Some basic considerations involved in the design process of permanent magnet synchronous motors

Time: 2019-09-07
Summary: Rare-earth permanent magnet ac motors generate magnetic fields that provide the same torque as ac induction motors, which are smaller and lighter.

Permanent magnet synchronous motors are a growing alternative to AC induction motors, and AC induction motors have been the mainstay of all motor applications for decades. Maintains the reliability and simplicity of AC induction motors while providing greater efficiency, simultaneous operation and the opportunity to use smaller frame sizes. The permanent magnet (usually made of a rare earth metal alloy) replaces the magnetic field induced in the rotor conductor, making its electrical resistance much lower than that of an AC induction motor because there is no induced current in the rotor. In order to replace mechanical commutation, a control system is needed to determine which coils are supplied with current to produce maximum torque. The magnetic field generated by the rare earth permanent magnet AC motor can provide the same torque as the AC induction motor, while the motor of the AC induction motor is smaller and lighter.

 

The motor design process involves some basic considerations. How long does it take for the starter, the application environment, and what torque and speed are needed? What is a work cycle? What are the environmental conditions such as temperature and pressure? Even the most efficient motor, if the motor is applied in the wrong field, it will not exert maximum efficiency. Many motors are used in combination with gear motors, gear reducers and motors. The gear motor provides high torque at low speed. In short, the gear motor absorbs the motor power and reduces the speed while amplifying the torque. The gear motor duty cycle affects the performance rating of the motor, such as a continuous duty cycle.

   Optimal cooling design housing

A better-cooled motor operates more efficiently, optimizing the design of the cooling fan and fan shroud for optimum airflow, ensuring tight coupling between the stator and the motor casing for optimum cooling performance. The electrical efficiency of the motor is much higher, but the power of the cooling fan is greater than the total loss. Optimization of the cooling fan size includes the use of the minimum power of the fan while providing adequate cooling. The optimized fan design reduces fan power requirements by 65% . An important design feature is the gap between the blade and the housing. The space between the outer casing and the fan blades should be as small as possible to prevent turbulence and reduce backflow.

   

Choose a low friction bearing suitable for working speed

Ball or roller bearings are used for high efficiency motors, which consist of an inner and outer ring and a cage containing steel or ceramic rolls or balls. The outer ring is connected to the stator and the inner ring is connected to the rotor. As the shaft rotates, the element also rotates and the friction of the shaft rotation is minimized. They have a long service life and low maintenance costs. High precision applications allow for a minimum air gap. Heat shrinkage and thermal expansion affect the fit of the shaft and housing and the internal bearing clearance. The power output controls the shaft size and bearing bore. The size and direction of the load determine the size and type of bearing. Consider additional forces such as asymmetric air gaps that cause magnetic pull, imbalance forces, gear pitch errors, and thrust loads. For bearing load calculations, the shaft is considered to be a beam supported on a rigid, torque-free bracket. Ball bearings are more suitable for high speed applications than roller bearings. High speed factors include cage design, lubricant, running accuracy, clearance, resonant frequency and balance.

   The bearings require a minimum load, so the rolling elements rotate to form a lubricating film rather than sliding, which increases the operating temperature and degrades the lubricant. The minimum allowable load is equal to 0.01 times the dynamic radial load rating of the ball bearing . This is especially important when the bearing is close to 70% of the recommended rating . Knowing the ambient temperature range and the normal operating temperature range will help determine the most effective lubrication method for the bearing: lubricating oil or grease, which is generally considered to have a normal operating temperature range of -25 to 40 °C . Synthetic greases provide excellent performance over a wide range of temperatures, and greases simplify maintenance, cleaning, and reduce leakage and contamination.


  Use high quality balancing machine, high standard and balance of motor running speed

When the axis does not coexist with the rotating axis, noise and vibration are generated. The balance has a limited effect on efficiency, but it will affect the running noise and life expectancy, which is also important for maximizing the use of resources. Bearing vibration readings are typically read in three planes: vertical, horizontal and axial. Vertical vibration may indicate installation problems, horizontal vibration may mean balance problems, and axial vibration may mean bearing problems. The balance at the operating speed is important because the centripetal force of the bearing can also cause an imbalance.

    Optimized design of sinusoidal magnetic field by rotor lamination

Synchronous motors with high-performance permanent magnets have sinusoidal flux distribution and electromotive force. For distributed windings, the stator windings are usually the same as asynchronous motor windings, which reduces vibration, noise and maintenance costs and improves overall performance.

    Selection of rare earth and ferrite (ceramic) magnets

Neodymium, rare earth, samarium cobalt magnets or ferrite (ceramic) magnets are used in the motor. The strength of the rare earth magnet is two to three times that of the ferrite or ceramic permanent magnet, but the price is relatively expensive. Samarium-cobalt magnets are the best choice for high-temperature applications because of their high energy density, temperature resistance from 250 to 550 ° C , small temperature reduction due to temperature rise, and oxidative protection. Choosing samarium or cobalt as a motor magnet is Based on operating temperature, corrosion resistance and required performance. If heated above 80 °C , low-grade neodymium magnets may begin to lose " strength " , and high-grade neodymium magnets work at temperatures below 220 °C . Ferrite or ceramic magnets are widely recognized for their strong electrical resistance, good demagnetization, high corrosion resistance and low cost. At 250 ° C of the magnetic loss will occur at temperatures above work, but the magnet of the magnetic loss will be restored to a lower temperature. Unless the circuit is designed for extreme conditions, a low temperature of -40 °C may result in permanent loss of permanent magnet strength.


   Motor needs inverter

The inverter drive unit can be used without loss in no-load operation / stationary state, and it is expected to save up to 30% of energy by replacing the existing line-powered three-phase drive . The drive features make it ideal for driving continuously operating pumps and fans. No additional components are required, such as an encoder. Up to 25% of the footprint allows for a more compact machine design. The motor has good control performance and is combined with a sensorless drive controller unit for excellent real-time performance even at low speeds and impressive dynamics in pulse load and speed changes.

Choose an inverter that provides sensorless operation

The drive 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 optimum torque for optimum efficiency. The lack of position or speed sensors reduces costs and increases the reliability of the drive system. As efficiency continues to increase, the importance of programming controller settings for a particular motor to achieve optimal efficiency is increasingly important.

in conclusion

When choosing a partner during the machine manufacturing process, remember that there are two ways to select the motor power supply, either by selecting a standard motor that may or may not be suitable for the specific application, or by selecting a qualified motor partner to design and manufacture a complete motor. Suitable for the application of the motor. If the design engineer does not have the time or engineering resources to design a custom version, or if a quick setup is required, a standard motor solution is appropriate. The new modular design and construction method enables manufacturing engineers to obtain custom motors at an affordable price, even in a limited number of cases. Regardless of the method of selecting the motor, the design / drive system is continually improved by comparing performance predictions with measurement results .

 

 

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