The Nuts and Bolts of Brushless DC MotorsAugust 23, 2018
Increasingly popular, a brushless DC (BLDC) motor is a permanent magnet synchronous electric motor that runs on DC power. BLDC motors operate similarly to conventional DC motors. When a current-carrying conductor is inserted into a magnetic field, its permanent magnet experiences an equal and opposite force. The conductor is stationary while the magnet moves.
Here is an overview of BLDC motors, their basic components, how they operate, how they differ and where they are used. Brushless motors are unique, complex, and arguably not commodity products.
BLDC motors, while similar to their brushed DC motor cousins, have distinct differences in their four main components: windings, magnets, sensors and controllers.
The stator of a BLDC motor features stacked steel laminations that carry windings. The windings are placed into slots that are arranged in either a star or delta connection. Based on stator windings, BLDC motors can have several physical configurations such as single-phase, two-phase or three-phase motors. Three-phase motors are the most common. Complete with permanent magnet rotor, they feature a three-phase, star-connected stator.
Each winding has several interconnected coils, with one or multiple coils in each slot. A stator winding is selected by voltage rating based on the power supply. BLDC motors with 48 V or less, for example, are used in robotics, automotive and small actuating applications, while select industrial and automation system applications may use 100 V or even higher-rated motors.
Electric motors generate a voltage potential, also called electromotive force (EMF), from the movement of the windings through the magnetic field. For a given motor with a fixed magnetic flux and number of windings, EMF is proportional to the angular velocity of the rotor. The induced EMF of rotation of the armature is known as back or counter EMF, which can be beneficial. When monitoring back EMF, a microcontroller determines the relative positions of stator and rotor without Hall effect sensors, simplifying motor construction, lowering cost and eliminating wiring and connections that typically support sensors. Reliability improves, especially in the presence of dirt and humidity.
Stationary motors do not generate back EMF and microcontrollers (MCUs) do not determine the position of the motor parts at start up. Here, the motor starts in an open loop configuration until sufficient EMF is generated. Sensorless BLDC motors are gaining in popularity.
BLDC motors feature a permanent magnet in the rotor. Historically, the number of poles in the rotor range from two to eight pole pairs, with alternate south and north poles based on the application. More recently, solutions may carry 14 or even 16 poles. For maximum torque, the flux density of the magnetic material must be high. Ferrite magnets, while inexpensive, have a low flux density. Rare earth alloy magnets are commonly used, including samarium cobalt (SmCo), neodymium (Nd), and ferrite and boron (NdFeB). Rotor configurations can vary, so that there can be a circular core with a permanent magnet on the periphery or circular core with rectangular magnets, among other setups.
When stator coils are electrically switched by a supply source, it is electromagnetic and produces a uniform field in the air gap. Due to the force created between electromagnet stator and permanent magnet rotor, the rotor continues to rotate.
In BLDC motors, Hall effect sensors provide data that synchronizes stator armature excitation with the position of the rotor. Before energizing a particular winding, the rotor position must be acknowledged. The Hall effect sensor embedded in the stator senses the rotor position and provides speed feedback information.
Electronic controllers may be a microcontroller unit (MCU), microprocessor unit (MPU), digital signal processor (DSP), field-programmable gate array (FPGA) or any other controller. The controller receives and processes signals, sending the control signals to the driver circuit. For a three-phase motor, three Hall effect sensors are embedded in the stator to indicate the relative positions of stator and rotor to the controller so that windings are energized in the correct sequence and correct time.
When rotor magnetic poles pass the Hall effect sensors, a high or low signal is generated and the controller decides which coils to energize. The electronic controller circuit energizes the appropriate motor winding by turning transistors or other solid-state switches, continuously rotating the motor.
Given that the rotor bears the magnets, it requires no power — no connections, no commutator, no brushes — and instead uses control circuitry. BLDC motors operate by an electronic drive, switching the supply voltage between the stator windings as the rotor turns. A transducer monitors the rotor position and provides data to the electronic controller. The electronic drive features two transistors for each phase operated by an MPU.
The magnetic field generated by the permanent magnets and the field induced by the current in the stator windings interact, creating mechanical torque. The electronic switching circuit switches the supply current to the stator.
While BLDC motors are relatively mechanically simple, they require sophisticated control electronics and regulated power supplies. An electronic controller replaces the brush assembly of brushed DC motors and performs comparative timed power distribution with a solid-state circuit.
