The principle and basic knowledge of frequency converter

1. What is a frequency converter?

The frequency converter is an electric energy control device that uses the on-off action of the power semiconductor device to transform the industrial frequency power supply into another frequency.

2. What is the difference between PWM and PAM?

PWM is the abbreviation of Pulse Width Modulation in English, which changes the pulse width of the pulse train according to a certain rule to adjust the output and waveform.

PAM is the abbreviation of Pulse Amplitude Modulation (Pulse Amplitude Modulation) in English. It 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 frequency converter can be roughly divided into two categories: the voltage type is the frequency converter that converts the DC of the voltage source into AC, and the filtering of the DC loop is the capacitor; the current type is the frequency converter that converts the DC of the current source into AC. The DC loop filter is an inductor.

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

The torque of an asynchronous motor is produced by the interaction between the magnetic flux of the motor and the current flowing in the rotor. Under the rated frequency, if the voltage is constant and the frequency is only reduced, the magnetic flux will be too large and the magnetic circuit will be saturated. The motor will be burned. Therefore, the frequency and the voltage should be changed in proportion, that is, the output voltage of the frequency converter is controlled while changing the frequency, so that the magnetic flux of the motor is kept constant, and the phenomenon of magnetic field weakening and magnetic saturation is avoided. This control method is mostly used for energy-saving frequency converters such as fans and pumps.

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

When the frequency drops (low speed), if the same power is output, the current increases, but under the condition of a constant 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 correspondingly increased, and the starting current is limited to 150% of the rated current (depending on the model, it is 125%~200%). When the power frequency power supply is used for direct starting, the starting current is 6 to 7 times, therefore, mechanical and electrical shocks will be generated. Using frequency converter drive can start 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 it can be started with full load.

7. What does V/f mode mean?

When the frequency drops, the voltage V also drops proportionally. This question has been explained in answer 4. The proportional relationship between V and f is determined in advance in consideration of the characteristics of the motor. There are usually several characteristics stored in the storage device (ROM) of the controller, which can be selected with a switch or a dial.

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

When the frequency drops, the voltage is completely reduced in proportion, then the AC impedance becomes smaller and the DC resistance remains unchanged, which will cause the torque generated at low speeds to have a tendency to decrease. Therefore, given V/f at low frequency, the output voltage should be increased to obtain a certain starting torque. This kind of compensation is called enhanced starting. It can be realized by various methods, such as automatic method, selection of V/f mode or adjustment of potentiometer.

9. It is stated in the manual that the speed range is 60~6Hz, that is, 10:1, then there is no output power below 6Hz?

The power can still be output below 6Hz, but according to the conditions such as the temperature rise of the motor and the size of the starting torque, the use frequency is about 6Hz. At this time, the motor can output the rated torque without causing serious heating 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, a certain torque is required above 60Hz, is it possible?

Usually not possible. Above 60Hz (there are also modes above 50Hz), the voltage remains unchanged, and it is generally a constant power characteristic. When the same torque is required at high speed, attention must be paid to the selection of the motor and inverter capacity.

11. What does the so-called open loop mean?

Set a speed detector (PG) for the motor device used, and feedback the actual speed to the control device for control, which is called "closed loop", and the one that runs without PG is called "open loop". Most general-purpose inverters are in open-loop mode, and some models can use options for PG feedback.

12. What should I do if the actual speed deviates from the given speed?

When the loop is open, even if the inverter outputs a given frequency, when the motor is running with a load, the speed of the motor will vary within the range of the rated slip (1%~5%). For the occasions that require relatively high speed regulation accuracy and require operation at a speed close to a given speed even if the load changes, a frequency converter with PG feedback function (optional) can be used.

13. If a 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 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 change of the inverter far exceeds the change of the speed (electrical angle frequency), the inverter will trip due to the overcurrent, and the operation will stop. This is called a stall. In order to prevent the motor from stalling and continuing to run, it is necessary to detect the magnitude of the current for frequency control. When the acceleration current is too large, slow down the acceleration rate appropriately. The same is true when decelerating. The combination of the two is the stall function.

15. There are models for which the acceleration time and deceleration time can be given separately, and the models for which the acceleration and deceleration time are given together. What is the significance of this?

