Due to the excellent torque characteristics, wide-speed range, high efficiency, reliability and high power to volume ratio, the brushless DC motor is used in several distinctive applications like consumer electronics, industrial engineering, robotics, medical equipments, automobiles, etc. A high-precision positioning drives are possible with outstanding controllability of this motor. As the name implies, these motors do not have the brushes, but these must be electronically commutated to have its functionality. So let us study in brief about this motor and its controlling.
Brushless DC Motor Principle of Working
A brushless DC (BLDC) motor is a type of synchronous motor that consists of a rotor that has a surface mounted permanent magnets and stator with polyphase armature windings. As compared with conventional DC motor, it doesn’t contain brushes, but the commutation is performed electrically using an electronic drive to energize the stator windings.
Mostly BLDC motor stator winding is placed in slots with stacked steel laminations along with inner periphery, which is connected in a star-like fashion. Numerous coils in the slots are interconnected to form a winding and to form an even number of poles, and these windings are distributed along the stator periphery. This motor winding rating is chosen depending on the controller power supply. With alternate North and South poles , the rotor is made with permanent magnets with a certain number of poles, that varies from 2 to 8 pole pairs. Traditionally ferrite materials are used as permanent magnets.
Generally a BLDC motor construction is done in two ways: either by placing windings in the core and rotor outside of it or by windings outside the core. In the first arrangement, the rate of heat dissipation gets reduced as the rotor magnets act as an insulator. So, it is typically used in fans. But in case of latter arrangement, heat dissipation in the motor is more and widely used in hard disk drives.
The principle of the working of a brushless DC motor is the mechanical torque development due to the interaction of the magnetic field produced by rotor magnets and stator coils. The electronic switching or commutation circuit switches the supply to the stator windings such that one winding is energized with positive power, and the second winding with a negative power and the third is non-energized to develop the torque. So the peak torque occurs when these two fields are at 90 degrees to each other and get reduced when they move together ( in terms of phase difference).
In BLDC motor, the feedback is achieved using multiple sensors, mostly hall-effect sensors which are mounted either on the stator or rotor. Thus, the rotor shaft position is determined by continuous high and low signals from these hall sensors as the rotor magnetic poles (North and South) pass to their vicinity. Thus, based on the signals from these sensors, the winding to be energized is decided.
Controlling a BLDC Motor
Since the brushless DC motor is used to run the loads at the desired speed, controlling its speed is essential with suitable controller. BLDC motor speed is controlled at the base, above and below rated speeds by armature voltage and flux weakening methods. By controlling the applied voltage, we can run the DC motor at the base and below-rated speeds, whereas by flux weakening above rated speed control is possible. Control unit design can be implemented with the use of several analog controllers to digital. In many of the cases, digital-integrated circuits with Pulse Width Modulation technique (PWM) are implemented to have control of this motor. This speed control can be open or closed-loop control.
- In an open-loop control, the input voltage applied is controlled to get the variable speed using different type of controllers like current limiters, potentiometers, chopper circuits, etc.
- In closed-loop control, the supply voltage is controlled by taking actual speed of the motor as a feedback signal. And depends on the error between desired and the actual speed motor voltage applied is varied or controlled. The closed-loop control consists of three basic elements: PWM circuit, sensing circuit and motor driver. PWM circuit can be implemented with timer or programmable controllers like microcontroller. Actual speed sensing is possible with different sensors like hall-effect sensors, optical encoders, IR sensors, etc. ,and motor driver drives the BLDC motor based on signals from the controller.
For a closed-loop control circuit operation, the following explanation describes a microcontroller based BLDC motor controller, which is made to run at desired speeds.
This system is implemented to design a brushless motor controller to provide precise and efficient speed control of the motor. You can observe the components used to implement this circuit in above figure, wherein the system uses a power supply circuit to power the entire circuit, a user input interface, and a controller circuit to program the desired speed for the low- rated brushless DC motor.
The block diagram below shows the operation flow of this circuit wherein the circuit connections are shown for each functionality. This circuit comprises hardware components like 8051 microcontroller, transformer, MOSFET, keypad, LCD, voltage regulator, capacitors, diodes, resistors, Opto-Isolators, IR photo diode, BLDC Fan and software requirements like Keil compiler with embedded C language.
- The power from the AC mains is stepped down to circuit operating range with a transformer which is rectified by a diode-bridge rectifier and regulated to a fixed-operating voltage using regulator IC. This regulated DC voltage is applied to the microcontroller and other circuit components.
- The matrix keyboard is interfaced to the microcontroller as a user input interface to enter the percentage of speed at which the user wants to run the motor. When the user enters the desired speed in keypad, the microcontroller receives and accordingly processes it.
- The Microcontroller is programmed using embedded C language in Keil software and in such a way that based on the input speed from user and actual speed of feedback speed sensor (IR sensor), it gives the appropriate signals to control the motor.
- PWM technique is implemented in the microcontroller program so the microcontroller switches the MOSFET at desired pulses to adjust the DC power applied to the motor to the desired speed.
This is about the brushless DC motor control with the appropriate circuit implementation. The information on this topic is certainly helpful for those who want to know about the DC motor drives. Furthermore, for any clarifications and technical help regarding brushless DC motor controller, you can contact us by commenting below.
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