0.Introduction
The no-load current and loss of a cage-type three-phase asynchronous motor are important parameters that reflect the efficiency and electrical performance of the motor. They are data indicators that can be directly measured at the use site after the motor is manufactured and repaired. It reflects the core components of the motor to a certain extent – The design process level and manufacturing quality of the stator and rotor, the no-load current directly affects the power factor of the motor; the no-load loss is closely related to the efficiency of the motor, and is the most intuitive test item for preliminary assessment of motor performance before the motor is officially put into operation.
1. Factors affecting the no-load current and loss of the motor
The no-load current of a squirrel-type three-phase asynchronous motor mainly includes the excitation current and the active current at no-load, of which about 90% is the excitation current, which is used to generate a rotating magnetic field and is regarded as a reactive current, which affects the power factor COS φ of the motor. Its size is related to the motor terminal voltage and the magnetic flux density of the iron core design; during design, if the magnetic flux density is selected too high or the voltage is higher than the rated voltage when the motor is running, the iron core will be saturated, the excitation current will increase significantly, and the corresponding empty The load current is large and the power factor is low, so the no-load loss is large. The remaining 10% is active current, which is used for various power losses during no-load operation and affects the efficiency of the motor. For a motor with a fixed winding cross-section, the no-load current of the motor is large, the active current allowed to flow will be reduced, and the load capacity of the motor will be reduced. The no-load current of a cage-type three-phase asynchronous motor is generally 30% to 70% of the rated current , and the loss is 3% to 8% of the rated power . Among them, the copper loss of small-power motors accounts for a larger proportion, and the iron loss of high-power motors accounts for a larger proportion. higher. The no-load loss of large frame size motors is mainly core loss, which consists of hysteresis loss and eddy current loss. Hysteresis loss is proportional to the magnetic permeable material and the square of the magnetic flux density. Eddy current loss is proportional to the square of the magnetic flux density, the square of the thickness of the magnetic permeable material, the square of the frequency and the magnetic permeability. Proportional to the thickness of the material. In addition to core losses, there are also excitation losses and mechanical losses. When the motor has a large no-load loss, the cause of the motor failure can be found from the following aspects. 1 ) Improper assembly, inflexible rotor rotation, poor bearing quality, too much grease in the bearings, etc., cause excessive mechanical friction loss. 2 ) Incorrectly using a large fan or a fan with many blades will increase wind friction. 3 ) The quality of the iron core silicon steel sheet is poor. 4 ) Insufficient core length or improper lamination results in insufficient effective length, resulting in increased stray loss and iron loss. 5 ) Due to the high pressure during lamination, the insulation layer of the core silicon steel sheet was crushed or the insulation performance of the original insulation layer did not meet the requirements.
One YZ250S-4/16-H motor, with an electric system of 690V/50HZ , a power of 30KW/14.5KW , and a rated current of 35.2A/58.1A . After the first design and assembly was completed, the test was carried out. The 4- pole no-load current was 11.5A , and the loss was 1.6KW , normal. The 16- pole no-load current is 56.5A and the no-load loss is 35KW . It is determined that the 16- pole no-load current is large and the no-load loss is too large. This motor is a short-time working system, running at 10/5min . The 16- pole motor runs without load for about 1 minute. The motor overheats and smokes. The motor was disassembled and re-designed, and re-tested after secondary design. The 4 -pole no-load current is 10.7A and the loss is 1.4KW , which is normal; the 16 -pole no-load current is 46A and the no-load loss is 18.2KW . It is judged that the no-load current is large and no-load The loss is still too large. A rated load test was performed. The input power was 33.4KW , the output power was 14.5KW , and the operating current was 52.3A , which was less than the motor’s rated current of 58.1A . If assessed solely based on current, the no-load current was qualified. However, it is obvious that the no-load loss is too large. During operation, if the loss generated when the motor is running is converted into heat energy, the temperature of each part of the motor will rise very quickly. A no-load operation test was conducted and the motor smoked after running for 2 minutes . After changing the design for the third time, the test was repeated. The 4- pole no-load current was 10.5A and the loss was 1.35KW , which was normal; the 16 -pole no-load current was 30A and the no-load loss was 11.3KW . It was determined that the no-load current was too small and the no-load loss was still too large. , conducted a no-load operation test, and after running for 3 minutes, the motor overheated and smoked. After redesigning, the test was carried out. The 4 -pole is basically unchanged, the 16 -pole no-load current is 26A , and the no-load loss is 2360W . It is judged that the no-load current is too small, the no-load loss is normal, and the 16 -pole runs for 5 minutes without load, which is normal. It can be seen that no-load loss directly affects the temperature rise of the motor.
