Brushless DC Motors Expand Reach, Take on New Challenges
By Bill Schweber
Historically, the only way to drive a motor from a DC power source was by using "brushes," usually made of graphite, to conduct power to contacts on the rotor as it turned. These brushes commutated (that is, reversed the apparent polarity of current), and so kept the motor turning despite application of DC rather than an alternating waveform. Thus, the motor became the switching control that changed the applied DC into an AC waveform.
This technique certainly worked in billions of motors from small to large, but also had unavoidable drawbacks:
- The brushes wore down with use from mechanical contact, and so they had to be replaced periodically.
- The make/break/make current flow between the brush and the contacts was a large source of EMI (if you looked into the vent slots of most brushed motors, you would see sparks) and was also a problem in explosive environments, where specially sealed motors were needed.
- Anything more than basic on/off control of the motor, such as speed or torque control, was difficult if not impossible, since the primary control parameter the user had was applied voltage value.
- Efficiency was low and very hard to increase, beyond using lower-friction bearings, different winding configurations, and careful mechanical design.
The limitations of brushed motor control led to the need for a better way to control a DC motor. Brushless DC (BLDC) motors, which are based on different physical construction and control, have provided the answer.
In most BLDC designs, the rotor of the motor has permanent magnets attached around it. Stationary electromagnetic coils are placed around the non-rotating part of the motor; usually three coils are used. By electronically controlling/ commutating the current flow to these coils, their magnetic fields are switched, which then causes the motor armature to rotate. In effect, the electronic commutation changes the DC source into an alternating voltage; this is not an AC line voltage, but nonetheless an alternating voltage.
The BLDC motor is made possible due to a combination of technical advances:
- Availability of high-strength permanent magnets.
- Development of efficient, low-loss, fast-turn on/off devices that can handle substantial current and voltage- primarily MOSFETs- which can act as electronic switches for the coil currents.
- High-performance microcontrollers that can implement the software and algorithms needed to control the current-switching MOSFETs.
BLDC motors can be much more efficient, and have a lighter rotor, than equivalent power brushed DC motors, since lower weight means faster acceleration. Even more important, the motor performance can be tailored to the applications, since firmware controls the commutation rather than the motor's rotation itself. Users may choose to emphasize motor position, accuracy, velocity, acceleration, torque, efficiency, or other parameters and trade different balances among these parameters, depending on their priorities, all with the same physical motor.
The virtues of BLDC motors and associated controllability, yielding optimum performance for a given application, have had an interesting effect: Many applications which previously used line-powered AC motors and AC drive controllers now use BLDC motors instead. The incoming line AC power is converted to DC first so that a BLDC motor can be used, of course.
Sensor versus sensorless designs
One of the issues in BLDC motor operation is how the controller knows the position of the rotor and thus when to switch the coils to achieve the desired motion profile for position, velocity, or acceleration. The obvious way is to use a sensor or encoder on the shaft as the feedback transducer. Depending on type, these encoders can provide relative or absolute position information to the controller. Many encoders are available, including those based on magnetic-coil resolver, optical, capacitive, and Hall-effect principles, each offering trade-offs in cost, ruggedness, and resolution.
In recent years, there has also been increased use of sensorless BLDC motor control. In this topology, the back EMF of the motor drive coils is sensed by the controller, and can be used to infer rotor position. When combined with techniques such as field-oriented control (sometimes called "vector control"), this approach can work well enough, especially in applications that do not need the highest accuracy, such as appliance or car window motors. However, without a discrete sensor to provide rotor information, the sensorless approach does not work well at low speeds, where there is little or no back EMF. That approach also requires a more powerful processor since the calculations of the inferred feedback can be complex and must be executed in real time. On the other hand, it saves the cost of the encoder, which may be substantial compared to the cost of a more powerful processor.
An important variation of the sensorless BLDC motor is the stepper motor. Steppers are used when precise positioning is the main requirement, more than velocity or mechanical power; they are usually relatively small and used for tasks such as moving print heads or advancing sheets of paper. The stepper is a BLDC design, but with many electromagnetic poles around the rotor, which itself has a toothed arrangement of permanent magnets.
