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Analysis Of The Reasons For The Low Efficiency Of The Motor

Analysis Of The Reasons For The Low Efficiency Of The Motor插图

1. Large stator copper loss
● Large stator winding resistance:

(1) The wire resistivity is large or the wire diameter is small, the wire diameter is uneven, or the number of windings is small;
(2) Wrong wiring or weak welding;
(3) The actual number of turns is more than the design value.

● Large stator current:

(1) Other losses are relatively large;
(2) The three-phase unbalance due to the asymmetry of the stator winding;
(3) The stator and rotor air gap is seriously uneven;
(4) Because the number of turns is less than the normal value, the resistance will be less than the normal value at this time;
(5) The winding wiring is incorrect.

2. Large rotor copper loss
● The resistance of the rotor winding (or bar) is large:

(1) The resistivity of aluminum (copper) is relatively large;
(2) There are air holes or impurities in the cast aluminum rotor guide bars or end rings, or local thin bars caused by casting defects;
(3) The stator slots are not neat (expressed as notch sawtooth), and there are wrong pieces and reverse pieces, resulting in the insufficient effective area of the rotor slots;
(4) Improper selection of cast aluminum parameters leads to loose aluminum structure, which directly leads to an increase in resistivity;
(5) The material does not meet the requirements, for example, the common aluminum rotor uses alloy aluminum;
(6) Using the wrong rotor, etc.

● Large rotor current:

(1) Using the wrong rotor;
(2) The wrong aluminum is used when casting aluminum, for example, the alloy aluminum rotor uses ordinary aluminum;
(3) The stacking of the rotor core is not solid, resulting in a large area of aluminum entering between the sheets, resulting in an excessive transverse current of the rotor.

3. Large stray loss

● Improper choice of stator winding type or pitch;
● Improper selection of stator and rotor slot fit;
● The air gap is too small or seriously uneven;
● Serious short circuit between rotor guide bar and iron core;
● Stator winding ends are too long, etc.

4. Large iron loss

● The quality of the silicon steel sheet is poor or the wrong material is used

● Poor insulation between stator core pieces:

(1) The insulation treatment is not carried out or the treatment effect is not good;
(2) When the iron core is laminated, the pressure is too large, which damages the insulation between the sheets;
(3) When the inner bore of the stator is turned or the iron core is repaired, it will cause a short circuit between the iron core pieces (this problem exists in most iron core manufacturers).

Analysis Of The Reasons For The Low Efficiency Of The Motor插图1

● The Insufficient number of iron cores and insufficient iron weight:

(1) The number of chips is insufficient (missing chips);
(2) The stacking pressure is small and not compacted, and the direct result is that the iron weight is insufficient;
(3) The burrs of the punching sheet are large, and the iron weight cannot be guaranteed when the iron length meets the requirements;
(4) The paint is too thick, which is a direct quality problem of the silicon steel sheet.

● The magnetic circuit is too saturated, and the relationship curve between no-load current and voltage is seriously bent.

● The no-load stray loss is relatively large because it is included in the iron loss during the test, which makes the iron loss appear larger.

● When the winding is removed by fire or electric heating, the iron core will be overheated, the magnetic conductivity will be reduced and the inter-chip insulation will be damaged.

5. Large mechanical loss

● The quality of the bearing or bearing assembly is not good. At this time, the bearing will be seriously heated or rotate inflexibly.
● The external fan is used wrongly (for example, a 4-pole fan is used for a 2-pole motor) or the blade angle is wrong; according to the conventional design, the fan of a 2P motor is relatively small, and the method of reducing loss by adjusting the fan is very effective, but the premise is that Ensure the temperature rise performance of the motor.
● The machine base and the bearing chambers of both end covers are not on the same axis;
● The diameter of the bearing chamber is small, which causes the outer ring of the bearing to deform under pressure, resulting in increased friction loss of the bearing; this situation may also lead to overheating and failure of the bearing.
● There is too much lubricating grease in the bearing chamber or the quality of the lubricating grease is not good. This problem is obvious in high-voltage motors. Ms. Shen once did a test, and the highest point of the bearing cap temperature was 10K higher than the lowest point. Open it and check that there is indeed a lot of grease accumulation at this position.
● Stator and rotor rub against each other, which is what we call sweeping. When the stator and rotor rub against each other, it will not directly cause the motor to stop rotating, but the loss of the motor will increase significantly.
● The axial size of the rotor is not correct, causing the two ends to be stuck, making the rotation inflexible.
● Oil seals or water-throwing rings and other components are improperly installed or deformed, resulting in large frictional resistance.
● With the fan motor, the fan rubs against the associated parts, causing the rotation to be difficult.

