Brushless motors for power tools
I first came across brushless motors in the form of outrunner motors a few years back powering model aeroplane propellers.
More recently I’ve heard about them being used for power tools such cordless drills, and on investigating I’ve learnt that Makita have been using them since 2003. The key advantages of this are:
- Higher Torque
- Cooler running
- Longer life
- Reduced battery consumption
- Better power to weight ratio
How brushless motors work
As the name suggests brushless electric motors don’t have brushes, so lets look at why brushes are used. The brushes are used in traditional DC electric motors to transfer the power to the coils. This allows the power to energise each coil in turn and hence generate rotation. There are a few problems with that approach. Firstly the brushes rub on the copper contacts and gradually wear down over time. The second is that the speed of rotation can only be indirectly controlled by adjusting the voltage to the coils. If the load on the motor increases the motor does not know that the speed has reduced and hence can’t compensate. Likewise if the load reduces the motor does not know that it can use less power. This wasted power is turned into heat which reduces the life of your drill.
A brushless motor gets around these issues by rotating the magnetic field using electronics. Each of the coils is energised in turn attracting a magnet(s) which then rotates. If you’ve ever looked at a stepper motor you will realise that at this point the operation is exactly the same. The difference for the brushless DC motor is that it also contains a sensor that can monitor the rotation. Typically a hall effect sensor is used as that can detect the rotating magnet. In simple controllers this would only be used to determine the speed but more sophisticated control electronics can adjust speed of rotation and torque based on the load and the trigger position.
Benefits and drawbacks
Because there are no brushes there are less parts to wear out, there’s nothing worse than smelling the ozone generated when your brushes are worn down to a minimum. The greater control over the power used means that battery life is longer, and the drill runs cooler. Makita claim upto 50% longer run time per charge for their LXT Brushless drills. Better torque and power use means that a smaller motor can be used and your cordless tool will weigh less. The main disadvantage is that the motor is more complex to make and the control electronics add to the cost. This increase in cost should be compensated for in greater life for your drill and battery so you could be better off with a brushless motor in your power tool.
Charles Atkinson writes articles for Drills and Drivers, a website that reviews the best cordless tools, explains the differences that distinguish quality power tools from the rest and helps you pick the right tool for your job.
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