What are the methods for reducing losses and improving efficiency in electric motors?

Since the distribution of losses in electric motors varies with power size and pole numbers, measures should focus on the main loss components for different power levels and pole numbers to reduce overall losses. The following are some methods to reduce losses:

1. Increase Effective Material to Reduce Winding Losses and Iron Losses:

According to the principle of similarity in electric motors, when electromagnetic load remains constant and mechanical losses are not considered, the losses in an electric motor are approximately proportional to the cube of the motor's linear dimensions, while the input power of the motor is approximately proportional to the fourth power of the linear dimensions. Thus, the relationship between efficiency and the amount of effective material can be approximated. To achieve a higher efficiency within a given installation size, it is important to provide more space for effective materials. Therefore, the outer diameter of the stator laminations becomes a crucial factor. In the same frame size, American motors generally have a larger output compared to European motors. To aid in heat dissipation and reduce temperature rise, American motors typically use larger diameter stator laminations, while European motors often use smaller diameter laminations due to considerations for explosion-proof motor structures, reducing copper usage at the winding ends, and lowering production costs.

2. Use Better Magnetic Materials and Process Measures to Reduce Iron Losses:

The magnetic performance of core materials (permeability and unit iron loss) has a significant impact on the efficiency and other performance aspects of the motor, and core material costs are a major part of the motor's cost. Therefore, selecting suitable magnetic materials is crucial for designing and manufacturing high-efficiency motors. In high-power motors, iron losses constitute a substantial portion of the total losses, so reducing the unit loss value of core materials will help lower iron losses. Due to the design and manufacturing processes, actual iron losses in motors often exceed the calculated values based on the unit iron loss provided by steel manufacturers. Typically, the unit iron loss value is increased by 1.5 to 2 times in design considerations to account for additional iron losses. The increase in iron losses is mainly due to the fact that the unit iron loss values provided by steel mills are measured using the Epstein frame method on sample strips, but the material undergoes significant stress after stamping and stacking, which increases losses. Additionally, the presence of slots causes air gaps and results in no-load losses due to slot harmonic fields on the core surface, significantly increasing the iron losses after motor assembly. Therefore, in addition to choosing magnetic materials with lower unit iron losses, controlling the stacking pressure and implementing necessary process measures are also required to reduce iron losses. Due to cost and process factors, high-grade silicon steel sheets and those thinner than 0.5mm are not widely used in high-efficiency motors; instead, low-carbon, non-silicon electrical steel sheets or low-silicon cold-rolled silicon steel sheets are more common. Some European manufacturers of small motors have used non-silicon electrical steel sheets with a unit iron loss value of 6.5 W/kg. Recently, steel mills have introduced Polycor 420 electrical steel sheets with an average unit loss of 4.0 W/kg, which is lower than some low-silicon sheets and also offers higher permeability. Recently, Japan has developed low-silicon cold-rolled steel sheets with the grade 50RMA350, which include a small amount of aluminum and rare earth metals, maintaining high permeability while reducing losses, with a unit iron loss value of 3.12 W/kg. These advancements could provide a better material foundation for the production and promotion of high-efficiency motors.

3. Reduce Fan Size to Lower Ventilation Losses:

For high-power 2-pole and 4-pole motors, windage losses account for a significant proportion; for example, the windage loss in a 90 kW 2-pole motor can be around 30% of the total losses. Windage losses are mainly due to the power consumed by the fan. Since high-efficiency motors generally have lower thermal losses, the amount of cooling air can be reduced, thereby reducing ventilation power. Ventilation power is approximately proportional to the fourth or fifth power of the fan diameter. Therefore, reducing fan size within acceptable temperature rise limits can effectively lower windage losses. Additionally, rational design of the ventilation structure is important for improving ventilation efficiency and reducing windage losses. Experiments show that high-efficiency motors with high power and 2 poles can achieve a reduction of around 30% in windage losses compared to ordinary motors. Since the reduction in ventilation losses is significant and does not require much additional cost, changing fan design is often one of the main measures for high-efficiency motors.

4. Reduce Stray Losses Through Design and Process Measures:

Stray losses in asynchronous motors are mainly high-frequency losses caused by high-order harmonics in the stator and rotor cores and windings. To reduce load stray losses, sinusoidal winding or other low-harmonic windings can be used to reduce the amplitude of harmonics, thereby lowering stray losses. Tests show that using sinusoidal windings can reduce stray losses by more than 30% on average.

5. Improve Die-Casting Process to Reduce Rotor Losses:

By controlling the pressure, temperature, and gas discharge path during rotor aluminum die-casting, the amount of gas in the rotor conductive bars can be reduced, thereby improving conductivity and reducing rotor aluminum losses. Recently, the U.S. has developed die-casting equipment and processes for cast-copper rotors, and small-scale trial production is underway. Calculations show that replacing cast-aluminum rotors with cast-copper rotors can reduce rotor losses by about 38%.

6. Apply Computer Optimization Design to Reduce Losses and Improve Efficiency:

In addition to increasing material, improving material performance, and refining processes, computer optimization design can be used to rationally determine various parameters under cost and performance constraints, thereby achieving the maximum possible improvement in efficiency. Using optimization design can significantly shorten motor design time and enhance the quality of motor design.

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