Invertor (DC to AC) for power tools
woodenfish3
| Posted in Power Tools and Machinery on
The kind of invertor I am talking about is the type that one plugs into the cigerette lighter of a car, one that converts DC to 120 V AC. They sell for $50-80 and many have peak of may be 900W and continuous duty of may be 700W. I heard that the AC generated is not sinusoidic but rectangular. I don’t know if this is still true now.
Would a 6 amp small trim saw run on such an invertor? The main purpose is to cut 4X8 panels, hopefully precisely, to ease transport.
Of course cordless is a possibility and I may very well opt for one, but I don’t like to buy batteries every two years if I can avoid it. In general I prefer corded.
Replies
As a purchasing agent I tend to let the manufacturer or dealer do my technical homework. That way if it doesn't work the way I want it to I can bring it back with no hassle. I would tend to post such a question at breaktime or an electrical forum in stead of a furniture forum. My preferred way would be to contact several manufacturers tech help and not even bother with forums.
Six amps at 120 volts translates into 720 watts (6 x 120 = 720) so you would be pushing the inverter you described right to its limits or slightly beyond. The consumer grade inverters don't hold up well to overloading, you would be better off go up in size on the inverter or you will most likely be replacing it in two years instead of a set of batteries for a cordless saw.
The inexpensive inverters don't produce a true sinusoidal current but this wouldn't affect the saw, it is only a problem with some electronic devices.
Also worth considering is that the cigarette lighter will have to deliver around 70 amps at 12 volts to power the inverter, which is well beyond what the lighter outlet is probably fused and wired for. What you need is an inverter wired directly to the battery with proper fusing and wire sizing to handle such a large load at low voltages.
John W.
Edited 1/11/2006 12:30 pm ET by JohnWW
But if you hook it up straight to the battery, how many auto charging systems could keep up with a 70 ampere drain? You'd need to vehicles hooked in parallel to keep the battery charged.
Or a high output alternator, either replacing the OEM one or an add-on. They're used in high powered audio systems all the time. If the vehicle is used for work, it's a write-off. H.O. alternators are expensive but if it solves the problem, it could be worth it.
"I cut this piece four times and it's still too short."
I missed the first part of this thread so may be making some false assumptions. Of course, you are right. Even so, it seems a waste to be running a big high powered engine for a long period of time to deliver electric power that a little portable generator could deliver.
It doesn't even make economical sense to assemble a small gas engine, an alternator and an invertor.
"I cut this piece four times and it's still too short."
How true! And if a cordless would work, why consider anything else. Pardon me for posting when I didn't understand the subject.
The vehicle's battery, even with the engine off, would be able to supply more than enough power for cutting up several sheets of material and the battery would be recharged the next time the vehicle was driven. With the amount of current drawing components on a typical modern vehicle, most cars and trucks already have fairly large alternator to keep up with the demand.
John W.
Most alternators are rated for the maximum current draw for the supplied accessories + a little more. A 150A alternator costs about $400. Unless the vehicle is a truck with hydraulics, tons of lights and other accessories, it's not likely to have an alternator rated for more than 100A. The battery on a full sized car or truck should have no problem with a 70A load for a short time if the device is connected directly. Once the load is too high while the motor is running, the bridge, bearings and slip rings overheat, and we all know alternators and batteries fail at the worst possible time. Jumping another vehicle is a great example of when the donor motor should be shut off. A good starter will draw a minimum of 125A on a small motor, 350A on a big one and there's no way an alternator will enjoy a long life with this kind of thing going on. A dying starter will draw even more.
"I cut this piece four times and it's still too short."
I am more inclined to get just a cordless and hope that the battery pack would last a while. Especially if I can get one from eBay for $80 or less.
But just for discussion, I don't think a 14.4v saw would draw 70 amps cutting 3/4 inch MDF or plywood. This would be close to 1000 Watts.
I think a 14.4 v cordless saw would not draw more than 30 amps, about 430 watts.
I think the drawback is in the unwieldiness of the cables etc. I think one would need 8AWG wires for the best performance. I think one can use 10 AWG for OK performance.
All and all, using a cars alternator for 14.4v cordless power tools is a possibility, IMO.
Edited 1/12/2006 11:32 pm ET by woodenfish3
Edited 1/12/2006 11:37 pm ET by woodenfish3
I'm confused by the 14.4V cordless AND alternator reference. If you mean recharging 14.4 batteries, you can do that with the car off. I have the 18V Milwaukee hammer drill and my invertor is rated at 700 watts, peak. Are you getting a 14.4V saw because you already have other 14.4V tools?
"I cut this piece four times and it's still too short."
I mean no battery pack at all, just use the alternator to run the tool by attaching two wires.
Sorry about the digression from the title with the invertor. I suddenly realise that one does not need an inverter for 14.4 volt, which is about what the alternator gives after voltage regulation for most cars.
Edited 1/14/2006 1:37 am ET by woodenfish3
Good luck with that.
"I cut this piece four times and it's still too short."
