Cordless power tools are constantly evolving — and fast. Some brands’ most popular tools have a generational lifespan as short as 18 months, as their designers work to make the latest models obsolete before they even hit the shelves. Although many of the improvements are minor changes in components, features and ergonomics, marketers tout each new iteration as a must-have.
Such rapid product proliferation means that trends associated with tools come and go. Some “advancements,” such as drill/drivers with super-duty 24-volt NiCad batteries, got a lot of hype but were not well received by tool users, so they quickly faded from the spotlight. Others, such as Li-Ion batteries, made a profound difference in tool performance.
The latest buzz is about tools powered by brushless motors. Here’s a look at what they offer to help you decide whether this technology is worth your tool-buying dollars.
Stationary shop equipment such as table saws, drill presses and air compressors are powered by AC induction motors, typified by their large size, quiet operation and relatively slow, fixed-speed output. These motors don’t use brushes in their operation.
Portable electric tools are powered by universal motors, so named because of their ability to be run by alternating current (AC) or direct current (DC) power supplies. These motors are small and light, run at high speed and are fairly noisy. Miter saws, electric drills and other corded tools run on AC power; cordless tool batteries consist of DC cells.
On the left is a universal motor rotor; on the right is a brushless-motor rotor.
The two most basic working components inside the universal motor are the rotor (the central rotating spindle) and the stator (the stationary field surrounding it). Electric current is converted into movement by the basic principle of magnetic attraction, so when magnets on the rotor pull toward opposing magnets on the stator, the attractive force creates torque, which causes the rotor assembly to spin. To keep pulling the rotor around, electromagnets are used; they can be activated in sequence to adjust the speed and direction of rotation. Electrical power is supplied to the electromagnetic windings of the spinning rotor through conductive carbon brushes’ rubbing against metal segments on the shaft that make up the commutator. The spaces between the bars of the commutator create intermittent electrical contact to control the timing of energizing the opposing rotor and stator electromagnets.
With line voltage from a wall outlet or generator, enough power is available to feed electromagnets on both the stator and rotor of a motor, so corded universal motors usually have field windings on both the rotor and stator.
Small DC motors found in cordless tools have windings only on the rotor; instead of electromagnetic windings around the stator, they have permanent magnets that save space and electric current — both of which are at a premium inside handheld tools. Such permanent-magnet motors are smaller and lighter than wound-field DC motors and can be highly efficient if strong rare earth magnets are used. These motors also have beneficial traits for cordless tools such as good torque at low speeds and magnetic self-braking action.
Taking the market by storm, Milwaukee was one of the first companies to have brushless-motor cordless tools on the market, with the March 2012 launch of the company’s flagship 18-volt brushless drill/driver. The lower-voltage subcompact Fuel tools became available in 2013.
No Brushes Required
Brushless motors — also called electronically commutated (EC) motors — have one fundamental difference from permanent-magnet motors: The rotors of brushless tools use permanent magnets rather than electromagnetic windings on the rotor, so no brushes are required. With the electromagnets in the surrounding stator only, brushless motors are basically an inside-out version of the permanent-magnet motor used in most cordless tools.
Because they lack the mechanical switching provided by a rotating commutator, brushless motors require a tiny computer to manage communication and provide external electronic switching. Sensors in the surrounding stator electronics determine the position of the rotor, and the computer times the electrical pulses that energize the electromagnets around the stator to synchronize with the rotor position, setting the direction and controlling the precise speed of the motor, even under varying load.
According to Bosch, the relatively constant low-torque demand placed on rotary hammers makes them a good match for brushless motors, even if the brushless feature was never promoted, as was the case with the 36-volt 3/4-in. rotary hammer released in 2008.
Benefits and Limitations
Replacing a standard motor with a brushless one doesn’t automatically boost a tool’s performance. To create an improved tool, designers must treat the motor, its communication and control electronics and the Li-Ion battery as an integrated system.
