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+Why is the efficiency of permanent magnet motors higher?

The permanent magnet synchronous motor is mainly composed of a stator, a rotor and a casing. Like ordinary AC motors, the stator core adopts a laminated structure to reduce the iron loss caused by the eddy current and hysteresis effects during the operation of the motor; the windings are usually three-phase symmetrical structure, but the choice of parameters varies greatly. The rotor part has many forms, including a permanent magnet rotor with a starting squirrel cage and a pure permanent magnet rotor embedded or surface mounted. The rotor core can be made into a solid structure or a laminated structure. The rotor adopts permanent magnet material, usually called magnet steel.

When the permanent magnet motor is operating normally, the magnetic fields of the rotor and stator are in a synchronized state. The rotor part has no induced current, no rotor copper loss, hysteresis and eddy current loss, and there is no need to consider rotor loss and heating. The general permanent magnet motor supplies power to the dedicated frequency converter and naturally has a soft start function. In addition, the permanent magnet motor is a synchronous motor, which has the characteristic of adjusting the power factor of the synchronous motor through the excitation intensity, so the power factor can be designed to a specified value.

From the starting point of view, because the permanent magnet motor is started by a variable frequency power supply or a supporting inverter, the starting process of the permanent magnet motor is easy to achieve; similar to the starting of a variable frequency motor, it avoids the starting defects of ordinary cage asynchronous motors.

In short, the efficiency and power factor of the permanent magnet motor can be very high, the structure is very simple, and the market has been very popular for the past ten years. However, loss of field failure is an inevitable problem of permanent magnet motors. When the current is too large or the temperature is too high, the temperature of the motor windings will rise instantaneously, the current will increase sharply, and the permanent magnet will quickly lose excitation. In the control of a permanent magnet motor, an overcurrent protection device is set to prevent the stator winding of the motor from burning out, but loss of excitation and equipment shutdown are inevitable.

Compared with other motors, permanent magnet motors are not very popular in the market. Motor manufacturers and users have some unknown technical blind spots, especially the matching with the inverter, which often leads to serious discrepancies between the design value and the test data, which must be verified repeatedly.

The design process of the motor involves some basic considerations, such as the requirements of the starter, the application environment, when and what torque and speed is needed, and how often? What is the working cycle? What are the environmental conditions, such as temperature and pressure? If the motor is used in the wrong field, it will not achieve the efficiency. Many motors are used in combination with gear motors, reducers, and motors. Gear motors provide high torque at low speeds. In short, while amplifying the torque, the geared motor absorbs the power of the motor and reduces the speed. The duty cycle of a gear motor will affect the performance rating of the motor, such as a continuous duty cycle.

Motors with good cooling performance run more efficiently. In order to obtain the air volume, the design of the cooling fan and the fan case is optimized to ensure the close combination of the stator and the motor case to provide cooling performance. The electrical efficiency of the motor has been greatly improved, but the power of the cooling fan accounts for a larger proportion of the total loss. The optimization of the cooling fan size includes the use of fan power while providing adequate cooling. The optimized fan design can reduce fan power demand by 65%. An important design feature is the gap between the blade and the shell. The space between the casing and the fan blades should be as small as possible to prevent turbulence and reduce backflow.

Ball or roller bearings are used in motors. They consist of an inner ring and an 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 element also rotates, so that the friction when the shaft rotates is small. Long service life and low maintenance cost. High-precision applications allow air gaps. Thermal shrinkage and thermal expansion will affect the fit between the shaft and the bearing seat and the internal clearance of the bearing. The power output controls the shaft size and bearing bore diameter. The size and direction of the load determines the size and type of the bearing. Consider additional forces such as magnetic tension caused by an asymmetric air gap, unbalanced force, gear pitch error, and thrust load. In the load-bearing calculation, the shaft is considered to be a beam supported on a rigid free support. Ball bearings are more suitable for high-speed applications than roller bearings. High-speed factors include cage design, lubricant, operating accuracy, clearance, resonance frequency and balance.