The only thing I would add, is that it will kill a battery prematurely. I have had an inverter for a long time for my Minivan and two kids so on trips they would watch movies (greatest thing they ever made). I killed two batteries in my van. I still love it, but it will shorten the life of a battery. Short term use, no problems.
Are you saying using the inverter for long terms would kill the battery with the engine is running or just when the engine is off and it runs the battery down?
Yea, I meant running. For the most part. Now days these little LCD TV's draw very little amperage and are DC. I was using a small 9" TV/VCR combo (120V) and a PS2 or Game cube running with the inverter. When I say that it killed two battaries, I mean their life was shortened significantly. The last battery I had was a Sears Die hard that has been going strong now for quite a while, however, the new video setup is two small lcd units on the backs of the the seats for each of my two boys. The amperage draw is significantly lower. The best inverters, if you have one is directly wired to the alternator. Those are more expensive. On a long trip, they are a godsend.
Of course the ultimate solution would be GMCs "Hybrid" pickup that has the 120 volt outlets :). Troy
Expensive solution, but that would be nice. I would rather have the Siverado 2500hd, but thats a topic for another forum..
For this use, I would recommend at least a 1500W inverter. The higher end mfgs produce units with more of a sine wave, but you pay for it.
With the cost of generators, you could get a small one for less than a good inverter.
Or, you could buy a cheap inverter and cordless, just use it to charge batteries. I have a 600W, 400W continous in my truck, it has a hard time keeping up with the charger. I have to have my truck running to supply enough power to the unit to convert.
If you only want to cut 4x8 panels, look on ebay for a used cordless saw. I just got a Milwaukee 18V for $63, including shipping. I already had the hammer drill, so batteries and charger aren't a problem, but you should be able to find a kit for a reasonable price. If you buy an invertor and it's not big enough, you will damage the motor and/or switch on a corded saw. Besides, if the saw isn't going to be used all day, every day, you won't need to replace the batteries that often. I have 7.2V Makita batteries that were used heavily for work for more than a year and still work. I don't use them much now but I bought them in '97. Also, if you want to save money on a cordless kit, Milwaukee has reconditioned kits available and they come with the full 5 year warranty. Contact one of their service centers and ask what is in stock.
If your plywood purchases are only occasional then why not pay the cutting charges to get them into the car? Your conscience won't be challenged by having to toss batteries (I don't like cordless for occasional use either) and you won't risk cooking marginal inverters. Another option although harder to find is to enlist the help of someone with a pickup or van. But that may cost you a case of beer which would end up the same as the cutting charge.
An idea came.
The automotive alternator gives close to 14.5v DC with proper voltage regulation.
I could get a 14.4V cordless saw and power it from the battery terminals of the car with the car running. All I would need is a couple of wires of enough length. I would need rather low gauge wires, may be AWG 10, I think. The current may be rather high since the voltage across the terminal is rather low at 15 volt. If the wattage of the saw is about 450, then the amp would be about 30.
Edited 1/11/2006 5:41 pm ET by woodenfish3
I have a RV that we rebuilt. Believe me the rating on those inverters are optimistic. The amperage involved in powering a saw will require heavy gauge wireing from the battery to the inverter. A high power inverter, the wire (and we wont consider your time) will cost you more than a Ryobi cordless saw. And here is a novel idea............. use a chalkline and a hand saw. Of course after the first time you will go the Home Depot and get a cordless saw. Ha ha.
Mike
I noticed at a recent go-cart meet that a lot of the guys there used old 12v drills wired to a couple of spade terminals onto their cart batteries . One or two had used the original drill battery pack less the batteries, others just soldered wires directly into the drill. So why not get a cordless saw & when the battery dies throw away the batteries (in an environmentally sensible fashion) & fix a wire to your car battery. Should you wish to use the saw with another battery later just unplug the hard wired canister & hey presto...............
Don
Buy a Ridgid cordless saw, a nice tool, and a lifetime warranty that covers the batteries.
John W.
I have a 600 watt invertor with alligator clams that clamp directly to my battery. I built a tree hunting platform out in the woods and I hooked up the invertor to the battery of my beater p/up truck. With my 6 amp DW drill plugged into the invertor, I screwed in 100 3" screws until the battery went dead (duh) but the job was finished. With the engine running I could have kept going 'til I run out of gas (another duh). I bought this invertor several years ago for $100.00 and the prices have come down quite a bit since then.
it is good to know that the universal motor can use rectangular wave AC just fine. Good knowledge.
Many corded tools draw more than 6 amps, however.
The full power13-15 amp saw would not work, or needs a much bigger invertor.
A small 6 amp corded trim saw would be nice.
Edited 1/16/2006 5:24 pm ET by woodenfish3
Edited 1/16/2006 5:24 pm ET by woodenfish3
You're right. I've plugged a larger tool into the invertor and it blew a fuse on the invertor.
The method to figure how big an invertor you would need for a given tool is: watts divided by volts = amps.
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