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What is a frequency converter? Principle and application of frequency conversion technology
Frequency conversion technology is a method used to convert direct current (DC) into alternating current (AC) at different frequencies. It can also convert AC into DC and then invert it back to AC with a different frequency, or directly convert DC into AC and then reconvert it into DC. In essence, the main purpose of this technology is to change the frequency of the electrical signal without altering the overall energy content.
**1. What is a frequency converter?**
A frequency converter is an electronic device that modifies the frequency of an incoming power supply by utilizing the switching capabilities of power semiconductor devices. This allows for precise control over the output frequency, making it ideal for applications where speed control is essential.
**2. What is the difference between PWM and PAM?**
PWM stands for Pulse Width Modulation, a technique where the width of the pulses in a pulse train is varied to control the output voltage and waveform. On the other hand, PAM stands for Pulse Amplitude Modulation, which adjusts the amplitude of the pulses instead of their width. Both are used in power electronics to shape the output signal efficiently.
**3. What is the difference between voltage type and current type?**
The main circuit of a frequency converter can be categorized into two types: voltage source and current source. A voltage-type inverter uses a capacitor for filtering in the DC link, while a current-type inverter uses an inductor for filtering. The choice between them depends on the application and the required performance characteristics.
**4. Why does the voltage and current of the inverter change in proportion?**
To maintain optimal motor performance, the voltage and frequency must be adjusted in proportion. If the frequency decreases but the voltage remains constant, the magnetic flux increases, potentially leading to saturation and overheating. By varying both voltage and frequency together, the motor’s magnetic field remains stable, preventing inefficiencies and damage.
**5. When the motor is driven by the commercial frequency power supply, the current increases when the voltage drops. For the inverter drive, if the voltage drops when the frequency decreases, does the current increase?**
When the frequency is reduced, the current may increase if the power output is kept constant. However, under a fixed torque condition, the current typically remains stable. This makes inverters more efficient and less stressful on the motor during operation.
**6. What is the starting current and starting torque of the motor when the inverter is running?**
With an inverter, the motor starts smoothly, with the frequency and voltage increasing gradually. The starting current is limited to around 150% of the rated current, significantly lower than the 6–7 times the rated current seen with direct-on-line (DOL) starting. The starting torque is about 70–120% of the rated torque, depending on the model.
**7. What does V/f mode mean?**
V/f mode refers to the proportional relationship between voltage and frequency. As the frequency decreases, the voltage is also reduced proportionally to maintain a consistent magnetic flux in the motor. This is pre-programmed based on the motor's characteristics and can often be adjusted using switches or dials.
**8. How does the torque of the motor change when V and f are changed proportionally?**
At lower frequencies, the motor's torque tends to decrease due to changes in resistance. To compensate, some inverters boost the voltage slightly at low frequencies to improve starting torque. This is known as "torque enhancement" and can be done automatically or manually.
**9. In the manual, the shift range is 60~6Hz, which is 10:1. Is there no output power below 6Hz?**
While the inverter can still output power below 6 Hz, it is generally not recommended due to potential overheating and reduced torque. The minimum usable frequency is typically around 6 Hz, though some models can start as low as 0.5–3 Hz.
**10. For the combination of general motors, the torque is required to be above 60 Hz. Is it ok?**
Operating above 60 Hz is usually not advisable for standard motors. At higher frequencies, the voltage remains constant, resulting in a constant power characteristic. If high torque is needed at high speeds, special considerations must be made for both the motor and inverter selection.
**11. What does it mean to open a ring?**
In motor control systems, "open loop" means the system does not use a feedback mechanism, such as a tachometer (PG), to monitor the actual motor speed. In contrast, "closed loop" systems use feedback to adjust the speed precisely.
**12. What should I do if the actual speed is different for a given speed?**
In open-loop systems, the actual speed may vary slightly due to load changes. For applications requiring high speed accuracy, a closed-loop system with PG feedback is recommended.
**13. If the motor with PG is used, can the speed accuracy be improved after feedback?**
Yes, using PG feedback improves speed accuracy, although the overall precision depends on the quality of the PG and the inverter's frequency control.
**14. What does the stall prevention function mean?**
Stall prevention is a safety feature that prevents the inverter from tripping due to excessive current during rapid acceleration or deceleration. It detects current levels and adjusts the acceleration/deceleration rate accordingly.
**15. What is the significance of the models that can be given separately for the acceleration time and deceleration time, and the acceleration and deceleration time?**
Some inverters allow separate setting of acceleration and deceleration times, offering greater flexibility for different applications. This is especially useful in precision manufacturing or systems with strict timing requirements.
**16. What is regenerative braking?**
Regenerative braking occurs when a motor acts as a generator during deceleration, converting mechanical energy back into electrical energy. This energy can be fed back into the system or dissipated through a resistor.
**17. Can you get more braking power?**
The amount of regenerative braking power depends on the inverter’s design. Most general-purpose inverters can provide up to 10–20% of the rated torque, but with additional options, this can be increased to 50–100%.