New inverter

[Soapman]

  So things aren't going too well. I posted before that my old inverter wasn't working. Probably the cold. It was just a 400w Canadian tire cheapy, but I'd had it for years and it didn't owe me anything. So last week the  1000w Xantrex inverters went on sale for $89 from $159. Minus a gift card I still had from Christmas and the Canadian tire money I've been saving it only cost me $33. I thought what the hell. I know it's still just a cheapy but I can't afford a really good one right now and  I figured it's better that nothing. I was wrong!

   I brought it home and hooked it up where the old one was. Turned it on and nothing happened. No power, nothing. So I thought maybe there's a bad connection in side or something. I brought it back and exchanged it for another one. This one worked fine. Well, for a day anyway. I've been using them in my garage to power some CFLs and a clock radio most of the time. This morning I went to the garage and turned it on. Everything seemed fine. I tried using my skill saw and all went wrong. Lots of sparks, a small flame and lots of smoke. 2 CFLs 13x2= 26 w , clock radio 25 w , skill saw 110x5.5 amps= 605 w. Total 656w right? Even if the peak is too much the surge should just shut it down shouldn't it? My old one did. I returned it and got a refund.

 So are these just that bad? I can't see any thing wrong with my system. 60w of panels into a 7amp charge controler - into a deep cycle 100 amp hr battery - into 1 guage cable - negative into inverter - positive cable into 80 amp fuse into inverter. This is a 12v system. Does this sound right? I made sure the cables weren't hooked up backwards so that's not it.

 I still need an inverter and still don't have alot of money. I don't know what to get. Hopefully this doesn't keep happening and it was just crapy inverters. Does anyone have any ideas for me?  What went wrong and/or what kind of inverter can I get that's good but won't break the bank.

Bill

[Tom in NH]

Mine failed the smoke test when I plugged in the power cord. I mistakenly connected the negative battery terminal to the chassis ground as it is done in automotive applications. Big mistake. You didn't do anything like that, did you? Xantrex sent me another unit under warranty. It has remained on for two years and has never given me a bit of problem (knock wood). This time it is ungrounded. --tom

[RobC]

Buy a Cobra inverter you will be much happier. RobC

[scottsAI]

Hello Soapman,

Motor starting current is 5-7x higher than run current, lasting a second or two and decaying down to normal current. Skill saws has a universal motor, which has very high starting currents, I would guess on the 7x side of things.

Inverters often have overloaded ratings 2x normal for a fraction of a second. (Overload times vary greatly on brands/models.)

600w * 5 = 3000w, so the inverter may not be rated high enough.

You need 1500w or better 2kw inverter to use that saw.

Yes, the inverter should have shutdowns, if not don't buy it. (over/under voltage, over current)

I used few cheap ones, worked OK for a while, never lasted long term.

Recently bough Tripp-lite 1kw, has 2kw 5 min rating! Works great.

Many other great brands out there.

Have fun,

Scott.

[RobC]

To clarify my comment let me say so far my cobra inverters have never had a problem with being overloaded. A friend of mine damaged his by hooking it up to live 110 power not only once but 3 times before it died and Cobra fixed it no questions asked. They are the only people I know who stand behind their products 100% at least so far. We just bought my brother a 2000 watt 4000 surge on sale on sale at Cabela's for less than 200 dollars  and it was marine grade. Good luck RobC

[nothing to lose]

I don't know about about newer ones or smallers ones, but I have an AIMS 5K 12V inverter that has been used and abused rough and never failed me.

This was about a $600 or more inverter severaal years ago when I bought it new for about $500.

It has run my Lathe, and everything else.

I know you don't want something that big or expensive, but perhaps in smaller sizes they are as good as the larger? Maybe not?

Something to remember also, although it used to be a person using things for a long period could give decent advice on stuff. NOW is not the same as the old days. Companies buy out companies, they farm out the work to the cheapest bidder etc...

What was an execellent product to buy 5 years ago may be crap today. Those of us that have the 5year old stuff and it's working great still love it, the new stuff made today may not last 2 years or 2 months?

 My Aims inverter is doing great still when I use it, now it's not used as much since I have it connected in a seldom used building.

[Norm]

  Of course a solid state electronic would work

but for a less skilled DIY'er...could you limit

the current on start up of the saw or motor that

takes a lot of current for start up?

  If the saw takes 600 watts to run under load

and you ...put a choke coil in series to limit

the current to maybe 800 watts?

  Of course you would have to know what size

wire and how many turns and the motor would be a

little slower on startup.

