• Robert

Cordless Tool Power - a longer discussion

Updated: Mar 1, 2019

What does the word power in power tools actually mean?


Without getting too technical, power tool mechanical power is all about rotational force and speed and power tool electrical power is about voltage and current. Rotational force is known as torque. It is a mistake to confuse force, torque or speed for the term power.

In power tool terms, a good example of how mechanical power works is a two gear power drill. The slow speed gear setting is not more powerful because it will drill larger holes and the faster gear setting is not less powerful because it can only drill smaller holes. The truth is that both gears produce approximately the same* power output because the motor power input is the same in both cases. However, in gear 1 the output torque (rotational force) is high and the output speed is low. In gear 2 the opposite is the case, the speed is high and the torque is low. When comparisons are made of how easily different power drills can turn a large drill bit, it is not how powerful the power tools are that is being compared but how much torque they can produce i.e. how torquey they are. (*Getting technical just for a moment: As with most things in life, this is an approximation and in reality because of the different efficiencies between the gears the power outputs may vary slightly but the principle is still correct).


An alternative example of how mechanical power works is to compare a powerful racing car, like a Formula 1 car or Indy car, with a tractor. Racing cars have very powerful engines but you wouldn't consider putting an F1 engine in a tractor to plough a field. A tractor engine, on the other hand is great for ploughing a field, but not much use for racing. Of the two, the racing car is likely to have much more power but the tractor engine is likely to be much bigger. High power is not the only thing required to plough a field, you need force, and lots of it for a long duration, the time it takes to plough the whole field and more! You get force from an engine or a motor in the form of rotational or twisting force which is known as torque. A tractor engine has lots of torque, much more than a racing car engine, but much less power. How so? The engine of a racing car turns at much higher speed than a tractor engine. Mechanical engine power is the product of torque and speed. You might think of it as a ratio thing. So, although the racing car engine has much less torque than a tractor engine, because it has much more speed, it can have much more power. For example, if the racing car has twice the power of the tractor engine, that power might be made up of only a tenth of the torque that the tractor engine can produce but 20 times the rotational speed, or rpm. (Getting a bit techy again and doing the maths, you end up with 20 x speed: 1/10 torque = 2 x power.) Of course the amount of power and torque a tractor engine produces compared to a racing car engine is not the whole story. If you were to fit a racing car engine in a tractor you would need to gear it down enormously and that would introduce all sorts of efficiency losses, quite apart from the fact that a high revving racing engine drinks fuel at a ferocious rate and would be horrendously expensive to run. An F1 or Indy racing car has a lightweight engine which is designed to complete a race and then be rebuilt, it is not designed for heavy duty work. A tractor engine is designed to work day in day out every day for years without a rebuild. A racing car engine is very lightweight, runs very fast producing lots of speed but not much torque, and wears out! A tractor engine is heavy, runs slowly producing lots of torque but not so much power, and keeps going. Which do you want for which role? You choose the best compromise and the same applies to power tools.


Most cordless power tools don’t display the power input or output on their rating plates. This is for a number of reasons: Firstly, the power varies according to the state of the battery charge level; secondly, professional-grade power tools can use different size batteries and the maximum power available from one size of battery may not be same as from another, even when they have the same voltage; thirdly, as they are not connected to the mains, cordless power tools don’t have to show the input power; fourthly, a comparison of the power input ratings for a corded power tool and a cordless power tool would not be helpful, at best, and probably misleading at worst.


The ratio, or comparison, of power input to power output is known as the efficiency of the power tool. Power input is always higher than power output. The higher the power output, compared to the power input, the greater the efficiency. A power tool with a 100% efficient power input to power output is the theoretical maximum but in reality it is only a dream!. A power tool with a power output that is 50% of the power input is feasible.


There are four main components of a cordless power tool that most determine the final power output and efficiency: the battery, the control system, the motor and the gearbox. Each of these parts behave in different ways when the power tool is in use.

The battery

The power provided by a battery is normally greatest when the battery is fully charged. As the battery charge depletes, the power the battery can provide also normally reduces. However, this is not the whole story. Cordless power tools often work their batteries very hard. For example when drilling a large hole into a wall the drill can be on full power for a few minutes. The thing is most batteries like an easy life! Providing full power for even a few minutes is not what a battery calls taking it easy! Normally the bigger the battery the easier its life will be when connected to the same power tool. To put it another way: If you have one power tool with a choice of batteries of the same quality, one physically small and one physically large, and you have two equal size largish holes to drill, in most cases the larger battery will work for longer and it will also work better and most likely have a longer life. Battery technology is very complex and the numbers written on a battery, the voltage in V and the capacity in Ah, can’t tell the whole story. Suffice to say, like with many things in life, it’s about the quality. For a more detailed discussion on batteries please see blog post: Talking Battery Performance.

