Why are permanent magnet motors more efficient?

Permanent magnet synchronous motor is mainly composed of stator, rotor and housing components. Like ordinary AC motors, the stator core is a laminated structure to reduce iron loss due to eddy current and hysteresis effects during motor operation; the windings are also usually three-phase symmetrical structures, but the parameter selection is quite different. The rotor part has various forms, including permanent magnet rotors with starting squirrel cages, and built-in or surface-mounted pure permanent magnet rotors. The rotor core can be made into a solid structure or laminated. The rotor is equipped with permanent magnet material, which is commonly called magnet steel.

Under the normal operation of the permanent magnet motor, the rotor and the stator magnetic field are in a synchronous state, there is no induced current in the rotor part, no rotor copper loss, hysteresis, and eddy current loss, and there is no need to consider the problem of rotor loss and heat generation. Generally, the permanent magnet motor is powered by a special frequency converter, and naturally has a soft start function. In addition, the permanent magnet motor is a synchronous motor, which has the characteristics of adjusting the power factor of the synchronous motor through the strength of the excitation, so the power factor can be designed to a specified value.

From the perspective of starting, due to the fact that the permanent magnet motor is started by the variable frequency power supply or the supporting frequency converter, the starting process of the permanent magnet motor is easy to realize; similar to the starting of the variable frequency motor, it avoids the starting defects of the ordinary cage-type asynchronous motor.

In short, the efficiency and power factor of permanent magnet motors can reach very high, and the structure is very simple. The market has been very hot in the past ten years. However, demagnetization failure is an unavoidable problem for permanent magnet motors. When the current is too high or the temperature is too high, the temperature of the motor windings will rise instantaneously, the current will increase sharply, and the permanent magnets will lose their magnetism rapidly. In the permanent magnet motor control, an overcurrent protection device is set up to avoid the problem of the motor stator windings being burned, but the resulting loss of magnetization and equipment shutdown are inevitable.

Compared with other motors, the application of permanent magnet motors in the market is not very popular. There are some unknown technical blind spots for both motor manufacturers and users, especially when it comes to matching with frequency converters, which often leads to design. The value is seriously inconsistent with the experimental data and must be verified repeatedly.

The motor design process involves some basic considerations, for starters, the requirements of the application environment, when do you need what torque and speed, and how often? What is a duty cycle? What are the environmental conditions such as temperature and pressure? Even the most efficient motor will not perform at its maximum efficiency if the motor is used in the wrong application. Many electric motors are used in combinations of gear motors, gear reducers and electric motors. Gear motors provide high torque at low speeds. In short, a gear motor absorbs power from the motor and reduces speed while amplifying the torque. The gear motor duty cycle affects the performance rating of the motor, such as the continuous duty cycle.

Optimal Cooling Design Case

A better cooled motor runs more efficiently, in order to get the best airflow, the design of the cooling fan and fan guard is optimized, ensuring a tight bond between the stator and the motor housing to provide the best cooling performance. The electrical efficiency of the motor has improved a lot, but the power of the cooling fan accounts for a larger proportion of the total loss. Optimization of cooling fan size involves using the minimum power of the fan while providing adequate cooling. An optimized fan design reduces fan power requirements by 65%. An important design feature is the clearance between the blades and the housing. The space between the housing and the fan blades should be kept as small as possible to prevent turbulence and reduce back flow.

Choose low-friction bearings for operating speeds

Ball or roller bearings are used in high-efficiency motors and they consist of an inner and outer ring and a cage containing steel or ceramic rollers or balls. The outer ring is connected to the stator, and the inner ring is connected to the rotor. When 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 minimal air gaps. Thermal shrinkage and thermal expansion affect the fit of the shaft and housing as well as the internal bearing clearance. Power output controls shaft size and bearing bore. The magnitude and direction of the load determine the bearing size and type. Consider additional forces such as asymmetrical air gaps causing magnetic pull, unbalanced forces, pitch errors of gears, and thrust loads. For bearing load calculations, the shaft is considered as a beam supported on rigid moment-free supports. Ball bearings are better suited for high-speed applications than roller bearings. High speed factors include cage design, lubricants, running accuracy, clearances, resonant frequency and balance.

Bearings require minimal load, so the rolling elements rotate to form a lubricating film instead of sliding, which raises operating temperatures and degrades the lubricating oil. 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 approaching 70% of the recommended rating. Knowing the ambient temperature range and the normal operating temperature range will help determine the most effective method of lubrication for the bearing: oil or grease, and the normal operating temperature range for geared motors considered in general is -25 to 40°C. Synthetic greases have good performance over a wide temperature range and greases allow for ease of maintenance, cleaning, reduced spillage and contamination protection.

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