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Power tools have evolved significantly over the past century. Today, they are cordless, lightweight, and battery-powered, making them a great addition to our lives. So what are the factors driving the evolution of power tools? In addition to power tool enthusiasts, the evolution of power tools, especially cordless power tools, can be largely attributed to advances in semiconductor technology.
Next, we will look at the key aspects of battery-powered cordless power tools, including the drivers of their development and the challenges they face along the way. We will also learn how microprocessors and brushless DC motors play a key role in transforming the power tools used today. We will also outline how the use of brushless DC motors in power tools can provide manufacturers with a competitive advantage.
The main components of a power tool
The first component of a power tool is the power source. All power tools can be divided into corded and cordless.
Corded power tools - Power is AC and need to be plugged into a power source to operate.
Cordless or cordless power tools — rely on electrical energy stored in batteries, which come in different chemistries such as nickel-cadmium (NiCd), nickel-metal hydride (NiMH), and lithium-ion (Li-Ion).
Lithium-ion batteries have become the most predominant battery due to their higher energy density and adaptability to hold a charge.
The second component is the actuator or motor that converts electrical energy into mechanical energy. This motor can be a common AC/DC brushed motor, a brushed DC motor, or a brushless DC (BLDC) motor. Many of today's tools already use a three-phase BLDC motor topology.
Finally, a switch is needed to control the transfer of energy from the power supply to the motor. This component can be as simple as a current cutout, controlling whether or not current flows. It can also be a slightly more complex component, such as a potentiometer, which allows the user to specify how much energy flows from the power supply to the motor.
Power Tool Challenge
For the first 100 years of power tool development, designing and manufacturing drills, grinders, thread grinders, screwdrivers, blowers, saws, etc. only required a power source, a motor, and a switch/potentiometer. However, in the 20th century, the advent of high energy density batteries changed this situation. In addition, green energy solutions have emerged in the market and are integrated into various forms of design.
The challenge was how to continue to use the potentiometer to control the speed of the tool without passing high current through its resistive element. As we will later discover, this is a relatively simple solution. The motor, on the other hand, proved to be a more significant and complex challenge.
In the early days of power tools, the motors used were either general-purpose brushed AC/DC motors for corded tools or brushed DC motors for cordless tools (Figure 1). Because both motor topologies are essentially brushed motors, the motor motion is achieved by using carbon brushes to pass current to a copper commutator, which creates an internal rotating magnetic field. By placing the electromagnet windings and commutator together in the rotor and the permanent magnets in the stator, we have two magnetic fields that constantly interact and produce the desired motion.
Unfortunately, this results in very high friction between the brushes and the commutator. Over time, this high friction will eventually destroy the motor. This friction is wasted energy in the form of heat. This is energy that flows away from the power source without producing any useful work. Systems that run around this topology are no more than 80% efficient (in the best case). This means that 20% of the energy inside the battery is being used to generate heat.
If you drill a hole with a battery-powered drill, one-fifth of the power will be used to generate heat, which seems inefficient.
Figure 1: A brushed DC motor.
Meeting the Challenges with BLDC Motor Technology
Given the challenges described above, it is clear that replacing or eliminating brushes and commutators is essential. This is highlighted in the three-phase BLDC motor topology (Figure 2). BLDC motors achieve the same rotational motion without the use of brushes or a mechanical commutator. Instead, we generate the rotating magnetic field electronically. Using electronic circuits, we can form two interacting magnetic fields to generate motor motion. The advantage is that there is no friction between the rotor and stator components, which improves reliability and energy efficiency.
The adoption of BLDC motors in manufacturing is accelerating...
Figure 2: In a three-phase BLDC motor topology, the brushes and commutator must be replaced or eliminated.
The efficiency of a three-phase BLDC motor can reach up to 96%. This means that only 4% of the energy in our battery is wasted as heat.
As with all designs, there are challenges with BLDC motors. Brushed DC motors can address the inherent problem of magnetic field alignment to achieve the most efficient motion profile. This can be achieved if the commutator sequence is designed and positioned so that the rotating magnetic field is always coordinated with the field of the permanent magnets. However, since BLDC motors do not have a physical commutator, this action is accomplished using commutation logic timing. In order to achieve the efficiency we are talking about, the two magnetic fields must be aligned as perfectly as possible using control circuitry (as shown in Figure 3).
Figure 3: BLDC motors do not have a physical commutator and must use commutation logic timing. To improve efficiency, control circuitry must be used to align the two magnetic fields as perfectly as possible.
This complex circuitry extracts the position of the rotor to electronically align the two magnetic fields. For a three-phase BLDC motor, this module typically consists of a microcontroller and a three-phase inverter power stage, with sensors (such as Hall sensors) used to extract the position of the rotor. Adding this circuitry does take up some space and lead to increased costs. However, manufacturers also see the benefits of being free of constraints, and consumers also demand these types of motor solutions. As a result, more and more power tools are being designed using three-phase BLDC motor topologies.
Complex power tools
Modern power tools still consist of a power source, a motor actuator and components to control the energy flow, such as a potentiometer. However, to make available all that energy reserve, we need to add intelligence.
Microprocessors provide this intelligence. Today, with microprocessors, we can monitor the power supply and provide the required drive. We can also monitor the value of a potentiometer and control the speed of a motor without having to pass current through its resistive element. We do this by using an analog-to-digital converter (ADC). The energy consumed in this operation is negligible.
However, the most important aspect of the microprocessor is to provide an efficient mechanism to properly energize the three-phase BLDC motor to obtain the efficiency improvements required for battery-powered tools. The microcontroller-based power stage provides all the tools required to generate a properly aligned rotating magnetic field, which translates into an optimal motion profile.
The PAC5527 includes a DC/DC converter that takes the battery voltage and steps it down to the different rails needed to power the different blocks of the system. It also includes the three high-current pre-driver stages needed to drive very high-power three-phase inverters (over 1kW). An ADC with a programmable sequencer allows the coordinated capture of multiple analog parameters without affecting the real-time performance of the central processing unit (CPU). It includes protection blocks to ensure that the system current remains within a certain range, preventing dangerous conditions that could cause damage to the tool while keeping the user away from hazards such as fire. Multiple general-purpose inputs/outputs (GPIOs) are provided for monitoring different signals. We can even generate a fully aligned rotating magnetic field through circuits for extracting the rotor position, which forms part of the tool library in a single PAC5527 device.
The PAC5527 produces one of the smallest three-phase inverter power drivers. As a result, power tools revolving around this solution can be designed with ergonomic efficiency while improving the energy efficiency of power tools. In addition, due to its small size and high integration, the cost structure of the entire application is optimized.
The next wave of power tool development
The adoption of BLDC motors in manufacturing is accelerating, and as technology continues to advance, these motors will become easier to use, more efficient, and more reliable. The advent of electronically controlled BLDC motors has made tools more powerful, efficient, compact, and lightweight. Qorvo will continue to deliver innovative products, such as the PAC5xxx series of components, to further advance the development of three-phase BLDC motor topologies.
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