A frequency converter is a control device that uses the switching action of power semiconductor devices to convert power frequency into electrical energy of another frequency. With the rapid development of modern power electronics and microelectronics technologies, high-voltage, high-power variable frequency speed control devices have become increasingly sophisticated. The high-voltage problem, which was previously difficult to solve, has been well resolved in recent years through device series connection or unit series connection.
1. Differences between voltage source and current source high-voltage frequency converters
The main circuit of a frequency converter can be broadly classified into two types: voltage source type and current source type. A voltage source type frequency converter converts DC voltage to AC voltage, and the filter element in the DC circuit is a capacitor; a current source type frequency converter converts DC current to AC current, and the filter element in its DC circuit is an inductor.
2. Why does the output voltage of a frequency converter change proportionally to its frequency?
The torque of an asynchronous motor is generated by the interaction between the motor's magnetic flux and the current flowing through the rotor. At the rated frequency, if the voltage is constant and only the frequency is reduced, the magnetic flux will be too large, causing magnetic circuit saturation, increasing the motor current, and potentially burning out the motor. Therefore, the frequency and voltage must be changed proportionally; that is, the output voltage of the frequency converter must be controlled while changing the frequency to keep the motor's magnetic flux constant and avoid magnetic saturation. This is the definition of VVVF (Voltage-Voltage-Frequency). Here, voltage refers to the effective value of the motor's line voltage or phase voltage.
3. When a motor is driven by a power frequency, the current increases when the voltage drops. For a frequency converter, if the voltage also drops when the frequency drops, will the current increase? When the frequency drops (low speed), if the output power is the same, the current will increase, but under the condition of constant torque, the current will remain almost unchanged.
4. When operating with a frequency converter, how do the motor's starting current and starting torque change?
When using a frequency converter, the frequency and voltage increase accordingly as the motor accelerates, limiting the starting current to below 150% of the rated current (125%–200% depending on the model). Direct starting with a mains frequency power supply results in a starting current 6–7 times the rated current, causing mechanical and electrical shocks. Frequency converters allow for smooth starting (though starting time is longer). The starting current is 1.2–1.5 times the rated current, and the starting torque is 70%–120% of the rated torque; for frequency converters with automatic torque boosting, the starting torque is over 100%, allowing for full-load starting.
5. What does V/f mode mean?
The voltage V decreases proportionally as the frequency decreases, as explained in answer 4. Maintaining a constant V/f ratio is the most basic control method for asynchronous motor variable frequency speed regulation. It controls the voltage output by the frequency converter while controlling the change in the motor's power supply frequency, keeping the ratio V/f constant, thus maintaining a constant magnetic flux in the motor. Under rated operating conditions, the voltage drop across the motor's stator resistance and leakage reactance is relatively small, and the motor's terminal voltage and induced electromotive force are approximately equal.
The main problem with constant V/f ratio control is its poor low-speed performance. This is because, firstly, at low speeds, the proportion of the stator resistance voltage drop in the asynchronous motor increases significantly and can no longer be ignored. The stator voltage and the motor's induced electromotive force can no longer be considered approximately equal, and constant V/f ratio control can no longer maintain a constant motor flux. The reduction in motor flux inevitably leads to a decrease in the motor's electromagnetic torque. Secondly, the dead time of the inverter's power devices is also a significant factor affecting the motor's low-speed performance. Dead time causes voltage drops and torque pulsations, and under certain conditions, it can also cause oscillations in speed and current.
Constant V/f ratio control is commonly used in general-purpose frequency converters. These converters are mainly used for speed regulation of fans and pumps, as well as in applications where the speed range requirement is not high. A key advantage of constant V/f ratio control is its ability to perform open-loop speed control of the motor.
6. How does the motor torque change when V and f are changed proportionally?
If the voltage is reduced proportionally as the frequency decreases, the torque generated at low speeds will tend to decrease because the AC impedance decreases while the resistance remains unchanged.
Therefore, at low frequencies, given a V/f, the output voltage needs to be increased to obtain a certain starting torque; this compensation is called enhanced starting. Various methods can be used to achieve this, including automatic methods, selecting a V/f mode, or adjusting a potentiometer.
7. What does "open loop" mean?
The process of equipping the motor with a speed sensor to feed back the actual rotational speed to the control device for control is called "closed loop". The process of operating without a speed sensor is called "open loop". Most general-purpose frequency converters are open loop.
8. Protection functions of the high-voltage frequency converter itself
Output overload, output overcurrent, mains overvoltage, mains undervoltage, mains power failure, DC bus overvoltage, DC bus undervoltage, transformer overheating, phase loss, control power failure, drive failure, power device overheating, cooling fan failure, external input disconnection, grounding failure, fiber optic failure, etc.
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