History of Power Tools
Until after World War II, the US power tool market was largely focused on the industrial metal working and wood working industries. The home and professional markets were largely left unaddressed. It wasn’t until the 1950s that equipment makers realized the potential of the home and professional sectors. Shown below is a photograph of three drill types. The image on the left is a brace and bit. Prior to the early 1950s, this was perhaps the most common drill type used by homeowners. (Yes, it was my dad’s and yes it still works!) On the right is a ¼-inch corded electric drill, typically used by homeowners around the 1950s, and in the center is a late 90s cordless electric drill. The technology only continues to progress!
Figure 1. Evolution of Drill Technology
This transformation in the capabilities of tools available to homeowners and professionals—from the extreme manual effort required to drill a hole to the need to drag around an extension cord to the almost effortless battery-powered device—is simply amazing. How did this transformation come about? It was through the R&D and marketing efforts of both Black & Decker and Milwaukee Tools. Today, powerful, low-cost drills, saws, and other tools are readily available.
In case you’re ever on Jeopardy, here are some fun facts:
- Ford Motor Company let a contract to what is now Milwaukee Tools to produce a ¼-inch drill for use on their assembly lines. This drill weighed less than 5 pounds and was invaluable to the manufacture of the Model A.
- Black & Decker patented the first trigger switch in the late 1910s. Up until that time, starting the device required pressing on a pressure sensitive pad on top of the drill.
- During the World War II years, Alonzo Decker discovered that workers were “borrowing” power tools to do their home jobs, leading to the development of their home line of power tools (as seen on the right of the photo above).
- Battery powered tools were first envisioned in the late 1950s. The most notable early battery powered tool was used by the Apollo 15 astronauts to drill a core sample on the moon.
Recent Advancements
Improvements in batteries (both their power density and ease of charging) has spurred the development of a diverse range of applications for homeowners and professional tradespeople, such as battery power drills, saws, nail guns, lawn mowers, edgers, and more. The list of potential applications continues to grow—especially with many tools now being designed around a common battery type, thus eliminating the need for numerous different batteries for your many devices. Although battery powered tools don’t provoke the range anxiety often associated with electric vehicles (EVs), longer run time is a constant target. This means less time spent changing and charging the battery and more time spent doing the task at hand. Imagine being an early riser and being able to cut your lawn at 7 AM without waking your neighbors. Battery power devices give this level of flexibility.
In addition to improvements in battery performance, overall device performance can be improved through advancements in motor design. Most modern battery powered devices still utilize a series-wound universal style electric motor. This motor is a proven design that has been upgraded from the old brushed design with electronic commutation.
The advantages of this motor are illustrated in the speed vs. torque curve below. This motor has a very high no-load speed (often 20,000 RPM or more) with the advantage of increasing torque with decreasing speed, as shown in Figure 2. The reason for the high no-load speed is the fact that the armature and field winding are connected in series. As the speed increases, the flux field creates a counter EMF in the armature, resulting in lower currents and higher rotational speeds. However, when a load is applied, RPM decreases and the current increases. Because the torque output of this motor is directly related to the current, higher currents at lower speeds generate more torque.
Figure 2. Speed vs. Torque Curve for Series-Wound Universal Style Motor
However, the high rotor speed of the universal motor necessitates a transmission to step down the motor speed and facilitate its use in many battery powered devices. This transmission is often quite complex with up to 30 individual gears to affect this variable speed. Another consequence of the transmission is added weight and overall length of the tool. Although not a serious complaint for the average homeowner, the professional contractor may begin to suffer fatigue after long periods of use. Thus, anything that can be done to further lighten the weight is a definite advantage.
One way to lighten and shorten the tool is by using an axial flux motor (AFM). Axial flux motors have the following advantages:
- Lighter weight for the same torque
- Higher power density
- Higher torque to weight ratios
- Potential rotor speeds up to 30,000 RPM
- Greater (often 90% or more) energy efficiency, enabling longer run times
- Reduced material usage (both copper and steel)
- Recyclability of copper
- Greater design flexibility
Figure 3. Speed vs Torque Curve for Axial Flux Motor
A distinct advantage of the axial flux design is relatively constant torque over a range of rotational speeds. This is an inherent advantage of all permanent magnet motors. The decrease in torque with increasing speed noted in Figure 3 results from the generation of back EMF. As the speed increases, the back EMF also increases, thus limiting both the speed and input current. Much the same as with the series motor, torque is directly related to the input current.
However, utilizing a properly designed AFM can achieve this relatively constant torque even at low speeds, eliminating or reducing the need for additional gears in a drill transmission. Additionally, AFMs exhibit reduced iron losses relative to a radial flux design of the same overall size, offering the potential for longer battery life and run time. Recent developments in the yokeless AFM provide enhanced design flexibility with the possibility of even greater torque; some report up to a four-fold increase.
How to Optimize Your Motor Designs
From a manufacturing perspective, AFMs can utilize pre-wound bobbins, implying easier assembly and the possibility for copper recycling. The topic of copper recycling is becoming more important with the increase in EVs. An EV is estimated to contain as much as five times the amount of copper of an internal combustion vehicle. Thus, to fully exploit the green advantages of electric mobility, there is a significant need to both reduce the amount of copper used in motors and find an easy method to efficiently recycle the copper.
If the transition to an axial flux geometry is practical in your design, you may be grappling with the manufacturing of the laminate steel stator core. Soft magnetic composites (SMCs) are the answer to this problem. SMCs offer the combined benefits of unique 3D design and flux carrying capability with even greater reduction in total iron losses. The net result is a more powerful electric motor with longer battery life and reduced material consumption. The ability to use pre-wound bobbins for the field current adds even greater manufacturing flexibility. Plus, the ability to simply grind up the SMC stator addresses the issue of copper recycling.
At the end of the day, SMCs give you higher performance and lower cost. This is achieved through net shape manufacturing and increased raw materials sustainability thanks to a simplified circular economy.
