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Analysis of variable frequency motor

Date：2024-02-18　　　Author：Shandong Xinda Motor Co., Ltd.

With the rapid development of power electronics technology and new semiconductor devices, AC speed control technology has been continuously improved and improved. The gradually improved frequency converters have been widely used in AC motors with their good output waveforms and excellent performance-price ratio.

For example : large motors and medium and small roller motors for steel rolling in steel plants, traction motors for railways and urban rail transit, elevator motors, hoisting motors for container lifting equipment, motors for water pumps and fans, compressors, and household appliances. Motors, etc. have all used AC variable frequency speed motors one after another, and have achieved good results. The use of AC variable frequency speed-regulating motors has significant advantages over DC speed-regulating motors : 1) Speed regulation is easy and energy-saving. 2) AC motors have simple structure, small size, small inertia, low cost, easy maintenance and durability. 3) Capacity can be expanded to achieve high speed and high voltage operation. 4) Soft start and quick braking can be achieved. 5) No sparks, explosion-proof, and strong environmental adaptability.

In recent years, variable frequency speed-adjustable transmission devices have developed at an annual growth rate of 13% to 16% internationally , and have a tendency to gradually replace most DC speed-adjustable transmission devices. Since ordinary asynchronous motors that operate with constant frequency and constant voltage power supplies have great limitations when used in variable frequency speed regulation systems, special variable frequency AC motors designed according to use occasions and requirements have been developed abroad. For example, there are motors with low noise and low vibration, motors with improved low-speed torque characteristics, high-speed motors, motors with tachometer generators, and vector control motors .

1.Construction principle of variable frequency motor

When the slip does not change much, the speed of an asynchronous motor is proportional to the frequency. It can be seen that changing the power frequency can change the speed of the asynchronous motor. During variable frequency speed regulation, it is always hoped that the main magnetic flux remains unchanged. If the main magnetic flux is greater than the magnetic flux during normal operation, the magnetic circuit will be oversaturated, causing the excitation current to increase and the power factor to decrease; if the main magnetic flux is less than the magnetic flux during normal operation, the motor torque will decrease.

Frequency converter is a power control device that uses the on-off function of power semiconductor devices to convert industrial frequency power into another frequency. In the early 21st century, the frequency converters used mainly adopt the AC-DC-AC method ( VVVF frequency conversion or vector control frequency conversion). First, the industrial frequency AC power is converted into DC power through a rectifier, and then the DC power is converted into frequency and voltage. Controlled AC power supply to the motor. The circuit of the frequency converter generally consists of four parts : rectifier, intermediate DC link, inverter and control . The rectifier part is a three-phase bridge uncontrollable rectifier, the inverter part is an IGBT three-phase bridge inverter, and the output is a PWM waveform. The intermediate DC link is filtering, DC energy storage and buffering reactive power.

2. Main features of variable frequency motors

The frequency conversion special motor has the following characteristics: 1 ) B -class temperature rise design, F -class insulation manufacturing. The use of polymer insulation materials, vacuum pressure impregnation paint manufacturing process and special insulation structure greatly improves the insulation voltage and mechanical strength of electrical windings. It is capable of high-speed operation of motors and resists high-frequency current impacts of frequency converters and damage to insulation caused by voltage. . 2 ) The balance quality is high, the vibration level is R level (vibration reduction level), the mechanical parts are processed with high precision, and special high-precision bearings are used, which can operate at high speed. 3 ) Forced ventilation and heat dissipation system, all adopt imported axial flow fans with ultra-quiet, long life and strong wind power. It ensures that the motor can effectively dissipate heat at any speed and can achieve long-term operation at high or low speed. 4 ) Compared with traditional variable frequency motors, it has a wider speed range and higher design quality. The special magnetic field design further suppresses high-order harmonic magnetic fields to meet the design indicators of broadband, energy saving and low noise. 5 ) It has wide range of constant torque and power speed regulation characteristics, smooth speed regulation and no torque pulsation. It has good parameter matching with various frequency converters, and with vector control, it can achieve zero-speed full torque, low-frequency high torque, high-precision speed control, position control and fast dynamic response control. 6 ) The frequency conversion special motor can be equipped with brakes and encoders, so that precise parking can be achieved, and high-precision speed control can be achieved through speed closed-loop control. Using "reducer + frequency conversion special motor + encoder + frequency converter" to achieve precise control of ultra-low speed stepless speed regulation. 6 ) The frequency conversion special motor has good versatility, its installation dimensions comply with IEC standards, and is interchangeable with general standard motors.

