted it will SHOCK YOU. ITS NOT ISOLATED. Its HV supply is referanced
to the incomming DC leads.
I am trying to pass some info along to help out. Hell man I have been
desinging circuits since 1972 in all fields.

Try it some models and touch one side of the AC out and case or DC ground
at the same time if it is so isolated as you state then you will feel
nothing Good luck on some units.
Then tell us all about it before you go off .

One way to provide the AC out is to have a + 180and a -- 180 volt supply
from ground pontental ,and switch the AC HOT lead from these voltage levels
either modifed sine wave or pure sinewave. This way the AC ground would be
where it should be.

 

[ptaylor]

Thats right but the HV CAP in some inverters are grounded to the input DC
supply to have a referenct to adjust the High Voltage Regulation to.

Exactly. and now your output is xxx volts above the input GND.

If they used optico isolators for their HI Voltage feedback with it woulb be
possible for isolation. Now the problem is an isolated supply for the
switching transistors . Rember the High Voltage FETS drive must come from
somewhere, and if it is powered from ICs or transistor drivers they must get
their power from somewhere. Most use the incomming DC and regulate to power
them. Now we are not isolated.

Indeed,most use a 7812 and/or 7805 to get the voltages for the driver
IC's,and circuitry straight from the DC input..And yep,isolation is
gone,unless they used optoisolators like you say..

I did not say there was no transformers used but some inverters are NOT
isolated. I am stating that there is a possibilty for a shock hazord on some
inverters. Here you are going off like you know so damn much and trying to
dispute my findings, but I bet you would not like touching my inverter as I
had noted it will SHOCK YOU. ITS NOT ISOLATED. Its HV supply is referanced
to the incomming DC leads.
I am trying to pass some info along to help out. Hell man I have been
desinging circuits since 1972 in all fields.

You said that most inverters do not have transformer isolation..The
transformers are isolated,but the rest of the circuit may not
be,depending on how it was designed..(above notes..)
(moot point,if the output isn't isolated anyways.)

I havn't been in electronics that long,but I've "been around the block"
so to speak.

Try it some models and touch one side of the AC out and case or DC ground
at the same time if it is so isolated as you state then you will feel
nothing Good luck on some units.
Then tell us all about it before you go off .

One way to provide the AC out is to have a + 180and a -- 180 volt supply
from ground pontental ,and switch the AC HOT lead from these voltage levels
either modifed sine wave or pure sinewave. This way the AC ground would be
where it should be.

Yes,that would work,and I think alot of larger sinewave inverters may do
just that,but i'm not sure..

 

[Bill Kaszeta / Photovoltaic Resources]

This a common problem with low cost, under 1000 watt inverters.
Some of the inverters, such as the Xantrex Portawattz 600, have a
warning in the user manual (but not on the box or on-line data) that
they are not to be used in conventional building wiring situations
wherein the neutral and ground are connected. Connecting neutral
to ground will destroy the inverter when input power is applied and this
damage is not covered under warranty.

I found the same problem with Coleman inverters, even after asking
factory engineers if the neutral could be grounded to the battery
negative and getting a email stating that it was okay to do so. At least
a copy of the email resulted in a refund after the inverter failed.

None of these low cost inverters are UL listed.

Most of these inverters use a circuit that boosts the +12V (or +24V) input
to about 160 volts using a switchmode up converter, then uses a
H-bridge of power FETs to switch this to the output to form a square
wave AC waveform. This is not isolated from the DC input.

