Variable frequency drive (VFD) technology is a conversion technology that inverts direct current (DC) into alternating current (AC) of different frequencies. It can convert AC to DC and then invert it back to AC of different frequencies, or convert DC to AC and then back to DC. In short, all of these processes involve only frequency changes, not changes in electrical energy.
1. What is a frequency converter?
A frequency converter is a control device that uses the switching action of electrical components to convert power frequency into electrical energy of another frequency.
2. What are the differences between PWM and PAM?
PWM is an abbreviation for Pulse Width Modulation, a modulation method that adjusts the output quantity and waveform by changing the pulse width of a pulse train according to a certain pattern. PAM is an abbreviation for Pulse Amplitude Modulation, a modulation method that adjusts the output quantity and waveform by changing the pulse amplitude of a pulse train according to a certain pattern.
3. What are the differences between voltage-source and current-source voltage ...
The main circuit of a frequency converter can be broadly divided into two categories: voltage-source converters, which convert DC voltage to AC voltage, use capacitors for DC circuit filtering; and current-source converters, which convert DC current to AC current, use inductors for DC circuit filtering.
4. Why does the voltage and current of a frequency converter change proportionally?
Asynchronous torque 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 only the frequency is reduced, the magnetic flux will be too large, causing magnetic circuit saturation, which can burn out the motor in severe cases. Therefore, the frequency and voltage must be changed proportionally; that is, the inverter's output voltage is controlled while changing the frequency to keep the motor's magnetic flux constant and avoid weak magnetic flux and magnetic saturation. This control method is often used in energy-saving inverters for fans, pumps, and other similar applications.
5. When a motor is driven by a mains frequency power supply, a decrease in voltage will increase the current; for a frequency converter drive, if a decrease in frequency also causes a decrease in voltage, will the current increase?
When the frequency decreases (low speed), the current increases if the same power is output, but the current remains almost unchanged under the condition of constant torque.
6. What are the starting current and starting torque of the motor when operating with a frequency converter?
When using a frequency converter, the frequency and voltage increase accordingly as the motor accelerates, limiting the starting current to below 150% of the rated current (125%~200% depending on the model). Direct starting with a mains frequency power supply results in a starting current 6~7 times the rated current, causing mechanical and electrical shocks. Frequency converters allow for smooth starting (though starting time is 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 boosting, the starting torque is over 100%, allowing for full-load starting.
7. What does V/f mode mean?
The voltage V decreases proportionally as the frequency decreases, as explained in answer 4. The ratio of V to f is predetermined by taking into account the motor characteristics. Several characteristics are usually stored in the controller's memory (ROM), which 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 proportionally. Since the AC impedance decreases while the DC resistance remains constant, the torque generated at low speeds tends to decrease. Therefore, at low frequencies, given a V/f ratio, the output voltage needs to be increased slightly to obtain a certain starting torque; this compensation is called enhanced starting. Various methods can be used to achieve this, including automatic methods, selecting a V/f mode, or adjustment.
9. The instruction manual states that the speed range is 60~6Hz, or 10:1. Does this mean there is no output power below 6Hz?
While the inverter can still output power below 6Hz, the minimum operating frequency is around 6Hz, taking into account factors such as motor temperature rise and starting torque. At this frequency, the motor can output rated torque without causing serious overheating problems. The actual output frequency (starting frequency) of the inverter ranges from 0.5Hz to 3Hz, depending on the model.
10. Is it possible to maintain a constant torque for general motor combinations even at frequencies above 60Hz?
Normally, this is not possible. Above 60Hz (and some modes above 50Hz), the voltage remains constant, exhibiting largely constant power characteristics. When requiring the same torque at high speeds, careful consideration must be given to the selection of motor and inverter capacities.
11. What does "open-loop" mean?
A closed-loop system is one that uses a speed detector (PG) to feed back the actual speed to the control device for control. A system that operates without a PG is called an open-loop system. Most general-purpose frequency converters are open-loop, although some models offer PG feedback via optional components.
12. What should be done if the actual rotational speed deviates from the given speed?
In open-loop operation, even if the inverter outputs a given frequency, the motor speed will vary within the rated slip range (1%~5%) when the motor is under load. For applications requiring high speed control accuracy and operation close to the given speed even with load variations, an inverter with PG feedback function (optional) can be used.
13. If a motor with a PG (Power Generator) is used, will the speed accuracy be improved after feedback?
Inverters with PG feedback function have improved accuracy. However, the speed accuracy depends on the accuracy of the PG itself and the inverter's output frequency.
14. What does stall prevention function mean?
If the given acceleration time is too short, the change in the inverter's output frequency will far exceed the change in speed (electrical angular frequency), causing the inverter to trip due to overcurrent and stop operation; this is called stall. To prevent stall and allow the motor to continue running, the current magnitude must be detected for frequency control. When the acceleration current is too large, the acceleration rate should be appropriately slowed down. The same applies during deceleration. The combination of these two measures constitutes the stall function.
15. What is the significance of models where acceleration and deceleration times can be set separately, and models where both acceleration and deceleration times can be set together?
For machine types where acceleration and deceleration can be set separately, this is suitable for short-time acceleration and slow deceleration applications, or for small machine tools where strict production cycle time needs to be set. However, for applications such as fan drives, where acceleration and deceleration times are relatively long, acceleration and deceleration times can be set together.
16. What is regenerative braking?
If the command frequency is reduced while the electric motor is running, the motor will operate as an asynchronous generator and act as a brake. This is called regenerative (electric) braking.
17. Is it possible to obtain greater braking force?
The energy regenerated from the motor is stored in the frequency converter. Due to the capacitance and voltage rating of the capacitor, the regenerative braking force of a general frequency converter is about 10% to 20% of the rated torque. With optional components, it can reach 50% to 100%.
