Introduction to brushless DC motors
A brushless DC motor consists of a motor body and a driver, and is a typical mechatronic product. A brushless motor refers to a motor without brushes and a commutator (or slip rings), also known as a commutatorless motor. As early as the 19th century, when motors were invented, the practical motors produced were brushless, namely AC squirrel-cage induction motors, which were widely used. However, induction motors had many insurmountable drawbacks, leading to slow development of motor technology. In the mid-20th century, the transistor was invented, and thus, DC brushless motors using transistor commutation circuits to replace brushes and commutators emerged. This new type of brushless motor is called an electronically commutated DC motor, which overcame the shortcomings of the first-generation brushless motors.
Working principle of brushless DC motor
A brushless DC motor consists of a motor body and a driver, and is a typical mechatronic product. The stator windings of the motor are often configured in a three-phase symmetrical star connection, very similar to a three-phase asynchronous motor. Magnetized permanent magnets are attached to the rotor, and a position sensor is installed inside the motor to detect the rotor's polarity. The driver, composed of power electronic devices and integrated circuits, functions as follows: receiving start, stop, and braking signals from the motor to control its operation; receiving position sensor signals and forward/reverse signals to control the switching of power transistors in the inverter bridge to generate continuous torque; receiving speed commands and speed feedback signals to control and adjust the speed; and providing protection and display functions, etc.
DC motors offer rapid response, high starting torque, and the ability to provide rated torque from zero speed to rated speed. However, this advantage is also their disadvantage. To generate constant torque under rated load, the armature and rotor magnetic fields must be maintained at a constant 90° angle, requiring carbon brushes and a commutator. These brushes and commutators generate sparks and carbon dust during motor rotation, potentially damaging components and limiting their applications. AC motors, on the other hand, are brushless, robust, and widely applicable. However, achieving DC-like performance requires complex control techniques. Rapid advancements in semiconductor technology have significantly increased the switching frequency of power components, improving drive motor performance. Furthermore, increasingly faster microprocessors allow for AC motor control within a rotating two-axis Cartesian coordinate system. By appropriately controlling the current components of the AC motor along both axes, similar control methods to DC motors can be achieved, resulting in comparable performance.
Features of brushless DC motors:
1. It can replace DC motor speed control, frequency converter + frequency converter motor speed control, and asynchronous motor + reducer speed control;
2. It has the advantages of traditional DC motors, while eliminating the need for carbon brushes and slip rings;
3. It can operate at low speed and high power, eliminating the need for a speed reducer and directly driving large loads;
4. Small size, light weight, and high output;
5. Excellent torque characteristics, good torque performance at medium and low speeds, large starting torque, and small starting current;
6. Stepless speed regulation, wide speed range, and strong overload capacity;
7. It features soft start and soft stop, and good braking characteristics, eliminating the need for traditional mechanical or electromagnetic braking devices.
8. High efficiency: The motor itself has no excitation loss or carbon brush loss, eliminating multi-stage reduction losses, and the overall power saving rate can reach 20%~60%.
9. High reliability, good stability, strong adaptability, and simple maintenance;
10. Resistant to bumps and vibrations, low noise, minimal vibration, smooth operation, and long service life;
11. It does not produce sparks, making it particularly suitable for explosive environments; explosion-proof versions are available.
12. Trapezoidal wave magnetic field motors and sinusoidal wave magnetic field motors can be selected as needed.
Introduction to PWM modulation method of brushless DC motor
Figure 1 shows a schematic diagram of the drive section of a three-phase brushless DC motor. It mainly includes the acquisition of Hall effect information and the corresponding modulation of the three-phase inverter based on the Hall signals. The switching sequence and duty cycle of the PWM in the three-phase inverter are the main modulator parameters. Different modulation methods have a significant impact on the operating performance of the BLDC motor. In recent years, with the increasing sophistication of motor control systems, sinusoidal pulse width modulation (SPWM) and space vector pulse width modulation (SVPWM) have emerged, based on the previously common square wave 120-degree pulse width modulation. These methods reduce motor pulsation and current waveform distortion. However, the algorithms for the latter two are more complex. This article will introduce the characteristics, principles, and calculation details of each of the three modulation methods. ON Semiconductor's LC08000M chip integrates these three modulation methods and is suitable for use in BLDC motor drives.
1. Square wave 120-degree pulse width modulation
By using Hall values (which change 6 times per electrical cycle), the current flow direction of the UVW phases can be changed, but the current flow direction remains unchanged within the same Hall value. At any given time, only the upper bridge of one phase and the lower bridge of the other phase can be turned on. This control method is simple, but it has a maximum torque deflection angle of 60 degrees, which reduces efficiency and is accompanied by rotational noise.
Figure 2: Correspondence between Hall state and PWM, three-phase back EMF, and three-phase current
By varying the PWM switching sequence between the upper and lower bridges, we can offer the following five optional modes.
To reduce torque fluctuations during commutation, the LC08000M employs a PWM value transition method, which effectively reduces rotational noise.
Figure 3: Correspondence between PWM and Hall effect of LC08000M square wave 120° pulse width modulation
2. Sinusoidal Pulse Width Modulation (SPWM)
The DC voltage superimposed on the MOSFET can be equivalent to a sinusoidal voltage through PWM switching control. Since the neutral point is 0, the phase voltage of the motor is also sinusoidal, thus making the motor phase current also sinusoidally variable, eliminating torque fluctuations. According to the area equivalence principle, a sine wave can also be equivalent to a PWM wave. As shown in Figure 5, we continuously adjust the duty cycle of the PWM to achieve the sinusoidal voltage effect.
Figure 4: Equivalent diagrams of sine wave and PWM wave
Sinusoidal pulse width modulation requires detailed information about ωt, but we can only read six approximate positional values (60°, 120°, 180°, 240°, 360°) from the Hall effect sensor. Therefore, we need to calculate the inner angle within 60 degrees based on the intervals between the first few Hall effect value changes. During a static start-up of the motor, we cannot calculate the inner angle information. Therefore, during startup, we still need to use square wave 120-degree pulse width modulation. However, once the motor has achieved stable rotation, we can calculate the inner angle and switch to sinusoidal pulse width modulation.
Method for calculating the inner angle: As shown in Figure 6-1, first calculate the time required for each 60°, divide it by the PWM cycle time to calculate the number of PWM cycles within 60°, and thus obtain the value of the increase in inner angle for each additional PWM cycle within 60°. Add the large angle value corresponding to the Hall value to obtain the current angle; the three phases UVW are 120° out of phase with each other.
