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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 back to DC. In essence, the core of this technology lies in changing the frequency while maintaining the same level of electrical energy.
1. **What is a frequency converter?**
A frequency converter is a power control device that adjusts the frequency of the input power supply by using the switching function of power electronic devices. It allows for precise control over motor speed and performance.
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 regulate output voltage and waveform. PAM, or Pulse Amplitude Modulation, changes the amplitude of the pulses instead, allowing for similar control over the output signal.
3. **What is the difference between voltage type and current type?**
Voltage-type frequency converters use a capacitor for DC filtering, converting DC into AC. Current-type converters, on the other hand, use an inductor for DC filtering, also converting DC into AC but with a different circuit design.
4. **Why does the voltage and current of the inverter change in proportion?**
To maintain stable magnetic flux in the motor, the voltage must be adjusted in proportion to the frequency. If the frequency decreases without adjusting the voltage, the magnetic flux may become too strong, leading to saturation and potential motor damage. This proportional control helps avoid weak magnetization and ensures efficient operation, especially in energy-saving applications like fans and pumps.
5. **When the motor is driven by commercial frequency power, the current increases when the voltage drops. For inverter drive, if the voltage drops when the frequency decreases, does the current increase?**
If the frequency drops and the same power is maintained, the current will increase. However, under constant torque conditions, the current remains relatively stable.
6. **What is the starting current and starting torque of the motor when the inverter is running?**
The inverter gradually increases the frequency and voltage as the motor accelerates, limiting the starting current to around 150% of the rated current. This is much lower than the 6–7 times the rated current seen in direct commercial power starts, reducing mechanical and electrical stress. The starting torque is typically 70–120% of the rated torque, and with automatic torque boosting, it can reach 100% or more.
7. **What does V/f mode mean?**
V/f mode refers to the proportional relationship between voltage (V) and frequency (f). As the frequency decreases, the voltage also decreases proportionally to maintain motor performance. This setting is pre-programmed based on motor characteristics and can be adjusted through switches or dials.
8. **How does the torque of the motor change when V and f are changed proportionally?**
At low frequencies, the AC resistance decreases, which can reduce torque. To compensate, some systems boost the voltage slightly at low frequencies to improve starting torque. This is known as enhanced starting 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?**
Power can still be output below 6 Hz, but the minimum usable frequency is generally around 6 Hz due to temperature and torque considerations. Some inverters can start as low as 0.5–3 Hz, depending on the model.
10. **For general motors, is it okay to require torque above 60 Hz?**
Typically not. Above 60 Hz, the voltage remains constant, and the motor operates in a constant power mode. If high torque is needed at higher speeds, both the motor and inverter must be carefully selected.
11. **What does it mean to open a ring?**
"Open loop" means the system does not use a speed sensor (PG) to monitor motor speed. "Closed loop" uses PG feedback for more accurate control. Most general-purpose inverters operate in open-loop mode, though some models include PG feedback.
12. **What should I do if the actual speed differs from the given speed?**
In open-loop operation, the motor speed may vary within the rated slip range (1–5%) under load. For high-speed accuracy, a closed-loop inverter with PG feedback is recommended.
13. **If a motor with PG is used, can the speed accuracy be improved after feedback?**
Yes, but the accuracy depends on the PG's precision and the inverter’s output frequency.
14. **What does the stall prevention function mean?**
Stall prevention detects excessive current during acceleration or deceleration and adjusts the rate to prevent overcurrent trips. This feature helps avoid motor stalling.
15. **What is the significance of being able to set acceleration and deceleration times separately?**
Setting separate acceleration and deceleration times allows for more precise control, especially in applications requiring fast acceleration and slow deceleration, such as machine tools.
16. **What is regenerative braking?**
Regenerative braking occurs when a motor acts as a generator, converting kinetic energy back into electrical energy. This happens when the motor is lowered or slowed down, providing a braking effect.
17. **Can you get more braking power?**
The energy generated during braking is stored in the inverter’s capacitors. Typically, regenerative braking provides about 10–20% of the rated torque. With additional options, this can be increased to 50–100%.
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