What is the mechanical loss of the motor? How to reduce the mechanical loss of the motor

What is the mechanical loss of the motor?

(1) Copper loss: stator copper loss; rotor copper loss; stray loss.

(2) Core loss.

(3) Mechanical loss: ventilation loss; friction loss.

The main methods to reduce the mechanical loss of electric motors

(1) Use high-efficiency fans (such as wing-type axial flow fans).

(2) Adjust the gap between the fan cover and the outer circle of the fan blade.

(3) For lightly loaded motors, the fan outer diameter should be appropriately reduced.

(4) Use high-quality bearings.

(5) Use high-quality lubricants.

(6) Improve the assembly quality of motors.

Specific measures to reduce mechanical loss of electric motors

First, let's look at how to save energy by changing the size of the fan. As we all know, the motor is a device that converts electrical energy into mechanical energy. Losses are generated during the conversion process. These losses appear in the form of heat, which makes the motor heat up. Customized windings generate copper losses when current flows through them. The heat is conducted to the stator core through the slot insulation material, and then from the stator core to the motor housing and dissipated into space. The heat of the rotor is generated by the rotor aluminum loss and its friction. It is transferred to the rotor core and the inner fan surface. The heat is dissipated in the motor space by the stirring of the inner fan. When the stator core, end cover, and base are rotated, the stator and rotor heat are blown away by the external fan. Therefore, the size of the external fan air volume is the key to determining that the temperature of the motor cannot exceed the temperature allowed by its insulation material grade.

The national standard stipulates the allowable temperature rise of motors of various insulation grades under rated operating conditions, requiring that the hottest point temperature in the motor is not allowed to exceed the limit temperature of its insulation grade. The commonly used insulation grades of motors are as follows: Wherein: Allowable temperature rise = Allowable temperature limit - Ambient temperature specified value - 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 unloaded or lightly loaded, the total loss of the motor is smaller than when it is rated, and the air volume is proportional to the total loss of the motor, so the fan is in a "big horse pulling a small cart" (because the ventilation loss is constant with the motor speed, the ventilation loss does not change with the load, so the air volume should be reduced to reduce the ventilation loss of the motor). Changing the shape of the fan blades can reduce the air volume, but it is more troublesome. It is simpler to directly reduce the diameter of the outer fan blades to reduce the air volume. We know that the mechanical loss of the fan itself is proportional to the 4-5th power of the blade diameter, and the air volume is proportional to the square of the blade diameter. Therefore, when the blade diameter is reduced, the air volume does not decrease much, but the ventilation loss is reduced a lot.

Due to the reduction of cooling air volume of the external fan, the temperature rise of the motor increases, but the temperature rise of the motor can still be kept within the range allowed by the insulation grade. At the same time, we must also see that the motor mechanical loss is reduced due to the reduction of the diameter of the outer fan blades, which will further reduce the temperature rise of the motor. Especially for high-speed motors such as 2 and 4, when the outer diameter of the fan is reduced by 14%-16%, the ventilation loss is reduced by 20%-40%. In addition, when changing the diameter of the fan blades, the size of the windshield or fan cover should be changed accordingly so that the sizes of their matching with each other meet the regulations. The gap between the fan blades and the fan cover should not be too large, generally between 10-15. Too large will cause leakage loss from the high-pressure area to the low-pressure area. At the same time, the angle between the fan blades and the fan cover should be maintained, which is to convert part of the dynamic pressure of the incoming air into static pressure and reduce losses. For example, the fan used in the No. 8 engine can be used on the light-loaded motor of the No. 9 engine after adjustment and trimming. In this way, the mechanical loss of the light-loaded motor of the No. 9 engine can be reduced, and the temperature rise can be increased to improve the efficiency and power factor of the motor.

3.1 Reduce friction to reduce mechanical losses

The normal operation, noise, vibration, overheating, life and other factors of motor bearings are all related to the reasonable selection of grease. At present, No. 3 lithium-based grease is usually used to reduce mechanical loss. However, with the rapid development of science and technology, many greases with superior performance have emerged, such as "small and medium-sized motor bearing grease". Its performance has been tested by relevant departments to meet the standards of Japanese JIS2220-80 rolling bearing grease, close to the level of Swiss SKF65C grease, with extremely low impurities, and the price is comparable to No. 3 lithium-based grease. The performance of American Esso grease currently appearing on the market is also good. We can use these greases with good performance to improve the operation of the motor to reduce mechanical loss. As professional motor maintenance personnel, we can also work hard on the reasonable selection of bearings, which can also save energy and reduce consumption; since we already know that the mechanical loss of motors with a small number is large, and the mechanical loss of motors with a large number of stages is small. We can selectively use bearings when repairing motors. For example, on high-speed motors, we can use imported bearings or domestic high-level B and C-level high-quality bearings to minimize the room loss. On low-speed motors, we can use domestic C and D grade products, so that we can save money on bearings and achieve the goal of energy saving and consumption reduction.

3.2 Improve power factor to reduce mechanical loss

95% of the rotating equipment in power plants are AC asynchronous motors, which are low power factor equipment. Let's discuss how to improve their power factor.

3.2.1 Rationally adjust operating equipment to improve power factor

Before performing artificial compensation, consideration should be given to inspecting and adjusting the existing equipment in the power plant to ensure that it operates reasonably and thus improve the power factor.

(1) Generally speaking, the equipment in the power plant is matched. We do not need to consider the capacity and selection. We only need to consider the running motor to avoid the "big horse pulling a small cart" phenomenon. When the load rate is lower than 40%, you can consider replacing the motor with a smaller capacity. When the load rate is lower than 30%, the winding of the delta connection can be changed to the star connection and operation mode, which will improve the efficiency and power factor of the motor.

(2) Old series of motors, such as J0 and J02 series, should be renovated or eliminated and replaced with Y series energy-saving motors.

(3) Use and maintain the motor correctly, pay attention to controlling its frequent starting (the power factor is very low when it is started frequently), and perform careful maintenance. It is also essential to adjust the center line and the gap between the 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 achieve the purpose of improving the power factor is also a common energy-saving method, that is, installing a frequency converter on an ordinary AC asynchronous motor. This method is simple to install, easy to operate, and has significant energy-saving effects.