Why is the efficiency of permanent magnet motor higher?
Permanent magnet synchronous motor is mainly composed of stator, rotor and shell. The stator core is laminated to reduce the iron loss due to eddy current and hysteresis effect; The winding is usually three-phase symmetrical structure, but the parameter selection is quite different. The rotor part has various forms, including permanent magnet rotor with starting cage and pure permanent magnet rotor with embedded or surface mounted. The rotor core can be made into solid structure or laminated. The rotor is equipped with permanent magnet material, which is commonly known as magnetic steel.is mainly composed of stator, rotor and shell. The stator core is laminated to reduce the iron loss due to eddy current and hysteresis effect; The winding is usually three-phase symmetrical structure, but the parameter selection is quite different. The rotor part has various forms, including permanent magnet rotor with starting cage and pure permanent magnet rotor with embedded or surface mounted. The rotor core can be made into solid structure or laminated. The rotor is equipped with permanent magnet material, which is commonly known as magnetic steel.
Under the normal operation of permanent magnet motor, the magnetic fields of rotor and stator are in the synchronous state, and there is no induced current in the rotor part, no rotor copper loss, hysteresis and eddy current loss, and no need to consider the rotor loss and heating problem. General permanent magnet motor is powered by special inverter, which has the function of soft start. In addition, permanent magnet motor belongs to synchronous motor, which has the characteristics of synchronous motor adjusting power factor by excitation strength, so the power factor can be designed to the specified value.
From the starting point of view, because of the fact that the permanent magnet motor is started by variable frequency power supply or matching inverter, the starting process of permanent magnet motor is easy to realize; Similar to the starting of variable frequency motor, it avoids the starting defects of common cage asynchronous motor.
In a word, the efficiency and power factor of permanent magnet motor can reach very high, the structure is very simple, and the market is very hot in recent ten years. However, the loss of excitation fault is an unavoidable problem of permanent magnet motor. When the current is too large or the temperature is too high, it will cause the temperature of motor winding to rise instantly, the current to increase sharply, and the permanent magnet to lose excitation rapidly. In the control of permanent magnet motor, the over-current protection device is set to avoid the burning of the stator winding, but the loss of excitation and equipment outage are inevitable.
Compared with other motors, the application of permanent magnet motor in the market is not very popular. For both motor manufacturers and users, there are some unknown technical blind spots, especially when it comes to the matching problem with the inverter, which often leads to serious inconsistency between the design value and the test data, and must be verified repeatedly.
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.
Optimum cooling design housing
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.
Choose low friction bearing suitable for working speed
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.
Use of high quality balancing machine, high standard and motor speed under the balance
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.
Optimal design of rotor lamination displaying sinusoidal magnetic field
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.
Selection of rare earth and ferrite (ceramic) magnets
Neodymium, rare earth, samarium cobalt magnets or ferrite 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 motor needs inverter
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.
Select the inverter that can provide sensorless operation
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.
Source: China Permanent Magnet Manufacturer – www.rizinia.com
