1. What is a frequency converter?
A frequency converter, also known as a variable frequency drive (VFD), is an electronic device that controls the speed of an AC motor by varying the frequency and voltage supplied to the motor. It works by using power semiconductor devices, such as IGBTs, to switch the input power on and off, effectively converting the fixed-frequency power supply into a variable frequency output. This allows precise control over motor speed and torque, making it widely used in industrial automation and energy-saving applications.
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 amount of power delivered to a load. This method helps regulate output voltage or current while maintaining a constant frequency. On the other hand, PAM stands for Pulse Amplitude Modulation, which adjusts the amplitude of the pulses instead of their width. Both methods are used in power electronics to control output characteristics, but they achieve this through different modulation techniques.
3. What is the difference between voltage type and current type?
Inverters can be classified into two main types based on their DC link: voltage source inverters (VSI) and current source inverters (CSI). A VSI uses a capacitor to smooth the DC voltage, allowing it to provide a stable voltage output. In contrast, a CSI uses an inductor to smooth the DC current, resulting in a more controlled current output. The choice between these two depends on the application, with VSI being more common due to its simplicity and efficiency.
4. Why does the voltage and current of the inverter change in proportion?
When operating an induction motor, the torque is determined by the interaction between the magnetic flux and the rotor current. If the frequency is reduced without adjusting the voltage, the magnetic flux increases, potentially causing core saturation and overheating. To prevent this, the voltage must be adjusted proportionally with the frequency, ensuring that the magnetic flux remains constant. This method, known as V/f control, is commonly used in energy-efficient systems like fans and pumps.
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?
If the frequency is lowered and the same power is maintained, the current may increase slightly. However, under a constant torque condition, the current should remain relatively stable. This is because the inverter adjusts both voltage and frequency to maintain proper motor performance and avoid excessive current draw.
6. What is the starting current and starting torque of the motor when the inverter is running?
When using an inverter, the starting current is typically limited to around 150% of the rated current, depending on the model. This is significantly lower than the 6–7 times the rated current seen in direct-on-line (DOL) starting. The inverter allows for a smoother start, reducing mechanical and electrical stress. Starting torque is usually between 70% and 120% of the rated torque, and some models with automatic torque boost can even reach 100% or more, enabling full-load starting.
7. What does V/f mode mean?
The V/f mode refers to a control strategy where the voltage and frequency are adjusted in a fixed ratio. As the frequency decreases, the voltage is also reduced proportionally to maintain a consistent magnetic flux in the motor. This relationship is pre-programmed in the inverter’s controller and can be selected from multiple preset profiles, depending on the motor’s characteristics.
8. How does the torque of the motor change when V and f are changed proportionally?
At low frequencies, the motor’s AC resistance becomes smaller, which can lead to a reduction in torque. To compensate, many inverters apply a slight voltage boost at low frequencies to improve starting torque. This is often referred to as "torque boost" and can be activated automatically or manually through settings on the inverter.
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 technically operate below 6Hz, it is generally not recommended due to potential overheating and reduced torque. Most motors can operate down to around 6Hz safely, with the actual minimum frequency typically ranging from 0.5 to 3Hz depending on the inverter model.
10. For the combination of general motors, the torque is required to be above 60 Hz. Is it ok?
Operating above 60 Hz is possible, but it usually means the motor is running in a constant power mode, where the voltage remains constant while the frequency increases. This can reduce available torque, so careful selection of the motor and inverter is necessary to ensure adequate performance at high speeds.
11. What does it mean to open a ring?
An open-loop system refers to a control setup where the inverter does not use a speed sensor (PG) to monitor the motor’s actual speed. Instead, it relies on the frequency command to estimate speed. In contrast, a closed-loop system uses feedback from the PG to adjust the output precisely. Most general-purpose inverters operate in open-loop mode, though some models offer optional PG feedback for improved accuracy.
12. What should I do if the actual speed is different for a given speed?
In an open-loop system, the motor’s speed may vary slightly due to load changes, typically within 1–5% of the rated slip. For higher speed accuracy, especially under varying loads, an inverter with PG feedback is recommended to ensure the motor runs closer to the desired speed.
13. If the motor with PG is used, can the speed accuracy be improved after feedback?
Yes, using a PG improves speed accuracy, but the overall precision still depends on the PG’s resolution and the inverter’s frequency control capability. Higher-resolution PGs and advanced inverters can achieve better performance.
14. What does the stall prevention function mean?
Stall prevention is a feature designed to avoid overcurrent trips caused by rapid acceleration or deceleration. If the inverter detects that the current is rising too quickly, it will slow down the acceleration rate to prevent damage. This function ensures smooth operation and protects both the inverter and the motor.
15. What are the meanings 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 settings for acceleration and deceleration times, offering greater flexibility for specific applications. This is useful for processes requiring fast acceleration and slow deceleration, such as small machine tools. In contrast, for applications like fan drives, longer acceleration and deceleration times may be more appropriate.
16. What is regenerative braking?
Regenerative braking occurs when a motor acts as a generator during deceleration, converting kinetic energy back into electrical energy. This energy is then returned to the inverter’s DC bus, helping to reduce energy consumption and improve system efficiency.
17. Can I get more braking power?
The braking power depends on the inverter’s ability to handle regenerative energy. Without a brake resistor, the energy is stored in the DC bus capacitor, limiting braking force to about 10–20% of the rated torque. With a brake unit, this can be increased to 50–100%, providing stronger braking capabilities.
18. Torque boosting problem
Frequency converters play a key role in automated systems by allowing precise control over motor speed and torque. For example, in the tobacco industry, they can regulate pump speed based on flow signals, ensuring uniform mixing of ingredients. They can also be integrated with production line controls, using start/stop signals to manage motor operations. Additionally, in case of equipment failure, the inverter can trigger an emergency stop to protect downstream machines. Some inverters, like the SANKEN MF, FUT, and FVT series, support multi-frequency presets, enabling complex automated sequences. By setting different frequencies and timing, the inverter can control the motor to run at various speeds in a programmed order, supporting efficient and flexible production processes.
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