The difference between variable frequency motor and ordinary motor
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The difference between variable frequency motor and ordinary motor

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  • Time of issue:2019-07-09 15:47
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(Summary description)Ordinary asynchronous motors are designed according to constant frequency and constant voltage, and it is impossible to fully adapt to the requirements of frequency control

The difference between variable frequency motor and ordinary motor

(Summary description)Ordinary asynchronous motors are designed according to constant frequency and constant voltage, and it is impossible to fully adapt to the requirements of frequency control

  • Categories:Information
  • Author:
  • Origin:
  • Time of issue:2019-07-09 15:47
  • Views:



First, the difference between variable frequency motor and ordinary motor


Ordinary asynchronous motors are designed according to constant frequency and constant voltage, and it is impossible to fully adapt to the requirements of frequency control. The following is the influence of the inverter on the motor, that is, the difference between the inverter motor and the ordinary motor:


1. The efficiency of the motor and the problem of temperature rise


Regardless of the form of the frequency converter, different levels of harmonic voltage and current are generated during operation, so that the motor operates under non-sinusoidal voltage and current. According to the data, taking the sinusoidal PWM inverter commonly used at present as an example, the lower harmonics are basically zero, and the remaining higher harmonic components that are about twice as large as the carrier frequency are: 2u+1 (u For the modulation ratio).


Higher harmonics cause an increase in the stator copper loss, rotor copper (aluminum) consumption, iron loss and additional losses, most notably the rotor copper (aluminum) consumption. Since the asynchronous motor rotates at a synchronous speed close to the fundamental frequency, the high-order harmonic voltage will cause a large rotor loss after cutting the rotor bar with a large slip. In addition, the additional copper consumption due to the skin effect needs to be considered. These losses will cause the motor to generate extra heat, reduce the efficiency, and reduce the output power. For example, if the ordinary three-phase asynchronous motor is operated under the non-sinusoidal power supply output of the inverter, the temperature rise will generally increase by 10%-20%.


2. Motor insulation strength problem


At present, many small and medium-sized inverters use PWM control. His carrier frequency is about several thousand to ten kilohertz, which makes the stator winding of the motor to withstand a high voltage rise rate, which is equivalent to applying a steep shock voltage to the motor, so that the inter-turn insulation of the motor is more resistant. A harsh test. In addition, the rectangular chopper surge voltage generated by the PWM inverter is superimposed on the operating voltage of the motor, which poses a threat to the insulation of the motor to the ground, and the ground insulation will accelerate the aging under the repeated impact of high voltage.


3. Harmonic electromagnetic noise and vibration


When the ordinary asynchronous motor is powered by the inverter, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complicated. Each time harmonic contained in the variable frequency power supply interferes with the inherent spatial harmonics of the electromagnetic part of the motor to form various electromagnetic exciting forces. When the frequency of the electromagnetic force wave coincides with or is close to the natural vibration frequency of the motor body, a resonance phenomenon occurs, thereby increasing noise. Since the operating frequency range of the motor is wide and the range of the rotational speed is large, the frequencies of various electromagnetic force waves are difficult to avoid the natural vibration frequency of each component of the motor. .


4. The ability of the motor to adapt to frequent starting and braking


Since the inverter is powered, the motor can be started with no inrush current at very low frequency and voltage, and can be quickly braked by various braking methods provided by the inverter, in order to achieve frequent starting and braking. The conditions are created, so that the mechanical system and the electromagnetic system of the motor are under the action of cyclic alternating force, which brings fatigue and accelerated aging problems to the mechanical structure and the insulating structure.


5. Cooling problem at low speed


First of all, the impedance of the asynchronous motor is not ideal. When the power frequency is lower, the loss caused by the higher harmonics in the power supply is larger. Secondly, when the normal asynchronous motor is reduced in speed, the cooling air volume is proportional to the cube of the rotational speed, which causes the low-speed cooling condition of the motor to deteriorate, and the temperature rise sharply increases, making it difficult to achieve constant torque output.


Second, the characteristics of the frequency conversion motor


Electromagnetic design


For ordinary asynchronous motors, the main performance parameters considered during redesign are overload capability, startup performance, efficiency, and power factor. The variable frequency motor, because the critical slip ratio is inversely proportional to the power supply frequency, can be directly started when the critical slip rate is close to 1. Therefore, the overload capability and the starting performance are not required to be considered too much, and the key problem to be solved is how to improve the motor pair. The ability to adapt to non-sinusoidal power supplies. The method is generally as follows:


1. Reduce the stator and rotor resistance as much as possible.


Reducing the stator resistance reduces the copper loss of the fundamental wave to compensate for the increase in copper consumption caused by higher harmonics.


2. In order to suppress the higher harmonics in the current, it is necessary to appropriately increase the inductance of the motor. However, the rotor slot leakage resistance is large, and the skin effect is also large, and the high-order harmonic copper consumption is also increased. Therefore, the size of the motor leakage reactance should take into account the reasonableness of the impedance matching in the entire speed range.


