Energy saving analysis of the motor during use

2019-07-10

This paper mainly analyzes the composition of energy loss during the use of the motor, and focuses on how to reduce the mechanical loss of the motor during operation to achieve energy saving and efficiency.
1. Asynchronous motor energy loss composition
(1) Copper loss: stator copper loss; rotor copper loss; stray loss.
(2) Core loss.
(3) Mechanical loss: ventilation loss; friction loss.
From the energy loss of the asynchronous motor listed above, copper loss and iron loss are not easily changed during maintenance. They are determined by design, and mechanical loss can be changed. Asynchronous motors generally have a large number of stages, that is, a motor with a high number of revolutions, which has a large mechanical loss and a small copper loss; and a motor with a large number of stages, that is, a motor with a low number of revolutions, has a larger copper loss than a mechanical loss. Therefore, we need to reduce the mechanical loss of the motor to reduce the mechanical loss of the motor.
2. The main method to reduce the mechanical loss of the motor
Reducing the mechanical loss of the motor is mainly solved from the following aspects:
(1) Use a highly efficient fan (such as a wing-type axial fan).
(2) Adjust the gap between the hood and the outer circumference of the blade.
(3) The light load motor appropriately reduces the outer diameter of the fan.
(4) Use high quality bearings.
(5) Use high quality lubricants.
(6) Improve the quality of motor assembly.
3. Specific measures to reduce motor loss
First, let's look at how to save energy by changing the size of the fan. Everyone knows that the motor is a device that converts electrical energy into mechanical energy. During the conversion process, losses are generated. These losses occur in the form of heat, which causes the motor to heat up. The custom winding has a copper loss after the current flows, and the heat is transmitted to the stator core through the slot insulating material, and then transmitted from the stator core to the motor casing to the space. The heat of the rotor is generated by the aluminum consumption of the rotor and its friction. It is transmitted to the rotor core and the inner fan surface. The internal fan is stirred to dissipate the heat in the motor space, and is transferred to the stator core, the end cover and the base. The two heats of the rotor are blown away by the external fan. Therefore, the amount of external fan air volume is the key to determining that the temperature of the motor cannot exceed the temperature allowed by its insulation grade.
The national standard stipulates the allowable temperature rise of the motor of various insulation grades under the rated operating conditions, and requires that the hottest temperature in the motor is not allowed to exceed the limit temperature of the insulation grade, and the common insulation grade of the motor is as follows: where: allowable temperature rise = Allowable temperature limit—ambient temperature specification—The hot spot temperature difference of the hot spot temperature difference winding makes the difference between the stable temperature of the winding hot spot and the average temperature of the winding when the motor is rated.
The numerical units in the table are all degrees Celsius
When the motor is at no load or light load, the total loss of the motor is smaller than the rated time, and the air volume is proportional to the total loss of the motor, so the fan is in the "large horse-drawn car" (because the ventilation loss and the motor speed are constant, 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 fan blade shape can reduce the air volume, but it is more troublesome. It is better to reduce the outer fan blade diameter to reduce the air volume. We know that the mechanical loss of the fan itself is proportional to the 4-5 power of the fan blade diameter, and the air volume is proportional to the square of the fan blade diameter. Therefore, when the fan blade diameter is reduced, the air volume is not reduced much, and the ventilation loss is much reduced.
As the cooling air volume of the external fan is reduced, the temperature rise of the motor is increased, but the temperature rise of the motor can be maintained within the allowable range of the insulation level. At the same time, we also need to see that the motor loss of the motor is reduced due to the reduction of the outer blade diameter, which will further reduce the temperature rise of the motor. Especially for such high-speed motors of 2, 4 grades, when the outer diameter of the fan is reduced by 14%-16%, the ventilation loss is reduced by 20%-40%. In addition, while changing the diameter of the blade, the size of the windshield or fan cover should be changed accordingly so that the dimensions of the mutual fit are in accordance with the regulations. The gap between the fan blade and the fan cover should not be too large, generally between 10 and 15, and the air leakage loss from the high pressure region to the low pressure region should be exceeded. At the same time, the angle between the fan blade and the fan cover should be maintained. Part of the wind's dynamic pressure is converted to static pressure to reduce losses. For example, the fan for the No. 8 base can be adjusted and trimmed by using the light-loaded motor of the No. 9 base. This will reduce the mechanical loss of the light-loaded motor of the No. 9 base and increase the temperature rise to improve the motor. Efficiency and power factor.
3.1 Reduce friction to reduce mechanical losses
The normal operation of the motor bearing, noise, vibration, overheating, life and other factors are related to the reasonable choice of grease. At present, it is usually better to use No. 3 lithium base grease, which can reduce the mechanical loss, but with the rapid development of science and technology There are many greases with superior performance, such as: “Small and medium-sized motor bearing grease”. Its performance has been tested by relevant departments to meet the Japanese JIS2220-80 rolling bearing grease standard, close to the level of Swiss SKF65C grease, with extremely low impurities, price It is equivalent to lithium grease No. 3. The performance of the American Aesop grease in the market is also good. We can use these good greases to improve the motor's operation to reduce mechanical losses. As a professional motor maintenance personnel, we can also work hard on the reasonable selection of bearings, but also save energy and reduce consumption; since we already know that the mechanical loss of the motor is small, the mechanical loss of the motor with many stages is small. We can use the bearings selectively when repairing the motor. For example, on high-speed motors, we can use imported bearings or domestic high-grade B, C-class high-quality bearing chamber losses to minimize. On the low-speed motor, we can use the domestic C, D grade, so that we can save a capital on the bearing, so as to achieve the mother of energy saving.
3.2 Improve power factor to reduce mechanical losses
95% of the transfer equipment in the power plant is an AC asynchronous motor, and the AC asynchronous motor is a low power factor device. If we raise the issue of its power factor, we will discuss it.
3.2.1 Reasonably adjust the operating equipment to improve the power factor
Before the manual compensation is carried out, the existing equipment of the power plant should be inspected and adjusted to make it operate reasonably, thereby improving 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 phenomenon of “large horse-drawn car”. When the load rate is lower than 40%, Consider replacing a small capacity motor. When the load rate is lower than 30%, the triangular wiring winding can be changed to the star connection and operation mode, so that the efficiency and power factor of the motor are improved.
(2) Renovate or eliminate the old series of motors such as J0, J02 series of old-fashioned motors, and replace the Y series of energy-saving motors.
(3) Proper use and maintenance of the motor, pay attention to controlling its frequent start (the power factor is very low when starting frequently), careful maintenance, adjustment of the center line and motor rotation, stator gap during motor installation is also essential, because the gas Uneven gaps cause an increase in no-load current and a reduction in power factor.
3.2.2 Adding a frequency converter to improve the power factor
Using the frequency converter for speed regulation to achieve the purpose of improving the power factor is also a common energy-saving method, that is, installing an inverter on an ordinary AC asynchronous motor. This method is simple to install, convenient to operate, and has remarkable energy saving effect.

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