I have been looking for a replacement of my older DC-AC inverter to go
camping and hook up inside my truck and wired up as code with the natural at
ground and case ground potential. Going over several DC-AC Sinewave Power
inverters there seems like there is only a few that DO a good job without
noise riding on the output, and provide the kind of output that I need. The
requirement I was looking for was that the NATURAL of the AC out must be at
case ground. I looked at some of the lower cost ( WAGAN sinewave) and others
inverters, and their so called natural was floating above ground as part of
the sinewave. The reason is that when I hooked these inverters up to my
camping trailer that normally plugs into the camping site AC power the AC
line going to the Camper is hooked up as it should be, natural and ground is
hooked to camper ground as required by code. I used this to power the small
AC lights and other devices plugged into the inside AC sockets running from
the Trucks DC battery supply. Now here is the problem that is very
DANGEROUS, and should be looked into. From the cheaper modified sine wave
or puresinewave inverters from many of manufactures, the NATURAL side of the
AC out is HOT ( part of the waveform to save a small cost in building) , the
problem is that if someone touches the truck, and the camper next to it
guess what BAD SHOCK. Keep this in mind if you are hooking the inverter to
something wired external. Some of the Xantrex Modified Sine Wave Inverters
,Prowatt,and Portawattz and their Prosine series,have this with grounded
natural output, and EXELTECK seem to also has a good grounded AC natural
with a clean sinewave output. I do not know about Samlex sinewave series but
I am looking for more input.

Bill Kaszeta
Photovoltaic Resources Int'l
Tempe Arizona USA
[email protected]

 

[fs]

Thanks for your input Bill as I have found the same from allot of the other
types. In my old manual the inverter never mentioned anything about any
external hookups. My 1500w Portawattz modifed sinewavw inverter is one of
the best I have seen in the build quality, manual, and also having the
output GFCI protected. Very good Unit . Now I am trying to find a lower
wattage sinewave unit ( 250-400) that is not too big but with a safe output,
netural grounded. I think ELEXTECH
makes it , but you pay the cost.
MY wife or others that might come along and might use it in a camping trip
I fell safer.

It comes down to cost in building them,H bridge outputs are a little cheaper
to build but parts cost ( cost in China $.35- $1.00 per unit) is just a
small amount for safety, maybe just a few more parts could have done it
right.

 

[Scott Willing]

Thanks for your input Bill as I have found the same from allot of the other
types. In my old manual the inverter never mentioned anything about any
external hookups. My 1500w Portawattz modifed sinewavw inverter is one of
the best I have seen in the build quality, manual, and also having the
output GFCI protected. Very good Unit . Now I am trying to find a lower
wattage sinewave unit ( 250-400) that is not too big but with a safe output,
netural grounded. I think ELEXTECH
makes it , but you pay the cost.
MY wife or others that might come along and might use it in a camping trip
I fell safer.

It comes down to cost in building them,H bridge outputs are a little cheaper
to build but parts cost ( cost in China $.35- $1.00 per unit) is just a
small amount for safety, maybe just a few more parts could have done it
right.

FYI the Samlex sine wave units are transformer isolated, and at least
some of their smaller non-sine units are as well. I bought a refurb
Samlex 600W non-sine to replace a 300W non-isolated unit for my
friend's little cabin set-up.

I have a Samlex 600W sine unit that I bought just to get a little
clean sine power for some audio applications. Then almost immediately
an opportunity arose to get into a sine inverter big enough to handle
the whole house, but I haven't wired it in yet.

The Samlex has been doing a fine job and is a real bargain IMHO. The
only precaution is that, if you're using temperature-compensated
battery charging and your batteries are subjected to cool
temperatures, the Samlex's max input voltage of 15.0 is going to be a
problem. It squawks and shuts down, but according to the manual it can
still be damaged if overvoltage persists. Some renewable energy
equipment dealers hammered this point home to Samlex until they
finally got the joke, and they will likely update the design to
provide higher max input voltage in future if they haven't already.

In an RV / cabin / non temp-compensated charging scenario this
wouldn't be an issue.

Cheers,
-=s

 

[Robert Morein]

[snip]
I do not know about Samlex sinewave series but

I am looking for more input.

I just checked my Samlex S1500 sinewave unit, and neutral is bonded to
ground.

 

[Robert Morein]

[snip]
I do not know about Samlex sinewave series but

I am looking for more input.

I just checked my Samlex S1500 sinewave unit, and neutral is bonded to
ground.

The AIM inverters will either smoke or shut down if there is a connection to
ground.