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Is low speed and high torque permanent magnet direct drive the optimal solution of motor driving?

Permanent magnet direct drive motor is mainly used in low-speed loads, such as belt conveyor, mixer, wire drawing machine, low-speed pump, replacing high-speed motor and mechanical deceleration mechanism composed of electromechanical systems. The permanent magnet direct drive motor has the advantages of large output torque, high system efficiency, fast dynamic response, strong impact and vibration resistance, which greatly improves the reliability of system operation and can effectively reduce maintenance costs. At present, it has been successfully applied in belt conveying, ore crushing, iron and steel metallurgy and other industries, and has broad market prospects.

There is no strict definition of low speed high torque direct drive motor, generally refers to the speed of less than 500r/min, torque greater than 500N•m, for direct drive motor, when the speed is less than 50r/min for ultra-low speed motor. The permanent magnet direct drive motor can be divided into two structural forms: external rotor and internal rotor, and the external rotor permanent magnet direct drive is mainly used in the belt machine. In the design and application of permanent magnet direct drive motor, it should be noted that permanent magnet direct drive is not suitable for particularly low output speed. The low speed high torque transmission system has a wide application prospect in industrial production, oil field exploitation, wind power generation, port lifting and ship propulsion.

The drive system of the traditional induction motor and mechanical deceleration mechanism has the disadvantages of complex structure, easy wear of the deceleration mechanism, lubricating oil leakage, poor operation reliability, high maintenance cost and low overall efficiency of the system, which does not meet the requirements of energy conservation and environmental protection. The adoption of direct drive motor to replace the traditional drive system has become a consensus. When the induction motor is designed at low rated speed, the number of poles is more, and the increase of excitation current seriously reduces the power factor and efficiency, so the induction motor is not suitable for low speed and high torque direct drive. The air gap field of the permanent magnet motor is excited by the permanent magnet, there is no excitation current, and the number of poles of the motor can be designed to be very high. The reactive power component in the armature current of the permanent magnet motor is small, the stator copper consumption is reduced, and the power factor and efficiency of the permanent magnet motor are higher than that of the induction motor. In addition, the permanent magnet motor can maintain good performance in a wide range of load changes, so it has been widely concerned in the low speed and high torque transmission system. Permanent magnet direct drive motor generally uses a surface mount permanent magnet rotor to improve the torque density and reduce the amount of material. Due to the low speed and small centrifugal force, there is no need to use built-in permanent magnet rotor structure, generally using batting, stainless steel sleeves, glass fiber protective sleeves to fix and protect the rotor permanent magnet. However, some motors with high reliability requirements, relatively small number of poles, or large vibration also use built-in permanent magnet rotor structure. The low speed direct drive motor is driven by frequency converter, and when the number of poles is designed to reach a upper limit, the speed will be reduced again, resulting in a lower frequency. When the frequency of the inverter is low, the PWM duty cycle is reduced and the waveform is poor, which will lead to fluctuation and speed instability. Therefore, especially low speed direct drive motor, control is also more difficult. At present, the ultra-low speed motor, there are magnetic field modulation motor scheme, in order to use a higher drive frequency. Although the use of permanent magnet direct drive motor can eliminate the original deceleration mechanism and reduce maintenance costs, but the design of the improper permanent magnet direct drive motor cost is too high, and the system efficiency is reduced. Generally speaking, increasing the diameter of the permanent magnet direct drive motor can reduce the cost per unit torque, so the direct drive motor can be made into a large disk, a larger diameter, and a shorter stack length. However, the increase of the diameter also has a limit, and too large a diameter will increase the cost of the casing and shaft, and even the structural material will gradually exceed the cost of the effective material. Therefore, the design of direct drive motor needs to optimize the selection of length-diameter ratio, so that the overall cost of the motor is low.

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