• Robert

Brushless Versus Brushed Cordless Power Tools - a longer discussion

Updated: Mar 1, 2019

Brushless and brushed power tools may look similar on the outside but inside their motors are very different.


First off, what are the differences between brushed and brushless power tools?


Aside from the obvious, that the motor in one has electrical brushes and the other doesn't, there is rather more to this answer. In cordless power tools both power tool types usually have what are known as open permanent magnet (PM) motors - open refers to the motor housing having vents to allow air to enter and cool the parts inside the motor, usually with the aid of a cooling fan.


The differences in more detail:


1. Perhaps one of the most obvious visual differences is how the permanent magnets are arranged.


Brushless motor

Brushless:

In most brushless permanent magnet motors, of the type most commonly used in power tools, the permanent magnets rotate in the centre of the motor and form the rotor. The stator is formed by multiple laminations and coil windings. The electrical route to the motor is via fixed connections to the static field windings of the stator.

Brushed motor

Brushed:

In a brushed permanent magnet motor, of the type used in power tools, the permanent magnets usually form a fixed field stator, the bit that doesn't move, and are normally arranged around the outer inside of the motor casing, often known as the can. The rotating part of the motor, known as the armature, is quite complex and is formed from multiple iron laminations and coil windings together with a commutator, a form of rotating electromechanical switching arrangement. Carbon brushes are held in place by the motor housing and slide on the commutator to provide the electrical route to the armature.


KwikPro power tools use brushless motors.


2. A fundamental difference is how each motor type makes the rotor move.


Brushless:

A brushless permanent magnet motor will not work if simply connected directly to a DC power supply, such as a battery. There is no mechanical commutator and therefore no way to switch current in the stator’s electromagnets. Instead of a mechanical commutator, the brushless permanent magnet motor relies on solid state power electronics (usually MOSFET transistors with control driver circuits) to switch the stator’s electromagnets. The power electronics are usually external to the motor and form part of the electrical power supply route to the stator. Many brushless permanent magnet motors used in power tools also use a speed feedback loop. This usually takes the form of some optical sensors mounted near, on or inside the motor housing. These measure the rotational speed of the permanent magnet rotor and the speed information is fed back to the power electronics control circuits.


Brushed:

A brushed permanent magnet motor will work straight out of the box, when connected to a suitable DC power supply, such as a battery of the right voltage. It uses its own integral electromechanical commutator to switch the current in the armature rotor’s electromagnets to cause rotation.


3. Power tool motors are often required to work very hard and in arduous conditions, especially in professional power tools. Brushed and brushless deal with these situations differently:


Brushed:

The weakest link in brushed permanent magnet power tool motors is usually where the electrical brushes slide on the rotating commutator parts of the motor. When the power tool is working hard, sparking and heat build-up can occur at the brush commutator interface and the motor can become much less efficient. When the work is hard for too long, or just too hard, the motor’s internal parts can get very hot indeed and parts of the motor can start to melt, erode or burn and eventually something will fail and the motor will burn out.


The efficiency of a permanent magnet brushed motor, of the type used in power tools, under ideal controlled speed and load conditions, can be relatively high. However, under heavier load conditions the efficiency can drop off dramatically. This does of course depend on how the motor has been designed and specified for the power tool. The quality of the power tool may go some way to determining how badly the efficiency will drop off under heavier loads. It may also be more reliable and efficient, overall, for a power tool to use a motor that may not have brilliant efficiency under ideal or lighter conditions, if it is likely to be more efficient under heavier load conditions, if they are likely to be encountered frequently. This is a design compromise, as is the case with many engineered products, and the best compromise for the situation is usually the best performer.


Brushless:

In a brushless permanent magnet power tool motor the brushed motor’s weak link is eliminated. The brushless motor can usually withstand heavier use and achieve longer run times, for a given size of motor and battery, mainly because there are no brushes sliding on a commutator with the attendant problems discussed above. In theory, battery power can spin the motor more easily and the heat can be controlled more effectively with much less chance of burn out. Greater efficiency is the theory, however, of course in practice nothing is ever quite that simple.


Whilst in a brushless motor there is no obviously visible potential weak spot, there are some penalties. The efficiency of the power electronics used with a brushless power tool motor can vary greatly according to working conditions. The way in which these electronics are specified, and how much heat they generate under different loads and motor speeds, can have a big effect on efficiency and reliability. Effective heat-sinking of the power electronics can be an important factor, especially under very heavy working conditions. If the manufacturer has got it right, then there is great potential for the power electronics to be very efficient over a good spread of speed and loads. Power electronics are not necessarily, however, a total panacea. Overall, under certain conditions, it can be questionable as to whether some power electronics are any more efficient than brushes sliding on a commutator in a brushed motor. The electrical losses through some power electronics can mean that under light loads a brushed motor may be more efficient. However, under heavy loads the brushless motor is most likely to be more efficient because the power electronics can be more easily designed to avoid unnecessary heating. Once again it comes down to quality and the compromises made for manufacture and perhaps, most particularly, in engineering design.


4. The reliability of a power tool is likely to be a significant factor in determining its usefulness.


Brushless:

One potentially great advantage of brushless power tools over brushed is the lack of sliding parts, other, of course, than the bearings common to both motor types. In conditions where dirt and dust may be encountered the brushless motor is less likely to be affected/damaged by particles entering with the cooling air and there are no brushes creating their own dust. Fewer moving parts also usually means less wear.


As an aside, as all commutation is solid state there should not be any sparking. Brushless power tools, therefore, also have the potential to be safer in semi hazardous environments; though it should be noted that no power tool with any form of open motor should be used in any highly hazardous atmospheric environments.


Brushed:

As previously discussed, the brushed motor’s brushes and commutator interface is a reliability weak spot. In addition, the brushes wear down during use and their quality can determine how long they last and how well the power tool performs. Build-up of dirt and or brush dust can also cause significant problems. Eventually, often even after brush replacements, the motor commutator can also fail through wear, spark erosion or uneven heating.


So, should you buy brushless or brushed power tools?


If you are likely to need your power tools for frequent and heavier duty work and want the most power and longest possible run times, with similar weight and size to lighter duty power tools, you should seriously consider buying brushless power tools. Brushed power tools may be cheaper but it can be a false economy to buy cheap. Good brushless power tools can normally be expected to be more reliable and to easily outlast brushed power tools.


KwikPro power tools have brushless motors.


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