Thermal dissipation is a byproduct of many modern semiconductor devices and has improved with low-leakage, low-power CMOS processes. But as designers cram more processing power into smaller spaces, heat will cause a rise in temperature if it’s left unchecked. Many consumer products utilize fans to cool devices such as processors, though they have been notoriously unreliable. The introduction of three-phase brushless dc motors has revolutionized the reliability and efficiency of fans, but with added complexity.
Electric motors have been around since the 1800s with notable improvements near the end of the 19th century, including the invention of the induction motor. Induction motors are found in just about every consumer appliance such as refrigerators, HVAC equipment, pumps, and mixers. Electric motors account for about half of all the energy consumed in the U.S.1
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Today, motors have improved in both performance and efficiency, especially brushless (no commutator) motors. Losing the commutator improved both efficiency (less drag due to friction) and reliability (loss of wear and eventual failure of the commutator brushes). Brushless motors are now extremely common with advances in integrated drive solutions and can be found in consumer and industrial applications.
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Mechanically Commutated DC Motors
Direct current (dc) motors with a commutator are the simplest rotating machines around. They can be found in many science fair projects made from a nail, magnetizing wire, magnets, and some paper clips. The rotating part of the motor is called the rotor, and the fixed part that holds the magnetic poles (either permanently magnetized or coils) is called the stator.
In a motor with a commutator, the rotor’s magnetic field is always out of phase with that of the stator. This is accomplished mechanically by passing the current through brushes held against the commutator pickup that automatically switches which pole is energized as the machine rotates (Fig. 1).
Simply applying dc to the motor causes it to rotate, limited only by load and mechanical friction. Springs force the brushes into the commutator to ensure continuous contact. However, the whole process of mechanical commutation causes several issues including RF noise and friction, which results in heat—the killer of all things mechanical.
A consequence of mechanical commutation is the making and breaking of the current path feeding the motor. The windings are inductors that store energy. As the commutator breaks the connection, current continues to flow, causing a voltage spike. This results in a small arc as the old connection is lost and a new one is made. This arc has a wide frequency spectrum that can interfere with RF products such as Bluetooth or Wi-Fi radios inside consumer devices. The arcing also degrades the brushes, reducing useful life.
Consumer products such as laptops, home computers, set-top boxes, and hundreds of other devices often are left running continuously. Depending on conditions, this may require the fans to run much of the time. If a motor using brushes and a commutator powers the fan, the frictional forces will wear out the motor long before the electronics fail. The answer to the frictional wear and RF noise caused by the brushes is simply to remove them and commutate electronically. Enter the brushless dc (BLDC) motor.
Brushless DC Motors
BLDC motors move the magnets onto the rotor and replace the stator magnets (or field coils) with windings that can be independently energized. By detecting the angle of the shaft, electronics can energize the proper stator coil to pull the motor around (Fig. 2).
Electronic commutators use FETs to steer current through the windings. As the shaft turns, the coils are energized to provide the opposite pole to the current position of the rotor magnet.
The shaft index is located between winding sets 1 and 3. Set 1 will be de-energized as the shaft angle lines up with the rotor, allowing the rotation to continue past that winding set. As it passes, polarity is reversed, pushing it toward winding set 2. Again, as it approaches set 2, this set is de-energized momentarily. The rotor then can pass this position and switch polarity to again push the rotor on to set 3. This is the basic principle of electronic commutation. But there’s more to the problem.
A key requirement of driving BLDC motors is knowing the angle of the shaft. Hall effect sensors often are placed between the stator coils to sense the rotor magnets and determine the angle. This becomes an input to a controller to figure out rotational speed and position. Unlike precision positioning required by many machines such as robots or machine tools, absolute encoding is not required for fans or other continuously rotating machines. Only rotations per minute (RPM) are required to manage the fan’s cooling capacity.
Fan speed often is managed to maintain an optimal (compromised) temperature of the equipment. This is done in consumer devices to mitigate noise generated by the fan blades. As fans speed up, they produce additional noise.
Another annoying problem with fans is beat frequencies, which occur when multiple fans rotate at slightly different speeds. The two frequencies mix to form additional lower (and higher) frequencies, resulting in an audible beat frequency. When multiple large fans are grouped together, as in the back of a computer chassis, fan speeds are synchronized to stay at the same RPM to reduce the noise and artifacts of differing fan speeds.
Integrated Driver Devices
To simplify the design of electronically commutating the windings, semiconductor manufacturers have begun to release integrated devices capable of directly driving small to medium-sized BLDC motors. Usually they have either integrated FETs or a means to drive external FETs along with inputs from the shafts of the Hall effect sensors embedded in the BLDC motor stator. Fans are much simpler machines and often are cost-sensitive, though. Consumer equipment manufacturers go to great lengths to avoid adding a fan at all, as well as the additional expense of the BLDC drive electronics.
For these applications, devices such as the DRV10866 from Texas Instruments integrate the FETs while using a novel back electromagnetic field (BEMF) to sense position. This completely removes the requirement for the Hall effect sensors, simplifying the connections to the motor while greatly reducing the cost of the fan.
These new driver devices are so small and low cost that they are also finding their way into appliances and toys such as radio control cars and helicopters where small BLDC motors are replacing brushed motors. The performance is so high that model cars can reach speeds exceeding 100 mph. Small quad-copters (four BLDC motors driving propellers) can fly for over 20 minutes to altitudes exceeding 400 feet. BLDC motors now can replace small brushed motors for improved efficiency and higher output torque. Integrated driver electronics have greatly reduced the cost and complexity of implementation.
BLDC motors have many advantages over their brushed motor cousins, but require additional circuitry to operate. Their increased reliability and higher performance will lead to cooling fans that run continuously. These fans most likely will be used for some time in consumer devices to control the temperature of processors and GP/GPU devices, along with moving air throughout a chassis. With the introduction of integrated BLDC-drive electronics, simpler and cost-effective implementations can now be realized.
2. Download this datasheet: www.ti.com/drv10866-ca
Richard Zarr is a technologist at Texas Instruments focused on high-speed signal and data path technology. He has more than 30 years of practical engineering experience and has published numerous papers and articles worldwide. He is a member of the IEEE and holds a BSEE from the University of South Florida as well as several patents in LED lighting and cryptography.