Basic knowledge of frequency converters and related concepts

The frequency converter is a power control device that converts the power frequency power source into another frequency by using the on/off function of the power semiconductor device. With the rapid development of modern power electronics technology and microelectronic technology, high-voltage and high-power variable frequency speed control devices have been continuously matured, and the high voltage problem that has been difficult to solve has been solved in recent years by device series or unit series.

1. The difference between voltage source type and current source type high voltage inverter

The main circuit of the inverter can be roughly divided into two categories: voltage source type and current source type. The voltage source type is a frequency converter that converts the direct current of the voltage source into an alternating current, and the filter element of the direct current loop is a capacitor; the current source type is an inverter that converts the direct current of the current source into an alternating current, and the direct current loop filter component is an inductor.

2. Why does the output voltage of the inverter change in proportion to the frequency?

The torque of the asynchronous motor is generated by the interaction between the magnetic flux of the motor and the current flowing in the rotor. At the rated frequency, if the voltage is constant and only the frequency is reduced, the magnetic flux is too large, the magnetic circuit is saturated, and the motor The current increases and the motor will burn out if it is severe. Therefore, the frequency and voltage should be changed in proportion, that is, the frequency of the inverter is controlled while changing the frequency, so that the magnetic flux of the motor is kept constant to avoid the occurrence of magnetic saturation. This is the definition of VVVF. The voltage here refers to the effective value of the line voltage or phase voltage of the motor.

3. 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, then if the current increases the frequency (low speed), if the same power is output, the current increases. However, under a certain torque condition, the current is almost constant.

4. What is the starting current and starting torque of the motor when the inverter is running?

The inverter is operated, and the frequency and voltage are increased correspondingly with the acceleration of the motor. The starting current is limited to 150% of the rated current (125% to 200% depending on the model). When starting directly with a commercial power supply, the starting current is 6 to 7 times, so a mechanical and electrical impact will occur. It can be started smoothly with the inverter drive (starting time becomes longer). The starting current is 1.2 to 1.5 times of the rated current, and the starting torque is 70% to 120% of the rated torque. For the inverter with the torque automatic enhancement function, the starting torque is 100% or more, and it can be started with full load.

5. What does V/f mode mean?

The voltage V also decreases proportionally as the frequency decreases. This problem has been explained in answer 4. Maintaining the V/f ratio constant control is the most basic control method for asynchronous motor frequency conversion speed regulation. It controls the voltage output of the inverter while controlling the change of the power frequency of the motor, and makes the ratio V/f constant. Keep the magnetic flux of the motor constant. Under the rated operation of the motor, the voltage drop of the stator resistance and leakage reactance of the motor is relatively small, and the terminal voltage of the motor and the induced potential of the motor are approximately equal.

The main problem with V/f ratio constant control is poor low speed performance. The reason is that the proportion of the stator resistance voltage drop of the asynchronous motor becomes large at low speed, which can not be ignored. It can no longer be considered that the stator voltage and the motor induced potential are approximately equal, and the motor flux can not be kept constant according to the V/f ratio control. The reduction of the motor flux will inevitably reduce the electromagnetic torque of the motor; in addition, the dead time of the inverter power device is also an important reason that affects the low-speed performance of the motor. The dead time causes the voltage to drop and also causes the torque ripple. Under the conditions, it will also cause the oscillation of the speed and current.

V/f ratio constant control is commonly used on general purpose inverters. This kind of frequency converter is mainly used for the speed control function of fans and pumps, and the occasions where the speed regulation range is not high. The outstanding advantage of the V/f ratio constant control is that the open loop speed control of the motor can be performed.

6. How does the torque of the motor change when V and f are changed proportionally?

When the frequency is lowered to completely reduce the voltage proportionally, since the AC impedance becomes small and the resistance does not change, the torque generated at the low speed tends to decrease.

Therefore, given V/f at low frequencies, the output voltage must be increased to obtain a certain starting torque. This compensation is called enhanced starting. It can be implemented in various ways, such as an automatic method, selecting a V/f mode, or adjusting a potentiometer.

7. What does it mean to open the ring?

The speed sensor is installed on the motor used, and the actual speed is fed back to the control device for control. It is called “closed loop”. If the speed sensor is not used, it is called “open loop”. The general inverter is mostly open loop.

8, high-voltage inverter's own protection function

Output overload, output overcurrent, grid overvoltage, grid undervoltage, grid power loss, DC bus overvoltage, DC bus undervoltage, transformer overheat, phase loss, control power failure, drive fault, power device overheating, cooling fan failure , external given drop, ground fault, fiber fault, etc.