Inverter Overloading

[howson]

So I ran into an unanticipated operational issue with my inverter setup--if shore power is lost it is very easy to overload the inverter. (Remember that a 3000VA inverter puts out 25amps of 120VAC.) Twice during a recent trip shore power shut down (campground lost power) and the inverter overloaded due to the air conditioners running and the washer and/or dryer running.

Below is a block diagram of my current setup. What the reader needs to understand is AMSolar's SmartATS (now called Smart Phase Selector, or SPS) allows power to ALL the circuits in the camper (both 120V lines in my 50A camper) so it is easy to inadvertently overload the inverter when power is suddenly lost. Just turning off the inverter is not an ideal solution because when I'm at a site with a 30A pedestal the inverter has the capability of assisting shore power when needed. (It's a very cool feature of the Victron inverter.)



Bottom line is I didn't anticipate this problem. The fix (if there is one) must meet the following criteria:

I want...
1) Full capability to use the Victron inverter's assist mode when shore power (through the external power connector) is available from a functioning 120V power source. (Worked great twice on a recent trip when connected to 30A sites.)

2) When shore power is lost (unintentionally) I want some circuits to disconnect automatically.

3) When shore power is disconnected intentionally, I want the ability to power circuits that are normally powered down when shore power is lost.

Here's why...
a) When towing, I want the refrigerator powered by the inverter (intentional loss of shore power). The refrigerator will *not* run on propane since I close the propane bottles prior to departure--the reason I installed an inverter in the first place!

b) When stationary, I want the refrigerator to switch automatically to propane when shore power is lost unintentionally (to save battery/inverter power). I open the propane bottles upon arrival, so the refrigerator will do this automatically when ac power is lost.

c) When shore power is lost unintentionally and we're away from the camper, the inverter is an emergency power source for the 13.5K bedroom air conditioner. The ac is set on low, auto and 72 degrees. By doing this my dogs don't end up trapped in a super-hot RV if shore power quits, at least for a couple of hours. (We're never gone more than 4 hours when we have the dogs with us.)

d) When we're at the trailer and shore power is lost unintentionally, I want high-amperage devices immediately turned off to keep our power load under the inverter's maximum output. The DW and I will then manually unplug chargers and anything else to maximize inverter run time. (We actually did this last trip when power was lost at night. The inverter kept the bedroom livable until power was restored.)

Any suggestions, feedback or issues I'm not considering?

Thanks in advance-
Howard

[Cate&Rob]

Hi Howard,

The concept that you are looking for is called "load shedding" and is very common in back-up generator systems . . . large and small. The idea is just as you describe . . . when the high capacity main power goes out, the lower capacity back-up power only supplies the selected critical devices. If you google "load shedding device" I expect that you will find one to suit your needs. Some rewiring will be required . . . but, you are good at that .

Rob

[OffToHavasu]

Howard,

This is what GD uses in the Momentum line for load shedding:

https://precisioncircuitsinc.com/cat...nagement-120v/

[howson]

Howard,

This is what GD uses in the Momentum line for load shedding:

https://precisioncircuitsinc.com/cat...nagement-120v/

THANK YOU! I know what I'll be reading for next several hours.

[howson]

I probably overlooked something but I could not find a load shedding device that will do everything outlined in the "I want..." section of post #1. The PDF attached shows what I intend to install. Should be interesting.

Wiring Schematic v4.pdf

It won't be for a week or so, but once everything is installed I'll follow up with any lessons learned and (hopefully) a video showing that it works. The alternative is you'll get a tale of woe on what I didn't anticipate. (Ugh.)

[howson]

Note the configuration below is for testing only!

Did a test today--it works!

For those that haven't read this entire thread the idea is when external power is lost the relay keeps 120V from passing to the L2 circuit. Without the relay the Victron inverter will attempt to run all of those components driving it into an overload condition.

For those that are curious and want more details, read on. (Don't miss the videos at the end.)

Here's what I'll be referencing:



For the first test I used the WFCO Converter as diagrammed in previous schematics but found it took a considerable amount of time (>5 seconds) before the converter's output dropped enough to turn off the Crydom Solid State Relay (SSR). I also found the WFCO was way too much of a power supply for an SSR that will draw 12mA. (At the most--normal draw is 7mA!) I'll have four of these in the final system but that is still only 48mA at the most.

What I ended up using was an old USB charger (I snipped off the USB end) rated for 5V at 150mA. The SSR will work with voltages from 4V to 32Vdc. I confirmed by testing the USB charger controls the SSR (closes and opens the relay as expected).

I powered the USB charger's 120v input by adding wires to the EMS' output that had a standard female 120V AC connector on the end. The charger plugs into the female connector. The DC output is wired into the SSR. (This is temporary--still waiting for the circuit breaker components to arrive in the mail.)

The video linked below shows what happens to the inverter WITHOUT the relay in the circuit. The following items were running during the test:
Refrigerator (on electric)
Hot water heater (electric)
Both air conditioners
Clothes Dryer

Here's what happens WITH the relay in the circuit. The refrigerator was not on but the other items were on as in the first test.

