Since the loss distribution of motors varies with power and number of poles, in order to reduce losses, measures should be taken to focus on the main loss components of different powers and numbers of poles. Some ways to reduce losses are briefly described as follows:
1. Increase effective materials, reduce winding loss and iron loss
According to the motor similarity principle, when the electromagnetic load remains unchanged and the mechanical loss is not considered, the motor loss is approximately proportional to the cubic power of the motor linear size, and the motor input power is approximately proportional to the 4th power of the linear size. From this, the relationship between efficiency and effective material usage can be approximated. In order to obtain a larger space under certain installation size conditions so that more effective materials can be placed to improve motor efficiency, the outer diameter size of the stator punching becomes an important factor. Within the same machine base range, American motors have greater output than European motors. In order to facilitate heat dissipation and reduce temperature rise, American motors generally use stator punchings with larger outer diameters, while European motors generally use stator punchings with smaller outer diameters due to the need for structural derivatives such as explosion-proof motors and to reduce the amount of copper used at the winding end and production costs.
2. Use better magnetic materials and process measures to reduce iron loss.
The magnetic properties (magnetic permeability and unit iron loss) of the core material have a great influence on the efficiency and other performance of the motor. At the same time, the cost of the core material is the main part of the cost of the motor. Therefore, the selection of suitable magnetic materials is the key to designing and manufacturing high-efficiency motors. In high-power motors, iron loss accounts for a considerable proportion of the total loss. Therefore, reducing the unit loss value of the core material will help reduce the iron loss of the motor. Due to the design and manufacturing of the motor, the iron loss of the motor greatly exceeds the value calculated according to the unit iron loss value provided by the steel mill. Therefore, the unit iron loss value is generally increased by 1.5~2 times during design to consider the increase in iron loss. The reason for the increase in iron loss is mainly because the unit iron loss value of the steel mill is obtained by testing the strip sample according to the Epstein square ring method. However, the material is subjected to great stress after punching, shearing and laminating, and the loss will increase. In addition, the air gap caused by the presence of the tooth slot leads to no-load loss caused by the tooth harmonic magnetic field on the surface of the core. These will lead to a significant increase in iron loss after the motor is manufactured. Therefore, in addition to selecting magnetic materials with lower unit iron loss, it is also necessary to control the lamination pressure and take necessary process measures to reduce iron loss. In view of price and process factors, high-grade silicon steel sheets and silicon steel sheets thinner than 0.5mm are not used much in the production of high-efficiency motors. Low-carbon silicon-free electrical steel sheets or low-silicon cold-rolled silicon steel sheets are generally used. Some manufacturers of small European motors have used silicon-free electrical steel sheets with a unit iron loss value of 6.5w/kg. In recent years, steel mills have launched Polycor420 electrical steel sheets with an average unit loss of 4.0w/kg, which is even lower than some low-silicon steel sheets. The material also has a higher magnetic permeability. In recent years, Japan has developed a low-silicon cold-rolled steel sheet with a grade of 50RMA350. A small amount of aluminum and rare earth metals are added to its composition, thereby maintaining a higher magnetic permeability while reducing losses. Its unit iron loss value is 3.12w/kg. These are likely to provide a better material basis for the production and promotion of high-efficiency motors.
3. Reduce the size of the fan to reduce ventilation loss
For larger power 2- and 4-pole motors, wind friction accounts for a considerable proportion. For example, the wind friction of a 90kW 2-pole motor can reach about 30% of the total loss. Wind friction is mainly composed of the power consumed by the fan. Since the heat loss of high-efficiency motors is generally low, the cooling air volume can be reduced, and thus the ventilation power can also be reduced. The ventilation power is approximately proportional to the 4th to 5th power of the fan diameter. Therefore, if the temperature rise permits, reducing the fan size can effectively reduce wind friction. In addition, the reasonable design of the ventilation structure is also important for improving ventilation efficiency and reducing wind friction. Tests have shown that the wind friction of the high-power 2-pole part of a high-efficiency motor can be reduced by about 30% compared with that of an ordinary motor. Since the ventilation loss is reduced significantly and does not require much additional cost, changing the fan design is often one of the main measures taken for this part of the high-efficiency motor.
4. Reduce stray losses through design and process measures
The stray losses of asynchronous motors are mainly high-frequency losses generated by high-order harmonics of the magnetic field in the stator and rotor cores and windings. To reduce load stray losses, the amplitude of each phase band harmonic can be reduced by using a Y-Δ series-connected sinusoidal winding or other low-harmonic winding, thereby reducing stray losses. Experiments have shown that the use of sinusoidal windings can reduce stray losses by an average of more than 30%.
5. Improve die-casting technology to reduce rotor losses
By controlling the pressure, temperature and gas discharge path during rotor aluminum casting, the gas in the rotor bars can be reduced, thereby increasing conductivity and reducing rotor aluminum consumption. In recent years, the United States has successfully developed die-casting equipment for copper rotors and corresponding processes, and is currently conducting small-scale trial production. Calculations show that if cast copper rotors replace cast aluminum rotors, rotor losses can be reduced by about 38%.
6. Apply computer optimization design to reduce losses and improve efficiency
In addition to adding materials, improving material properties and improving processes, computer optimization design is used to reasonably determine various parameters while meeting cost, performance and other constraints to achieve the greatest possible improvement in efficiency. Adopting an optimized design can significantly shorten the time of motor design and improve the quality of motor design.
