1. The difference between voltage source type and current source type high voltage inverter.
The main circuit of the inverter can be roughly divided into two categories: voltage source type and current source type. The voltage source type is an inverter that converts the DC of the voltage source to AC, and the filter element of the DC loop is a capacitor; the current source type is an inverter that converts the DC of the current source to AC, and the DC loop filter element is an inductor.
2. Why does the output voltage of the inverter change in proportion to the frequency?
The torque of the asynchronous motor is generated by the interaction between the magnetic flux of the motor 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 If the current increases, the motor will be burned in severe cases. Therefore, the frequency and voltage should be changed in proportion, that is, the output voltage of the frequency converter should be controlled while changing the frequency, so that the magnetic flux of the motor is kept constant, and the phenomenon of magnetic saturation is avoided. This is the definition of VVVF. The voltage here refers to the effective value of the line voltage or phase voltage of the motor.
3. When the motor is driven by power frequency power supply, the current increases when the voltage drops; for the inverter drive, if the voltage also decreases when the frequency decreases, does the current increase?
When the frequency decreases (low speed), if the same power is output, the current increases, but under the condition of a certain torque, the current is almost unchanged.
4. When the inverter is used, what is the starting current and starting torque of the motor?
The inverter is used for operation. With the acceleration of the motor, the frequency and voltage are increased accordingly. The starting current is limited to less than 150 rated current (125 ~ 200 depending on the model) When the power frequency power supply is used to directly start, the starting current is 6 to 7 times, therefore, mechanical and electrical shock will occur. The inverter drive can start smoothly (starting time becomes longer). The starting current is 1.2 to 1.5 times the rated current, and the starting torque is 70 to 120 rated torque. For the inverter with automatic torque enhancement function, the starting torque is more than 100, and it can be started with full load.
5. What does V / f mode mean?
When the frequency decreases, the voltage V also decreases proportionally. This question has been explained in answer 4. Maintaining a constant V / f ratio control is the basic control method of variable frequency speed regulation of asynchronous motors. It controls the output voltage of the inverter while controlling the power frequency of the motor, and makes the ratio V / f constant, so that The magnetic flux of the motor remains constant. Under the rated operation of the motor, the voltage drop of the stator resistance and leakage reactance 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 constant control is poor performance at low speeds. The first reason is that the proportion of the stator motor voltage drop at low speed becomes larger and can no longer be ignored. It can no longer be considered that the stator voltage and the induced electric potential of the motor are approximately equal, and the constant control of the V / f ratio can no longer keep the motor flux constant. The reduction of the motor magnetic flux will inevitably cause the reduction of the electromagnetic torque of the motor; in addition, the dead time of the inverter power device is also an important reason for the low-speed performance of the motor. The dead time causes a voltage drop and also causes torque ripple. Under conditions, it can also cause speed and current oscillations.
Constant V / f ratio control is commonly used on general inverters. This type of frequency converter is mainly used for the speed regulation function of fans and water pumps, and occasions where the range of speed regulation is not high. The outstanding advantage of constant V / f ratio control is that the motor's open-loop speed control can be performed.
6. When changing V and f proportionally, how does the torque of the motor change?
When the frequency decreases, the voltage is reduced proportionally, then the AC impedance becomes smaller and the resistance remains unchanged, which will cause the torque generated at low speed to have a tendency to decrease.
Therefore, given V / f at low frequencies, the output voltage should be increased to obtain a certain starting torque. This compensation is called enhanced starting. Various methods can be used, including automatic methods, V / f mode selection, or potentiometer adjustment.
7. What does open loop mean?
It is called "closed loop" to install a speed sensor to the motor used to feed back the actual speed to the control device for control, and it is called "open loop" if it does not operate with a speed sensor. Most general-purpose inverters are open loop.
8. The protection function of the high-voltage inverter
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