The brushless DC (BLDC) motor is becoming increasingly popular in sectors such as automotive (particularly electric vehicles (EV)), HVAC, white goods and industrial. This is because it does away with the mechanical commutator used in traditional motors, replacing it with an electronic device that improves the reliability and durability of the unit. Major advantage of a BLDC motor is that it can be made smaller and lighter than a brush type with the same power output, making the former suitable for applications where space is tight. This is made possible with the help of a BLDC motor controller unit. In this report, we will give a short description of brushless DC motor theory of operations, different methods to control a brushless DC motor and approximate BOM cost for developing a sensorless BLDC controller for controlling the BLDC motor of 1KW in EV.
Types and methods to control BLDC motor
In an electric vehicle the main electronic components that make the drive are motor, controller, harness, batteries and throttle. A MCU uses the input from position sensors or by coils back emf measurements, which indicates the position of the rotor, to energize the stator coils at the correct moment. Precise timing allows for accurate speed and torque control, as well as ensuring the motor runs at peak efficiency. Commutation ensures the proper rotor rotation of the BLDC motor, while the motor speed only depends on the amplitude of the applied voltage. The amplitude of the applied voltage is adjusted using the PWM technique. The required speed is controlled by a speed controller, which is implemented as a conventional Proportional-Integral (PI) controller. The difference between the actual and required speeds is input to the PI controller which then, based on this difference, controls the duty cycle of the PWM pulses which correspond to the voltage amplitude required to maintain the desired speed.
The major advantages of brushless DC (BLDC) motors are its low rotor inertia, fast response, high reliability and low maintenance. This is due to the result of replacing brushes/mechanical commutator with an electronic commutation controller and rotor feedback device. A BLDC motor can be of two kinds, an in runner and an out runner type, and there are two methods by which we can control a BLDC motor.
Sensor method works well, but add cost, increase complexity (due the additional wiring), and reduce reliability (due in part to the sensor connectors that are prone to contamination from dirt and humidity). Sensorless control addresses these drawbacks.
In sensor mode, Brushless DC motors usually consist of three main parts: a Stator, a Rotor and Hall Sensors. For the estimation of the rotor position, the motor is equipped with three hall sensors. These hallsensors are placed every 120° in fig 1. With these sensors, 6 different commutations are possible. Phase commutation depends on hall sensor values. Power supply to the coils changes when hall sensor values change. With right synchronized commutations, the torque remains nearly constant and high. In short a BLDC motor using position sensors uses the output from the devices – controlled by a MCU and operating through a driver – to switch MOSFET or IGBT to correctly sequentially energize the coils. The transistors are triggered (and coils energized) when the Hall sensor output changes state.
The benefits to sensor-based BLDC motors include that they are able to detect the absolute rotor position allowing for the application to have a high starting torque. Also sensor-based BLDC motors do not require as much software development. Finally, sensor-based BLDC motors are the best for applications in which the torque varies greatly, easily transitioning from high to low in torque. The downsides to sensor-based BLDC brushless motors include that they have a singular point of failure; making it so that is one part of the motor fails, the entire system stop. Additionally, they have more components and, as such, there is less flexibility in their construction.
In the sensorless variant of the BLDC motor, there are no Hall-effect sensors (fig 2). Instead, as the motor rotates, the back EMF in the three coils varies in a trapezoidal waveform. In every commutation step, one phase winding is connected to positive supply voltage, one phase winding is connected to negative supply voltage and one phase is floating. The back-EMF in the floating phase will result in a “zero crossing” when it crosses the average of the positive and negative supply voltage. The zero crossing occurs right in the middle of two commutations. At constant speed, or slowly varying speed, the time period from one commutation to zero-crossing and the time period from zero-crossing to the next commutation are equal. This is used as basis for this implementation of sensorless commutation control. Unlike sensor-based BLDC motors, sensorless BLDC motors have fewer components and have greater flexibility in their construction because of the fact that they do not have sensors. One disadvantage is that no back EMF is generated when the motor is stationary, so startup is affected by operating in open loop. Consequently, the motor can take a short time to settle and run efficiently. A second disadvantage is that at low speeds the back EMF is small and difficult to measure, which can result in inefficient operation.
The motor power used in most EV in India is 650W to 1250W, the latest government norms allow it to be 650W, most of them run on 850W and higher, which adversely affects the battery life. The controller used is ~40A, 48V output, uses around 24 MOSFETs and powers all electric components. Infact, many of the pros to sensor-based BLDC motors become cons for sensorless BLDC motors, beginning with finding the exact rotor position. However the sensorless BLDC motors are simpler, advanced and potentially more reliable than units that use Hall-effect sensors, especially if the application is in a dirty and humid environment. The motors rely on measurement of back EMF to determine relative positions of stator and rotor so that the correct coil energizing sequence can be implemented. A good quality controller must be used to increase performance of the motor and other overheating issues. Also the controller should be installed with electrical brake cut off and vehicle body is kept light in weight in order to increase life of the battery.