- Categories:Industry application
- Time of issue:2020-05-28 00:00:00
According to the performance and power level of the application, as well as the specific control and isolation schemes, motor drive has a variety of system designs. Figure 3 shows a block diagram of isolated communication commonly used in inverters or low-end motor drivers. In this system, the controller potential is the same as the power level, and the communication interface is isolated because this is usually a lower speed and simpler interface. In such systems, the power inverter may have low-side gate drivers that do not need to be isolated because they share the same ground as the motor control module. High-end drivers can be isolated, but techniques such as level shifting can also be used, especially when the power inverter voltage is not too high. In this block diagram, the motor controller does not use isolation and is directly connected to the inverter feedback. When the power level is high, the use of this architecture has limitations. The additional noise generated by the switching signal on the motor may overwhelm the feedback signal used to monitor the motor current, which may cause the motor to lose control.
Figure 3 Block diagram of isolated communication motor control
For higher performance drives, such as large multiphase drives used in industrial motors and train traction motors, isolated control and communication will be required, as shown in Figure 4. In this system block diagram, control and communication are located on the safety side of the isolation barrier for reasons of noise immunity and increased communication speed. Because the motor control module is located on the safe side of the isolation barrier, all gate drivers need to be isolated. The specific isolation voltage and safety requirements are determined by the specific architecture and the location of the isolation barrier. In the block diagram, inverter feedback is used to help control the motor drive and is one of the most important aspects of motor control. As shown in the figure, the inverter is feedback connected to the current measurement nodes iV and iW in the two phases of the three-phase AC motor. In the isolated control and communication system diagram, the inverter feedback must be connected across the isolation barrier, so isolation is also required here. In many high-power motor applications, the architecture will require enhanced isolation of the high voltage of the three-phase motor to prevent users from being exposed to the high voltage.
Figure 4 Block diagram of isolated control and communication motor control
In addition, in large motor applications, when the motor control switching circuit produces a step change in the bridge voltage, the common mode voltage change on the isolation barrier may generate noise. The ability of the isolator to withstand this high-voltage slew-rate voltage transient and the output of the isolator is not disturbed is the common-mode transient immunity (CMTI). The CMTI of the optocoupler may not be very high because its receiving element is very sensitive and susceptible to capacitive coupling effects. The capacitive coupling of the optocoupler is a single-ended structure, and there is only one path for signal and noise across the isolation barrier. This requires that the signal frequency must be much higher than the expected noise frequency, so that the isolation barrier capacitance provides low impedance to the signal and high impedance to noise. When the frequency of the motor control signal is low (usually below 16 kHz), the high frequency component of the common mode transient will be higher than the signal frequency, and its amplitude may be sufficient to disturb the output of the optocoupler.
Figure 5 Transformer-coupled digital isolator
The digital isolator based on a micro-transformer shown in Figure 5, the transformer has a differential input structure, which provides different transmission paths for the input signal and noise, so it must have greater common-mode noise immunity, and there is no light The coupler requires the signal frequency to be higher than the noise frequency limit. The improved electrical noise immunity allows the device to work reliably in high noise environments.
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