There are several methods used to implement the control unit, including using a microcontroller, a dedicated microcontroller, a hard-wired microelectronic unit or a PLC, among others.
Motor speed is based on controlling the input DC voltage — the higher the voltage, the greater the speed. Speed control can be closed loop or open loop.
- Open-loop speed control involves simply controlling the DC voltage applied to motor terminals by current limiting.
- Closed-loop speed control involves controlling the input supply voltage through the speed feedback from the motor, controlling supply voltage based on an error signal. The BLDC motor can rotate at any desired speed.
There are many inherent advantages to BLDC motors. They have fewer parts that wear out, long life expectancies (greater than 10,000 hours) and higher reliability and efficiency. They can run at speeds above 10,000 rpm and operate with less noise, given the lack of brushes and reduced electromagnetic interference (EMI).
Rare earth magnets generate greater flux density, allowing the rotor to be smaller for a given torque. The magnets deliver higher power than a brush-type DC motor of the same size. They also feature smaller motor geometry and are lighter in weight than brushed DC and induction AC motors, as well as higher dynamic response.
While BLDC motors require additional sensors and complex drive circuitry, causing them to be more expensive, robust electronic devices are available for motor control, resulting in simple and inexpensive circuit design. BLDC motors can run in a basic configuration without a microcontroller when using a three-phase sine- or square-wave generator.
As costs decline, there are a greater number of applications that are integrating BLDCs or expanding their use.
BLDC motors are increasingly popular in applications where small, light, reliable, durable and high-power motion control is necessary. They are for varying loads, constant loads and positioning. Top applications include:
- Sleep apnea equipment
- Medical analyzers
- Surgical hand tools
- Dental hand tools
- Electric/hybrid vehicles
- Power windows
- Power seats
- Small transport
- Industrial automation
- Motion control and positioning
- Small robots
- HVAC and refrigeration
- Valve control
- Fans, pumps, blowers
- PC cooling fans
- Media players
- Direct-drive turntables
- Model aircraft
- Motorized toys
- Cordless power tools
- Electric bicycles
- A/V equipment
Commodity Solutions? BLDC Motor Customization
There are a substantial number of options available with BLDC motors. They clearly are not one-size-fits-all, although the marketplace often makes it seem so. The decision to use a BLDC motor and the choice of supplier should be based on many considerations.
Central to the decision should be the philosophy of the manufacturer. If, for example, you are led to a product that is “good enough” to meet your requirements — keep on looking. Instead, your application should have a product that meets all necessary requirements, while taking advantage of unique customization capabilities.
Naturally, the manufacturer should offer DC motors, sensors and encoders, as well as OEM assembly, industrial robotics and other ancillary support. The underlying corporate philosophy must be based on high-quality product offerings, quality control that is more stringent than current standards, as well as sustainable business practices. Sustainable practices, for example, focus on environmental manufacturing based in part on reducing CO2 emissions and hazardous materials.
Canon Motors meets and exceeds these requirements. It provides custom products that solve challenging and complex requirements with unique customization solutions. Canon’s BN22 BLDC, for example, is ideal for medical devices, small robotics and industrial and consumer applications.
Featuring 7 percent higher torque compared to conventional 22 mm motors at 7.5 to 8 W, the BN22 integrates a number of components to reduce manufacturing cost. Specifications include rated voltages spanning DC 12 V, 24 V, 36 V and 48 V. It also has a rated speed from 5,440 rpm at low speed to 15,000 rpm at high speed, a rated torque of 5.88 mN/m and a weight of 45 grams.
Options include a built-in driver, a ball-bearing option for high-speed applications, a planetary gearbox with 1/20, 1/62, 1/107, 1/242 and 1/410 ratios and an optical encoder option with resolutions of 100 P/R and 200 P/R.
The BN22 is based on Canon production technologies, including:
- Permanent magnets attached to the rotor shaft and aligned to achieve an evenly distributed magnet field.
- An automated assembly system for faster and more predictable manufacturing.
- Parts bonding with Canon’s proprietary tooling and jigs for each fixing and bonding process.
The result is greater accuracy and finer tolerances. Given Canon’s expertise in the development, manufacturing and sale of precision micromotors and control systems, optical semiconductor sensors and energy-efficient small motors, Canon delivers all of the necessary capabilities to deliver BLDCs that are not one-size-fits-all, but products that can be customized into the unique motor technology that is best suited to a particular application.