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 converting the fixed-frequency power from the grid into a variable-frequency output using semiconductor switches like IGBTs. This allows for precise control of motor speed and torque, making it ideal for applications requiring energy efficiency and process control.
2. What is the difference between PWM and PAM?
PWM stands for Pulse Width Modulation, a technique where the width of each pulse in a pulse train is varied to control the average power delivered to the load. This method is commonly used in power electronics to regulate voltage and current. On the other hand, PAM stands for Pulse Amplitude Modulation, where the amplitude of each pulse is adjusted instead of its width. While both methods are used in signal processing and power conversion, PWM is more widely used in modern inverters due to its efficiency and ease of implementation.
3. What is the difference between voltage type and current type?
Inverters can be categorized into two main types based on their DC link: voltage-source inverters (VSI) and current-source inverters (CSI). A VSI uses a capacitor in the DC link to smooth out the voltage, allowing it to deliver a controlled AC output. In contrast, a CSI uses an inductor in the DC link to maintain a steady current, which makes it suitable for high-power applications. The choice between the two depends on the application requirements, such as power level, efficiency, and dynamic response.
4. Why does the voltage and current of the inverter change in proportion?
When controlling an AC motor, maintaining a constant ratio between voltage and frequency (V/f) is essential to prevent magnetic saturation or weak magnetization. At lower frequencies, if the voltage remains constant while the frequency decreases, the magnetic flux increases, potentially causing overheating or damage. By adjusting the voltage proportionally with the frequency, the motor’s magnetic field remains stable, ensuring safe and efficient operation. This method is commonly used in energy-saving applications 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 inverter reduces the frequency but maintains the same voltage, the current may increase, especially at low speeds. However, under constant torque conditions, the current typically remains stable. The relationship between voltage, frequency, and current depends on the motor's load and the control strategy used in the inverter.
6. What is the starting current and starting torque of the motor when the inverter is running?
When using an inverter to start a motor, the starting current is significantly reduced compared to direct-on-line (DOL) starting. The inverter limits the starting current to around 150% of the rated current, depending on the model. This results in a smoother start with less mechanical and electrical stress. The starting torque is usually around 70–120% of the rated torque, and some inverters offer automatic torque boosting to improve performance during startup.
7. What does V/f mode mean?
The V/f mode refers to the proportional control of voltage and frequency in an inverter. As the frequency decreases, the voltage is also reduced in a predefined ratio to maintain a stable magnetic field in the motor. This mode is ideal for applications where constant torque is required over a wide speed range. The V/f curve is often stored in the inverter’s memory and can be selected via switches or dials depending on the motor’s characteristics.
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 because the AC resistance becomes smaller while the DC resistance remains the same. To compensate for this, many inverters boost the voltage slightly at low frequencies to ensure sufficient starting torque. This is known as torque enhancement and can be achieved through various methods, including automatic control, pre-set V/f curves, or manual adjustments.
9. In the manual, the shift range is 60~6Hz, which is 10:1. Is there no output power below 6Hz?
Although the inverter can technically operate below 6Hz, it is generally not recommended due to potential overheating and reduced torque. The minimum usable frequency is typically around 6Hz, where the motor can still provide rated torque without excessive heat. Some models allow operation down to 0.5–3Hz, but performance may be limited depending on the motor and load conditions.
10. For the combination of general motors, the torque is required to be above 60 Hz. Is it ok?
Operating a motor above 60 Hz is generally not advisable unless the motor is specifically designed for high-speed operation. Above 60 Hz, the voltage remains constant, resulting in a constant power characteristic. This means that the torque decreases as the speed increases. If high torque is needed at higher speeds, special motors and inverters with higher capacity must be used.
11. What does it mean to open a ring?
"Open loop" refers to a control system where no feedback is used to monitor the actual speed of the motor. In contrast, "closed loop" systems use a speed sensor (PG) to provide real-time feedback to the inverter, allowing for more accurate speed control. Most general-purpose inverters operate in open-loop mode, though some models include PG feedback for improved performance.
12. What should I do if the actual speed is different for a given speed?
In open-loop operation, the motor’s speed may vary slightly due to load changes or slip. If precise speed control is required, a closed-loop system with PG feedback is recommended. This ensures that the motor runs at the desired speed even under varying loads.
13. If the motor with PG is used, can the speed accuracy be improved after feedback?
Yes, using a PG with the inverter improves speed accuracy. However, the overall precision depends on the quality of the PG and the resolution of the inverter’s frequency output. High-resolution PGs and advanced inverters can achieve very tight speed control.
14. What does the stall prevention function mean?
Stall prevention is a feature that protects the inverter from overcurrent during rapid acceleration or deceleration. If the acceleration time is too short, the inverter may trip due to excessive current. The stall prevention function detects this and adjusts the acceleration rate accordingly, preventing damage to the motor and inverter.
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 in controlling motor movement. This is useful in applications requiring precise timing, such as small machine tools. In contrast, for applications like fan drives, longer acceleration/deceleration times are often preferred.
16. What is regenerative braking?
Regenerative braking occurs when the motor acts as a generator, converting kinetic energy back into electrical energy. This happens when the motor is slowed down or lowered, and the energy is returned to the inverter’s DC bus. This method is efficient and helps reduce energy consumption.
17. Can I get more braking power?
The amount of braking power depends on the inverter’s design. Without a braking unit, most inverters can only regenerate about 10–20% of the rated torque. However, with an optional braking resistor or unit, this can be increased to 50–100%, providing stronger braking capabilities.
18. Torque boosting problem
Modern frequency converters allow for flexible control of motor speed and torque through programmable settings. This enables precise management of industrial processes. For example, in the tobacco industry, frequency converters can adjust pump speeds based on flow signals, ensuring uniform mixing of ingredients. They can also be used to control motor direction and speed based on production line signals, integrating seamlessly into automated systems. Additionally, in case of equipment failure, the inverter can trigger an emergency stop to protect downstream machines. Many inverters, such as those in the SANKEN, MF, FUT, and FVT series, support multiple preset frequencies, enabling complex automation sequences. After setting the operating frequency and time, the inverter can control the motor to run at different speeds at different intervals, forming a complete automated process.
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