Re: 9.6V 18V 24V what's the advantage in hand tools?

On Mon, 29 Aug 2005 00:05:14 -0500, RP <no_mail_no_spam@xxxxxxxxx>
wrote:

>
>
>Pi wrote:
>> On Tue, 09 Aug 2005 20:22:59 GMT, John C. Polasek
>> <jpolasek@xxxxxxxxxx> wrote:
>>
>>>On Mon, 08 Aug 2005 21:27:39 -0500, RP <no_mail_no_spam@xxxxxxxxx>
>>>wrote:
>>>
>>>>John C. Polasek wrote:
>>>>
>>>>>The ads for hand drills push higher and higher voltages at higher and
>>>>>higher prices but I fail to see the logic. What can be done at 24V can
>>>>>be done at 9.6, just use up the battery space and size the motor
>>>>>windings properly.
>>>>>High volt batteries have many more cells and many solder joints. A
>>>>>24v battery has 20 cells and 21 joints; a 9.6V has 9 joints (for Nicad
>>>>>at 1.2V/cell). Further more there are than many more metal walls to go
>>>>>bad.
>>>>>High voltage only has an advantage when there are long runs of wire or
>>>>>the load cannot be adjusted, but there are none that I can see here.
>>>>>Are the manufacturers putting something over on what they perceive to
>>>>>be stupid customers?
>>>>>John Polasek
>>>>
>>>>Higher voltage drills have more cells, thus providing longer use between
>>>>chargings. That is, more cells means more total stored energy.
>>>>The greater power of the higher voltage drills is a result of the fact
>>>>that if you load the lower voltage pack to the same extent, then the
>>>>effiiciency will be greatly reduced. This in turn is due to the physics
>>>>of batteries; higher load equals lower efficiency. The reason for this
>>>>is that the battery pack has an internal resistance. For maximum
>>>>efficiency of energy transfer the external resistance should be much
>>>>higher than the internal battery resistance. Thus you basically want to
>>>>keep the ratio above a certain level, regardless of voltage, that is the
>>>>ratio targets a minimum desireable efficiency. Higher voltage at the
>>>>same efficiency means greater torque, hp, and length of use.
>>>>Richard Perry
>>>
>>>Poppycock. The intrinsic energy storage of Nicads joules/volume is a
>>>constant.
>>>And a 24V needs 20 cells whose probability of failing is twice that
>>>for a 12v battery. I've had a number of such tools and when I throw
>>>them away it's always because the $20 or $45 battery has flunked, and
>>>it's always 1 cell (how could it be 2?).
>>>If I have a 24v motor with 100 turns carrying 1 amp, I can duplicate
>>>what's going on with a 12 volt battery.
>>>Just take 50 of the turns and solder them in parallel with the other
>>>50 and the 12v will deliver exactly the same flux and power,
>>>The 24v power is 24^2/R equal to 12^2/(R/4). The original winding
>>>resistance is cut by 4 in parlleling the 2 50 turn portions.
>>>No there's more to this subterfuge.
>>>John Polasek
>>
>>
>> It is true that the energy storage in Joules/volume is constant. But
>> there is another factor to consider:
>>
>> Lets suppose two drill handtools. If they really were resistive this
>> would give (both tools are of the same power, lighly loaded 48 Watt):
>> Tool1: 12V, so I = 48/12 = 4 A, R seems to be 12/4 = 3 Ohm
>> Tool2: 24V, so I = 48/24 = 2 A, R seems to be 24/2 = 12 Ohm
>>
>> BUT: a motor does not act as resistor. In fact the resistance of the
>> windings is very low. For example 0.5 Ohm (measure it if you do not
>> believe this). So whats happening that makes it look like a resistor?
>> The answer is that the motor is a the same time a generator, dynamo.
>> When you do not load the motor heavily, the motor generates an EMF
>> (electrical feedback). A DC motor with magnets can actually be used as
>> generator. This feedback voltage depends on the rotation-velocity.
>>
>> So the 12V drill at 48 Watt load generates 12V-4A*0.5 Ohm (resistive
>> losses) = 12-2 = 10 Volt. 2 Volts are lost in heat!! When you load it
>> heavily where this drill almost stops (so no generatorfunction), the
>> current is 12V/0.5 Ohm = 24 A. This gives 24A*12V = 288 Watt max peak
>> power (Try it, but don't do this too long at home, burns out the
>> drill). Remember now that the torque = current * windings.
>>
>> The 24V drill at 48 Watt has because it has twice the windings also
>> twice the internal resistance, so 0.5*2 = 1 Ohm. In our example at a
>> 48 Watt load the gerator delivers 24 - 2A*1 Ohm = 24 - 2 = 22 Volt.
>> Notice that at light load the losses are the same. But now we have to
>> drill a concrete wall where the drill almost comes to a stop. The
>> generator voltage is near to zero. The current is 24V / 1 Ohm = 24 A.
>> But the power is 24V*24A = 576 Watt!!!!!. Twice the power!The current
>> is the same as in the 12V model, but twice the windings, so twice the
>> torque (Note that the real technician is not interested in Watts, but
>> wants the torque at heavy load).
>>
>> So twice the power (I know, torque) at full load where you are in fact
>> using the device a bit out of spec. When you load it not too heavily,
>> the battery will last the same time in both devices as the same power
>> (I know, Joules) is used. Only at heavy load the higher voltage model
>> gets an advantage. But only when they have the same quality of motor
>> (it is a bit tempting for the manufacterer to use a somewhat cheaper
>> motor in the 48V devices, degrades performance not so much as in the
>> 12V device). And of course the same gearbox.
>>
>> Do not try this at home, or make at least a video of it and post that
>> to give us a good laugh.
>
>He was suggesting splitting the 24v motors windings in two, wiring them
>in parallel and sending 12v to each. This would provide, as he says,
>exactly the same performance with 12v as with 24v.
>
>The advantage of the 24 over the 12 is that the 24 *usually* has twice
>as many cells, thus twice the staying power.

Beg to differ. A 24 volt nicad has 20 cells:
UUUUUUUUUUUUUUUUUUUU
That's 20 cells and 38 walls (vs 10 & 18) that not only take up
space, but have to be soldered together. The probability of failure is
twice as high. There is in fact a loss in "staying power" just due to
the extra walls.

>If OTOH both packs were
>identical except for the parallel wiring in the 12v version, he is
>correct, a motor could easily be manufactured to equal the 24v version
>by simply paralleling the winding. OTOH, there is still this advantage
>of the higher voltage version, that the wire size, and any other control

Forget wire size. The only advantage of lower current is in the
(admittedly flakey) battery contacts and switch surfaces.

>components, can be halved in ampere rating with the 24v version, thus
>saving a few dimes on every unit, while also eliminating IR losses in
>the delivery circuit. So IOW, by Polasek's reasoning,

I think you mean by YOUR reasoning (or whatever process you use to
come up with for example "twice the staying power").

>we shouldn't need
>high voltage cross country transmission lines, the same power can be
>delivered to the homes at a much lower voltage. Sure, and even if this
>were magically correct what would the advantage of a lower voltage drill
>be?
Lower failure rate.
>He was just pissed because his battery packs keep crapping out on
>him. He probably doesn't know that you aren't supposed to over-torque
>the drill or battery damage can occur:)
>
>hvacrmedic
John Polasek
John Polasek
http://www.dualspace.net
.

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