Brushless DC Motors Turn to Integrated Controllers

Even though controllers and regulated power supplies serve as essential links in operating brushless DC motor systems, their size, weight, placement, and cost tend to run contrary to industry trends. The marketplace is moving steadily toward smaller, lighter, less obtrusive, and more efficient motor technology, leaving little enthusiasm for external equipment and complicated connections.

Supported by advances in technology, motor control engineers have been able to relocate controllers inside motor frames, replacing traditional external boxes, wires, and bulk. Electronic components have been designed into the internally integrated motor controllers. The time is not far off when power supply components will also be able to move inside small motor packages.

Electronic Commutation
Compared with brush-commutated counterparts, brushless DC motors (BLDCs) can achieve rapid acceleration and higher speed, generate less audible noise and less electromagnetic interference, and perform more reliably over time. Applications include equipment for the medical, automation, and semiconductor industries.

BLDCs achieve commutation electronically by utilizing a permanent-magnet rotor, wound stator, and rotor-position sensing scheme instead of a mechanical commutator or brushes found in brushtype DC motor designs.

BLDCs produce torque through the interaction of two magnetic forces. In permanent-magnet motors, because the field force is generated by magnets, the controlling electronics need only regulate the electromagnetic field in the stator. Once the rotor starts to turn, however, current has to be changed in the stator to keep it moving. The process of switching the current as the motor rotates is known as commutation.

Here is how electronic commutation generally works. The motor’s rotor consists of a steel shaft with permanent magnets or a magnetic ring fixed around the circumference. As the rotor turns, the magnet’s poles pass each of the three Hall-effect sensors (usually mounted in or around the stator structure), where they sense the polarity of the permanent magnet field. These sensors read the motor’s rotor position and enable the amplifier to switch the three winding phases on and off in the proper sequence to produce rotary motion.

The switching is accomplished as the sensors generate signals to a control chip, also called the commutation chip or commutation controller, which decodes the rotor’s position and simultaneously provides signals into six “states” and outputs these signals to six solidstate power switches. The power switches (see Figure 2) then steer current into the windings, keep the winding currents in synch with the rotor, and cause the motor to turn.

Role of Controllers
A controller’s basic role is to convert the six-state outputted signals into desired operating functions for the motor’s shaft. Specifically, controllers govern shaft direction, speed, and brake. Controllers also feature a feedback device (digital tachometer output), whose signal monitors shaft speed to confirm it is running at required velocity.

In the effort to design smaller, more efficient DC motor systems, controllers have found new internal homes inside conventional brushless motors in frame sizes as large as 2KW motors and as small as NEMA23. Controller PCBs can be designed in a circular shape for mounting easily inside a motor’s frame.

Integration of a controller inside the motor frame provides a capability to convert a multi-wire brushless DC motor into a simple two-wire (single polarity) motor with open loop speed control in voltages from 12 V to 48 V and currents to 8 amps, depending on windings used. Dual polarity and bi-directional motors also can be engineered.

Controllers housed inside brushless DC motors also allow for closed loop motor speed controls. In open loop systems, speed will be impacted by load and rate of speed will fluctuate during motor operation. Adjustable closed loop speed controls (see Figure 2) offer a steady solution by utilizing an external analog signal to maintain constant speed, regardless of load torque variations. The internal rotor position sensors serve as the feedback elements to the electronics to maintain motor velocity. The speed/signal voltage ratio is dependent on motor windings.

Advances in Electronics
With controllers integrated, their real estate opens new doors and advantages. Controllers can be packed with an array of electronic components to perform a variety of functions, yielding faster, more versatile, more controllable, and more cost-effective motor performance. “Onmotor” controllers today represent completely integrated and sealed solutions for commutation and motor control.

Virtually every relevant electronic component has undergone dramatic transformation over the years. Among those currently being designed into integrated motor controllers for BLDCs are control chips, support chips, and commutation sensor systems.

Control chips have become smaller, denser, more intelligent, more feature-packed, and otherwise enhanced. They have been developed in a wide range of variations to meet functional requirements. A single chip can now perform standard functions (on/off, direction, brake, and speed control) accomplished in the past by multiple chips. In addition, a series of advanced support chips can extend power supply range, convert analog to digital (or digital to analog), enable electrical interface, allow for linking to computer and/or communications systems, and govern temperature control and current limits.

Control and support chips will continue to evolve and more features will become standard even as chips become smaller. While the expected proliferation of chip functions and choices may tend to make the chip-selection process more involved or complicated, a motor application will suggest relevant features. At the outset, several factors are best considered before choosing a control chip: voltage range, current rating, and speed/torque requirements.

Commutation sensor systems continue to play a major role in BLDC system function and performance. Hall-effect sensors remain the most widely used method. Other alternatives for angular position sensing systems can include optical encoders to allow absolute shaft position for a higher level of control.

Integrated Power Supply
With integration of motor controllers and control electronics in full swing, motor control engineers are setting their sights on power supply components as the next to be located inside the motor frame.

Complete motor power supplies already have been paired with internal controllers or embedded within the motor housings for large-scale brushless DC motors (56 frame and higher). Compared with small motors, these larger sizes have more space for components and exhibit enhanced heat dissipation capabilities.

In the case of small motor frames, though, realizing the integration of power supply components presents particularly challenging sets of specifications. As the electronics industry strives for faster, low-power operation, methods have been sought to reduce operating voltage. These have led to development of low voltage power supplies and recent improvements in Schottky diodes (AC to DC rectification devices). As these diodes (see Figure 2) become smaller and more efficient, they will likely become the first power supply components designed inside motor frames. New higher-density capacitor technology allows filter capacitors to be integrated into controllers as well (see Figure 2). Other components will follow, and external power supplies will be able to go the way of external motor controllers.

Looking Ahead
While the basic design of BLDCs will probably change little in the years ahead, advances in electronics and the methods to make motors smarter will change the motorscape. Among the more highly anticipated developments:

• Smaller control chips embedded with more standard, intelligence features.

• Sophisticated support chips will promote complex motion-positioning applications.

• Greater chip communications, enhancing programmable motion control.

• Motor power capabilities will grow, i.e. higher current and voltage, and faster switching speeds.

In addition, an increasing use of digital signal processors (DSPs) to control motors suggests that motor control engineers will broaden their focus beyond hardware to software. While users will clearly have more motor options at their disposal, it will be the particular application demands that establish how the motor is designed and which components would be best engaged.

Jon Haas is sr. project engineer at PennEngineering® Motion Technologies, 343 Godshall Drive, Harleysville, PA 19438-0003. E-mail the author at jhaas@pittmannet.com. For more information, call (877) 748-8626 (toll-free) or (215) 256-6601. Visit Penn-Engineering Motion Technologies online at www.pennmotion.com.