Two technological developments are reshaping cordless tool lines: the ongoing battery amp-hour escalation race and the rise of brushless motors. Modern brushless motors have been in use for 50 years, but only recently have they found their way into portable power tools.
For the tradesman, there are a couple of questions: What are the benefits of brushless motors, and do they outweigh the added costs? In this article, I will explain the technology and the related claims of manufacturers. Be sure to see the tool test on page 16, because it addresses some of the claims made for brushless impact drivers, which are the most widely distributed brushless cordless products on the market.
Standard universal motors, like this one from a trim router, have electromagnetic windings on both the rotor and the stator. The electromagnets create magnetic attraction, which makes the motor go.
Corded handheld (and most bench-top) power tools have universal motors — meaning they can be powered by alternating current (AC) or direct current (DC). However, most of these tools are configured to run on AC power only.
Aside from the bearings and wiring, universal motors have two basic components: a rotor and a stator. The rotor is the center part of the motor that spins; the stator is the stationary component surrounding it. Both the rotor and the stator contain field windings, which are electromagnets that come on when power is applied. The magnetic fields from these electromagnets are what make the motor go.
Magnetic attraction pulls the rotor toward opposing magnets on the stator, creating torque and causing it to spin. The electromagnets are activated in sequence by commutation to keep the rotor spinning and to adjust the speed and direction of rotation.
Brushed commutation. The stator is fixed, so power can be supplied to it through a hard-wired connection. But the rotor spins, so power is supplied to it through conductive carbon brushes that ride against the surface of the commutator — which is basically a rotary switching device. The commutator is segmented, and the gaps between the segments create the intermittent contact that times and sequences the energizing of the rotor electromagnets. This is true of any brushed motor, corded or cordless.
Power and space are at a premium in cordless tools, so the stators in brushed cordless models contain permanent magnets instead of windings because magnets are smaller and do not require power. Such permanent-magnet motors can be highly efficient if they contain strong rare-earth magnets. Among the qualities of permanent-magnet motors are good low-speed torque and magnetic self-braking action.
Electronic commutation. A brushless motor — also called an electronically commutated (EC) motor — is essentially a permanent-magnet motor turned inside-out; it has electromagnetic windings on the stator and permanent magnets in the rotor. The magnets require no power, so there is no need for brushes and a commutator.
Since it lacks the mechanical switching provided by a rotating commutator, a brushless motor enlists a microprocessor (a tiny onboard computer) to provide electronic switching in a process called electronic commutation. The position of the rotor is monitored by the electronics, which time the electrical pulses to the electromagnets in the rotor, thereby controlling the direction and speed of the motor.
Brushless Motor Qualities
I interviewed tool-company engineers and product managers to get their take on brushless cordless tool motors. Some of what they told me was in conflict, in part because each person sought to promote his own company’s products. That said, everybody generally agreed with the following.
Efficiency. Brushless motors are more efficient because there is no friction loss from brushes rubbing against a commutator, and because electronic commutation (computer control) is more precise than mechanical commutation. The higher efficiency of these motors makes for greater power and longer runtime than a conventional motor of the same size.
Performance. Brushless motors run cooler, quieter, and with less vibration. The absence of brush arcing reduces radio frequency and electrical noise that can interfere with electronics. Unlike brushed motors, which are often wired to be more powerful running forward, brushless motors are equally powerful in either direction.
Durability. Brushless motors are more durable because there are no brushes to wear out. The lack of electrified windings on the rotor reduces heat at the core of the tool, where it’s more difficult for it to dissipate. To prevent the electronic controls from being damaged, some manufacturers encase them in plastic (or “pot” them) to protect them from dust and moisture for the life of the tool.
Design. Brushless motors have a greater power density, so for a given level of output they can be smaller and lighter than conventional motors. Since these motors are electronically controlled, it’s easy to add performance features such as multiple speed settings, soft-start, and soft-fade when the tool is switched off.
Simply replacing a standard motor with a brushless one won’t automatically boost performance. To improve the tool, it is necessary to treat the motor, control electronics, and battery as an integrated system.
Most cordless tools have permanentmagnet brushed motors. The rotor has windings, but the stator contains permanent magnets. This is a more compact and efficient design, because magnets require less space than windings and do not consume power.
The brushes in this four-pole motor transfer power to the spinning rotor by riding against the commutator. The commutator functions as a rotary switching device, reversing polarity by directing power in sequence to individual windings on the rotor.
A brushless motor is like a permanent-magnet motor turned inside-out; it has windings on the stator and permanent magnets on the rotor. Brushes are not required because the rotor consumes no power.
Because it lacks brushes and a commutator, a brushless motor must be electronically commutated. A controller senses the position of the rotor and relays that information to the microprocessor that regulates the tool.
The microprocessor functions as an onboard computer, controlling the speed and direction of rotation while monitoring the condition of the motor and battery.
Credit: David Frane
The controller on this brushless impact driver has been “potted” — encased in a thick rubber membrane that will protect it from dust and moisture for the life of the tool.
Credit: David Frane
Because they are more compact than brushed motors, brushless motors allow manufacturers to design smaller, lighter tools.
Electronic control made it possible for the manufacturer of this brushless impact driver to add advanced features like digital speed and impact settings.
Credit: David Frane
Brushless motors are not the best choice for every tool — and thus far, no manufacturer has a brushless-motor cordless tool that will out-muscle one of its own standard-motor models. Most brands only put them in tools that operate under light, near-constant loads where there are no rapid changes in rpm. These would be tools that are difficult or impossible to stall: rotary hammers, impact drivers, impact wrenches, and the like.
Manufacturers have generally avoided putting these motors in tools where loads vary greatly and there are sudden demands for peak torque and high current draw (like recip and circular saws). But that may be changing; the latest generation of brushless cordless tools contains some heavy-duty drill/drivers and a chain saw.
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 similar brushed models (instead of replacing them outright). It is also why brushless motors are limited to the most popular cordless tools like drill and impact drivers, and are not used in less frequently used add-on tools. While it’s impossible to predict the future, I expect to see larger lines of brushless-motor tools from the major brands, and brushless tools at more competitive prices.
—Michael Springer is the former executive editor of Tools of the Trade.