Basic knowledge of high voltage VFD (Variable-frequency Drive)

1. The difference between voltage source type and current source type high voltage VFD (Variable-frequency Drive).

The main circuit of VFD (Variable-frequency Drive) can be roughly divided into two types: voltage source type and current source type. The voltage source type is a VFD (Variable-frequency Drive) that converts the DC of the voltage source into an AC, and the filter element of the DC circuit is a capacitor; the current source type is a VFD (Variable-frequency Drive) that converts the DC of the current source into an AC. Its DC loop filter element is an inductor.

2. Why does the output voltage of VFD (Variable-frequency Drive) 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 the frequency is only reduced, the magnetic flux will be too large, the magnetic circuit will be saturated, and the motor will If the current increases, the motor will be burned in severe cases. Therefore, the frequency and voltage should be changed proportionally, that is, the output voltage of VFD (Variable-frequency Drive) should be controlled while changing the frequency, so as to keep the magnetic flux of the motor constant and avoid the occurrence of magnetic saturation. 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 frequency power supply, the current increases when the voltage drops; for VFD (Variable-frequency Drive) drive, 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 constant torque, the current is almost unchanged.

4. What is the starting current and starting torque of the motor when VFD (Variable-frequency Drive) is used?

Adopting VFD (Variable-frequency Drive) operation, the frequency and voltage are increased correspondingly with the acceleration of the motor, and the starting current is limited to below 150% of the rated current (125% to 200% depending on the model). 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 VFD (Variable-frequency Drive) transmission 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 VFD (Variable-frequency Drive) with automatic torque enhancement function, the starting torque is more than 100%, which can Start 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. Keeping the V/f ratio constant control is the most basic control method for variable frequency speed regulation of asynchronous motors. It controls the voltage output by VFD (Variable-frequency Drive) while controlling the power supply frequency of the motor, and makes the ratio V/f between the two f is constant so that the flux of the motor remains constant. Under the condition of 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 potential of the motor.

The main problem of constant V/f ratio control is poor low-speed performance. The first reason is that the proportion of the voltage drop of the stator resistance of the asynchronous motor becomes larger at low speed, which can no longer be ignored. It can no longer be considered that the stator voltage and the induced 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 lead to the reduction of the electromagnetic torque of the motor; in addition, the dead time of the VFD (Variable-frequency Drive) power device is also an important reason affecting the low-speed performance of the motor. Torque ripple, under certain conditions, will also cause the oscillation of speed and current.

Constant V/f ratio control is often used in general VFD (Variable-frequency Drive). This type of VFD (Variable-frequency Drive) is mainly used for the speed regulation function of fans and water pumps, and occasions that do not require high speed regulation range. The outstanding advantage of constant V/f ratio control is that the open-loop speed control of the motor can be carried out.

6. When V and f are changed proportionally, how does the torque of the motor change?

When the frequency drops, the voltage is reduced completely proportionally, and the torque generated at low speed tends to decrease because the AC impedance becomes smaller and the resistance remains unchanged. Therefore, given V/f at low frequency, the output voltage should be increased to obtain a certain starting torque. This 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?

Installing a speed sensor to the motor used to feed back the actual speed to the control device for control is called "closed loop", and the operation without a speed sensor is called "open loop". The general VFD (Variable-frequency Drive) is mostly Open loop mode.

8. High voltage VFD (Variable-frequency Drive) own protection function

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 optic fault, etc.2