Frequency conversion technology knowledge
1. What is a frequency converter?
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.
2. What is the difference between PWM and PAM?
PWM is the abbreviation of English Pulse Width ModulaTIon, which changes the pulse width of the pulse train according to a certain rule to adjust the output value and waveform. PAM is the abbreviation of Pulse Amplitude ModulaTIon in English. It is a modulation method that changes the pulse amplitude of pulse train according to a certain rule to adjust the output value and waveform.
3. What is the difference between voltage type and current type?
The main circuit of the frequency converter can be roughly divided into two types: the voltage type is a frequency converter that converts the direct current of the voltage source into an alternating current, and the filtering of the direct current circuit is a capacitor. The current type is a frequency converter that converts the direct current of the current source into an alternating current, and the direct current loop filtering is an inductance.
4. Why does the voltage and current of the inverter change in proportion?
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, and the magnetic circuit is saturated. The motor will be burned. 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 weak magnetic and magnetic saturation. This type of control is mostly used for energy-saving inverters such as 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?
When the frequency drops (low speed), if the same power is output, the current increases, but under a certain torque, the current hardly changes.
6. 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 the commercial frequency power supply, the starting current is 6 to 7 times, so mechanical and electrical impact will occur. It can be started smoothly with the inverter drive (starting time becomes longer). The starting current is 1.2~1.5 times of the rated current, and the starting torque is 70%~120% of rated torque. For the inverter with automatic torque boosting function, the starting torque is 100% or more, and it can start with full load.
7. What does V/f mode mean?
The voltage V also decreases proportionally as the frequency decreases. This problem has been explained in answer 4. The proportional relationship between V and f is predetermined in consideration of the characteristics of the motor. Usually, there are several characteristics in the memory device (ROM) of the controller, which can be selected by a switch or a dial.
8. 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 resistance becomes small and the DC resistance does not change, the torque generated at the low speed tends to decrease. Therefore, given V/f at low frequencies, the output voltage is increased somewhat in order to obtain a certain starting torque, which is called an enhanced starting. It can be implemented in various ways, such as an automatic method, selecting a V/f mode, or adjusting a potentiometer.
9. In the manual, the shift range is 60~6Hz, which is 10:1. Is there no output power below 6Hz?
The power can still be output below 6 Hz, but according to the conditions of the motor temperature rise and the starting torque, the minimum use frequency is about 6 Hz. At this time, the motor can output the rated torque without causing serious heat generation. The actual output frequency (starting frequency) of the inverter is 0.5~3Hz according to the model.
10. For the combination of general motors, the torque is required to be above 60 Hz. Is it ok?
Usually not allowed. Above 60Hz (also in the mode above 50Hz) the voltage is constant, generally constant power characteristics, when the same torque is required at high speed, you must pay attention to the choice of motor and inverter capacity.
11. What does it mean to open a ring?
A speed detector (PG) is provided for the motor device to be used, and the actual speed is fed back to the control device for control, which is called "closed loop". If the PG is not operated, it is called "open loop". General-purpose inverters are mostly open-loop, and some models use PG feedback.
12. What should I do if the actual speed is different for a given speed?
When the ring is open, even if the inverter outputs a given frequency, the motor's speed will fluctuate within the range of rated slip (1%~5%) when the motor is running with load. For the requirement that the speed regulation accuracy is relatively high, even if the load is changed, it is required to operate at a speed close to the given speed, and the inverter (optional) with the PG feedback function can be used.
13. If the motor with PG is used, can the speed accuracy be improved after feedback?
The inverter with PG feedback function has improved accuracy. However, the speed accuracy depends on the accuracy of the PG itself and the resolution of the inverter output frequency.
14. What does the stall prevention function mean?
If the given acceleration time is too short, the output frequency of the inverter changes far more than the change of the speed (electrical angle frequency), the inverter will trip due to the overcurrent, and the operation stops. This is called stall. In order to prevent the stall from running, the magnitude of the current is detected for frequency control. When the acceleration current is too large, the acceleration rate is appropriately slowed down. The same is true when decelerating. The combination of the two is the stall function.
15. What is the significance of the models that can be given separately for the acceleration time and deceleration time, and the acceleration and deceleration time?
Acceleration and deceleration can be given separately. It is suitable for short-time acceleration, slow deceleration, or for small machine tools where the production tact time is strictly required. However, for fan drive and other occasions, the acceleration/deceleration time is longer. The acceleration time and deceleration time can be given together.
16. What is regenerative braking?
If the motor is lowered during operation, the motor becomes an asynchronous generator and operates as a brake. This is called regenerative (electrical) braking.
17. Can you get more braking power?
The energy regenerated from the motor is stored in the filter capacitor of the inverter. Due to the relationship between the capacity of the capacitor and the withstand voltage, the regenerative braking force of the general-purpose inverter is about 10% to 20% of the rated torque. If the optional brake unit is used, it can reach 50%~100%.
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