Application of permanent magnet synchronous motor system in electric forklift

In recent years, with people's deep understanding of the harm of environmental pollution, environmental protection has become the focus of common concern in the world. Therefore, various battery vehicles such as electric forklifts that use batteries as power sources have developed rapidly. Electric forklifts have the advantages of high energy conversion efficiency, low noise, no exhaust emissions, and convenient control. They have been widely used in workshops, warehouses, automatic storage systems, large supermarkets and other places with high environmental conditions. At present, the proportion of electric forklifts in developed countries such as Europe and the United States has reached 60% of the total forklifts, while the proportion of domestic electric forklifts is only about 20%.

Electric forklifts have now broken through the limitation that they can only be used for small tonnage operations, and have gradually moved from indoors to outdoors, and market demand has increased year by year.

1. Electric forklift drive motor
The development of electric forklifts has promoted the continuous innovation of its drive motor and its control system. There are many types of motors and their control systems. Domestic electric forklifts mainly use DC motors, including series-excited DC motors, parallel-excited DC motors, and compound-excited DC motors. Although DC motors have many characteristics, such as good speed regulation performance, wide speed regulation range, large starting torque, and easy control, DC motors contain contact brushes, which makes commutation failures and other mechanical failures easy to occur during operation, with high noise, low life, and frequent maintenance. Therefore, the application of asynchronous motors has been increasing in recent years, and permanent magnet synchronous motors and switched reluctance motors are also used. From the characteristics of various motors, asynchronous motors have high reliability, and permanent magnet synchronous motors have excellent comprehensive performance, but both asynchronous motors and permanent magnet synchronous motor control systems face the problem of high controller costs.
The comprehensive performance of permanent magnet synchronous motors exceeds that of other types of motors, and it is an ideal electric forklift drive motor. With the development of computer technology, sensor technology and power electronics technology, permanent magnet synchronous motor systems will have wider applications in the field of electric vehicles. In view of the above analysis, this paper introduces a permanent magnet synchronous motor drive system for electric forklifts, and compares its performance with DC motors of the same power level.

2. Permanent magnet synchronous motor
The operating principle of permanent magnet synchronous motor is the same as that of electric excitation synchronous motor, but it replaces the excitation winding with permanent magnet, which makes the motor structure simpler, reduces the processing cost, and eliminates the commutator and brushes that are prone to problems, improving the reliability of the motor; and because there is no excitation loss, the efficiency and power density of the motor are improved. Figure 1 shows a rotor magnetic circuit diagram of a permanent magnet synchronous motor for electric forklifts. The motor adopts a magnetic steel built-in rotor magnetic circuit structure.
The advantages of the magnetic steel built-in rotor are small leakage coefficient and good dynamic performance, which is particularly suitable for permanent magnet synchronous motors for electric forklifts with relatively high dynamic performance requirements. The magnetic resistance torque generated by the asymmetry of its magnetic circuit also helps to improve the overload capacity and power density of the motor, and is easy to increase the speed by "weak magnetic field". The motor has the characteristics of low speed and high torque to ensure the starting torque required for the vehicle at low temperature; it has a wide operating speed range to meet the needs of vehicle power generation and power assistance; and it has high working efficiency. Table 2 lists the performance comparison of a 6.5 kW permanent magnet synchronous motor and a commonly used DC motor.

3. Permanent magnet synchronous drive motor control system
The permanent magnet synchronous motor controller system adopts direct torque control, space vector control and speed sensorless vector control and other control methods. It has high efficiency, high performance and high reliability, and has the function of feeding back the braking energy of the electric forklift to the power battery pack. The control system has a CAN communication module with other vehicle systems.     Equipping the control system of electric vehicles with a controller area network (CAN) is one of the latest control technologies in the world today. The electric forklift control system based on the CAN bus has become a hot topic for research by major forklift manufacturers in the world.     The permanent magnet synchronous motor controller realizes the following functions through effective control of the drive motor: ① Power generation operation: used for energy recovery during braking; ② Electric operation: drive the vehicle, and can also be used for assisting during acceleration.     Motor drive hardware part: The motor controller is mainly composed of a high-performance Infineon XC2000 series embedded chip with integrated DSP, an IGBT module, a drive circuit module and a signal detection circuit module. It realizes closed-loop control of the drive motor by detecting the feedback current, and uses a variety of sensors to monitor the entire system to ensure the reliability and safety of the motor. Figure 2 shows the hardware schematic diagram of the drive motor controller.
The main protection functions of the drive motor controller include motor temperature fault protection, motor controller temperature fault protection, DC bus overvoltage fault protection, DC bus undervoltage fault protection, motor overload fault protection, motor overspeed fault protection, motor controller IPM fault protection, 12 V control power supply fault protection, CAN communication fault protection and vehicle control command abnormal fault protection. The technical indicators and functions that the permanent magnet synchronous motor controller can achieve are as follows: (1) Input voltage fluctuation range: UN±30%; (2) Communication control interface: two CAN2.0B buses; (3) Insulation resistance: each terminal to the shell is greater than 25 MΩ; (4) It has fault self-diagnosis function: overvoltage/undervoltage, overload, controller over-temperature limited power output, communication failure, extreme temperature shutdown, etc.; (5) High efficiency working area: 75% of the working area efficiency is greater than 90%; (6) Mean time between failures: >3,000 h; (7) Torque control accuracy is less than or equal to 5%; (8) The ratio of maximum control speed and inflection point speed is greater than 3; (9) Peak power volume density is greater than or equal to 8 kW/L; (10) Electromagnetic compatibility meets the requirements of GB/T18655-2002 and GB/T17619-1998. Permanent magnet synchronous motor and its control system are a kind of vehicle drive system with excellent comprehensive performance. In the field of electric forklifts, they tend to gradually replace DC motor and asynchronous motor systems.