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I live in an off grid property running on 6 12v batteries. We have a 2000w inverter and are trying to get a fridge to run on our system. We have enough power to run it however the issue I am having is on the initial start up the compressor draws around 100amps for around a second. This draw drops the voltage of the batteries to the point that anything else we were using (TV, computer) turns off and has to be rebooted.

I am looking into a soft start device to distribute that instant 100amp draw over a few seconds (such as this one https://www.jaycar.co.nz/240v-10a-motor-speed-controller-kit-with-soft-start/p/KC5526

I was wondering if anyone has any experience with this and if they believe that it will work or any other suggestions. My main concern is that the soft start will mess up the timing of the electric motor of the fridge and it will just stall out.

Thanks in advance!

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The linked speed controller clearly states that it is for a universal motor and only a universal motor. A universal motor is a series-field type DC motor with brushes, that has been designed to run on either AC or Dc power. It is unlikely that a refrigerator will have a universal motor. If you can not see brushes and a commutator inside, or two round insulated covers for changing brushes, it is not a universal motor.

There are variable-voltage AC speed controllers that are designed to power fans and centrifugal pumps. The torque required to drive a fan or centrifugal pump increases in proportion to speed squared. They require very little torque to start and reach their maximum torque requirement at full speed. One of those might be able to to start a refrigerator, but it is rather unlikely.

Your best alternative is probably to buy an inverter that is designed to start an AC motor that has a hard-starting load. The fridge may need its own inverter. If the inverter you have can start the inverter, buy a second inverter for everything else.

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  • \$\begingroup\$ Thank you for your reply would this be more beneficial? jaycar.co.nz/soft-start-kit-for-power-tools/p/KC5511 \$\endgroup\$ – Nectar-Bomb Nov 19 '18 at 19:36
  • \$\begingroup\$ There does not seem to be enough information to tell. \$\endgroup\$ – Charles Cowie Nov 19 '18 at 20:20
  • \$\begingroup\$ I agree with J. Raefield. An AC soft starter is not going to work or at least there is only a slim chance it will work. The device linked in your comment may be a DC device like the first one you proposed. It is not as completely described, but it is recommended for power tools that usually have universal motors. With what you have, you can disconnect half of the batteries, see if the fridge starts and note the battery voltage drop. I also like the new fridge idea by DrMoishe Pippik. \$\endgroup\$ – Charles Cowie Nov 21 '18 at 13:47
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If you mean 100 amps from the 12 volt batteries (in parallel, I assume), ~1,000 W out from the inverte, that is to be expected for a full-sized household refrigerator. The initial current draw for a refrigerator is high because the motor must turn over the compressor against gas pressure. There is no way to distribute that initial load over time, because enough force must be applied against the "springy" gas. In addition, household refrigerators consume a lot of energy over time, perhaps 1 kWh/day for a small, highly-efficient ("Energy Star") appliance to 5kWh for older models. How are you keeping those batteries charged?

Most likely, you need a smaller, more efficient refrigerator, e.g. one designed for a recreational vehicle, or more batteries, or perhaps a separate inverter with its own batteries.

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The average household refrigerator uses a hermetically sealed single phase AC induction motor that uses a capacitor for starting. It is the charging of that capacitor that you are seeing as that 10x spike in current and after that, the motor will pull approximately 6x current until it gets to around 80-90% speed. That entire process usually takes a second or less, but if your system cannot supply it, you have the problem you are seeing.

An AC soft starter is not going to work for this, in fact there really isn't a solution because anything that interferes with that capacitor starting routine ultimately interferes with the motor starting at all. Even VFDs designed for SOME types of single phase motors will not work for this type, the capacitor charging current looks and acts like a short circuit to the VFD transistors and the VFD harmonics gets trapped in the starting capacitor, so it is a race to see which one fails first.

The viable solution, as mentioned by DrMoishe Pippik at the end of his first response, is to attain a new refrigerator designed to operate off-grid.

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A “soft start”, even if appropriate for the motor, will only lead to a stall of the compressor. You will need an inverter (and batteries) capable of handling the start-up surge.

It’s possible that there are special fridges that have less start-up surge designed for your application and similar ones such as RVs.

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Soft starter is the one which slowly changes the applied voltage to an induction motor which results in reduction of electrical surge current of motor during starting.

The product is an Universal motor speed controller.

soft starting is just an another method of starting an induction motor. it will not change any of its operating parameters

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  • \$\begingroup\$ that is incorrect: when a compressor motor first starts, particularly with high back-pressure, as in a warm refrigerator, it must draw a large current to overcome the initial mechanical resistance. \$\endgroup\$ – DrMoishe Pippik Nov 19 '18 at 5:41
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    \$\begingroup\$ not only Compressor.. Any electrical motor will draw almost 10 times its rated current at start to overcome its inertia...that is the place where we are going to use soft starter to reduce initial current draw \$\endgroup\$ – Satish Singupuram Nov 19 '18 at 5:52
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    \$\begingroup\$ Again, reduce the current and the compressor wont start. TANSTAAFL. \$\endgroup\$ – DrMoishe Pippik Nov 19 '18 at 6:07
  • \$\begingroup\$ it's actually a very fast process.. i had preciously done some work on soft starters for squirrel cage induction motors... but not tried on refrigerator \$\endgroup\$ – Satish Singupuram Nov 19 '18 at 6:11
  • \$\begingroup\$ yeh.. got your point. \$\endgroup\$ – Satish Singupuram Nov 19 '18 at 6:16
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Just a thought, you might try a UPC backup (for computers) inline for the fridge. These units have batteries and power conditioners. I believe that an APC1500 UPS can provide about 900 watts, which is about half of what a refrigerator requires to start.

Conversely, you could run your television and other sensitive equipment with the UPC inline.

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No it won't cost I tried it ....your best bet and it's the only possible way is to add an 12v capacitor bank before your inverter.

Checkout lasersaber's youtube channel ~ he uses high induction power tools directly from portable solar panels via a super capacitor bank and inverter.....that system could also be incorporated into your existing system too.

https://youtu.be/eJ1OztoGprU

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  • \$\begingroup\$ Answers that rely on an external link are not encouraged here. If the link dies then the information is lost. Please try to summarize the guidance provided in the video. In particular, can you calculate the required size of the capacitor bank that would provide 100 A for 1 s? \$\endgroup\$ – Elliot Alderson Dec 30 '18 at 3:01

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