Sep 04, 2020
Basic knowledge of high voltage inverter
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 a frequency converter 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 a frequency converter that converts the DC of the current source to AC, and its 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 an asynchronous motor is produced by the interaction between the magnetic flux of the motor and the current flowing in the rotor. Under the rated frequency, if the voltage is constant and the frequency is only reduced, the magnetic flux will be too large, the magnetic circuit will be saturated, and the motor If the current increases, the motor will be burnt in severe cases. Therefore, the frequency and the voltage should be changed proportionally, that is, the output voltage of the frequency converter is controlled while changing the frequency, so that the magnetic flux of the motor is kept constant and the occurrence 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 a commercial power supply, the current will increase when the voltage drops; for inverter driving, if the voltage also drops when the frequency drops, does the current increase?
When the frequency drops (low speed), if the same power is output, the current increases, but under the condition of a constant torque, the current hardly changes.
4. What is the starting current and starting torque of the motor when the inverter is used for operation?
Using frequency converter operation, with the acceleration of the motor, the frequency and voltage are correspondingly increased, the starting current is limited to 150% of the rated current (depending on the model, it is 125% to 200%). When starting directly with a power frequency power supply, the starting current is 6 to 7 times, so mechanical and electrical shocks will occur. Using frequency converter 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% of the 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 decreases proportionally. This question has been explained in answer 4. Maintaining a constant V/f ratio control is the most basic control method for 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. Keep the magnetic flux of the motor constant. In 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 is approximately equal to the induced electric potential of the motor.
The main problem of V/f ratio constant control is poor low-speed performance. The first reason is that the proportion of the stator resistance voltage drop of the asynchronous motor at low speed becomes larger, which 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 cannot keep the motor flux constant. The reduction of motor flux will inevitably reduce the electromagnetic torque of the motor; in addition, the dead time of the inverter power device is also an important factor affecting the low-speed performance of the motor. The dead time causes a voltage drop and also causes torque pulsation. Under the conditions, it will also cause the oscillation of speed and current.
V/f ratio constant control is often used in general-purpose inverters. This type of frequency converter is mainly used for the speed control function of fans and water pumps, and occasions where the speed control range is not high. The outstanding advantage of V/f ratio constant control is that it can carry out open loop speed control of the motor.
6. How does the torque of the motor change when V and f are changed proportionally?
When the frequency drops, the voltage is completely reduced in proportion, then the AC impedance becomes smaller and the resistance remains unchanged, which will cause the torque generated at low speeds to decrease. Therefore, given V/f at low frequencies, the output voltage should be increased to obtain a certain starting torque. This kind of compensation is called enhanced starting. It can be realized by various methods, such as automatic method, selection of V/f mode or adjustment of potentiometer.
7. What does the so-called open loop mean?
Install a speed sensor for the motor used, and feedback the actual speed to the control device for control, which is called "closed loop", and the operation without a speed sensor is called "open loop". General inverters mostly use open loop mode.
8. The protection function of the high-voltage inverter
Output overload, output overcurrent, grid overvoltage, grid undervoltage, grid power loss, DC bus overvoltage, DC bus undervoltage, transformer overheating, phase loss, control power failure, drive failure, power device overheating, cooling fan failure , External reference drop, ground fault, fiber fault, etc.