  Actually I think for the price of a more

expensive  larger converter I'd just get an

18 volt cordless saw.

               ( :>) Norm.

[Drives]

Norm:

This has been exactly my plan...place an inductor on the output of my inverter.  We do this on the output of VFDs all the time.

It appears 2mH for a 15 amp circuit should work well.  This approx. 3-4% impedance.

I lost my program that calculated how many turns of what wire around certain cores would produce the rated inductance value.  The program was a free download off the web, I will have to search for it.  

I am guessing 12-14 AWG air core inductor should be good.  I like air core vs. iron incases of potential saturation, like large motor starting currents.  

Just another project to add to my list!

Yea!

[scottsAI]

Hello Norm,

I agree.

Series resistors are used to limit starting current. Inductor not a good choice.

Universal motors have high starting current, that is why the saw jumps in your hand when it starts. One or two step current limiting resistors are used. Rather simple controller... :-)

Nice thing with universal motors, they work great on AC or DC, induction motors do not like so much the mod sin the lower cost inverters provide.

Yes, I think the cordless saw may be a cheaper option assuming it's the only large thing he needs to run. If not then a larger inverter is in order.

Have fun,

Scott.

[Drives]

Scott:

I don't disagree that a reduced voltage starter whether via resistors, SCR's, autotransformer, wye/delta, two speed starter, one arm bandit, etc. is a good idea to reduce starting currents.....

And your right, you can install resistors in series with the output of the inverter, along with a switch or relay in parallel to short them out once the motor is up to speed.  Or better yet, place an AC current meter in series with the output, and close the shorting switch once the current has settled, and the motor is up to speed.  To me this sounds labor intensive, and I would worry about forgetting to close the shorting switch (I'm getting old :-).  If the shorting switch was not closed, overheating of the resistors and potential fire is only one of many possible scenarios.....scary thoughts.  

However, if you are familiar with failure mode of inverters, their current sensing circuits, and fault trip circuitry, you will realize that to say inductors are not a good choice is not entirely correct.  IMHO

An inductor by definition "resists a rapid change in current".  It is this rapid change in current (Di/Dt) that can cause damage to the output transistors, especially when switching into a low impedance load (stopped motor for example).

By placing an inductor in series with the output of the inverter, we are reducing the rate of rise of this high current spike each time the output transistors are "fired on".  (I'm talking about milliseconds and faster speeds).  

This is easier on the output transistors, and slows & limits the current spike to allow the current sensing and fault trip sensing circuit time to react before damage usually occurs.  Picture a line graph where the amps rate of rise was near vertical, and after an inductor it becomes sloped, more 45 degreeish.

In my limited single phase inverter experience, it appears the more expensive the inverter, the more these output "filters" are included.  What's to say we can't add them on a cheap inverter?  :-)

An additional benefit to placing an inductor on the output of a MSW inverter is the effect that it will "clean up" the output waveform some.  At only 3-4% impedance, you will not achieve a clean sine wave, but an induction motor will be much happier with the improved waveform.

In conclusion, I agree a 3-4% impedance inductor will not reduce the high motor starting currents from 700% down to 100%, yet, an inductor will help to reduce the damaging peak currents that kill power transistors commonly.

As I have seen it said here before "your mileage may vary".

[Nando]

Coils will not work in this case, VFD may require choke for other reasons and this due to the PWM frequency generation.

Do not make the mistake to place an inductor in series with the inverter.

If the inverter is MSW the internal switching is done, in principle, with two Mosfets to change the "DC voltage" supply polarity to the external world.

For this reason the CHEAP MSW inverters can not have the "NEUTRAL" of the MSW to the Neutral of the house wiring ( the Neutral is connected to Ground at the main Breaker box and other places).

Though, there are MSW inverters with 4 MosFet switching that allows the Neutral connection to a house wiring since the circuit does keep the Neutral as Neutral and the polarity switching is done using the HOT wire only.

The starting of motors does require an inverter with a capability of at least 5 times the running current,a nd this capability is NOT for 1 or 2 seconds, it could be as high as 8 - 10 seconds, all depending on the motor structure and its load.

One can improve the Power Factor of the motor if a capacitor in parallel with the motor is applied during the heavy current starting time, I have used a current transformer to fire TRIACS to place the needed capacitor, which can be calculated for each specific motor that for this type of AC power supplies may be with some advantage -- but the price to pay if more than one motor, the cost of this arrangement may be as much or more than buying a much higher capacity Inverter.

Nando

[Drives]

 I'll state that I politely disagree.

