Characteristics of permanent magnet synchronous motor

Permanent magnet synchronous motor is a growing substitute for AC induction motor. For decades, AC induction motor has been the main force of all motor applications. It maintains the reliability and simplicity of AC induction motors, while providing opportunities for higher efficiency, synchronous operation and smaller frame sizes. The permanent magnet (usually made of rare earth metal alloy) is used to replace the magnetic field induced in the rotor conductor, so that the resistance loss is much lower than that of AC induction motor, because there is no induced current in the rotor. In order to replace the mechanical commutation, a control system is needed to determine which coils are supplied with current to produce the maximum torque. The magnetic field produced by rare earth permanent magnet AC motor can provide the same torque as AC induction motor, while the motor of AC induction motor is smaller and lighter.

The motor design process involves some basic considerations, such as the requirements of starter, application environment, when, what torque and speed, and how often? What is a work cycle? What are the environmental conditions such as temperature and pressure? Even the most efficient motor, if the motor application in the wrong field, it will not play the maximum efficiency. Many motors are used in combination of gear motors, gear reducers and motors. Gear motor provides high torque at low speed. In short, gear motor will absorb motor power and reduce speed while amplifying torque. Duty cycle of gear motor will affect performance rating of motor, such as continuous duty cycle.

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A motor with better cooling has higher efficiency. In order to obtain the best air flow, the design of cooling fan and fan cover is optimized to ensure the close combination between stator and motor shell and provide the best cooling performance. The electrical efficiency of the motor has been improved a lot, but the power of the cooling fan accounts for a larger proportion of the total loss. The optimization of cooling fan size includes using the minimum power of the fan while providing sufficient cooling. The optimized fan design can reduce the fan power demand by 65%. An important design feature is the clearance between blade and shell. The space between the enclosure and the fan blades should be as small as possible to prevent turbulence and reduce backflow.
Ball or roller bearings are used in high efficiency motors and consist of an inner and outer ring and a cage containing steel or ceramic rollers or balls. The outer ring is connected with the stator and the inner ring is connected with the rotor. When the shaft rotates, the components also rotate, and the friction of shaft rotation is minimized. They have long service life and low maintenance cost. High precision applications allow minimum air gap. Thermal shrinkage and thermal expansion will affect the fit of shaft and bearing seat and internal bearing clearance. Power output control shaft size and bearing bore. The size and direction of the load determine the size and type of the bearing. Additional forces, such as asymmetric air gap, unbalanced force, pitch error and thrust load, are considered. For the calculation of bearing load, the shaft is regarded as a beam supported on a rigid torque free support. Ball bearings are more suitable for high speed applications than roller bearings. High speed factors include cage design, lubricant, running accuracy, clearance, resonance frequency and balance. The bearing requires minimum load, so the rolling element rotates to form a lubricating film instead of sliding, which will increase the working temperature and degrade the lubricating oil. The minimum allowable load is equal to 0.01 times of the dynamic radial load rating of ball bearings. This is particularly important when the bearing is close to 70% of the recommended rating. Understanding the ambient temperature range and normal operating temperature range will help to determine the most effective lubrication method for bearings: lubricating oil or grease. Generally, the normal operating temperature range of gear motor is – 25 to 40 ° C. Synthetic grease has good performance in various temperature ranges. Grease can simplify maintenance, clean, reduce leakage and pollution protection.

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When the axis and the rotating axis do not coexist, there will be noise and vibration. The impact of balance on efficiency is limited, but it will affect the operating noise and life expectancy, which is also very important to maximize the use of resources. Bearing vibration readings are usually taken in vertical, horizontal and axial planes. Vertical vibration may indicate installation problems, horizontal vibration may indicate balance problems, and axial vibration may indicate bearing problems. Balance at operating speed is important because the centripetal force of the bearing can also cause imbalance.
The synchronous motor with high performance permanent magnet has sinusoidal flux distribution and electromotive force. For the distributed winding, the stator winding is usually the same as the asynchronous motor winding. It reduces the vibration, noise and maintenance costs, and improves the overall performance.
Neodymium, rare earth, samarium cobalt magnets or ferrite (ceramic) magnets are used in the motor. The strength of rare earth magnets is two to three times that of ferrite or ceramic permanent magnets, but the price is higher. SmCo magnets are the best choice for high temperature applications because of their high energy density, temperature resistance from 250 to 550 ° C, small parameter reduction due to temperature rise and oxidation protection. SmCo or neodymium magnets are selected according to the working temperature, corrosion resistance and required performance. If heated to above 80 ℃, the low-grade neodymium magnets may begin to lose “strength”, and the high-grade neodymium magnets may work at temperatures below 220 ℃. Ferrite or ceramic magnets are widely recognized because of their strong resistance, good demagnetization, strong corrosion resistance and low price. Magnetic loss will occur when the magnet works above 250 ° C, but it will recover when the magnet drops to a lower temperature. Unless the circuit is designed for extreme conditions, a low temperature of – 40 ° C may result in permanent loss of magnetic strength.
The inverter drive unit can be lossless under no-load operation / static state. By replacing the existing three-phase drive device powered by line, it is estimated that up to 30% of energy can be saved. The characteristics of the drive make it very suitable for driving pumps and fans in continuous operation. No additional components, such as encoders, are required. Up to 25% of the floor space allows for a more compact machine design. The motor has good control performance, and combined with sensorless drive controller unit, it has excellent real operation performance even at low speed, and has impressive dynamic characteristics when pulse load and speed change.
The actuator can “self detect” and track the permanent magnet position of the rotor. This is essential for a smooth start of the motor, while also allowing the best torque to be generated to achieve the best efficiency. The lack of position or speed sensors reduces the cost and improves the reliability of the drive system. With the continuous improvement of efficiency, it is more and more important to program the controller settings of a specific motor to obtain the best efficiency.
When choosing a partner in the machine manufacturing process, please remember that there are two ways to choose the motor power supply, either choose a standard motor that may or may not be suitable for a specific application, or choose a qualified motor partner to design and manufacture a motor that is completely suitable for the application. If design engineers do not have the time or engineering resources to design custom versions, or if quick setup is required, standard motor solutions are appropriate. New modular design and construction methods enable manufacturing engineers to obtain custom motors at reasonable prices, even in limited quantities. No matter how the motor is selected, the design / drive system should be improved continuously by comparing the performance prediction with the measurement results.
Source: China Motor Magnets Manufacturer



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