1. What is a frequency converter?
A frequency converter, also known as a variable frequency drive (VFD), is an electronic device used to control the speed of an AC motor by varying the frequency and voltage supplied to the motor. It works by converting the incoming power from the grid into a different frequency using semiconductor switches like IGBTs or MOSFETs.
2. What is the difference between PWM and PAM?
PWM stands for Pulse Width Modulation, where the width of each pulse in a pulse train is varied to control the output waveform and magnitude. PAM, or Pulse Amplitude Modulation, changes the amplitude of the pulses instead. Both are methods used in power electronics to regulate energy transfer efficiently.
3. What is the difference between voltage type and current type?
Voltage-type inverters use a DC source with a capacitor for filtering, while current-type inverters use an inductor for filtering. The main distinction lies in how they handle the DC link—capacitors store energy for voltage-type, while inductors limit current in current-type systems.
4. Why does the voltage and current of the inverter change in proportion?
To maintain the magnetic flux in an induction motor at a safe level, the voltage must be adjusted proportionally with the frequency. If the frequency decreases but the voltage remains constant, the magnetic flux increases, leading to saturation and potential damage. By keeping V/f proportional, the motor operates efficiently without overheating or losing torque.
5. When the motor is driven by commercial power, the current increases when the voltage drops. For inverter-driven motors, if the voltage drops with frequency, does the current increase?
When frequency decreases, the motor runs slower. If the same power is maintained, the current may rise slightly. However, under constant torque, the current should remain relatively stable. This depends on the load and control method used.
6. What is the starting current and starting torque of the motor when running on an inverter?
The inverter gradually increases both frequency and voltage as the motor accelerates, limiting the starting current to around 150% of the rated value. In contrast, direct-on-line (DOL) starting can cause currents up to 6–7 times the rated current, leading to mechanical stress. Inverter-driven starts are smoother, with lower current and higher torque for better performance.
7. What does V/f mode mean?
V/f mode refers to maintaining a proportional relationship between the voltage and frequency supplied to the motor. This ensures that the motor’s magnetic field remains stable across different speeds. The ratio is pre-programmed based on the motor's characteristics and can be adjusted via settings on the inverter.
8. How does the motor torque change when V and f are changed proportionally?
At low frequencies, reducing both V and f can lead to reduced torque due to increased AC resistance. To compensate, some inverters boost the voltage slightly at low frequencies, improving starting torque. This feature helps the motor start more effectively under load.
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 operate below 6Hz, the minimum usable frequency is typically around 6Hz to prevent overheating. Some models allow operation down to 0.5–3Hz, depending on the application and motor specifications.
10. For general-purpose motors, is it okay to require torque above 60Hz?
It’s generally not recommended. Above 60Hz, the inverter usually operates in constant power mode, meaning the torque decreases as speed increases. If high torque is needed at high speeds, you need to ensure the motor and inverter are properly sized.
11. What does "open loop" mean?
Open loop means the inverter does not use feedback from a speed sensor (PG). It controls the motor based on frequency and voltage alone. Closed-loop systems use PG feedback to improve accuracy and stability.
12. What should I do if the actual speed differs from the set speed?
In open-loop mode, speed variations are normal due to slip. For precise speed control, a closed-loop system with PG feedback is recommended. This allows the inverter to adjust the output to maintain the desired speed.
13. Can speed accuracy be improved with PG feedback?
Yes, PG feedback improves speed accuracy, but the overall precision also depends on the quality of the PG and the resolution of the inverter.
14. What does the stall prevention function mean?
Stall prevention prevents the inverter from tripping due to overcurrent when the acceleration time is too short. It monitors the current and adjusts the acceleration rate to avoid excessive load on the motor, ensuring smooth operation.
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