Basic questions about frequency converters

1. What is a frequency converter?

A frequency converter is an electric energy control device that uses the on-off function of power semiconductor devices to convert industrial frequency power into another frequency.

2. What are the differences between PWM and PAM?

PWM is the abbreviation of Pulse Width Modulation, which is a modulation method that changes the pulse width of a pulse train according to a certain rule to adjust the output value and waveform. PAM is the abbreviation of Pulse Amplitude Modulation, which is a modulation method that changes the pulse amplitude of a pulse train according to a certain rule to adjust the output value and waveform.

3. What is the difference between voltage type and current type?

The main circuit of the inverter can be roughly divided into two categories: the voltage type is an inverter that converts the DC of the voltage source into AC, and the DC circuit filter is a capacitor; the current type is an inverter that converts the DC of the current source into AC, and its DC circuit filter is an inductor.

4. Why does the voltage of the inverter change in proportion to the current?

The torque of an asynchronous motor is generated by the interaction between the motor's magnetic flux and the current flowing through 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 in serious cases, the motor will be burned. Therefore, the frequency and voltage must be changed proportionally, that is, the inverter output voltage must be controlled while the frequency is changed to keep the motor's magnetic flux constant and avoid the occurrence of weak magnetic field and magnetic saturation. This control method is mostly used in energy-saving inverters for fans and pumps.

5. When the motor is driven by industrial frequency power supply, the current increases when the voltage decreases; for inverter drive, if the voltage decreases when the frequency decreases, does the current increase?

When the frequency decreases (low speed), if the output power is the same, the current increases, but under the condition of constant torque, the current remains almost unchanged.

6. When the inverter is used for operation, what are the starting current and starting torque of the motor?

When using a frequency converter to operate, the frequency and voltage are increased accordingly as the motor accelerates, and the starting current is limited to less than 150% of the rated current (125%~200% depending on the model). When starting directly with an industrial frequency power supply, the starting current is 6~7 times, so there will be mechanical and electrical impacts. Using a frequency converter drive can start smoothly (the starting time becomes longer). The starting current is 1.2~1.5 times the rated current, and the starting torque is 70%~120% of the rated torque; for frequency converters with automatic torque enhancement function, the starting torque is more than 100%, and it can start with full load.

7. What does V/f mode mean?

When the frequency decreases, the voltage V also decreases proportionally. This problem has been explained in answer 4. The proportional relationship between V and f is predetermined by considering the motor characteristics. Usually, several characteristics are stored in the controller's storage device (ROM) and can be selected using a switch or dial.

8. How does the motor torque change when V and f are changed proportionally?

When the frequency decreases, the voltage is reduced in full proportion. Since the AC impedance decreases and the DC resistance remains unchanged, the torque generated at low speed tends to decrease. Therefore, when the V/f is given at low frequency, the output voltage should be increased to obtain a certain starting torque. This compensation is called enhanced starting. It can be achieved by various methods, such as automatic method, selecting V/f mode or adjusting potentiometer.

[page]9. The manual says the speed range is 60~6Hz, which is 10:1. So is there no output power below 6Hz?

Power can still be output below 6Hz, but according to the motor temperature rise and starting torque, the minimum operating frequency is about 6Hz. At this time, the motor can output rated torque without causing serious heating problems. The actual output frequency (starting frequency) of the inverter is 0.5~3Hz depending on the model.

10. Is it possible to require a certain torque for general motor combinations above 60Hz?

Normally, this is not possible. Above 60Hz (there are also modes above 50Hz) the voltage remains constant, which is generally a constant power characteristic. When the same torque is required at high speed, attention must be paid to the selection of motor and inverter capacity.

11. What does open loop mean?

The motor device used is equipped with a speed detector (PG) to feed back the actual speed to the control device for control, which is called "closed loop", and the operation without PG is called "open loop". Most general-purpose inverters are open loop, and some models can use options to perform PG feedback.

12. What should I do if the actual speed deviates from the given speed?

In open loop, even if the inverter outputs a given frequency, the motor speed will change within the rated slip rate range (1%~5%) when the motor is running with load. For applications requiring higher speed regulation accuracy and requiring the motor to run at a speed close to the given speed even when the load changes, an inverter with PG feedback function (optional) can be used.

13. If a motor with PG is used, can the speed accuracy be improved after feedback?

The inverter with PG feedback function has improved accuracy. However, the speed accuracy depends on the accuracy of the PG itself and the resolution of the inverter output frequency. 14. What does the stall prevention function mean?

If the given acceleration time is too short, the output frequency change of the inverter far exceeds the change of the speed (electrical angular frequency), and the inverter will trip due to overcurrent, and the operation will stop. This is called stalling. In order to prevent stalling and keep the motor running, the current size must be detected to control the frequency. When the acceleration current is too large, the acceleration rate should be appropriately slowed down. The same is true for deceleration. The combination of the two is the stall function.

15. There are models where the acceleration time and deceleration time can be given separately, and models where the acceleration and deceleration time can be given together. What is the significance of this?

Machines whose acceleration and deceleration can be given separately are suitable for situations where short-time acceleration and slow deceleration are required, or for small machine tools where the production cycle time needs to be strictly given. However, for situations such as fan drives, the acceleration and deceleration times are relatively long, so the acceleration time and deceleration time can be given together.

16. What is regenerative braking?

If the command frequency is reduced during the operation of the motor, the motor will change to an asynchronous generator and work as a brake. This is called regenerative (electrical) braking.