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Inrush protection ac side

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Boron

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Oct 26, 2023
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UK SE Kent
Hi Guys
I dont wish to hijack @milopalmer thread

EG4 6500 Inrush Amp Limit​

but my post is related - hope this is proper etiquette

Has anyone made use of thermistor devices (eg Ametherm or others out there) intended for current inrush protection on the load ac side - typically for small domestic appliances, eg fridges, freezers, old style ballast fluorescent tubes, filament lamps, Microwaves, and so on. These can have surprisingly high inrush of 6 to 10x fl current for half cycle 10ms enough to upset HF inverters or otherwise stress LF inverters that may be undersized (these very rapidly increase in cost with surge capacity). Cheap local thermistors might help?

Anyone experienced this here?
Boron

OBTW this may not be applicable to crude AC units as these are beasts
 
chrisski said:
If you look in this resource on this forum, you'll see @FilterGuy has used a PTC in a pre-charge switch. In the Download button top right of the page, you can download the schematic.
Click to expand...
@chrisski Precharging the caps on the INPUT of an inverter is already recognised as necessary and devices are out there to deal with this. My post deal specifically with the ac OUTPUT of the inverter
 
The devices you list might DEPEND on getting the high inrush current they need to start up. Screwing with it may have interesting and unintended consequences.

Might be better to save up and buy an inverter that can supply what's necessary in the first place.

Most cheap (non-inverter) fridges already have a PTC "relay" internally to handle startup.
 
My first concern with those thermistors is they run HOT. Need to be mounted where that is OK.

I've measured capacitor inrush of SMPS and transformer inrush at work. One time we turned off a machine, and when turned back on it tripped the breaker. Work-around was to turn off several switches in the machine, turn on main power, turn on the several switches one at a time. I've got a power strip (PDU) that sequences its outlets.

Is the inrush actually a problem for your system? Maybe you can reduce the extreme peak sufficiently with just some resistance, even length of wire. Or a thermal-magnetic circuit breaker, which has some built-in resistance.

For grid connected systems, the usually recommendation is to size breakers sufficiently. Especially for transformers, which have variable inrush depending on timing.

My greater concern is power factor. The diode-capacitor front end of VFD is enough to upset my GT PV inverter, when operating off-grid. I want PFC front end. We built a test setup with TDK-Lambda PFC modules, which draw synthesized sine wave current from input. They would also have inrush, until their capacitors are above Vpeak of line, so they have a relay and precharge resistor (either internal to module or external, depending on model.)
 
v_green57 said:
The devices you list might DEPEND on getting the high inrush current they need to start up Yes indeed, more work needed. Screwing with it may have interesting and unintended consequences Yes probably not start and the PTC will burn out as per spec - in secs.

Might be better to save up and buy an inverter that can supply what's necessary in the first place. Thats the purpose of my post - to economise on the cost and size of an LF inverter by using a HF type for the sake of a $1 PTC thermistor - maybe.

Most cheap (non-inverter) fridges already have a PTC "relay" internally to handle startup. Really, I knew it had a contactor but I didnt know it had a PTC element as well hmm thnx, I wonder how effective it is?
Click to expand...
I guess RV guys would know about being careful what piece of kit to start up and in what order, but a domestic household with kids? Solarists with deep pockets can afford BIG inverters $$$$ :ROFLMAO:
 
I'm getting hung up imagining a guy trying to living off grid but just loves his old florescent lights too much.
 
The PTC element IS the contactor.

Google this:

Refrigerator PTC relay wiring diagram

for more info.
 
Hedges said:
My first concern with those thermistors is they run HOT. Need to be mounted where that is OK.
Click to expand...
Yes they do - 150C even, but then so do power resimy hf inverter lyena 500W.jpg

I bought one of these 500W sine wave PRC jobs £60 in panic at the thought of being isolated by grid power cut during Covid. I only intended it to keep my router powered and run my laptop to stay on line ca 40W max. I tested the system but never had to use it for real, so its on standby with my 24V LAB battery and will give me 10 hours allegedly - well I was pressed for time and our grid supply is very reliable normally. It does say specifically to uprate inductive loads, refrigerators etc 10x so my kitchen fridge 150W would need 3x size of what I have so you the cost benefit begins to pale when faced with that criterion - hence my post about PTC thermistors.
BTW a reputable 3kW LF inverter starts at £1500 (vs £150 for a 3kW HF inverter) my household base load is 250W (after drastic economy measures following the Ivans assault on our gas supplies)
So If I could devise some thermistor help for my fridge and freezer hence my post

Only my wash m/c might be a problem but I have an OH with a wash board and tub for that - she mourns the demise of the village square with its public fountain and stone slabs where all the wimmin gathered to gossip every morning - sigh. Real off-gridders have washing lines and a Mountain Girl heh heh?
 
Yep, the 2.7 c/f Haier fridge I use when camping only needs 55W in "run" mode, but needs a 1kW inverter to start it.
 
v_green57 said:
The PTC element IS the contactor.

Google this:

Refrigerator PTC relay wiring diagram

for more info.
Click to expand...
Now you see, a half truth here, the relay is a contactor and a PTC Thermistor connection goes to the start winding, so as it heat up it de energises the start winding. That all makes sense. It also shows that this PTC relay is not there to limit current inrush as you first implied.

Hope everyone understands this

More work need here
 
Boron said:
Now you see, a half truth here, the relay is a contactor and a PTC Thermistor connection goes to the start winding, so as it heat up it de energises the start winding. That all makes sense. It also shows that this PTC relay is not there to limit current inrush as you first implied.

Hope everyone understands this

More work need here
Click to expand...

Yes, correct. I was not clear on that.
 
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