Cheap Centech Inverters. Stackable?

[Tedd]

I have a pair of Centech (yeah ... I know. Harbour Freight) 2000/4000 inverters.

Does anybody know if they are "stackable" ... or if their 120VAC hots and commons can be joined together at appropriate bus bars in the AC panel?

http://legacy.outbackpower.com/pdf/t..._Explained.pdf

1. After all I've read, I'm pretty certain that all the GROUNDS can go to the chassis.

2. After all I've read, I THINK I want to join all the commons from the inverters to one leg of a transfer switch which will lead to the common bus bar in the AC Panel. The other leg of that transfer switch will be fed by the output from the transfer switch for the common from Shore Power and Generator.

Refer to the WONDERFUL diagram from Familywagon found at the top of page 16 of this thread: http://www.skoolie.net/forums/f10/ho...ac-448-16.html

Thanks!

T

[Dapplecreek]

If it were DC voltage, you might be able to get away with it - like running batteries in parallel.

But you're dealing with alternating current, meaning the hot wire could be anywhere from about 150 V to about -150 V (roughly - it comes out to 120 volts or so when it's kinda-sorta-averaged). So you could spectacularly blow things up if it happened to be off just right, or merely start a fire and have minor explosions if it were just a bit off.

If I'm understanding your question. Others may wish to chime in.

HOWEVER, you could certainly use one inverter for the front half of the bus and the other for the back half. Or left and right. But be sure they're separate all the way back to the inverter.

[ashandrik]

The only way you can "stack" the AC side of inverters is if they are grid-tie inverters. Those sense the phase of the AC wave and sync them together.

[Tedd]

Originally Posted by Tedd

2. After all I've read, I THINK I want to join all the commons from the inverters to one leg of a transfer switch which will lead to the common bus bar in the AC Panel. The other leg of that transfer switch will be fed by the output from the transfer switch for the common from Shore Power and Generator.

After re-reading this, I see I've made a HUGE error. It should read:

After all I've read, I THINK I want to join all the HOTS (Black Wire) from the inverters to one leg of a transfer switch which will lead to the HOTS (Black Wire) in the AC Panel. The other leg of that transfer switch will be fed by the output from the transfer switch for the HOTS (Black Wire) from Shore Power and Generator.


Here's the diagram I referenced earlier.:

Originally Posted by family wagon

Since there haven't been any other replies, I'll take a swing. Here's a block diagram for "some day." Protection devices (fuses, over-current/arc/ground-fault breakers) are omitted for clarity.

ALL of the commons would go to the common bus bar ... all of the grounds would go to the chassis.

[Tedd]

Originally Posted by Dapplecreek

If it were DC voltage, you might be able to get away with it - like running batteries in parallel.

But you're dealing with alternating current, meaning the hot wire could be anywhere from about 150 V to about -150 V (roughly - it comes out to 120 volts or so when it's kinda-sorta-averaged). So you could spectacularly blow things up if it happened to be off just right, or merely start a fire and have minor explosions if it were just a bit off.

I'm definitely into trying to avoid a fire.

I am pretty sure that I could do it with VDC. In a VDC circuit, if I connected everything in series, I wouldn't be messing with volts ... I'd be increasing amps. I'd have to up the wires to handle the load ... but otherwise ... no problem.

Originally Posted by Dapplecreek

HOWEVER, you could certainly use one inverter for the front half of the bus and the other for the back half. Or left and right. But be sure they're separate all the way back to the inverter.

Yeah ... that's kinda what I was afraid of.

Unfortunately, I want to work shore power and power from the genset into the system. I fear a setup like this would require TWO transfer switches and TWO VAC panels. Messy and complicated.


Lets see what other VAC mavins have to say ...

[Tedd]

About stacking inverters, I included this in my first post:

http://legacy.outbackpower.com/pdf/t..._Explained.pdf

This .pdf seems to suggest that inverters CAN be stacked ... but VAC is tricky and not something I deal with alot ... I'm a DC kinda guy. The ,pdf could be total BS, after all, I found it on the internet!

I figured I'd throw it out here to the experts.

[family wagon]

Yes, some inverters, even non-grid-tie inverters, can be stacked. Consider parallel-capable inverter-based generators and this "GTSUN" inverter I dug up on Amazon. Unfortunately it doesn't seem to be a very common feature.

The suggestion from Dapplecreek to divide the bus so that each inverter handles only part is interesting. If you had just two inverters, you could use a conventional 120/240 volt breaker panel for the distribution and wire one inverter to each hot leg in that panel. It would only add one more switch to the diagram quoted earlier. Add a switch for the second inverter, just like the one shown between the first inverter and AC panel. The "upstream" side of the two switches, going to the shore/generator selector, could be tied together. In this way the two hot legs in the panel would be isolated when fed from inverters, but tied together when fed from other sources.

