In a variable frequency servo system for motor control, the sampling accuracy and real-time performance of the current sensor largely determine the system's dynamic and static performance. Accurate current detection is crucial for improving the system's control precision, stability, and speed, and is also key to achieving a high-performance closed-loop control system. Simultaneously, when the motor stalls or experiences overload, the system detects overcurrent in the line through the current sensor, which can then drive the IPM module to implement a shutdown protection function. Figure 1 below is a block diagram of a variable frequency servo system for motor control:
Figure 1 : Block diagram of a variable frequency servo system for motor control
Current sensors can be categorized according to their working principle into : Hall effect current sensors, current transformers, magnetoresistive current sensors, resistive current sensors, and fiber optic current sensors. In frequency converter servo systems, common current sensors include Hall effect current sensors, current transformers, and resistive current sensors. Hall effect current sensors, utilizing the Hall effect principle, can directly convert current (unidirectional or bidirectional) signals into DC voltage signals, which are then converted into digital signals by an A/D converter (either independent or integrated within the MCU). As one of the system control parameters, it works alongside other parameters such as voltage and speed to achieve the system's control function. A brief introduction will be given using the MELEXIS Hall effect current sensor as an example.
MELEXIS's Hall current sensors are divided into traditional Hall current sensor solutions and MELEXIS's proprietary patented triaxial planar Hall current sensor solution, as shown on the left side of Figure 2. Traditional Hall current sensor solutions require an additional shielding magnetic ring or shield. They offer excellent anti-interference performance and can detect larger currents. The triaxial planar Hall current sensor solution, shown on the right side of Figure 2, integrates an integrated magnetic collector (IMC). The IMC has high permeability and low hysteresis, converging the magnetic lines of force generated by a current-carrying conductor, making them perpendicular to the chip. Therefore, no additional shield is needed. This results in a small product size, simple installation and use, and suitability for low-to-medium power frequency converter servo systems for low-current detection applications.
Figure 2 : Traditional Hall current sensor scheme and triaxial planar Hall current sensor scheme
MELEXIS' traditional Hall current sensors include the MLX91207 and MLX91209 , while its triaxial planar Hall current sensors include the MLX91205, MLX91206 , and MLX91208, as shown in Figure 3. They also include the MLX91210, a third-generation current sensor product from MELEXIS with an integrated current carrier. This sensor offers detection current ranges of ±20A, ±30A, and ±50A. It is ideally suited for current detection in low-to-medium power variable frequency servo systems.
Figure 3: Models of MELEXIS Traditional Hall and Planar Hall Current Sensors
MLX91210 key features:
• Small size
• Low power consumption and minimal heat dissipation
• Differential technology has strong resistance to external interference.
• Temperature drift compensation
• High bandwidth, 5µs response time
• High insulation voltage
Figure 4: Schematic diagram of the internal structure of MLX91210
Table 1: MLX91210 Model and Specifications
Figure 5: Internal structure diagram of MLX91210
The MLX91210, distributed by Worldway, integrates a current carrier, allowing current to flow directly into the chip. The maximum current is ±50A, and the output voltage is 1/2VDD when the input current is zero. The 5V version integrates a power regulator, eliminating concerns about power supply voltage fluctuations affecting output accuracy. Nonlinearity across the entire measurement range reaches 0.5 %FS.
For the MLX91210, the recommended application circuit is shown in the figure below: It can be seen that the MLX91210 is very simple to use. Only a few filter capacitors need to be connected to the outside. The RC circuit in the figure is to improve the resolution and reduce the output noise.
Figure 6: Recommended application circuit for MLX91210
Yu Biao, Marketing Manager of MELEXIS, summarized that current sensors are indispensable in variable frequency servo systems used for motor control. Their accuracy, reliability, anti-interference performance, and ease of use are all crucial. MELEXIS current sensors happen to have outstanding advantages in these aspects, making them ideal for such applications.