3. The main magnetic circuit of the variable frequency motor is generally designed to be unsaturated. One is to consider the higher harmonics to deepen the saturation of the magnetic circuit, and the other is to increase the output voltage of the inverter in order to increase the output torque at low frequencies.


2. Structural design


In the re-structural design, the main consideration is also the influence of non-sinusoidal power supply characteristics on the insulation structure, vibration and noise cooling mode of the variable frequency motor. Generally pay attention to the following problems:


1. Insulation grade, generally F grade or higher, strengthen the insulation strength of the ground insulation and the turns, especially the ability of the insulation to withstand the impact voltage.


2. For the vibration and noise of the motor, the rigidity of the motor components and the whole should be fully considered, and the natural frequency should be improved as much as possible to avoid resonance with each force wave.


3, cooling method: generally use forced ventilation cooling, that is, the main motor cooling fan is driven by an independent motor.


4, to prevent the shaft current measures, for the capacity of more than 160KW motor should be used for bearing insulation measures. Mainly due to the asymmetry of the magnetic circuit, the shaft current is also generated. When the currents generated by other high-frequency components are combined, the shaft current will increase greatly, resulting in bearing damage, so insulation measures are generally taken.


5. For constant power variable frequency motor, when the speed exceeds 3000/min, special grease with high temperature resistance should be used to compensate the temperature rise of the bearing.


Third, the difference between the key points


Ordinary motor can be used to install the inverter, but pay attention to heat dissipation. Especially in the low frequency state, the ordinary motor does not have a special fan. It is the air volume generated by the motor to rotate the tail with the tail. When the low frequency is turned, the time is changed. Slow, the wind blades will not produce air volume and cause the motor to heat up. In addition, the frequency of the ordinary motor can not be set too high, the frequency of the domestic ordinary motor is 50HZ, so the normal short time is within 100HZ, can be used.


Fourth, other questions and answers


1. What is a frequency converter?


The frequency converter is a power control device that converts the power frequency power source into another frequency by using the on/off function of the power semiconductor device.


2. What is the difference between PWM and PAM?


PWM is an abbreviation of English Pulse Width Modulation, which changes the pulse width of a pulse train according to a certain rule to adjust the output amount and waveform.


PAM is the abbreviation of Pulse Amplitude Modulation in English. It is a modulation method that changes the pulse amplitude of pulse train according to a certain rule to adjust the output value and waveform.


3. What is the difference between voltage type and current type?


The main circuit of the inverter can be roughly divided into two types: the voltage type is a frequency converter that converts the direct current of the voltage source into an alternating current, the filtering of the direct current circuit is a capacitor, and the current type is a frequency converter that converts the direct current of the current source into an alternating current. Its DC loop filter stone inductance.


4. Why does the voltage and current of the inverter change in proportion?


The torque of the motor is generated by the interaction between the magnetic flux and the current flowing in the rotor. At the rated frequency, if the voltage is constant and only the frequency is reduced, the magnetic flux is too large, the magnetic circuit is saturated, and the motor will be burnt when it is severe. . Therefore, the frequency and voltage should be changed in proportion, that is, the frequency of the inverter is controlled while changing the frequency, so that the magnetic flux of the motor is kept constant to avoid the occurrence of weak magnetic and magnetic saturation. This type of control is mostly used for energy-saving inverters such as fans and pumps.


5. When the motor is driven by the commercial frequency power supply, the current increases when the voltage drops. For the inverter drive, if the voltage drops when the frequency decreases, does the current increase?


When the frequency drops (low speed), if the same power is output, the current increases, but under a certain torque condition, the current hardly changes.


6. What is the starting current and starting torque of the motor when the inverter is running?


The inverter is operated, and the frequency and voltage are increased correspondingly with the acceleration of the motor. The starting current is limited to 150% of the rated current (125% to 200% depending on the model). When starting directly with the commercial frequency power supply, the starting current is 6 to 7 times, so mechanical and electrical impact will occur. It can be started smoothly with the inverter drive (starting time becomes longer). The starting current is 1.2~1.5 times of the rated current, the starting torque is 70%~120% of rated torque; for the inverter with automatic torque boosting function, the starting torque is more than 100%, and it can start with full load.


7. What does V/f mode mean?


The voltage V also decreases proportionally as the frequency decreases. This problem has been explained in answer 4. The proportional relationship between V and f is predetermined in consideration of the characteristics of the motor. Usually, there are several characteristics in the memory device (ROM) of the controller, which can be selected by a switch or a dial.


8. How does the torque of the motor change when V and f are changed proportionally?


When the frequency is lowered to completely reduce the voltage proportionally, since the AC resistance becomes small and the DC resistance does not change, the torque generated at the low speed tends to decrease. Therefore, given V/f at low frequencies, the output voltage is increased somewhat in order to obtain a certain starting torque, which is called an enhanced starting. It can be implemented in a variety of ways, including automated methods, selecting V/f mode, or adjusting potentiometers.