[Dapplecreek]

Originally Posted by family wagon

... you could use a conventional 120/240 volt breaker panel for the distribution and wire one inverter to each hot leg in that panel....

But if you do that, whatever you do, DON'T TRY TO ADD A 220V BREAKER! And I would worry that the next owner might try it... this is why we have codes, and why we all should at least be within horseshoe distance of 'em - or better. Prevents time bombs.

[family wagon]

It's an interesting thought exercise to wonder what might happen if one did try to use a two-pole/240v breaker.. when connected to the shore/generator power with the two hot legs tied together the load wouldn't do much of anything. Supposing it were a range, oven, or clothes dryer, then the controls might come on but the appliance wouldn't heat. On the other hand, when supplied by the inverters whose phases aren't synchronized, the controls would probably power on as before and there'd be some heat but not so much as would be expected. It would vary from full heat to no heat as the phase difference between the inverters varied. A resistive heater might not mind that so much, but if instead the load included a 240 volt motor then operation would be strange and possibly damaging to the motor.

I agree that it's non-conventional and would require leaving something written in the panel to explain the situation. On the other hand it does avoid having two breaker panels (which can have its own set of "I thought I turned off all the breakers!" problems). Also it seems 120/240 volt main lug load centers are much easier to find than the 120 volt-only type are.

[Carytowncat]

Originally Posted by ashandrik

The only way you can "stack" the AC side of inverters is if they are grid-tie inverters. Those sense the phase of the AC wave and sync them together.

By stacking you mean to tie the outputs together, correct?

These harbor freight (i shop there often) inverters and most others do not sync up.
One may be outputting a low phaze (-60v ) while at the same time another is outputtings a high phaze (pos 60) equalling zero volts. Surely they will be pushing and pulling on each other and destroy themselves and electonic attachments. It's a phaze thing, AC is complex.

[aaronsb]

Unless they are specified to sync their sine signal, I think there is an element of danger here.

The neutral wire in an AC circuit is intended as the return path to the current source. With a pair of non synchronized inveters you are creating the distinct possibility of some sort of bastardized 2 leg ac system, especially if you share or tie neutrals intentionally or not at any point.

An actual 2 leg common neutral system has the current out of phase, thus as one current path crosses the voltage drop threshold, the other rises, utilizing the "slack" in the current use.

This would give you a reliable 240 volts between your two legs. If you have the bastard system with the phasing wrong, you are possibly sending far more current on your return neutral than the wire is specified to load. This will fry items powered from the line, and possibly set the wiring on fire by overloading your neutral since its not a neutral anymore.

Don't do it, unless you can ensure it is being done right.

[turf]

have the rules of grounding AC changed?

imo, an AC ground needs to be a wired floating ground on the system. not a system grounded to the 12v vehicle. with out a ground rod, there is no place for the current to go.

i can only see the probability of this arrangement hot skinning the bus any time the ground circuit is energized.

and with the half phase of the set up, i think that means hot skin most of the time.

[Tedd]

Originally Posted by turf

have the rules of grounding AC changed?

imo, an AC ground needs to be a wired floating ground on the system. not a system grounded to the 12v vehicle. with out a ground rod, there is no place for the current to go.

i can only see the probability of this arrangement hot skinning the bus any time the ground circuit is energized.

and with the half phase of the set up, i think that means hot skin most of the time.

Mike Sokol explains why this is not correct in his book RV Electrical Safety and for us he explains this starting on page 9, post 88 of this thread: http://www.skoolie.net/forums/f10/ho...ac-448-14.html

I'd read the thread from page 9 to the end.

ALL grounds go to the chassis.

[Tedd]

Originally Posted by family wagon

The suggestion from Dapplecreek to divide the bus so that each inverter handles only part is interesting. If you had just two inverters, you could use a conventional 120/240 volt breaker panel for the distribution and wire one inverter to each hot leg in that panel.

It would only add one more switch to the diagram quoted earlier. Add a switch for the second inverter, just like the one shown between the first inverter and AC panel.

The "upstream" side of the two switches, going to the shore/generator selector, could be tied together. In this way the two hot legs in the panel would be isolated when fed from inverters, but tied together when fed from other sources.

I understand. Keep the hots (black wire) from the inverters separate, but join ALL the neutrals (white wires) to the neutral bus bar in the panel and run grounds to the chassis.

One more switch, a panel and a TWO 20 amp breakers.

I think I can even draw it.

[Tedd]

Just for clarity ... we are ALL agreed, even me, that "stacking", or bonding the hots (black wire) from the inverters is a big NO-NO!

Don't do it.

[aaronsb]

With that said, I'd be OK with inverters supplying separated circuits. I would require independent disconnects for the circuits when connecting to shore power. (Some savings is lost by doubling your disconnect hardware)

Originally Posted by Tedd

Just for clarity ... we are ALL agreed, even me, that "stacking", or bonding the hots (black wire) from the inverters is a big NO-NO!

Don't do it.