Motor Science Series丨Understanding Motor Control Devices

石榴姐

Motor Science Series丨Understanding Motor Control Devices [Copy link]

Introduction: Consumers are demanding more power, smaller size, and higher efficiency from their home appliances, garden tools, and motor-driven products. Like many consumer electronics products, consumers expect these products to be less expensive, more reliable, and easier to use. Brushless DC (BLDC) motors help meet these demands. To meet this demand, fully optimized, highly integrated system-on-chip (SOC) devices are needed. Today's SOC devices are fully programmable motor controllers that provide efficient, compact solutions that help meet the stringent green energy efficiency requirements of 21st century manufacturers. This book details valuable information on how these SOCs improve efficiency and where to use them.

This book is written for both technical and non-technical readers. If you are an executive, salesperson, or design engineer, this book is for you. As long as you are curious about DC motor controller power management, you can read this book. In the first chapter "Electric Motor Science Series丨DC Motor Controller Basics", we gave a basic popular science about it. In today's report, we will take a deep look at motor control devices.
There are many types of brushless motors. The most widely used are single-phase and three-phase brushless DC motors (BLDC)/permanent magnet synchronous motors (PMSM). Both brushless DC motors and permanent magnet synchronous motors are based on the working principle of synchronous motors. As the stator phase sequence switches to generate a rotating magnetic field, the rotor poles try to keep pace with it, causing the motor to start running. The rotor continues to follow the stator at each commutation, so the motor continues to run.

However, the two DC motors have different stator winding geometries, so they produce different back electromotive force (BEMF) responses.

The Brushless DC (BLDC) motor back EMF response is trapezoidal. This means that the control waveforms required to control each of these motors are different, as the control should be based on the motor type. Figure 2-1 compares the waveforms for the two types of motors. In contrast, in a permanent magnet synchronous motor, the coils are wound sinusoidally, resulting in a sinusoidal back EMF signal (similar to three sine waves spaced 120 degrees apart). To maximize performance, these motors typically use sinusoidal commutation.

Back EMF

Brushless DC motors/ Permanent Magnet Synchronous Motors generate back EMF through their windings when they are in operation. If a current carrying conductor is placed in a magnetic field or if the conductor is moved in a magnetic field cutting magnetic flux lines, an EMF is induced or generated in the conductor. If a closed path is provided, current will flow through it. In any motor, the EMF generated due to the movement of the motor is called back EMF because the EMF generated in the motor is opposite to the EMF generated in the generator.

Field Oriented Control

To achieve a sinusoidal waveform for controlling a permanent magnet synchronous motor, a field oriented control (FOC) algorithm is required. Field oriented control or vector control is a technique for variable frequency control of the stator in a three-phase motor using two orthogonal components. One orthogonal component defines the magnetic flux produced by the stator, while the other corresponds to the torque defined by the motor speed, which is determined by the rotor position.

Field-oriented control algorithms are commonly used to maximize the efficiency of three-phase permanent magnet synchronous motors operating in sinusoidal mode. In sinusoidal commutation, three wires are permanently powered with sinusoidal currents that are 120 degrees apart. This creates a rotating north-south magnetic field within the motor cage. Field-oriented control algorithms require motor position and speed for calculations.

Sinusoidal controllers for permanent magnet synchronous motors are more complex and therefore more expensive than trapezoidal controllers for brushless DC motors. The increased cost does bring some advantages, such as lower noise and fewer harmonics in the current waveform. However, the main advantage of brushless DC motors is that they are easier to control. The choice of which motor is best depends on the specific application.

Either type of commutation scheme will work with either type of motor. However, a brushless DC motor may perform better with the six-step trapezoidal algorithm, while a permanent magnet synchronous motor will perform better with a sinusoidal commutation algorithm.

Choose between a sensored or sensorless motor

In this section we will take a closer look at two very important types of brushless DC motors and permanent magnet synchronous motors: sensored motors and sensorless motors.

