Brushless Motor Engineering Manager
Senior Project Engineer
Edited by Leland Teschler
It's become common knowledge among engineers that brushless dc motors have taken motion-control applications by storm. They can accelerate rapidly and hit high speeds, generate minimal audible noise and electrical interference, and have a long service life with little maintenance.
Nevertheless, brushless motors present their own set of design challenges. One of these involves the cabling that runs between the motor and amplifier. For reasons that are probably obvious, many applications would benefit from having as little cabling as possible. Fortunately, advances in solid-state electronics have allowed manufacturers to eliminate complicated hookups between the motor and amplifier. The development that makes this possible is integrated driver-control electronics small enough to fit inside the motor frame.
The circuit boards holding driver electronics can be designed in the same circular form factor as the motor frame. In some cases they can even be configured for mounting in a standard incrementalencoder housing. This strategy converts what would otherwise be a multiwire motor into one which uses just a simple two-wire (single polarity) connection.
The brushless motor driver ICs that are being built into motor frames provide a few standard functions. They implement speed control using input voltages ranging from 12 to 48 V. Outputs can range from 8A, depending on the motor windings. (Dual polarity and bidirectional motors with a 3-A limit also can be engineered.)
In performing its tasks, the chip accepts inputs from Hall sensors that detect the position of the permanent-magnet rotor. The chip uses sensor data to synthesize coil currents in the proper sequence and duration to drive the motor at the desired speed.
It may be useful to review the role of commutation in brushless motors. Brushless dc motors produce torque through the interaction of two magnetic forces (the stator and the rotor). Magnets generate the field force in permanent-magnet motors. Thus the controlling electronics need only regulate the electromagnetic field in the stator. Once the motor starts to turn, however, current must be changed in the stator to keep it moving. The process of switching the current as the motor rotates is known as commutation.
Recall that a brushless motor rotor consists of a steel shaft with permanent magnets or a magnetic ring fixed around the circumference. As the rotor turns, the shaft's poles pass each of three Hall-effect sensors (usually mounted in or around the stator structure). The sensors detect the polarity of the permanent-magnet field in the air gap. This serves as a measure of motor-rotor position. The amplifier electronics uses this information to switch the three winding phases on and off in the proper sequence to produce rotary motion.
A control (or commutation) chip reads the rotor position and generates six signals in the right sequence and duration. These six signals go to six solid-state power switches. The switches in turn steer current into the windings, keep the winding currents in sync with the rotor, and cause the motor to turn.
The trend toward smaller and denser integrated circuits has also had an impact on commutation electronics. It is typical to now find these chips handling functions such as braking, speed, and direction control. In addition, advanced support chips can extend power supply range, handle analog/digital conversion tasks, help limit motor current and mitigate temperature problems, and provide an interface to communication networks.
Some controller chips incorporate pulse-width-modulation (PWM) inputs to control brushless motor speed. Closed-loop applications may incorporate circuitry to read signals from Hall-effect sensors or encoders. These sensors provide velocity feedback for speed-control or as an additional source of feedback in servosystems for stability.
Finally, commutation sensor systems continue to be important in brushless systems. Hall-effect sensors remain the most widely used and cost-effective method. Early brushless motor designs sometimes located the Hall sensors such that they saw high temperatures. Heat generated by the stator windings, particularly in times of high peak loads, would affect sensor-switching performance. The impact was sometimes severe enough to limit the duration of peak loads the motor could handle.
But Hall-sensor technology has resolved the location/stator temperature issue. Today's silicon sensors can perform optimally in winding temperature ranges from 40 to 150°C. New Hall sensors also are highly sensitive and can sit close to the rotor if magnets are relatively weak.