John T. Lee, Miguel Mendoza and Carlos Ribeiro; Micrel
Today's electronic manufacturers are looking for motor solutions that are practical, implementable, energy-efficient and cost-effective. For automotive applications, any technology that also contributes to fuel savings offers an added bonus. Brushless DC (BLDC) motors deliver the best of both worlds. In these motors, digital and analog technologies come together to overcome industry challenges such motor speed control, rotation direction, drift, and motor fatigue. At the same time, BLDC motors are slightly more than half the size of AC motors and 25 percent smaller than brushed DC motors for the same power ratings. This translates to less weight and reflects an innovative approach to achieve fuel savings. Furthermore, BLDC motors represent the ideal choice for achieving the next-generation power savings in mechanical systems because they improve devices' thermal characteristics when compared to brush and induction motors.
Key applications for BLDC motors include automotive, home appliance, industrial systems and white goods where they replace mechanical and hydraulic pumps/movement controls. Automotive applications ideally suited for these motors include electrical pumps, power steering, wipers and window lifts (see Figure 1). With approximately 22 motors in the current generation of high-end automobiles any reduction in weight helps with fuel economy. In fact, using brushless motors can achieve fuel savings of up to 1 mile/gallon while offering higher efficiency and reliability.
Click image to enlarge
Figure 1. Sample Automotive Implementation of a Window Lift Functional Block Diagram
Driving BLDC motors
The implementation of microcontroller units (MCU) has given today’s engineers the ability to dynamically control motor actions so that they respond to environmental stresses and conditions. The MCU contains the control algorithms, which then controls the circuit via MOSFET drivers with appropriate response times such as prop delays, rise and fall times and drive capability. Combined, these things help to ensure longer operational lifetime and minimized maintenance, which translates into lower cost.
Since most BLDC motors are a closed loop control system, they require a MCU that offers construability and flexibility for optimizing the algorithms for the application (see Table 1). Typically 8 Bits or higher, they require an EEPROM to store the firmware that performs the algorithms which set the desired motor speeds, direction and maintain motor stability. These MCUs often feature ADCs that that allow for a sensorless motor control architecture that saves cost and board space. The analog device offers the MCU a power efficient power supply, voltage regulation, voltage references, ability to drive MOSFETs and fault protection (see Figure 2).
Click image to enlarge
Table 1: System requirements to drive a BLDC motor
Click image to enlarge
Figure 2. Typical Functional Block Diagram of a 24V Brushless DC Motor Control
3-phase motor control adds additional complexity to any design compared to brushed DC or AC motors. Thus, the collaboration between digital and analog components becomes very important. BLDC motors are synchronous motors; this means they have permanent magnets residing on the rotor and coil windings that produce electrical magnets on the stator of the motor. Using permanent magnets on the rotor offers a mechanical advantage that reduces size and weight. Electrical terminals are directly connected to the stator windings. Therefore, it is through this design that brushes or mechanical contacts to the rotor are not needed (unlike brushed motors). These motors use DC power and a switching circuit consisting of a high side switch and low side switch for each winding (a total of six switches for one BLDC motor), which produce bi-directional currents on the stator windings.
Depending upon the voltage and motor speed, most motor designs use solid-state switches such as MOSFETs. Switching currents produce the necessary magnetic field polarity to attract opposite polarities and repel equal polarities; and this is what produces the magnetic force needed to rotate the rotor.
Using six-step commutations is typical in BLDC motors. The current is bi-directional with each phase is broken into two steps per each conducting interval. Each conducting stage is called one step, with two phases conducting current at any time, and a third phase floating. This unenergized winding can be used as a feedback control providing the basis for sensorless control algorithms.
For optimal torque, transitioning from one sector to another must occur at precise rotor positions. This allows the magnetic field in the stator to continue advancing ahead of the rotor. Maximum torque is achieved with the switching circuit commutating every 60 degrees. With all switching control algorithms embedded in the microcontroller, it can control the switching circuit via MOSFET drivers.
Determining the right instant to commutate the motor winding is based on rotor position. Depending on the precision needed for the target application, options to perform these calculations range from hall sensors or tachometers (for precision applications) to back-EMF (for cost-sensitive applications).
What are the benefits?
Automotive manufacturers are under continued pressure to find energy efficient solutions and thereby reduce mileage so the ability to offer reliable motors with higher power output while reducing energy consumption and heat generation is a key focus in today's market. There are many key benefits of using brushless motors (see Table 2) that justify their adoption, including:
• 75 percent efficiency (compared to just 40 percent for an AC motor)
• Less heat generated
• Higher reliability since they do not have brushes that can wear out.
• Lower system weight that translates to higher fuel economy in automotive applications
Click image to enlarge
Table 2. Advantages of Brushless DC Motor
The trend to move towards using higher efficiency BLDC motors in the automotive market, among others, is due to the key benefits highlighted above, including efficiency (lower power consumption), reliability and safety. Additionally, using BLDC motors in key sub-systems reduces system weight to achieve more miles per gallon. Since the BLDC motor is totally commutated electronically, using analog and micro components, it is much easier to control the torque and RPM of the motor and at much higher speeds. BLDC implementation is enabling automotive manufacturers to achieve higher power output while keeping manufacturing and operating costs at a minimum, thus increasing the accessibility of these devices.