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Inverter with "best" surge capability for fridge startup consumption

Sipma02

Sipma02

New Member
Joined
May 27, 2020
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141
Hello all,

I'd like to tap into everyone's experience here to see if there is any general knowledge of an inverter with a strong surge capability. I'm hoping to run a fridge which, upon startup, draws about 1000-1100 watts (as measured by my Kill-a-watt meter). When running, it's rarely above 80-100 watts! My thought is to buy an inverter with around 500 watts of nominal power output, with a surge of 1000. Otherwise, is there a way to modify an inverter for higher surge capacity or bypass the high draw shutdown? I'm fairly handy with a soldering iron and could potentially replace a component or two.

Any other ideas to use a more efficient (smaller and cheaper) inverter? Maybe some way to use one inverter to "start" the fridge, and one to keep it running?

The inverter will be dedicated to the fridge, no other loads.
 
Personally, I would not trust that the kill-a-watt is actually capturing the surge or in-rush current unless it explicitly states it has this capability.

Second, small/cheap/light inverters are high frequency (HF) inverters with multiple small transformers. A 500W HF inverter may say it has a 1000W surge capability, but what they don't tell you is that the surge can only be maintained for milliseconds.

If using a cheap HF inverter, size it according to the inrush/surge, not the running load. IMHO a 500W unit would probably not suffice and you should go for a 1000W continuous unit.
 
Only thing I can think of to save max power is have thermostat turn of and on inverter when needed
 
smoothJoey said:
Get one of these https://www.amazon.ca/Klein-Tools-CL800-Auto-Ranging-Digital/dp/B019CY4FB4
and one of these https://www.amazon.ca/Line-Splitter-Klein-Tools-69409/dp/B01B7RTNSO
Then you will have a very good idea what size inverter to get.
Click to expand...

I actually just got gifted that exact same clamp meter from my dad, a career electrician! I will try using that.

Edit: Joy also diminished.

snoobler said:
Personally, I would not trust that the kill-a-watt is actually capturing the surge or in-rush current unless it explicitly states it has this capability.

Second, small/cheap/light inverters are high frequency (HF) inverters with multiple small transformers. A 500W HF inverter may say it has a 1000W surge capability, but what they don't tell you is that the surge can only be maintained for milliseconds.

If using a cheap HF inverter, size it according to the inrush/surge, not the running load. IMHO a 500W unit would probably not suffice and you should go for a 1000W continuous unit.
Click to expand...

That's good to know. Is there any way to increase the capacity of said inverter? What limits them for only a millisecond, software or hardware?
 
Sipma02 said:
I actually just got gifted that exact same clamp meter from my dad, a career electrician! I will try using that.

Edit: Joy also diminished.



That's good to know. Is there any way to increase the capacity of said inverter? What limits them for only a millisecond, software or hardware?
Click to expand...

Yes. Return it and buy one with a higher continuous rating. HF inverters don't have high surge because they can't, not because the manufacturer chose to make them that way. It's the big single heavy transformer in the low frequency (LF) inverters that provide excellent surge.
 
To measure in-rush, you need a specific in-rush feature. A "max" capability won't cut it. But, if the fridge is one of those new hyper-efficient inverter driven fridges, then there is no in-rush (or it's minimal).
 
snoobler said:
It's the big single heavy transformer in the low frequency (LF) inverters that provide excellent surge.
Click to expand...

Yet I'm not sure why.
Except for some self-resonance between L and C holding up the output for a few milliseconds, any power needs to be drawn from battery as current through transistors.
Maybe those LF inverters just have more power transistors.

Except for the older "modified sine wave" inverters that switched 12V DC into a transformer, everything is high[er] frequency. That is how you synthesize sine wave, e.g. my VFD operates at a user selectable frequency up to 15 kHz, applying pulses of varying with from 400 VDC rail (rectified line) to motor, creating a sine-wave current at zero to 60 (or up to 400) Hz.

You can send power through big iron at low frequency, or much smaller iron at high frequency.
A high frequency inverter with transistors and inductors capable of supplying the surge current would do it, and continuous rating of the inverter could be lower due to thermal resistance, junction to ambient.

The other implementation decision (HF vs. LF, I believe) is whether you put a battery-voltage sine wave into a step-up transformer, or use a boost converter to make a line-voltage sine wave. And then there are high voltage battery inverters, using 400V lithium. Those don't need to step up the voltage.
 
Don't forget peak power for defrosting cycle. Some refrig's have high power heater (1 kW) with short duty cycles, some are lower power (300 W) that stays on longer.
 
svetz said:
.... But, if the fridge is one of those new hyper-efficient inverter driven fridges, then there is no in-rush (or it's minimal).
Click to expand...
Yes, maybe instead of worrying about inverter overhead, consider the energy savings of a new or used inverter driven refrigerator with no startup surge. Long term you will be money ahead.
 
Last edited:
RCinFLA said:
Don't forget peak power for defrosting cycle. Some refrig's have high power heater (1 kW) with short duty cycles, some are lower power (300 W) that stays on longer.
Click to expand...

Good call. This fridge doesn't have a defrost cycle, so one less thing to think about!

Ampster said:
Yes, maybe instead of worrying about inverter overhead, consider the energy savings of a new or used inverter driven refrigerator with no startup surge. Long term you will be money ahead.
Click to expand...

Also interesting. I did a quick Google search for "inverter driven fridge" and "inverter fridge". Not much, save a few articles. How can I search or find those?
 
My Samlex sine wave inverter manual lists sizing factors. For refrigeration; "Multiply the Running Active Power Ratting (Watts) of the appliance by the Factor of 5 to arrive at the Continuous Power Rating of the inverter for powering this appliance." The Running Active is the Watts on the appliance tag. Not what you measure or guess at. Also note the word Continuous Rating of inverter. I can also recommend those new "soft start" devices for any compressor.
 
Since the compressor motors are small for refrig they typically have a PTC thermistor in series with their start capacitor. As it gets hot it goes to high resistance taking starter cap effectively out of circuit. PTC is thermally clamped to compressor housing to keep it hot.

The lowest cost solution is a 2kW inverter that can be fairly sure to make startup surge. Just make sure you give it time to equalize refrigerant and let the PTC thermistor and compressor to cool off between run cycles.
 
Last edited:
RCinFLA said:
Since the compressor motors are small for refrig they typically have a PTC thermistor in series with their start capacitor. As it gets hot it goes to high resistance taking starter cap effectively out of circuit. PTC is thermally clamped to compressor housing to keep it hot.
Click to expand...

Wow! What the purpose of the PTC thermistor?
 
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