With the rapid development of the national economy, AC motors play a vital role in industrial production, serving as the primary drive motors in various industries and enjoying widespread application. It is well known that when an AC motor starts at full voltage, the starting current reaches 5 to 7 times the rated current, which can impact the power grid; the starting torque is approximately twice the rated torque, accelerating wear on the mechanical structure. To solve the starting problem of AC motors, a soft starter is typically used for soft starting.
Currently, soft starter monitoring is typically performed by individual monitoring units, which fails to provide comprehensive control and monitoring of the entire system. Therefore, establishing a data communication system to connect the various soft starter monitoring nodes and form a complete monitoring network is essential.
Compared to communication standards such as RS-485, the CAN (Controller Area Network) bus has been widely used in many fields due to its numerous advantages. The CAN bus, introduced by Bosch in the early 1980s, is a multi-master local area network. The CAN communication network is a fully distributed, fully digital, intelligent, bidirectional, multi-variable, multi-point, and multi-station distributed communication system, possessing advantages such as high reliability, good stability, strong anti-interference capability, fast communication speed, and low cost. The CAN bus is a communication method very suitable for harsh industrial environments. This paper introduces a CAN bus-based AC motor soft-start communication system.
CAN communication system architecture
The AC motor soft starter communication system uses an industrial computer as the monitoring host. The industrial computer is connected to the CAN bus via a CAN bus interface. Each node is an AC motor soft starter controlled by a microcontroller with CAN bus data transmission and reception capabilities. The system structure is shown in Figure 1.
Figure 1 Block diagram of the communication system
The communication system comprises various components that provide different functions. The industrial control computer controls the operating status of the AC motor soft starter, such as soft start and soft stop, and displays various soft start parameters, such as starting voltage and starting current. The soft starter controls the AC motor to start softly according to the commands from the industrial control computer, and collects parameters such as motor starting current and voltage, transmitting them to the industrial control computer via the CAN bus.
CAN communication protocol
In this communication system, the length of transmitted data is not fixed. The control commands issued by the industrial control computer to the soft starter are only a few bytes, while the real-time soft start data uploaded by the soft starter to the industrial control computer is generally tens of bytes. Therefore, the CAN communication protocol needs to have the ability to transmit large amounts of data while also being fast and efficient. Since CAN communication uses a short frame structure, the effective data length of each frame is at most 8 bytes. When transmitting a large amount of data, the data must be split into multiple frames for transmission. If all information and data are placed in the 8-byte data field, the effective data in the transmission frame will be less, and the communication information will be more, reducing the efficiency of CAN bus communication. Therefore, this paper adopts a method that uses the arbitration field in the message identifier to contain communication control information and the data field to contain the actual data to improve communication efficiency.
In this system, CAN communication uses the extended frame with a 29-bit message identifier in the CAN 2.0B standard. The message identifier includes an emergency flag, destination address, source address, frame type, and frame number, and its format is shown in Table 1.
The emergency flag has only one bit, which is used to mark the priority of the current frame. For general information, this bit is set to 1, and for emergency information (such as alarm signals such as three-phase voltage imbalance, overcurrent, soft starter component failure, etc.), this bit is set to 0, so that it has a high priority and can be transmitted to the destination as soon as possible.
The destination address consists of 7 bits, indicating the destination to which the frame is to be reached. It can be a specific address or a broadcast address (mainly used by industrial control computers to transmit data, such as time synchronization data, to various soft starters via broadcast).
The source address is 7 bits long and indicates the source address of the frame.
The frame type has 4 bits, which indicate the type of the frame, such as industrial control computer control commands, soft start real-time data, etc.
The frame number is 8 bits long and is used to mark the sequence number of the frame in multi-frame transmission.
Each CAN communication node has a unique address number, represented by a 7-bit binary number. The highest priority address is assigned to the industrial control computer, the lowest priority address is used as the destination address for broadcasting, and the other addresses are assigned to the soft starters in sequence.
CAN nodes employ a dual-filtering method to obtain the necessary information. Both filters use 7 bits of valid data, corresponding to the node address and the address in broadcast mode, respectively. This ensures that only frames destined for the node and broadcast frames can reach the application layer for processing, while other irrelevant communication content is filtered out, improving system efficiency.
The industrial control computer's control commands for the soft starter are represented by corresponding control command codes. The main control commands include: soft start start/stop, soft stop start/stop, starting mode (step, pulse-triggered, constant current) setting, and starting time setting. Some control commands include related parameters, such as the duration in the starting time command. The control command codes and command parameters are stored in the data field.
Soft starter CAN communication hardware design
The CAN communication of the soft starter consists of a P89V51RD2 microcontroller, a CAN controller SJA1000, a CAN bus transceiver PCA82C250, and an optocoupler 6N137. The circuit principle is shown in Figure 2.
Figure 2 Schematic diagram of the CAN communication circuit of the soft starter
The SJA1000 is an independent CAN controller with both BasicCAN and PeliCAN operating modes. The PeliCAN mode supports the CAN 2.0B protocol, which features many new characteristics. The PCA82C250 is a bus transceiver, primarily used to increase communication distance, improve the system's instantaneous interference immunity, protect the bus, and provide thermal protection. To enhance the interference immunity of the CAN bus nodes, the SJA1000 is connected to the PCA82C250 via a high-speed optocoupler 6N137, thus effectively achieving electrical isolation between CAN nodes on the bus. The power supplies on both sides of the optocoupler are completely electrically isolated, with the isolation power provided by the low-power power isolation module B0505.
Soft starter CAN communication program design
After the soft starter is powered on, it first performs a self-test and sends the self-test result to the industrial control computer upon completion. If the self-test is successful, the industrial control computer sends soft start initialization parameters and a "start allowed" command to the soft starter, and the motor begins soft starting. During the soft start process, the soft starter sends soft start data every 100ms, including information such as three-phase voltage, three-phase current, and motor speed. After the motor has finished running, the industrial control computer sends a "soft stop start" command, and the soft starter begins to control the motor to soft stop. When the soft starter malfunctions, the industrial control computer receives a fault alarm signal and displays the cause of the fault.
The soft starter CAN communication program mainly consists of three parts: CAN node initialization, message sending, and message receiving. The CAN node initialization part is crucial; correct initialization ensures the normal operation of message sending and receiving. The CAN initialization program flowchart is shown in Figure 3.
Figure 3 CAN initialization procedure flowchart
When sending a message, simply combine the data to be sent into a message frame according to the communication protocol, send it to the sending buffer, and then start sending. The program flowchart is shown in Figure 4.
Figure 4 Flowchart of CAN message sending procedure
During message reception, the process first involves handling issues such as bus disconnection and error alarms, then reading data from the buffer, and finally releasing the buffer and related registers to complete the reception and proceed to the data processing program. The program flowchart is shown in Figure 5.
Figure 5 Flowchart of CAN message receiving program
6. Conclusion
This paper designs a reliable and efficient communication system for AC motor soft starters. Utilizing CAN bus technology, it realizes remote control and monitoring functions for motor soft starters. The system has been successfully applied to the soft starter system of a water pumping station. Practice has proven that the system has strong anti-interference capabilities, good real-time performance, and stable operation, meeting the design requirements.