How to define the high-efficiency motor?

Analysis Of The Reasons For The Low Efficiency Of The Motor插图2

Ordinary motor: The motor is a device that converts electrical energy into mechanical energy. 70%-95% of the electrical energy absorbed by the motor is converted into mechanical energy. This is the efficiency value of the motor, which is an important technical indicator of the motor. The 30%-5% part is consumed by the motor itself due to heat generation, mechanical loss, etc., so this part of electric energy is wasted.
High-efficiency motor: A motor with a high utilization rate of electric energy is called a high-efficiency motor.
For ordinary motors, it is not easy to increase the efficiency by 1 percentage point, and the material will increase a lot, when the efficiency of the motor reaches a certain value, no matter how much material is added, it cannot be improved. Most of the high-efficiency motors on the market are replacement products of three-phase asynchronous motors, that is to say, the basic working principle has not changed.

The high-efficiency motor mainly improves the efficiency of the motor through the following ways:

  1. Increase the outer diameter of the iron core, increase the length of the iron core, increase the size of the stator groove, and increase the weight of the copper wire to achieve the purpose of improving efficiency.
  2. The silicon steel sheet with good magnetic conductivity is used. In the past, the hot-rolled sheet with large iron loss was used, but now high-quality cold-rolled sheet with low loss is used, such as DW470. Even lower DW270.
  3. Improve machining accuracy and reduce mechanical loss. Replace the small fan to reduce fan loss and use high-efficiency bearings.
  4. Optimize the design of the electrical performance parameters of the motor, and optimize the parameters by changing the groove shape.
  5. Adopt cast copper rotor (complicated process and high cost)
    Therefore, to make a real high-efficiency motor, the cost of design, raw materials, and processing is much higher, so that electricity can be converted into mechanical energy to the greatest extent.

Energy-saving measures for high-efficiency motors:

Motor energy saving is a systematic project, which involves the whole life cycle of the motor. From the design and manufacture of the motor to the selection, operation, adjustment, maintenance, and scrapping of the motor, the effect of energy-saving measures must be considered throughout the entire life cycle of the motor. Domestic In this aspect, the main consideration is to improve efficiency from the following aspects:
The design of an energy-saving motor refers to the use of modern design methods such as optimization design technology, new material technology, control technology, integration technology, test and detection technology, etc., to reduce the power loss of the motor, improve the efficiency of the motor, and design a high-efficiency motor.
When the motor converts electrical energy into mechanical energy, it also loses a part of its energy. Typical AC motor losses can generally be divided into three parts: fixed loss, variable loss, and stray loss. Variable loss varies with load, including stator resistance loss (copper loss), rotor resistance loss, and brush resistance loss, and fixed loss has nothing to do with load, including iron core loss and mechanical loss. The iron loss is composed of hysteresis loss and eddy current loss, which is proportional to the square of the voltage, and the hysteresis loss is also inversely proportional to the frequency; other stray losses are mechanical losses and other losses, including friction losses of bearings and fans, rotors and other windage losses due to rotation.

Features of high-efficiency motors:

  1. Save energy and reduce long-term operating costs. It is very suitable for textiles, fans, water pumps, and compressors. The motor purchase cost can be recovered by saving electricity for one year;
  2. Start directly or adjust the speed with a frequency converter, and the asynchronous motor can be completely replaced;
  3. The rare earth permanent magnet high-efficiency energy-saving motor itself can save more than 15% of electric energy than ordinary motors;
  4. The power factor of the motor is close to 1, which improves the quality factor of the power grid without adding a power factor compensator;
  5. The motor current is small, which saves power transmission and distribution capacity and prolongs the overall operating life of the system;
  6. Power-saving budget: Taking a 55Kw motor as an example, the high-efficiency motor saves 15% of electricity compared with the general motor, and the electricity fee is calculated at 0.5 yuan per kilowatt-hour. Using an energy-saving motor can recover the cost of replacing the motor by saving electricity within one year.

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