The bearing requires a small load, so the rolling elements rotate to form a lubricating film instead of sliding, which will increase the operating temperature and reduce the lubricating oil. The 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. Understanding the ambient temperature range and normal operating temperature range will help determine the effective lubrication method of the bearing: lubricating oil or grease. It is generally believed that the normal operating temperature range of gear motors is -25 ~ 40°c. Synthetic grease has good performance in various temperature ranges. Grease can simplify maintenance, cleaning, reduce leakage and pollution protection.

When the shaft and the rotating shaft do not coexist, noise and vibration will occur. The impact of balance on efficiency is limited, but it will affect operating noise and life expectancy, which is also important for resource utilization. Bearing vibration readings are usually taken on three planes: vertical, horizontal and axial. Vertical vibration may indicate installation problems, horizontal vibration may indicate balance problems, and axial vibration may indicate bearing problems. It is important to maintain balance at operating speeds because the centripetal force of the bearing may also cause imbalance.

High-performance permanent magnet synchronous motors have sinusoidal magnetic flux distribution and electromotive force. For distributed windings, the stator windings are usually the same as asynchronous motors. Reduce vibration, noise and maintenance costs, and improve the performance of the whole machine.

The motor uses neodymium, rare earth, samarium cobalt magnets or ferrite (ceramic) magnets. The strength of rare earth magnets is 2 to 3 times that of ferrite or ceramic permanent magnets, but they are expensive. Samarium cobalt magnets are the choice for high-temperature applications because they have a higher energy density, resistance to temperatures of 250-550°C, temperature rise and oxidation protection caused by small reduction parameters. The motor magnets can be selected from samarium cobalt or neodymium according to the working temperature, corrosion resistance and required performance. If heated to above 80°C, low-grade neodymium magnets may begin to lose their "strength", while neodymium magnets work at temperatures below 220°C. Ferrite or ceramic magnets are widely recognized for their strong resistance, good demagnetization performance, strong corrosion resistance, and low price. When working at a temperature above 250°C, magnetic loss will occur, but when the magnet drops to a lower temperature, the magnetic loss will recover. Unless the circuit is designed for conditions, the low temperature of -40°C may cause the loss of permanent magnet strength.

The drive unit of the frequency converter can remain undamaged in no-load operation/static state. By replacing the existing three-phase drive device powered by this line, it is expected that up to 30% of energy can be saved. The characteristics of the drive unit 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 makes the machine design more compact. The motor has good control performance, combined with a sensorless drive controller unit, has good actual running performance even at low speeds, and has impressive dynamic characteristics when pulse loads and speed changes.

The drive can "self-check" and track the permanent magnet position of the rotor. It is essential to start the motor smoothly and also allows the efficiency of generating torque. The lack of position or speed sensors reduces costs and improves the reliability of the drive system. With the continuous improvement of efficiency, in order to obtain efficiency, it becomes more and more important to program the controller settings of a specific motor.

Permanent magnet synchronous motors, to be precise, should be called asynchronous start synchronous permanent magnet motors. This kind of motor can replace the original y, Y2, and Y3 motors with the same size. Reduce the trouble of the replacement process. Compared with ordinary motors, permanent magnet motors have their own characteristics:

1. The speed is constant, that is, the synchronous speed. The speed is slightly higher than that of ordinary motors. For example, the four-pole speed of an ordinary motor is above 1400N/min, while the speed of a permanent magnet synchronous motor is 1500N/min, which has a small rotation loss.

2. High power factor. In the normal operation of the permanent magnet motor, the rotor speed is consistent with the stator magnetic field speed. The magnetic poles of the rotor are made of permanent magnet steel, without current, the induced current on the stator is reduced, and the power factor is high. Through reasonable design, it can work under lagging power factor, unit power factor and leading power factor. Generally, the lagging power factor can reach or exceed 0.95. The wide application of permanent magnet motors can save equipment such as reactive power compensators.

3. High efficiency, especially high operating efficiency. During the normal operation of the permanent magnet motor, since the magnetic pole of the rotor adopts permanent magnet neodymium iron boron magnet, the permanent magnet field can ensure the normal operation of the motor, so the rotor has no winding loss. The rotor has no iron loss and the efficiency is much higher than that of ordinary motors. At present, the energy efficiency of the general design of permanent magnet synchronous motors can easily reach the second level or even the indicators specified in GB/t18613-2012; the design of ordinary motors is more difficult to achieve the corresponding performance, which is especially obvious in low-power motors.