 I guess I need provide some proof that my suggestion is beneficial.  I will be busy consulting & teaching the rest of this week, but I will try to gather the waveforms this weekend.

[Nando]

Drives:

I welcome your information and if you have technical info, please send it to me directly, do correct the anti spam changes.

One needs to remember that in an AC environment :

V= L* dI/T

Nando

[TomW]

Nando;



I welcome your information and if you have technical info, please send it to me directly, do correct the anti spam changes.

I vigorously disagree he should NOT do this in private. This forum is here to teach many people as much as possible.

I often wonder why you constantly attempt to lure folks into private dialog. I finally decided you like being "The Expert" and perhaps fear someone might show you that you are not always correct.

Frankly, Dean works with motor control every day and he is willing to share that knowledge and I hope he refuses to take it into closed discussion.

The whole point of this forum is to disseminate large amounts of information to as broad an audience as possible. Open discussion often brings a lot of new ideas.

Lets stick with what works. Please stop luring people into private discussion. We do appreciate your input here but you need to realize why it is very public and keep it that way.

Speaking as both user and editor here.

Cheers.

TomW

[(unknown)]

Use a thermistor on the input to the skill saw. Very commonly used to limit inrush currents on large amateur radio amplifier power supplies. High resistance when cold that goes to a lower resistance as they warm up. The amplifier power supplies typically draw 15-20 amps at 240 volts and the inrush current charging the capacitors needs to be limited.

[ghurd]

Something is being overlooked.  Circular saws start so slow, the surge may be a full second or more on a MSW inverter.

Last fall we were running a small normal circular saw from one of those $250 PepBoys generators.  I just remember it was orange.

About half the time, when the trigger was pulled the blade just turned a couple degrees then stopped, and the generator slowed way down.  The saw only hummed.

Gave it a flick on the side of the blade to get it turning, the generator sped up, and it worked.  But it wouldn't get to speed for a few seconds, maybe 5 seconds.

With Scotts numbers, 600W x 7 = 4200W.  And for far longer than a fraction of a second.

A 1000W inverter should not be expected to handle it.

Meaning I don't believe the ability to power a 600W circular saw is a fair way to rate a 1000W inverter.

G-

[TomW]

G-;

Good point. my table saw is similar. Old direct drive Craftsman. To get it to start on even a 2500 watt inverter you need to coax it to start by flipping it on for a millisecond [fast toggle] then off then before the blade stops turning flip it on. Occasionally it can take a couple "shots". Not exactly inverter friendly but a way to get past a bit of the startup surge.

Funny thing is that once its running it handles heavy loading just fine. It is a one horse motor according to the nameplate.

Just my experience with my cheapie old [now dead] inverter.

Cheers.

TomW

[Spdlmt150]

I have a Tripp-Lite 1800w inverter which was $289 new. I also have a pair of 375w Tripp-Lite inverters. All 3 have been heavily abused. The 1800w can run my DeWalt circular saw with no problems, providing the batt voltage is up. I think once you get over 500w the battery reserve & cabling has a lot to do with running heavy loads. The 375w are self resetting, self protecting. The 1800w will stay tripped until manually reset. All three have been pushed to overload many times with no problems. The 375w come with a constant-on cooling fan. Thermal switches can be found for about $7. The 1800w has dual fans with thermal switching. For the money, I'll never complain.

[Soapman]

 This is certainly alot more than I expected. I never thought the peak on a circular saw would be that high but it makes sence now that I think about it. I think I'm going to save up and buy a bigger inverter. I want to start upgrading my system and it seems like I might as well start with the inverter since I need a new one anyway. The whole point of my system for now is to play with in the garage and back up power when the lights go out. Thanks for all the input everyone.

Bill

[Drives]

Just an update.  As is usual with me, I have been swamped at work, and one of my wonderful clients was kind enough to give be the FLU with Strept Throat.  

I did purchase a new inverter from Wally Mart just for the testing, but have not had the time to take it out of its box.

My work bench is buried!  :-)

Now I am behind at the office!  This will have to be a project for the upcoming months.

I will post results in a Diary, sorry for the delay.

[Drives]

Nando:

Please read the following link...I don't think Xantrex thinks it is a "mistake"

http://www.xantrex.com/web/id/730/docserve.asp

In the attached link they refer to a 5 mHenry inductor in series, I think that is a better value than my 2-3 mHenries.  I was concerned about too much voltage drop, yet I am sure Xantrex knows what they are doing.

I think the formula is X sub L = 2 X pi X Freq X L

Hope this info helps.