Still more testing and configuration to do--I'll update when complete.

-Howard

[howson]

Please note this is not a time-tested installation or configuration. If you choose to copy what I've done you do so at your own risk!

30 Aug 2020 Update: I now consider this mod "time tested"--it works exactly as expected and designed. Please remember I'm not an engineer, electrician or certified in any way, shape or form so if you copy what I've done you do so at your own risk!

Install is complete. To keep this post short see the PDF then watch the two videos.

Wiring Schematic v6 Small Version.pdf

Video showing the wiring (before I closed up all the panels)

Video showing an actual test showing what happens when shore power is lost:

Any questions--fire away.

[Gyrogearloose]

Hey Howard,

Love solid state relays...... Question: Are you using appropriate sized heat sinks with these and thermal paste? They are derated considerably without heat sinks.

Pat

[howson]

Hey Howard,

Love solid state relays...... Question: Are you using appropriate sized heat sinks with these and thermal paste? They are derated considerably without heat sinks.

Pat

The SSRs each have a thermal pad attached specifically designed for this purpose https://www.onlinecomponents.com/crydom/hsp2-45159125.html

I did not get the specified heat sink--I am using the metal case of the SmartATS for that purpose. The SSRs are bolted to the SmartATS (bonded by the thermal pad).

I don't "know" if this will actually work--I guess I'll find out soon enough. The 50A SSR is the one that will fail first if there's an issue. (Kind of like running a computer CPU without a proper heat sink with thermal paste--you can fry that thing in a heartbeat!)

[Gyrogearloose]

I did not get the specified heat sink--I am using the metal case of the SmartATS for that purpose. The SSRs are bolted to the SmartATS (bonded by the thermal pad).

Cool..(I think I just made a play on words).... Then if the SmartATS has an aluminum case and internal fan and/or it looks like it could be top vented natural draft, it might work out. Make sure the 50amp one is easy to get to.

Pat

[howson]

Cool..(I think I just made a play on words).... Then if the SmartATS has an aluminum case and internal fan and/or it looks like it could be top vented natural draft, it might work out. Make sure the 50amp one is easy to get to.

Pat

I've already got an "emergency fix plan" ready just in case. I'll tie together the "in" and "out" on the relay effectively eliminating it from the circuit. Then I'll get a higher rated SSR (90A?) along with the proper heat sink so there's no more doubt. (I rarely do something just once!)

[howson]

7 Nov 19: Follow-up on this issue for those that may be interested: this setup is working perfectly. Shore power kicked off at least three times while camping during my recent two month trip. In each case the relay system performed exactly as intended. The inverter was able to handle the momentary power surge requirement until the relays turned off the circuits that aren't supposed to run on inverter-only.

With the "tow" mode I was able to run any system I wanted on demand. (We stopped at a "Cell Phone Rest Area" in New York, and made breakfast with all Fran's appliances running off the inverter. It was awesome.)



Very, very pleased with the results.

[Nordicsoul56]

Hi Howard,

Your posts, diagrams and content are terrific.

Maybe I need to start another thread but it seems the Crydom relay idea might just be able to replace the SPS as I see it?

With the way you have the Solid State Relays(Crydom's) wired the SPS appears to no longer be needed.

Seems it would work the same whether a 50 amp service or a 50 to 30 amp dogbone (or plugged into a generator) is used.

I'd be happy to cancel this post and start a new one...

30+ years Metrology & 13+ years Test Engineering experience here.

What do you think?

Best regards,

Jon

[howson]

Hi Howard,

Your posts, diagrams and content are terrific.

Maybe I need to start another thread but it seems the Crydom relay idea might just be able to replace the SPS as I see it?

With the way you have the Solid State Relays(Crydom's) wired the SPS appears to no longer be needed.

Seems it would work the same whether a 50 amp service or a 50 to 30 amp dogbone (or plugged into a generator) is used.

I'd be happy to cancel this post and start a new one...

30+ years Metrology & 13+ years Test Engineering experience here.

What do you think?

Best regards,

Jon

Hi Jon,
First, thank you for the kind words and welcome to Grand Design's Technical Forum. There's quite a few (retired) engineers here (not me!) so you'll fit right in.

I'm reading your post early in the morning so my brain is still a little foggy--what does "SPS" reference?

EDIT: Now that I'm awake (and I was working on a schematic) it accorded to me that "SPS" is AMSolar's Smart Phase Selector. The relays do not, IMO, alleviate the need for the SPS. Without the SPS only one leg of the camper's 120v circuits will have access to power from the Victron inverter.

Howard

P.S. Continuing this thread is fine.

[bertschb]

Holy cow Howard! You take electrical mods to a whole new level. My wife thinks I'm a genius when I add my Progressive Industries EMS and re-do the wiring in the battery compartment. No way I'm showing her your work :-)