By having the microcontroller sequentially energize the appropriate poles, the core can be directed to step quickly and crisply, without overshoot, to a desired angular position; no mechanical brake is needed. A stepper motor usually has angular resolution of about 1° depending on design, but there are ways to direct it to "microstep" and achieve even finer angular resolution without adding more poles. The stepper can be controlled via the electromagnetics to actively maintain the desired position as well, despite external forces, up to specified force limit. Stepper motors have lightweight rotors so that they can accelerate and move quickly, and also not have momentum from their mass that will carry them past the desired stopping point.
Controlling the BLDC coils
There are three distinct elements in the path to controlling the current flow to the BLDC coils: the microcontroller that develops the PWM signal for the motor control, the gate drivers for the power switches, and the switches themselves, which are almost always MOSFETs. For very small motors, it is possible to get a microcontroller with built-in gate drivers and even some with integral MOSFETs. In most cases, however, the BLDC motor requires external discrete drivers and MOSFETs.
The most common connection configuration for the MOSFETs is the H-bridge or full bridge (Fig. 1) used with a bipolar supply. By turning on the MOSFET pairs located across diagonal corners, using transistor pair 1 and 3 or pair 2 and 4, the coils are energized in one direction or the other, so the motor can be reversed. Note that in the conventional H-bridge, the motor itself is often not connected to ground, so the MOSFET drivers may need to be electrically isolated from the microcontroller, which is usually done with a pulse transformer or optocoupler.
Figure 1: The basic H-bridge uses an array of four switches to control current flow to a load, often from a bipolar supply; when transistors 1 and 3 are on, a motor coil is energized one way; with transistors 2 and 4 on, the coil polarity and thus direction of rotation is reversed.
The BLDC motor coils can be positioned in various mechanical arrangements to achieve different performance attributes. The selection of the appropriate MOSFET is a complicated subject, and is dependent on voltage rating, current rating, switching speed, efficiency, losses, and many other factors. The MOSFET driver must be appropriately matched to the MOSFET's size and characteristics.
Due to the high volume of BLDC motor applications, vendors offer many suitable and focused controller ICs. The PSoC 3 from Cypress Semiconductor is an example (Fig. 2). Note the complexity and sophistication of function blocks within the PSoC as controller, yet the cost for such ICs is low and the benefits of BLDC motors worth it. (Also note that since Cypress is focused on the PSoC, the H-bridge driver, the H-bridge itself, and the motor are shown without detail.)
Figure 2: The PSoC 3 microcontroller from Cypress Semiconductor is a good fit for sensorless BLDC motor control, as it contains all the functional blocks needed except for the MOSFET drivers and MOSFETs. (Source: Cypress)
For the interface between the microcontroller's PWM signals and the bridge's MOSFETs, an IC such as the MCP8024 3-phase BLDC Power Module from Microchip Technology (Fig. 3) can be used. Its internal complexity simplifies the interface task by handling the details that would otherwise require additional circuitry. This IC integrates many features: three 12 V/0.5 A half-bridge drivers for external high-side/low-side MOSFETs, a comparator, a voltage regulator to provide bias to a companion microcontroller, power-monitoring comparators, an overtemperature sensor, two level translators and three operational amplifiers for motor phase-current monitoring and position detection, among others. In addition, the drivers have shoot-through, overcurrent, under- and over-voltage lockout, and short-circuit protection, all essential features in a motor-driver applications. Despite this high level of functionality and features, it is compact, and available in 40-lead 5×5 mm QFN and 48-lead 7×7 mm TQFP style packages.
Figure 3: The MCP8204 3-phase BLDC Power Module from Microchip Technology provides a complete interface between the microcontroller and the BLDC motor's MOSFETS; in addition to the supporting the basic motor-drive features, it also incorporates critical monitoring and protection functions which would otherwise require considerable external discrete circuitry. (Source: Microchip Technology)
The BLDC motor is versatile, efficient, and highly controllable source of rotary motion from subwatt to kilowatt power ranges. Due to advances in MOSFETs, MOSFET drivers, processors, and high-energy magnets, it is often a superior alternative to the older brushed DC motor technology. Its technology is also the basis for the stepper motor, widely used for precise positioning at carefully defined angles.