2. Main influencing factors of motor core loss
In low-voltage, high-power and high-voltage motor losses, motor core loss is a key factor affecting efficiency. Motor core losses include basic iron losses caused by changes in the main magnetic field in the core, additional ( or stray ) losses in the core during no-load conditions, and leakage magnetic fields and harmonics caused by the working current of the stator or rotor. Losses caused by magnetic fields in the iron core. Basic iron losses occur due to changes in the main magnetic field in the iron core. This change can be of an alternating magnetization nature, such as what occurs in the stator or rotor teeth of a motor; it can also be of a rotational magnetization nature, such as what occurs in the stator or rotor iron yoke of a motor. Whether it is alternating magnetization or rotational magnetization, hysteresis and eddy current losses will be caused in the iron core. The core loss mainly depends on the basic iron loss. The core loss is large, mainly due to the deviation of the material from the design or many unfavorable factors in production, resulting in high magnetic flux density, short circuit between the silicon steel sheets, and a disguised increase in the thickness of the silicon steel sheets. . The quality of the silicon steel sheet does not meet the requirements. As the main magnetic conductive material of the motor, the performance compliance of the silicon steel sheet has a great impact on the performance of the motor. When designing, it is mainly ensured that the grade of the silicon steel sheet meets the design requirements. In addition, the same grade of silicon steel sheet is from different manufacturers. There are certain differences in material properties. When selecting materials, you should try your best to choose materials from good silicon steel manufacturers. The weight of the iron core is insufficient and the pieces are not compacted. The weight of the iron core is insufficient, resulting in excessive current and excessive iron loss. If the silicon steel sheet is painted too thickly, the magnetic circuit will be oversaturated. At this time, the relationship curve between no-load current and voltage will be seriously bent. During the production and processing of the iron core, the grain orientation of the punching surface of the silicon steel sheet will be damaged, resulting in an increase in iron loss under the same magnetic induction. For variable frequency motors, additional iron losses caused by harmonics must also be taken into consideration; this is what should be considered in the design process. All factors considered. other. In addition to the above factors, the design value of the motor iron loss should be based on the actual production and processing of the iron core, and try to match the theoretical value with the actual value. The characteristic curves provided by general material suppliers are measured according to the Epstein square circle method, and the magnetization directions of different parts of the motor are different. This special rotating iron loss cannot currently be taken into account. This will lead to inconsistencies between calculated values and measured values to varying degrees.
3. Effect of motor temperature rise on insulation structure
The heating and cooling process of the motor is relatively complex, and its temperature rise changes with time in an exponential curve. In order to prevent the temperature rise of the motor from exceeding the standard requirements, on the one hand, the loss generated by the motor is reduced; on the other hand, the heat dissipation capacity of the motor is increased. As the capacity of a single motor increases day by day, improving the cooling system and increasing the heat dissipation capacity have become important measures to improve the temperature rise of the motor.
When the motor operates under rated conditions for a long time and its temperature reaches stability, the allowable limit value of the temperature rise of each component of the motor is called the temperature rise limit. The temperature rise limit of the motor has been stipulated in the national standards. The temperature rise limit basically depends on the maximum temperature allowed by the insulation structure and the temperature of the cooling medium, but it is also related to factors such as the temperature measurement method, the heat transfer and heat dissipation conditions of the winding, and the heat flow intensity allowed to be generated. The mechanical, electrical, physical and other properties of the materials used in the motor winding insulation structure will gradually deteriorate under the influence of temperature. When the temperature rises to a certain level, the properties of the insulation material will undergo essential changes, and even Loss of insulating ability. In electrical technology, the insulation structures or insulation systems in motors and electrical appliances are often divided into several heat-resistant grades according to their extreme temperatures. When an insulation structure or system operates at a corresponding level of temperature for a long time, it will generally not produce undue performance changes. Insulating structures of a certain heat-resistant grade may not all use insulation materials of the same heat-resistant grade. The heat-resistant grade of the insulation structure is comprehensively evaluated by conducting simulation tests on the model of the structure used. The insulating structure works under specified extreme temperatures and can achieve an economical service life. Theoretical derivation and practice have proven that there is an exponential relationship between the service life of the insulation structure and temperature, so it is very sensitive to temperature. For some special-purpose motors, if their service life is not required to be very long, in order to reduce the size of the motor, the allowable limit temperature of the motor can be increased based on experience or test data. Although the temperature of the cooling medium varies with the cooling system and cooling medium used, for various cooling systems currently used, the temperature of the cooling medium basically depends on the atmospheric temperature, and is numerically the same as the atmospheric temperature. Much the same. Different methods of measuring temperature will result in different differences between the measured temperature and the temperature of the hottest spot in the component being measured. The temperature of the hottest spot in the component being measured is the key to judging whether the motor can operate safely for a long time. In some special cases, the temperature rise limit of the motor winding is often not entirely determined by the maximum allowable temperature of the insulation structure used, but other factors must also be considered. Further increasing the temperature of the motor windings generally means an increase in motor losses and a decrease in efficiency. The increase in winding temperature will cause an increase in thermal stress in the materials of some related parts. Others, such as the dielectric properties of the insulation and the mechanical strength of the conductor metal materials, will have adverse effects; it may cause difficulties in the operation of the bearing lubrication system. Therefore, although some motor windings currently adopt Class F or Class H insulation structures, their temperature rise limits are still in accordance with Class B regulations. This not only takes into account some of the above factors, but also increases the reliability of the motor during use. It is more beneficial and can extend the service life of the motor.
4 . in conclusion
The no-load current and no-load loss of the cage three-phase asynchronous motor reflect the temperature rise, efficiency, power factor, starting ability and other main performance indicators of the motor to a certain extent. Whether it is qualified or not directly affects the performance of the motor. Maintenance laboratory personnel should master the limit rules, ensure that qualified motors leave the factory, make judgments on unqualified motors, and carry out repairs to ensure that the performance indicators of the motors meet the requirements of product standards.a
Post time: Nov-16-2023