9. In the manual, the shift range is 60~6Hz, that is, 10:1. Is there no output power below 6Hz?


The power can still be output below 6 Hz, but according to the conditions of the motor temperature rise and the starting torque, the minimum use frequency is about 6 Hz. At this time, the motor can output the rated torque without causing serious heat generation. The actual output frequency (starting frequency) of the inverter is 0.5~3Hz according to the model.


10. For the combination of general motors, the torque is required to be above 60 Hz. Is it ok?


Usually not allowed. Above 60 Hz (also in the mode above 50 Hz), the voltage is constant and is generally constant power. When the same torque is required at high speed, the motor and inverter capacity must be selected.


11. What does it mean to open a ring?


A speed detector (PG) is provided for the motor device to be used, and the actual speed is fed back to the control device for control, which is called "closed loop". If the PG is not operated, it is called "open loop". General-purpose inverters are mostly open-loop, and some models use PG feedback.


12. What should I do if the actual speed is different for a given speed?


When the loop is opened, even if the inverter outputs a given frequency, the motor's speed will fluctuate within the range of the rated slip (1%~5%) when the motor is running with load. For the case where the required speed regulation accuracy is relatively high, even if the load is changed, it is required to operate at a speed close to a given speed, and an inverter (optional) having a PG feedback function can be used.


13. If using a motor with PG, can the speed accuracy be improved after feedback?


The inverter with PG feedback function has improved accuracy. However, the speed accuracy depends on the accuracy of the PG itself and the resolution of the inverter output frequency.


14. What does the stall prevention function mean?


If the given acceleration time is too short, the output frequency of the inverter changes far more than the change of the speed (electrical angle frequency), the inverter will trip due to the overcurrent, and the operation stops. This is called stall. In order to prevent the stall from running continuously, it is necessary to detect the magnitude of the current for frequency control. When the acceleration current is too large, the acceleration rate is appropriately slowed down. The same is true when decelerating. The combination of the two is the stall function.


15. What is the significance of the models that can be given separately for the acceleration time and deceleration time, and the acceleration and deceleration time?


Acceleration and deceleration can be given for different models. It is suitable for short-time acceleration, slow deceleration, or for small machine tools where the production tact time is strictly required. However, for fan drive and other occasions, the acceleration and deceleration time is longer. The acceleration time and deceleration time can be given together.


16. What is regenerative braking?


If the command frequency is lowered while the motor is running, the motor becomes an asynchronous generator and operates as a brake. This is called regenerative (electrical) braking.


17. Can you get more braking power?


The energy regenerated from the motor is stored in the filter capacitor of the inverter. Due to the relationship between the capacity of the capacitor and the withstand voltage, the regenerative braking force of the general-purpose inverter is about 10% to 20% of the rated torque. If the optional brake unit is used, it can reach 50%~100%.


18. Please explain the protection function of the inverter?


Protection functions can be divided into the following two categories:


(1) Automatically correct the operation after detecting the abnormal state, such as overcurrent stall prevention and regenerative overvoltage stall prevention.


(2) After detecting the abnormality, the PWM control signal of the power semiconductor device is blocked, so that the motor stops automatically. Such as overcurrent cutoff, regenerative overvoltage cutoff, semiconductor cooling fan overheating and instantaneous power failure protection.


19. Why does the protection function of the inverter operate when the clutch is continuously loaded?


When the load is connected by the clutch, at the moment of connection, the motor changes sharply from the no-load state to the region where the slip ratio is large, and the large current flowing causes the inverter to over-current trip and cannot operate.


20. In the same factory, the large motors move together, and the inverter stops during operation. Why?


When the motor starts, the starting current corresponding to the capacity will flow, and the transformer on the stator side of the motor will produce a voltage drop. When the motor capacity is large, the voltage drop will have a large influence. The inverter connected to the same transformer will make an undervoltage or instantaneous stop. The judgment, and thus the protection function (IPE) action, causes the operation to stop.


21. What is the frequency conversion resolution? What is the point?


For digitally controlled inverters, even if the frequency command is an analog signal, the output frequency is given in stages. The smallest unit of this level difference is called the frequency conversion resolution.


The frequency conversion resolution usually takes a value of 0.015~0.5Hz. For example, the resolution is 0.5Hz, then the upper surface of 23Hz can be changed to 23.5, 24.0 Hz, so the motor's action is also stepped. This poses a problem for applications like continuous take-up control. In this case, if the resolution is about 0.015 Hz, and the level difference of the four-stage motor is 1 r/min or less, it can be sufficiently adapted. In addition, some models have different resolutions and output resolutions.


22. Is there a limit to the installation direction when installing the inverter?


The structure of the inside and the back of the inverter takes into account the cooling effect. The relationship between the upper and lower sides is also important for ventilation. Therefore, for the unit type, the longitudinal position is placed in the disc and hung on the wall, and installed as vertically as possible.


23. Is it possible to use the soft start without directly applying the motor to a fixed frequency inverter?

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