The control system

If your cordless power tool has variable speed then it will have a control system. Even if it doesn’t have variable speed, quite rare for many types of power tool these days, it will have an on-off switch which is still a very basic form of control. On brushed power tools the control system regulates the electrical power to the motor at the start. On some brushed power tools it also continues to do so all the way up to, and including, full power. However, on many higher performance brushed power tools, when the throttle trigger is pressed fully the control system is bypassed by a switch. This has the advantage of improving the efficiency of the control system so that when the power tool is working hardest one area of inefficiency is all but eliminated. On a brushless power tool it is not possible to bypass the control system, as the brushless motor will not work without the control system. However, as the control system is a vital part of the motor it can have a very significant effect on the efficiency of the brushless power tool. A well-engineered brushless control system can be much more efficient than a poorly designed system with the result that both maximum power and battery run times can both be significantly affected by the design and engineering of the control system. Bottom-line, don’t assume that all power tools have control systems that have the same efficiency. KwikPro motor handles have control systems that have been optimised for the best power and efficiency compromise. KwikPro Gold for frequent heavy duty work and KwikPro silver for more occasional and medium duty use.

The motor

Motors are not 100% efficient at converting electrical power into mechanical power, with the possible debatable exception of those with supercooled magnets, perhaps. Generally, if a motor is running hot then it is converting electrical power into heat instead of mechanical power. Under these conditions it is wasting power. There are not many things that convert one form of power into another that don’t waste some power in the process. However, electric motors can be far better, far more efficient, than many other systems. As discussed in blog Brushed Versus Brushless Motors, the efficiency of the motor in a power tool is often an engineering compromise. Different motors can be designed to work more efficiently under different conditions. The skill of the power tool manufacturer's motor designer is to decide under what conditions the power tool will mostly be used and juggle that with making the motor work best under those conditions, while not working too badly under the less frequently used conditions. For example, if a power drill is likely to be required to work mostly at full power, then the motor will need to be most efficient at full power. Conversely, if another power tool is most likely to be used most often at, say, mid power then it is likely that this is where it should be designed to be most efficient. Generally, the compromise is made when the motor is operating at performances outside where peak efficiency is required. In some power tools this compromise can make the overall efficiency of the power tool much lower if it is not used as intended, with consequent reduction in battery run times.

The gearbox

The quality of a cordless power tool gearbox can make a massive difference to how well the power tool performs and how long it runs for before discharging a battery. Consider that a gearbox is a series of gears with shafts and bearings. All these parts have to be turned by the motor. This absorbs power even before the power tool has done any work, i.e. before you have used it to do the job you want it to do! When all the parts in a gearbox turn they slide together and encounter friction forces that are trying to stop the sliding and turning. Oils and greases improve matters but can’t entirely eliminate friction. Higher quality gearboxes will often have lower friction than lower quality gearboxes. In addition to friction, inertia has to be overcome when the power tool starts and or speeds up, which further drains the battery. And, as usual that isn’t the whole story. Like all the other parts of a cordless power tool, the gearbox is just one more part that has to be optimised for efficiency for the conditions the power tool is likely to be used most. In addition a balance between strength and efficiency also has to be achieved. A heavy duty gearbox may well have higher friction and heavier components with greater inertia than one designed for less arduous work but it won't break when put to the use intended. Performance, efficiency and longevity all force design compromises.


Conclusions

So, the power output from cordless power tools is perhaps more complicated than it might at first seem. Power is often confused with torque. The most powerful power tool may not have the most torque. A very torquey power tool may be heavy and difficult to hold one-handed. The efficiency of power input to power output is a compromise influenced by many factors. The cordless power tool with the greatest efficiency may not be the power tool that will do the job you need it to do. It may use parts that, to be lightweight and efficient, are not strong enough for heavy duty work. So when you decide on which cordless power tools to buy, choose the system that is designed for the kind of use you are likely to need it for. The power tool manufacturer has optimised the power tool for that purpose. It would be unfair to expect a cheap, lightweight cordless power tool with a small battery, designed for occasional light duty use, to perform as well for frequent heavy jobs as a power tool designed for heavy duty work.


KwikPro power tool systems use motors, gearboxes, control systems and batteries that have all been carefully specified to provide the optimum power and torque performance and to work as efficiently as possible as drills, drivers, grinders, saws or multi-tools, according to the duties for which they are intended to be used. KwikPro Gold for frequent heavier duty work and KwikPro Silver for more occasional medium duty use.

© Copyright 2018-2019 - Power Tool Systems Limited - UK - www.kwikprotools.com

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