Sensored Brushless DC Motor

Sensored motors/PMSMs are used in applications where the motor needs to start under load. They use Hall sensors embedded in the motor stator. The sensor is essentially a switch with a digital output that is equivalent to the sensed magnetic field polarity (i.e. HI for North, LO for South). A separate Hall sensor is required for each phase of the motor. A single-phase BLDC/PMSM requires only one Hall sensor; a three-phase BLDC/PMSM requires three Hall sensors. Using these sensors, the controller can derive the rotor position, determine which sector (i.e. magnetic field polarity) requires excitation, and determine when to apply the excitation scheme.

Recently, Hall sensors have become available that provide the absolute position of the rotor using an increasing number of position points.

Sensorless motor

Hardware-based sensing increases the cost of sensors, wiring, and manufacturing, and also reduces motor manufacturing yields. For these reasons, sensorless motors are becoming more and more popular in many applications.

Sensorless motors require algorithms to operate the motor as a sensor. They rely on back-EMF information. In the traditional six-step trapezoidal commutation algorithm for controlling a brushless DC motor, only two phases are energized at any given time, as shown in Figure 2-2. The other phase is left floating, providing a window for the motor's back-EMF. By sampling this back-EMF, the rotor position can be inferred, eliminating the need for hardware-based sensors.

Regardless of the motor topology, controlling these machines requires knowledge of the rotor position so that the motor can be effectively commutated. If the rotor's rotating magnetic field is generated by interaction with the stator's permanent magnets, the system's motion and efficiency will be affected. Some motors use sensors and sensored algorithms to obtain the rotor position, while others are sensorless and derive the rotor position from a mathematical model (sensorless algorithm).

One drawback of the sensorless algorithm occurs at startup, when the motor speed is zero. Because back EMF is proportional to motor speed, when the motor speed is zero, the back EMF is also zero. Without a back EMF value, the rotor position cannot be inferred. This problem is solved by a new algorithm that injects a high-frequency signal into the three phases to infer the rotor position.

The choice between sensored and sensorless is usually based on cost. Generally speaking, the choice between a brushless DC motor and a permanent magnet synchronous motor is based on performance, cost, and other factors.

Sensorless control reduces costs because it requires no additional hardware and allows motor manufacturing yields close to 100%. Therefore, sensorless motor control is common in low-cost, variable-speed motor applications such as fans, refrigerator compressors, air conditioners, and many garden tools. However, applications that require high torque at start-up, such as electric bicycles and many power tools, require sensored motors.

Permanent magnet synchronous motors combined with field-oriented control algorithms generally provide the highest performance. However, permanent magnet synchronous motors generally cost more than brushless DC motors (although the gap is narrowing) and are more complex to control. Robotics and servo applications may benefit from permanent magnet synchronous motors.

Exploring miniaturized motor controllers

Many of today's integrated motor control and drive devices are very complex. They require analog circuits such as differential amplifiers to sample the phase currents and analog-to-digital converters (ADCs) to transfer these values to the digital domain. In addition to these two blocks, they also require comparators for current sampling to protect the system from overcurrent. They use programmable digital-to-analog converters (DACs) as references and other analog blocks such as single-ended amplifiers to sample the phase voltages.

Now, instead of using discrete components to implement all of these functions, these blocks can be integrated into a single device. Doing so ensures a compact solution for all applications. Product engineers no longer have to piece together many separate components; instead, they can use a plug-and-play system-on-chip (SOC) with flexible software configurability.

As shown in Figure 2-3, the microprocessor core has analog front end, power driver, power management, pulse width modulation (PWM) generator, and sequence driven data acquisition functions. The power manager also handles some system functions, including internal reference generation, timers, sleep mode management, and power and temperature monitoring.

In the next article, we will take a deeper look at the motor control algorithm. Welcome to pay attention.

Jacktang

How to understand this, one of the shortcomings of the sensorless algorithm occurs at startup, when the motor speed is 0?

Why is it 0?