Inverter Topologies

[travelite]

Folks,

The evolution of DC-AC inverter technology over the years has left many of us dazed and confused. The attached article from Trace Engineering (circa 2000) describes each topology along with their advantages and disadvantages. Amazing that after 23 years the venerable Trace SW series still competes well with the latest offerings from Magnum, Outback, Victron and the others in the mobile market. In fact, the only inverter that really competes with the SW in terms of surge capability, total harmonic distortion, idle power consumption, voltage regulation, and efficiency is the Hybrid style inverter, such as Magnum's MSH4024M. The only real relative disadvantage of the Trace SW technology is cost to produce.


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[Gil_J]

David,

Great information on inverter technology. Even for those that find the document to in-depth, it raises the awareness that not all inverters are created equally.

For those of us Trace 4024 inverter/chargers, and it's the vast majority of us with late 90s and newer coaches, the only possible drop-in replacement inverter/charger is the Magnum MSH4024RE you referenced. I say possible because I'm not sure what each converter may have used the extra relays for. If your Trace 4024 is your generator autostart trigger, then you will need a new AGS unit as well. That's no big deal given the Magnum unit is far superior to the Trace trigger or anything developed by a converter.

Trace 4024 owners do have other alternatives that will vary based on how your AC distribution is wired and how your converter sheds AC loads when you are on inverter supplied powered rather than shore or generator power.

[Gil_J]

I should add that most service centers probably aren't looking at the Magnum MSH4024RE because it's designed for the renewable energy market. Although I haven't went through its configuration options, I suspect it has features we would never use and shouldn't. Otherwise, its features are comparable to our Trace 4024 units.

[travelite]

Unfortunately neither the Magnum MSH4024M (Mobile) nor the MSH4024RE (Renewable Energy) are drop in replacements for the Trace SW's used in my Vantare. The MSH4024M has only one AC-in and one AC-out whereas the Trace SW has two AC-ins and one AC-out. This means the Magnum won't phase-shift from shore to generator power transitions, or vice versa, like the Trace does. IOWs, house power will be interrupted during transitions whereas with the Trace house power is transferred seamlessly by the SW inverting during the phase-lock period. Turning to the MSH4024RE; this inverter doesn't have an internal DPDT relay to bond neutral to ground as required by code in mobile applications. This bonding of neutral to ground is a feature incorporated into the Trace SW MC2 versions. Code requires bonding to occur at the power generating device such that the bond is made at the moment power is provided. I'm afraid there's nothing currently in the mobile inverter market that solves these issues to the extent that the Trace SW does.

[Gil_J]

David,

Great catch on the RE. There's good reason to limit the bonding of neutral and ground at the service entrance. That would be some point before or at the shore power plug when using shore power, at the generator when using generator power, and at the inverter when using inverter power.

From a practical standpoint, all 3 could exist with little risk. Shore ground has no potential to generator or inverter ground. Some could argue the ground strap does provide a path between the two. That path has so much resistance I don't believe it could lead to a ground loop. Inverter ground and generator ground are both bonded to the chassis. Therefore, the resistance between the two grounds should have so little resistance that they are one in the same.

Fortunately, some wiring schemes can support more common inverters. Although the features of the Trace SW series are highly beneficial, there will come a day when they are no longer available. Maybe, before that day comes a fully complint drop-in replacement will become available.

[travelite]

Frankly, I wonder how many converters get this neutral to ground bonding issue right. The issue as I see it is neutral current finding an alternative path back to the source via two or more neutral to ground connections (a loop), one at the source and one at the generator for instance. If this path has high impedance than voltage potential exists along the ground path; i.e., hot skin. I'm glad my coach is plastic! LOL! Regardless of what we think, the code is clear and without question it behooves the converters to follow the code.

I need to correct what I said early. I inspected the Trace SW MC2's I have in my possession and I don't see any relay controlled neutral to ground bonding/switching capability. As with the MSH4024RE, this needs to be done externally.

[Gil_J]

I'm betting the converters are depending on the owners. The ground loop to the generator, in my opinion, is a non issue. The likelihood of a difference from the inveter's ground attachment to the chassis and the generator's attachment to the chassis are slim. As long as the shore cable is unplugged, the other ground loop is eliminated.

I've recently worked on 3 Trace 4024 inverters on two different conversions and can tell you all had the 3 neutral connections bonded. You can see the bonding plate electrically making all neutrals one.

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Switching neutral also has some risks. The biggest risk is extensive damage to electrical systems due to the potential for excess voltage. If neutral is going to be switched, there really needs to be a detection circuit to verify neutral has actually switched.

[travelite]

Gil,

Your pic is identical to my MC2's here at the house. I still need to see how my Vantare handles this. Having the neutrals all tied together is a dead giveaway that the MC2 is not bonding neutral and ground within the inverter via an SPDT relay thereby switching ground and neutral simultaneously. This function needs to be implemented external to the inverter. Sigh... :)

From the Trace manual:

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