The frequency conversion special motor has high efficiency, reaching the second level value of the European CEMEP-EU efficiency grade motor standard, and conforms to the national standard GB18613-2002 energy efficiency limit value for small and medium-sized three-phase asynchronous motors. Dual bandwidth voltage, voltage range 220V~690V , suitable for 50Hz and 60Hz power supplies. Low noise. By optimizing electromagnetic design, ventilation conditions, structural dimensions and other technologies, the M2JA series motors have low noise. The bearing load capacity is high, and the motor uses deep groove ball bearings with long service life. The 80-132 center height motor is permanently lubricated, and the 160-355 is equipped with a refueling device. It has good reliability, the motor is a fully enclosed air-cooled structure, the protection level is IP55 , and the materials and processes meet environmental requirements. The motor has high mechanical strength, is sturdy and durable, and has strong anti-rust and anti-corrosion properties.

The winding has good reliability, adopts Class F insulation structure, and has Class B assessment. And PTC thermistor or thermal switch can be added according to user needs . The power of the three-phase asynchronous motor dedicated for frequency conversion ranges from 0.25KW to 315KW , and the center height of the base ranges from 71mm to 355mm . It can be widely used in light industry, textile, chemical industry, metallurgy, machine tools and other industries that require speed-regulating rotating devices. It is an ideal speed-regulating power source.

3. Frequency conversion motor application

Frequency control motors have become the mainstream speed control solution and can be widely used in continuously variable speed transmissions in various industries. Especially with the increasingly widespread application of frequency converters in the field of industrial control, the use of variable frequency motors has also become increasingly widespread. It can be said that due to the superiority of variable frequency motors over ordinary motors in terms of frequency control, wherever frequency converters are used, we It is not difficult to see the figure of variable frequency motor. The social benefits of domestically produced high-voltage frequency conversion devices are significant, including energy saving, resource conservation and environmental pollution reduction. Eliminate the starting impact of the motor and the impact on the power grid, and reduce the failure rate of the motor and equipment. Improve control accuracy and automation. The economic benefits of variable frequency speed regulation are also very significant. For pumps and fans, the fluid flow is proportional to the square power of the speed, the torque is proportional to the square power of the speed, and the power is proportional to the cubic power of the speed. When the speed decreases, the motor power The power consumption decreases by the third power, so the power saving effect of frequency conversion speed regulation is very significant.

If the flow rate is reduced, the rotation speed will be reduced, the current will be reduced, and the power consumption of the motor will be reduced, which theoretically saves energy. If dampers and valves are originally used for adjustment, the flow rate will decrease, the pressure head will increase, and the motor power will decrease. In this way, variable frequency speed regulation is more energy-saving than dampers and valves. In addition to energy saving and efficiency improvement, there are some indirect economic benefits for different loads. The main ones are improved power factor, soft start, reduced electrical and mechanical damage to the motor caused by the starting torque, and smooth, stable and high-precision control. In the 21st century, variable frequency speed regulation has become the mainstream speed regulation solution and can be widely used in continuously variable transmission in various industries. Especially with the increasingly widespread application of frequency converters in the field of industrial control, the use of variable frequency motors has also become increasingly widespread. It can be said that due to the superiority of variable frequency motors over ordinary motors in terms of frequency control, we have used frequency converters wherever they are used. It is not difficult to see the presence of variable frequency motors.

The difference between variable frequency motors and ordinary motors The difference between variable frequency motors and ordinary motors is that the variable frequency motor can adjust the speed arbitrarily within its speed range, and the motor will not be damaged. Generally, most domestic ordinary motors can only operate under the condition of AC380V/50HZ In operation, ordinary motors can be used with reduced or increased frequency, but the range cannot be too large, otherwise the motor will heat up or even burn out. The cooling fan and the fan movement in an ordinary fan are on the same line, but in the variable frequency fan, these two are separate.

Therefore, when the frequency of an ordinary fan is too low, it may burn due to overheating. Since variable frequency motors have to withstand high-frequency magnetic fields, their insulation level is higher than that of ordinary motors. In principle, ordinary motors cannot be driven by frequency converters. However, in practice, in order to save money, ordinary motors are used in many situations where speed regulation is required. It can replace variable frequency motors, but the speed regulation accuracy of ordinary motors is not high. This is often done in the energy-saving transformation of fans and water pumps. When using an ordinary motor instead of a variable frequency motor, the carrier frequency of the frequency converter should be as low as possible to reduce insulation damage to the motor caused by high frequency.

Analysis of variable frequency motor

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