FAQs about Variable Frequency Converters

2022-07-21

1. What is the difference between a high-voltage frequency converter and a low-voltage converter?

Inverters can be roughly divided into two categories: high-voltage frequency converters and low-voltage converters. In terms of use, the applicable objects are different; in terms of speed regulation principle, both are the same; in terms of topology, the difference between the two is It is very large, mainly due to the insufficient withstand voltage of the currently widely used IGBT modules. Due to the high voltage of the high-voltage frequency converter, the interference to the driving circuit is also large, and the optical fiber is usually used for isolation.

2. Why does the output voltage of the variable frequency converter 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 The current increases, and the motor will be burned in severe cases. Therefore, the frequency and voltage should be changed proportionally, that is, the output voltage of the inverter should be controlled while the frequency is being changed, so that the magnetic flux of the motor is kept constant and the occurrence of magnetic saturation is avoided. The voltage here refers to the rms value of the line voltage or phase voltage of the motor.

3. What is the starting current and starting torque of the motor when the variable frequency converter is used for operation?

The inverter is used for operation, and the frequency and voltage are correspondingly increased 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 using the power frequency power supply to start directly, the starting current is 6 to 7 times, so there will be mechanical and electrical shocks. Using 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% of the rated torque; for the inverter with automatic torque enhancement function, the starting torque is more than 100%, and it can start with full load.

4. What does variable frequency converter V/f mode mean?

When the frequency drops, the voltage V also drops proportionally. Keeping the V/f ratio constant is the most basic control method for variable frequency speed regulation of asynchronous motors. The ratio V/f is 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 is approximately equal to the induced potential of the motor.

The constant V/f ratio control is often used in general-purpose inverters. This type of inverter is mainly used for the speed regulation function of fans and water pumps, as well as occasions where the speed regulation range is not high. The outstanding advantage of constant V/f ratio control is that it enables open-loop speed control of the motor.

5. How does the torque of the variable frequency converter motor change when V and f are changed proportionally?

When the frequency drops, the voltage is completely proportionally reduced, so 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, it is necessary to increase the output voltage 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.

6. What does the so-called variable frequency converter open loop mean?

The speed sensor is installed on the motor used, and the actual speed is fed back to the control device for control, which is called "closed loop", and the operation without the speed sensor is called "open loop". The general-purpose inverters are mostly open loop.

7. The protection function of the high-voltage frequency converter itself

Output overload, output overcurrent, grid overvoltage, grid undervoltage, grid loss, DC bus overvoltage, DC bus undervoltage, transformer overheating, phase loss, control power failure, drive failure, power device overheating, cooling fan failure, the external given drop of high-voltage frequency converter, ground fault, optical fiber fault and so on.