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The common frequency setting methods of frequency converter are: operator keyboard setting, contact signal setting, analog signal setting, pulse signal setting and communication method setting. These frequency setting methods have their own advantages and disadvantages, and must be selected and set according to actual needs.

Related question and answer

Frequency converter

The frequency converter is a power control device that uses the power semiconductor device's on-off function to convert the power frequency power supply to another frequency. It can achieve soft start for the AC asynchronous motor, frequency conversion speed regulation, improve operating accuracy, change the power factor, and overcurrent. Overvoltage/overload protection and other functions.

Differences between PWM and PAM

PWM is the abbreviation of English Pulse Width Modulation (Pulse Width Modulation), a modulation method that changes the pulse width of the pulse train according to a certain rule to adjust the output volume and waveform. PAM is the abbreviation of English Pulse Amplitude Modulation (Pulse Amplitude Modulation), which is a modulation method that changes the pulse amplitude of the 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 inverter can be roughly divided into two categories: the voltage type is the inverter that converts the DC of the voltage source to AC, the filter of the DC loop is the capacitor; the current type is the inverter that converts the DC of the current source to AC, The DC loop filter is inductance.

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

The electromagnetic torque of any motor is the result of the interaction of current and magnetic flux. The current is not allowed to exceed the rated value, otherwise it will cause heating of the motor. Therefore, if the magnetic flux is reduced, the electromagnetic torque must also be reduced, resulting in reduced load capacity.

It can be seen from the formula E=4.44*K*F*N*Φ that during frequency conversion speed regulation, the magnetic circuit of the motor changes within a relatively large range with the operating frequency fX, it is very easy to make the magnetic circuit of the motor serious Saturation causes severe distortion of the excitation current waveform, resulting in a spike current with a very high peak.

Therefore, the frequency and voltage should be changed in proportion, that is, the output voltage of the inverter should be controlled while changing the frequency, so that the magnetic flux of the motor is kept constant, and the phenomenon of weak magnetic and magnetic saturation is avoided. This control method is mostly used in energy-saving frequency converters for fans and pumps.

5. When the motor is driven by power frequency power supply, the current increases when the voltage drops; for the inverter drive, if the voltage also decreases when the frequency decreases, does the current increase?

When the frequency decreases (low speed), if the same power is output, the current increases, but under the condition of a certain torque, the current is almost unchanged.

6. What is the starting current and starting torque of the motor when the inverter is used for operation?

The inverter is used for operation. With the acceleration of the motor, the frequency and voltage are increased accordingly. The starting current is limited to less than 150% of the rated current (125% to 200% depending on the model). When the power frequency power supply is used to start directly, the starting current is 6 to 7 times the rated current, therefore, mechanical and electrical shock will occur. The inverter drive can be started smoothly (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 the rated torque; for the inverter with automatic torque enhancement function, the starting torque is more than 100% and can be started with full load.

7. What does V/f mode mean?

When the frequency decreases, the voltage V also decreases proportionally. This question 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 stored in the storage device (ROM) of the controller, which can be selected with a switch or a dial.

8. When changing V and f proportionally, how does the torque of the motor change?

When the frequency decreases, the voltage is reduced proportionally, then the AC impedance becomes smaller and the DC resistance does not change, which will cause the torque generated at low speed to have a tendency to decrease. Therefore, given V/f at low frequencies, the output voltage should be increased to obtain a certain starting torque. This compensation is called enhanced starting. Various methods can be used, including automatic methods, V/f mode selection, or potentiometer adjustment.

9. The instruction manual states that the speed range is 60~6Hz, that is, 10:1. Is there no output power below 6Hz?

It can still output power under 6Hz, but according to the temperature rise and starting torque of the motor, the minimum use frequency is about 6Hz. At this time, the motor can output rated torque without causing serious heat problems. The actual output frequency (starting frequency) of the inverter is 0.5~3Hz according to the model. .

10. For the combination of general motors above 60Hz, it also requires a certain torque, is it possible?

Normally it is not possible. When the voltage is above 60Hz (there is also a mode above 50Hz), the voltage is unchanged, and it is generally a constant power characteristic, when the same torque is required at high speed

11. What does open loop mean?

The speed detector (PG) is set for the motor device used, and the actual speed is fed back to the control device for control. It is called "closed loop", and it is called "open loop" without PG operation. Most general-purpose inverters are open-loop, and some models can use options to perform PG feedback. The speed sensorless closed-loop control method is based on the established mathematical model to estimate the actual speed of the motor based on magnetic flux, which is equivalent to using a virtual speed sensor Form a closed-loop control.

12. What if the actual speed deviates from the given speed?

In open loop, even if the inverter outputs a given frequency, when the motor is running with load, the motor speed changes within the range of the rated slip (1% to 5%). For the occasions where the precision of speed regulation is required to be high, even if the load fluctuates, it is required to run at a speed close to the given speed, an inverter with PG feedback function (optional) can be used.

13. If the motor with PG is used, can the speed accuracy be improved after feedback?

The frequency converter with PG feedback function has improved accuracy. But the value of speed accuracy depends on the accuracy of the PG itself and the resolution of the output frequency of the inverter.

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 speed (electrical angular frequency), the inverter will trip due to overcurrent, and the operation will stop. This is called a stall. In order to prevent the stall from causing the motor to continue running, it is necessary to detect the magnitude of the current and perform 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 are the meanings of models with acceleration time and deceleration time that can be given separately, and models with acceleration and deceleration time given together?

The acceleration and deceleration models can be given separately. It is suitable for short-time acceleration and slow deceleration, or for small machine tools where strict production time is required, but for fan transmission and other occasions, the acceleration and deceleration time are longer. The acceleration time and deceleration time can be given together.

16. What is regenerative braking?

If the command frequency is lowered while the motor is running, the motor becomes an asynchronous generator and operates as a brake. This is called regenerative (electrical) braking.

17. Can I 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 inverter is about 10% to 20% of the rated torque. If the optional braking unit is adopted, it can reach 50%~100%.

18. Please explain the protection function of the inverter?

Protection functions can be divided into the following two categories:

(1) Corrective actions are automatically performed after detecting abnormal conditions, such as overcurrent stall prevention and regeneration overvoltage stall prevention.

(2) After detecting the abnormality, the PWM control signal of the power semiconductor device is blocked, and the motor is automatically stopped. Such as overcurrent cutoff, regeneration overvoltage cutoff, semiconductor cooling fan overheating and instantaneous power failure protection.

19. Why does the protection function of the inverter act when the clutch is connected to the load?

When the load is connected with a clutch, at the moment of connection, the motor changes rapidly from the no-load state to a region with a large slip rate. The large current flowing causes the inverter to overcurrent trip and cannot run.

20. The large motors move together in the same factory, and the inverter stops during operation. Why?

When the motor starts, the starting current corresponding to the capacity will flow, and the transformer on the stator side of the motor will generate a voltage drop. When the motor capacity is large, the voltage drop will also have a large impact. The inverter connected to the same transformer will make an undervoltage or instantaneous stop Therefore, the protection function (IPE) may be activated and stop the operation.