Brushless motors are smaller and lighter than standard tool motors with a similar power output. They are also more efficient. In addition to saving the considerable frictional losses of brushes’ rubbing against a commutator, a rotor without windings has less mass and is therefore easier to accelerate from a stopped position. The precise computerized control of the motor adds to the overall efficiency. In cordless tools, this efficiency gain provides greater power delivery and longer runtime on a battery charge.
Brushless motors run cooler and quieter with less vibration than standard motors. Avoiding the friction, heat, vibration and noise caused by rubbing brushes and their constant sparking makes a big difference, and the absence of brush arcing reduces radio-frequency and electrical noise that can interfere with electronics. In addition, the lack of electrified windings on the rotor reduces heat in the tool’s core, where it’s more difficult to dissipate.
Released in early 2012, the 20-volt Max impact driver with three speed/power settings and a fully recessed bit-holder design is DeWalt’s first brushless tool.
Brushless motors are more durable and last longer because there are no brushes to eventually replace and no wear to parts other than the shaft bearings. Because the rotor has no windings, there are no vulnerable areas on this fast-moving component to be sandblasted by airborne grit. And the delicate control electronics are encased in plastic to protect them from dust and moisture for the life of the tool.
As you might expect, these advantages come at a price. With the requisite electronics package, a tool with a brushless motor costs significantly more to manufacture than one with a standard motor. This cost premium is why some brands’ brushless models are sold alongside their standard models instead of replacing them. It is also why brushless motors are limited to the most popular cordless tools, such as drills and impact drivers, not add-on tools that are used less frequently, such as cordless reciprocating saws. In the future, you can expect to see more competitive prices and larger lines of brushless-motor tools from the major brands.
Festool’s TI15 hybrid switches from impact operation to smooth drilling mode automatically when the chuck is attached. In both modes, it uses a 16-position dial to electronically fine-tune its speed and power.
Uses of Brushless Motors
Brushless motors have been in use for 50 years; they’re commonly found in computers and electronic devices for disc drives, cooling fans and videotape drives. Their precise speed control, cool operation and lack of electrical noise and radio-frequency interference make them ideal for such uses. Furnace motors and ceiling fans are growing applications that take advantage of the motors’ variable speed capability and economical and quiet running characteristics. And their high efficiency and battery-runtime benefits make brushless motors an important component in electric vehicles.
However, brushless motors are not the best solution for every tool — yet. Many companies are only using these motors for tools with near-constant, lighter loads without rapid changes in speed: tools that are difficult or impossible to stall, such as rotary hammers and impact drivers and wrenches. Loads that vary greatly or sudden demands for peak torque and high current pose a significant challenge to brushless technology. The drill/drivers and the chain saw in the latest generation of brushless tools seem to be stretching that limitation, but so far no manufacturer has a brushless-motor cordless tool that will out-muscle one of its standard-motor models.
Panasonic was the first company with a brushless impact driver (complete with three speed/power settings) launched with its first Li-Ion batteries in 2006. The 14.4-volt model (shown) set a high standard early on for this class of tool.
High-end airless paint sprayers, direct-drive lathes, electric jackhammers, random-orbit sanders and a growing range of cordless power tools and yard tools are some of the latest applications of brushless technology. Although these new offerings may seem tempting, they’re not the best investment for every DIYer. Unless you squeeze every drop of power and runtime out of a contemporary, professional-grade cordless tool whenever you use it, you will probably benefit by waiting until the technology becomes more mainstream and the prices drop. Now there are not enough tangible benefits to the average user to offset the price premium of brushless tools over standard-motor tools of similar specifications. However, if it’s time to replace an old tool, or if you just have to have the latest and greatest model, splurging on a new brushless version will net you a tool at the current pinnacle of cordless performance, one with a more efficient motor, the newest battery chemistry and the latest advanced control features.
In 2011 Stihl launched a line of 36-volt brushless outdoor power equipment, including a very impressive cordless chain saw that cuts just like a 31cc gas-powered saw.