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Frank Hagan

Woodworker's Journal: Error, but Why?

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The August 2007 Woodworker's Journal has an article about Lithium Ion battery packs on tools, and it notes that the higher voltage cordless tools last longer than lower voltage tools when performing a certain task (driving screws, for instance.)  OK, I've heard that before.  But then it uses an analogy that I know is wrong:

A high voltage cordless tool, say, a 36-volt model, uses less current (amperage) when preforming any given task than the same tool running on 18 volts (similarly, a table saw with a dual voltage motor is more efficient running on 220 volts, where it draws half the amps it needs at 110 volts).

Single phase, dual voltage electrical motors use the exact same amount of energy, have the same torque, and in all other ways perform the same on either 115v or 230v.  When you look at the motor label, you'll see that the amperage is indeed half for the higher voltage, but you don't pay for amps on your electric bill.  You pay for watts.  And the formula for figuring out what an hour of electrical usage is with your electric motor is to multiply "volts times amps" to obtain the watts used.  A motor running on 115V may pull 12 amps (115 x 12 = 1380 watts), while the same motor re-wired to run on 230V will pull 6 amps (230 x 6 = 1380 watts). 

There may be differences elsewhere ... 230V motors can use smaller wire to supply the circuit, for instance.  And if your wiring is undersized and your motor is overheating, changing a dedicated circuit to a higher voltage might be easier than pulling new wire.  But there is no difference in the motors we use between 230v and 115v.

I suspect the author has confused battery life ... the amp hour rating of the battery ... with the amp draw a motor in the shop has.  I think, but I'm not sure at all on how you rate a battery in this regard.  Anyone know?

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There may be a real difference in the particular models checked but not necessarily because of the battery.

As you said - Twice the voltage, half the current, same wattage.  But maybe they have similar gauge wires delivering the energy from the battery.  The greater current of the lower voltage would result in greater losses due to resistance (as you said).

I'm not sure how it translates to the wire gauge of the windings in the motor itself but maybe that has something to do with it.

Or maybe I'm giving the reviewer too much credit for understanding efficiency.  I think you may have spotted something there, Frank.

There often seems to be a confusion between volts/amps/watts.

A possible analogue to that is an initiative I heard about today to require gas stations to install temperature compensation to the pumps because gas is sold by volume but the energy density is temperature dependent.  If you buy gas at a high temperature you get less bang for the buck (except that gas stations store gas underground at a fairly stable temperature).  Rather than install all the complicated electronics to compensate for temperature why don't they just sell gas by the pound (I believe that's how they do it in aviation, but I'm not sure)?

Batteries are funny because some (like lead acid) are pretty efficient and durable as long as they are not discharged below 85 (or so) percent or left in a discharged condition for long, but others (maybe lithium ion, I can't remember) can sit at a 30 percent charge for months and then be quick-charged with little or no damage so it may be that lithium ion batteries stand up to abuse better than other (coincidentally lower votage) batteries.

  It sounds like the article may have been asking and answering the wrong questions because efficiency and battery life in actual use really depends on the care and feeding of the battery.

  Now that I've rambled on and taken the time to re-read your post I agree completely that the analogy is absolutely wrong. But I'm no expert :)

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The article was pretty well written, and had the "air of authority" that these types of articles have.  But when you see something that you know isn't true ... well, it makes you question the rest of the claims.  Even if the thing is not really one of the main points of the article.

The article basically said the Lithium Ion batteries are superior for giving more power per ounce, so you could have a higher voltage battery for the same weight as a lower voltage batteries.  And weight is definitely a factor in a portable hand held tool.  The energy consumption curve is given, and while lesser batteries have a fall off in voltage as they discharge, the LIon batteries give full power until right before they are completely discharged.  So I think the point the author was making was really that LIon batteries allow the manufacturers to provide longer duration, higher power tools.  But I don't think its all related to the voltage rating of the batteries as his analogy suggests. 

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There is a loss of efficiency at high amperages due to heat loss.  The higher the amperage the higher the heat.  I don't know if that is significant but it is a factor. 

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Guest alanj

My real world experience with 110/220 tools is that, invariably, the tools run better and much more powerfully on higher voltage.

My first tablesaw, maybe 30 years ago was a Rockwell/Beaver 9". In my unheated shop on the frigid Canadian prairies, it would barely start on 110, when it was cold. Even when warm, 3/4 oak solids were about all it could rip, and it was a struggle. This was a 3/4 horse motor (3450 rpm) and pulleyed appropriately.

