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How can I reduce or supress the in-rush current for an inverter?

I have a pair of 100 watt solar panels feeding into a solar regulator (TPS 1230) which is connected to a 12v 100 Ah lead acid battery and a 2Kw inverter connected to a 50watt fridge.

The solar regulator claims to be ’10 amp’ and the inverter draws 4-5 amps when the fridge is on. So, in theory the two ought to be compatible. In practice whenever the fridge switches itself on and the inverter kicks in then the solar regulator trips out and needs to be manually reset.

To cure this I tried upgrading to a 30 amp regulator (MPPT30 like this), the good news is that this one only trips out 50% of the time, the bad news is that it still trips out 50% of the time.

How can I cure this? My guess is that the regulator is tripping out due to a large in rush current when the inverter is switched on, I can measure the steady-state current (about 4-5amps) with a multi-meter, but have no idea what the in-rush current might be.

I know that I could connect the inverter directly to the 12v battery (I’ve tried doing that and it works fine) but by doing that I loose all the battery health and charge status information that the solar regulator gives me (with the load going via the regulator I get to monitor the inverter current and the health of the battery, I lose this if I connect the load straight to the battery)

Is there anyway to suppress the initial current surge of the inverter and prevent the regulator from tripping?

Thanks.

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2 Answers 2

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The inverter may or may not have an inrush, the refrigerator certainly will. They will typically have a vicious starting current, sometimes even 10x the nominal running current.

There's not a lot you do about this at the fridge motor. Most folks simply oversize the inverter and its supplies sufficiently.

As the fridge inrush is mostly reactive, some smart people build a two-stage inverter. They use an initial DC-DC converter, sized correctly for running, and then a DC-AC H-bridge capable of handling the awful power factor.

As your inverter is over-sized sufficiently, it should be enough to handle the surge between your batteries and inverter. As you've found, 30A is almost enough. Could you bypass the battery to inverter components while the fridge is starting up, perhaps with a beefy FET and a circuit that senses the start of the current surge?

While you could build a current-limiter, this would probably cause your fridge to fail to start.

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  • \$\begingroup\$ The fridge is about 50watts, the inverter is 2000watts, so should be enough of an oversize shouldn't it? \$\endgroup\$ Jul 3, 2019 at 17:33
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There's a very convenient and very inexpensive way of limiting inrush currents (especially where the average load is much less than the peak).

You simply need to add a specialist NTC (negative temperature coefficient) thermistor, often referred to as a Surge Guard, in series with your load.

When inactive i.e. with the load switched off, the Surge Guard has a highish (cold) resistance. Depending on your selection it may be, say, 20 ohms. Any switch-on surge has to overcome this resistance and so, the surge is reduced in magnitude. Very quickly however, the passage of current heats up the Surge Guard and the resistance reduces, reducing the resistive losses that you would otherwise have.

You'll need to select a part suitable for your application. Examples can be found here. http://www.farnell.com/datasheets/18517.pdf

In higher power circuits, the losses from having any extra resistance in circuit can be removed by a relay whose contacts (across the surge guard) close some seconds after switch on. This also allows the part to cool down, in order to allow it to immediately resist subsequent further inrush surges following switch off. In many/most cases this is an unnecessary refinement.

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