This article mainly analyzes the composition of energy loss in electric motors during use, and focuses on how to reduce mechanical losses in electric motors during operation to achieve energy saving and efficiency improvement.
1. Components of energy loss in asynchronous motors
(1) Copper loss: stator copper loss; rotor copper loss; stray loss.
(2) Core loss.
(3) Mechanical losses: ventilation losses; friction losses.
From the energy losses of asynchronous motors listed above, we can see that copper and iron losses are difficult to change during maintenance, as they are determined by the design. Mechanical losses, however, can be modified. Generally, asynchronous motors with fewer stages (i.e., higher speeds) have greater mechanical losses and less copper losses; conversely, motors with more stages (i.e., lower speeds) have a higher proportion of copper losses than mechanical losses. Therefore, to reduce the energy losses of an asynchronous motor, we must start by reducing its mechanical losses.
2. Main methods for reducing the mechanical losses of electric motors
Reducing the mechanical losses of electric motors can be addressed by focusing on the following aspects:
(1) Use high-efficiency fans (such as airfoil axial flow fans).
(2) Adjust the gap between the fan cover and the outer circle of the fan blade.
(3) For light-load motors, the outer diameter of the fan should be appropriately reduced.
(4) Use high-quality bearings.
(5) Use high-quality lubricant.
(6) Improve the quality of motor assembly.
3. Specific measures to reduce the mechanical losses of electric motors
First, let's look at how to save energy by changing the fan size. As we all know, an electric motor is a device that converts electrical energy into mechanical energy. During this conversion, losses occur, and these losses manifest as heat, causing the motor to heat up. When current flows through the custom windings, copper losses are generated. This heat is conducted to the stator core via the slot insulation material, and then from the stator core to the motor casing, dissipating into the space. The rotor's heat is generated by the rotor's aluminum losses and friction. This heat is transferred to the rotor core and the surface of the internal fan. The internal fan's agitation dissipates the heat into the motor space. Finally, the external fan dissipates the heat from both the stator and rotor, passing through the stator core, end covers, and frame. Therefore, the airflow of the external fan is crucial in ensuring that the motor's temperature does not exceed the allowable temperature range specified by its insulation material grade.
National standards specify the allowable temperature rise of motors with various insulation classes under rated operating conditions. The standards require that the temperature of the hottest spot in the motor must not exceed the limit temperature of its insulation class. The commonly used insulation classes of motors are shown in the table below: Where: Allowable temperature rise = Allowable temperature limit - Specified ambient temperature - Hot spot temperature difference The hot spot temperature difference of the winding refers to the difference between the stable temperature of the hot spot of the winding and the average temperature of the winding when the motor is under rated load.
All values in the table are in degrees Celsius.
When the motor is under no-load or light-load conditions, its total losses are lower than at rated speed. Since airflow is directly proportional to total motor losses, the fan is essentially "overpowered" (because ventilation losses are constant with motor speed, they don't change with load; therefore, airflow should be reduced to minimize motor ventilation losses). Changing the fan blade shape can reduce airflow, but it's cumbersome. Reducing the diameter of the external fan blades is simpler. We know that the fan's mechanical losses are proportional to the 4th to 5th power of the blade diameter, while airflow is proportional to the square of the blade diameter. Therefore, reducing the blade diameter doesn't significantly reduce airflow, but it greatly reduces ventilation losses.
Because the cooling airflow of the external fan is reduced, the motor temperature rises, but it still remains within the allowable range of the insulation class. Simultaneously, the reduced mechanical losses due to the smaller external fan blade diameter further decrease the motor temperature rise. Especially for high-speed motors like those with poles 2 and 4, a 14%–16% reduction in fan outer diameter reduces ventilation losses by 20%–40%. Furthermore, when changing the fan blade diameter, the dimensions of the baffle or fan cover must be adjusted accordingly to ensure they fit properly. The gap between the fan blades and the fan cover should not be too large, generally between 10 and 15 mm. Excessive gaps will significantly increase leakage losses from high-voltage areas back to low-voltage areas. Maintaining the angle between the fan blades and the fan cover is crucial; this converts some of the dynamic pressure of the incoming air into static pressure, reducing losses. For example, a fan used in a No. 8 frame can be adjusted and modified to be used in a lightly loaded No. 9 frame motor. This reduces mechanical losses in the No. 9 frame motor and slightly increases its temperature rise, thereby improving its efficiency and power factor.
3.1 Reduce friction to lower mechanical wear
The normal operation, noise, vibration, overheating, and lifespan of electric motor bearings are all related to the proper selection of lubricating grease. Currently, No. 3 lithium-based grease is generally preferred to reduce mechanical wear. However, with the rapid development of science and technology, many high-performance lubricating greases have emerged, such as "small and medium-sized electric motor bearing grease." Its performance, as tested by relevant departments, meets the Japanese JIS2220-80 standard for rolling bearing grease and is close to the level of Swiss SKF65C grease. It has extremely low impurity content and its price is comparable to No. 3 lithium-based grease. American Esso grease, currently available on the market, also performs well. We can certainly use these high-performance lubricating greases to improve the operating condition of electric motors and reduce mechanical wear. As professional electric motor maintenance personnel, we can also focus on the proper selection of bearings to save energy and reduce consumption. Since we already know that electric motors with fewer poles have higher mechanical wear and those with more poles have lower mechanical wear, we can selectively use bearings when repairing electric motors. For example, in high-speed electric motors, we can use imported bearings or high-grade domestic B and C grade bearings to minimize wear. For low-speed motors, we can use domestically produced C and D grade motors, which saves us money on bearings and thus achieves energy saving and consumption reduction.
3.2 Improve power factor to reduce mechanical losses
In power plants, 95% of the rotating equipment is driven by AC asynchronous motors, which are devices with low power factors. We will discuss how to improve their power factor.
3.2.1 Adjust operating equipment appropriately to improve power factor
Before implementing artificial compensation, the existing equipment in the power plant should be inspected and adjusted to ensure its proper operation, thereby improving the power factor.
(1) Generally speaking, the equipment in the power plant is matched, so we do not need to consider the capacity and selection. We only need to consider the motor in operation to avoid the phenomenon of "oversized motor for small vehicle". When the load rate is less than 40%, we can consider replacing it with a smaller capacity motor. When the load rate is less than 30%, the delta-connected winding can be changed to a star-connected operation mode, which will improve the efficiency and power factor of the motor.
(2) Modify or eliminate old-style motors such as J0 and J02 series motors and replace them with energy-saving motors of Y series.
(3) Use and maintain the motor correctly, pay attention to controlling its frequent starts (the power factor is very low when it starts frequently), and carefully inspect it. It is also essential to adjust the center line and the gap between the motor rotor and stator when installing the motor, because uneven air gap will cause the no-load current to increase and the power factor to decrease.
3.2.2 Install a frequency converter to improve the power factor
Using a frequency converter to adjust the speed to improve the power factor is a common energy-saving method. This involves installing a frequency converter on an ordinary AC asynchronous motor. This method is simple to install, easy to operate, and has a significant energy-saving effect.