4. Permanent magnet synchronous motors have a wide range of economic operation. The economic operating range of ordinary motors is generally 60 ~ 100% of the rated load. When the load is lower than 60%, the efficiency and power factor curve of the motor drop sharply, and the operating efficiency and power factor are very low. The economic operating range of permanent magnet synchronous motors is much larger than that of ordinary motors. It not only has higher efficiency under rated load, but also has higher efficiency in the range of 25 ~ 120% of rated load. The efficiency curve is relatively smooth with little change. The motor efficiency is basically not less than 80% of the rated efficiency. The efficiency of ordinary motors drops rapidly when the rated load is close to 35%, and can be as low as 30-40%. At 25% load, the power factor of the permanent magnet motor can also reach more than 0.9. The lighter the load, the higher the power factor; ordinary motors quickly drop from about 0.85 to below 0.5 under the rated load.

5. Small size and light weight. Due to the application of rare earth permanent magnet materials on the rotor of the permanent magnet motor, the loss is low, the efficiency and the power factor are high, and the same power can be achieved. On the basis of ensuring efficiency and power factor, it can be smaller and lighter than ordinary motors. Compared with ordinary motors, this has incomparable advantages. In some cases, small seats and high power are required.

6. The locked rotor torque multiple is high. The locked rotor torque multiples of ordinary motors are generally 1.6 to 2.3 times the rated torque, while the locked rotor torque of permanent magnet motors can generally reach more than 2.4 times, and some specifications can even reach more than 3.5 times. Some fields collectively refer to permanent magnet motors as "permanent magnet synchronous motors with high starting torque". When the starting torque of some equipment is high, many use high-torque motors, but the efficiency is very low; moreover, increasing the capacity increases the starting torque, but in actual operation, the load rate is very low, and the efficiency and power factor are very low, resulting in Waste of facilities and energy. The use of permanent magnet motors to achieve the same torque can appropriately reduce the motor capacity. The permanent magnet motor has high power factor, high efficiency and obvious energy saving effect.

7. The speed is slow and the efficiency is high. There are few motors with more than 10 poles than ordinary motors, which is not technically impossible, but the lower the speed, the lower the efficiency, the number of frames is large, and the power is small. In the past, it was considered uneconomical. The number of poles of a permanent magnet motor can be very high. Asynchronous start permanent magnet motors have 24 poles or even 32 poles. The speed is very low. Part of the equipment can be directly driven, eliminating the need for deceleration facilities. From an energy-saving point of view, this can improve efficiency. Moreover, because the rotor loss of the permanent magnet motor is small, although the number of poles is high, the efficiency can be high, and the energy saving prospect is very good.

8. The cost of permanent magnet motor is high and the processing technology is complicated. Due to the use of high-performance rare earth permanent magnet NdFeB, the manufacturing cost is high. The permanent magnet is placed inside the rotor, the design and installation process is complicated, and the manufacturing cost also increases. Of course, with the continuous innovation of new technologies, new materials, and new processes, the cost is much lower than the cost of the permanent magnet synchronous host at the beginning.

9. The starting of permanent magnet motors has its own characteristics. Generally, permanent magnet motors cannot be started under reduced voltage, because when the voltage of ordinary permanent magnet motors (380V, 50Hz) drops to 330V, it is difficult to start and the rotor shakes seriously. Low-power permanent magnet motors usually adopt direct start mode. When the transformer capacity is large enough and the mechanical shock requirements of the equipment are not strict, the high-power permanent magnet motor can also be started directly. Otherwise, it is recommended to use the soft start mode driven by the inverter

10. The drive of the three-phase AC permanent magnet synchronous motor can adopt the "stator winding star seal" mode to provide the electromagnetic torque generated during braking. The motor itself does not have an elevator in the non-driving state to suppress "rapid sliding" in accidents. In the state, but the function of this connection method cannot be connected with the elevator's upward overspeed protection device, and the elevator accidental action protection device is confused.

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