Changing to 220v, made it into a new saw. Ramped up to full speed instantly, and no problem with two inch material of any kind. And temperature made little difference. Torque as needed, kinda like the difference between  gas motor and a diesel.

I will add that the wiring in that shop was new and completely to code. Distances from the box were insignificant  as the shop was attached to my house and the breakers were in the basement a few feet away. All 15 amp. and I guess that motor was no more than 12 amps.

Just my experience. Repeated thru' the years.

Alan

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I spent a career as a Controls Engineer, and still work daily with motors.  I think Frank is correct in doubting the rest of the author's claims and/or work, but then I am always a skeptic.  Twice the voltage, 1/2 the current is still the same power and does not change the HP rating of the motor.  I like the higher voltages because the motors seem to run better and supply power over a better power curve....stretches it out a bit more in relation to load, and is therefore better at delivering power.  I know that is not a good way to describe the process but it works for thinking of how to use it.  Higher voltage does mean lower line loss, as was pointed out, and motors run a bit cooler too, which could be considered higher efficiency.  Maybe that is what the author was meaning?

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I think the author was trying an analogy to explain something on the order of reserve capacity or whatever you call it with batteries, but failed when he employed an urban myth as the subject of his analogy.  But I don't know enough about batteries to make the distinction.

I found the "unit watts out" rating of DeWalt's cordless tools ... and I'm assuming that phrase is somehow similar to the watts we are used to in an electric motor:

  • 9.6v = 130 Unit Watts Out
  • 12v = 230 Unit Watts Out
  • 18v = 510 Unit Watts Out

I think that means that the "equivalent" to the power rating I'm used to seeing for electric motors would work out this way:

  • 9.6 volts X 13.54 amps = 130 Unit Watts Out
  • 12 volts X 19.16 amps = 230 Unit Watts Out
  • 18 volts X 28.33 amps = 510 Unit Watts Out

Instead of seeing half the amps on the battery twice the voltage of the 9.6v battery, we see the 18v battery is more than twice the amps.  That's more like putting a 2hp motor on your 1hp saw.

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It does make sense for batteries.  Generally the higher a voltage a battery can provide, the higher its available amps too, the trade off being how large the battery's storage capacity is.  This is why the tools using batteries have the sizes increase with voltage level.

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Voltage drop as a percentage is less with higher voltages.  Less efficiency losses.

Same with motor wired for 110 or 220. 

Under what load conditions and are you referring to attenuation due to wire sizes, etc.?  Another wrinkle is the motors used in the battery powered tools could be either DC with brushes, or brushless AC (servo type).  More likely the latter.

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Voltage drop as a percentage is less with higher voltages.  Less efficiency losses.

Same with motor wired for 110 or 220. 

True, as evidenced by the wire size charts for 115 or 230v; but I don't think it makes much difference inside a motor. As long as you have the voltage and the current carrying capacity of the wire, you get the same power output on 115 or 230v. 

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I found this at:      http://www.faqs.org/faqs/electrical-wiring/part2/

Subject: Is it better to run motors at 110 or 220?

Theoretically, it doesn't make any difference.  However, there

is a difference is the amount of power lost in the supply

wiring.  All things being equal, a 110V motor will lose 4 times

more power in the house wiring than a 220V motor.  This also

means that the startup surge loss will be less, and the motor

will get to speed quicker with 220V.  And in some circumstances,

the smaller power loss will lead to longer motor life.

This is usually irrelevant unless the supply wires are more

than 50 feet long.

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I found this at:      http://www.faqs.org/faqs/electrical-wiring/part2/

Subject: Is it better to run motors at 110 or 220?

Theoretically, it doesn't make any difference.  However, there

is a difference is the amount of power lost in the supply

wiring.  All things being equal, a 110V motor will lose 4 times

more power in the house wiring than a 220V motor.  This also

means that the startup surge loss will be less, and the motor

will get to speed quicker with 220V.  And in some circumstances,

the smaller power loss will lead to longer motor life.

This is usually irrelevant unless the supply wires are more

than 50 feet long.

Yep, that's my understanding on the issue.  It gets more complicated with larger motors and 3 phase power, but for what we run in our shops, it usually doesn't make a difference.  It CAN if the wire size is undersized some; the higher voltage has less loss, so the motor is getting more of what it needs to run.  But with properly sized wire, it makes no difference.

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