Why inverters explode

[papaof2]

You may have seen someone showing off their new, high power DC-AC inverter by repeatedly starting a large chop saw or the like.

Here's a picture of why the inverter being tested often fails: www.jecarter.us/images/soft-start.jpg

Notice the red curve - that's what you get when the power source has adequate power to start the saw. That curve peaks at about 55 amps. 55 amps at 120 volts is 6600 watts. At that power level, the inverter may be a split 240 volts, with two 120 volt outlets - which means each 120 outlet only has 4000 watts of power. How many 8000 watt inverters will be able to start that saw? How many times will they be able to start that saw repeatedly?

The blue curve shows how the NTC (negative temperature co-efficient) resistor in the soft-start device resists that initial surge and holds it down to just a bit more than the power used during normal cutting. Think that might be a better way of doing things? It should make the saw last longer and also allow you to run that saw on an inverter or generator that's maybe 30%-50% bigger than the saw's nameplate power requirement.

You'll likely be building your own, as www.raymondinnovations.com/collections/soft-starters is mostly out of stock on their soft-start products, with the 15 amp, 120 volt GS10 being the only one currently available and it's now $159.99 (more than twice what it cost me a few years ago). The GS10 is probably aimed at circular saws, as most of them are within its 15 amp capacity. There are some how-to videos on youtube so that's one place to look for making your own.

[feralferret]

Most people have no clue about surge current, especially when electric motors are involved.

I used to work with equipment for blending the different ingredient for making the several different vinyl compounds for making vinyl siding and skirting for a major manufacturer. The batches were up to 2100 lbs per batch depending on which of the compounds was being mixed. The electric motor on the intensive mixer was a 500 horsepower motor. The cabinet containing the soft start equipment was about 12 feet long, 7 feet high, and three feet deep. The operating power consumption was around 375,000 watts. That is around 285 amps for each of the three legs of 440 VAC feeding the motor. The inrush current if uncontrolled would have been over 2000 amps per leg (6000 amps total). That would have dimmed the lights of the whole town if not controlled.

[papaof2]

It seems that those people don't notice that the saw dims the lights when it's on grid power. Folks, that's a big hint that the saw is a power hungry beast when it starts up and a lot of non-grid power sources do NOT have the reserves to start it. The fact that an alternate source has the same mouse-faced outlet does NOT mean it has the same power capability as a grid-based outlet.

I think those people and the ones trying to load 20 sheets of drywall on a Prius are all from the same family...

I have a neighbor that I woud never offer even a single outlet extension cord to for their fridge. There would be a triple tap on the single outlet and a 6 or 8 outlet power strip on each of the triple tap's outlets - and complaints about "the fridge sounds funny" and/or "the heater isn't very warm" and/or "the TV picture isn't full size".