1. What is the difference between high-voltage inverter and low-voltage inverter?
Frequency converters can be broadly divided into two categories: high-voltage frequency converters and low-voltage frequency converters.
In terms of use, the applicable objects are different, and the high-voltage motor is the same from the principle of speed regulation. From the perspective of topology, the two are very different, mainly because of the lack of withstand voltage of IGBT modules, which are widely used. Due to the high voltage, the high-voltage inverter also has a large disturbance to the driving circuit, etc., and the optical fiber is generally used to block it.
2. Why does the output voltage of the inverter change in proportion to the frequency?
The torque of the asynchronous motor is the interaction between the magnetic flux of the motor and the current flowing in the rotor, under the additional frequency, if the voltage must only drop the frequency, then the magnetic flux is too large, the magnetic circuit is saturated, the motor current increases, and the motor will be burned out when it is severe. Therefore, the frequency and voltage should be changed in parts, that is, the output voltage of the inverter is controlled together with the changed frequency, so that the magnetic flux of the motor is maintained and the occurrence of magnetic saturation is avoided. This is the VVVF theory. The voltage here refers to the line voltage or phase voltage RMS of the motor.
3. When the motor is driven by power frequency power supply, the current is added when the voltage drops; Regarding the inverter drive, if the voltage drops when the frequency drops, does the current increase?
When the frequency drops (low speed), the current is added if the same power is output, but the current is simply unchanged when the torque is constant.
4. When the inverter is selected, what is the starting current and starting torque of the motor?
The inverter is selected to work, and the starting current is limited to less than 150% of the additional current (125%-200% according to the different models) with the acceleration of the motor. When the industrial frequency power supply is started directly, the starting current is 6-7 times, therefore, the mechanical and electrical shock will occur. The inverter drive can be used to start smoothly (the starting time becomes longer). The starting current is 1.2-1.5 times of the additional current, and the starting torque is 70%-120% of the additional torque; For the inverter with the active torque enhancement function, the starting torque is more than 100%, and it can start with full load.
5. What does the V/f form mean?
When the frequency drops, the voltage V also decreases in shares, and this question has been clarified in Answer 4. Maintaining the stable control of the V/f ratio is the most fundamental control method of the frequency conversion speed regulation of the asynchronous motor, which controls the voltage output of the inverter together with the change of the power supply frequency of the control motor, and makes the ratio of the two V/f stable, so that the magnetic flux of the motor remains stable. In the case of additional operation of the motor, the voltage drop of the stator resistance and leakage impedance of the motor is relatively small, and the terminal voltage of the motor and the induced potential of the motor are approximately equal.
The main problem with V/f ratio stability control is poor low-speed function. The reason is that the share of the stator resistance voltage drop of the asynchronous motor at low speed becomes larger, which can no longer be ignored, and the stator voltage and the induced electric potential of the motor can no longer be considered to be approximately equal, and the motor magnetic flux can no longer be maintained stable according to the V/f ratio. The decrease of the magnetic flux of the motor must lead to the reduction of the electromagnetic torque of the motor; In addition, the dead time of the inverter power equipment is also an important reason for the low-speed function of the motor, and the voltage drop at the dead time will also cause torque ripple, and the oscillation of speed and current will also be caused under certain conditions.
V/f ratio stabilization is often used on general-purpose inverters. This kind of inverter is mainly used for the speed regulation function of fans and pumps, as well as for occasions where the speed regulation scale is not high. The outstanding advantage of V/f ratio stable control is the ability to operate the open-loop speed of the motor.
6. How to change the torque of the motor when changing V and f according to shares?
If the voltage drops completely and proportionally when the frequency drops, the communication impedance decreases while the resistance does not change, and the torque that occurs at low speeds tends to decrease. Therefore, given V/f at low frequency, the output voltage should be improved a little in order to obtain a certain starting torque, and this compensation is called enhanced starting. It can be achieved by various methods, such as active methods, V/f form selection or potentiometer adjustment.
7. What does the so-called open loop mean?
The speed sensor is installed on the motor used, and the practical speed is fed back to the control device for control, which is called "closed loop", and the speed sensor is called "open loop" if it does not work, and the general inverter is mostly open loop method.
8. The protection function of the high-voltage inverter itself
Output overload, output overcurrent, grid overvoltage, grid undervoltage, grid power loss, DC bus overvoltage, DC bus undervoltage, transformer overheating, phase loss, power control power failure, drive failure, power equipment overheating, heat dissipation fan failure, external given disconnection, grounding failure, optical fiber failure, etc.
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