Acceleration and deceleration can be given separately for the machine type. It is suitable for short-time acceleration and slow deceleration, or for small machine tools that require strict production tact time. However, for fan transmission and other occasions, the acceleration and deceleration time is longer. Acceleration time and deceleration time can be given together.

16. What is regenerative braking?

If the motor reduces the command frequency during operation, the motor becomes an asynchronous generator and operates as a brake. This is called regenerative (electric) braking.

17. Can it get more braking force?

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 brake unit is used, 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) After detecting the abnormal state, it will automatically carry out corrective actions, such as over-current stall prevention and regenerative over-voltage stall prevention.

(2) After detecting the abnormality, block the PWM control signal of the power semiconductor device to make the motor stop automatically. 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 operate when the clutch is used for continuous load?

When the load is connected with the clutch, the motor changes sharply from the no-load state to the area with large slip at the moment of connection, and the large current flowing causes the inverter to trip over current and cannot operate.

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

When the motor starts, the starting current corresponding to the capacity will flow. The transformer on the stator side of the motor produces 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 cause undervoltage or instantaneous shutdown. Therefore, sometimes the protection function (IPE) acts and causes the operation to stop.

21. What is the frequency conversion resolution? what sense?

For digitally controlled inverters, even if the frequency command is an analog signal, the output frequency is also given in stages.　The frequency conversion resolution is usually 0.015~0.5Hz. For example, if the resolution is 0.5Hz, then the upper part of 23Hz can be changed to 23.5 and 24.0 Hz, so the action of the motor is also followed in stages. This causes problems for applications such as continuous winding control. In this case, if the resolution is about 0.015Hz, one step difference for a 4-level motor is 1r/min or less, it can also be fully adapted. In addition, the given resolution of some models is different from the output resolution.

22. Is there any restriction on the installation direction when installing the inverter?

The internal and rear structure of the inverter takes into account the cooling effect, and the relationship between the top and bottom is also important for ventilation. Therefore, for the unit type in the panel and hanging on the wall, take the vertical position and install it as vertically as possible.

23. Is it possible to directly input the motor to a fixed frequency inverter without using soft start?

It is possible at a very low frequency, but if the given frequency is high, the conditions for direct starting with the industrial frequency power supply are similar. A large starting current (6~7 times the rated current) will flow, and the motor cannot start because the inverter cuts off the overcurrent.

24. What problems should be paid attention to when the motor is running at more than 60Hz?

Pay attention to the following matters when operating at more than 60Hz:

(1) It must be possible for machinery and equipment to operate at this speed (mechanical strength, noise, vibration, etc.).

(2) When the motor enters the constant power output range, its output torque must be able to maintain its work (the output power of the shaft of fans, pumps, etc. increases in proportion to the cube of the speed, so pay attention to a slight increase in the speed).

(3) Bearing life issues must be fully considered.

(4) For motors with medium capacity and above, carefully discuss with the manufacturer when operating above 60Hz.

25. Can the inverter drive a gear motor?

Depending on the structure and lubrication method of the reducer, several issues need to be paid attention to. Generally, 70~80Hz can be considered in the structure of the gear. When using oil lubrication, continuous operation at low speed is related to the damage of the gear.

26. Can the inverter be used to drive a single-phase motor? Can single-phase power be used?

Basically unusable. For the single-phase motor of the speed governor switch-start type, the auxiliary winding will be burnt when the speed range is below the operating point; for the capacitor start or capacitor operation mode, the capacitor will be induced to explode. The power supply of the inverter is usually 3-phase, but for small-capacity models, there are also models that operate with a single-phase power supply.

27. How much power does the inverter consume?

It is related to the inverter model, running status, frequency of use, etc., but it is difficult to answer. However, the efficiency of the inverter below 60Hz is about 94%~96%, and the loss can be estimated based on this, but the built-in regenerative braking (FR-K) inverter, if the loss during braking is also taken into account, the power consumption It will become larger, so attention must be paid to the design of the operation panel.

28. Why can't it be used continuously in the whole area of 6~60Hz?

Generally, the motor uses the outer fan mounted on the shaft or the blades on the rotor end ring for cooling. If the speed is reduced, the cooling effect will be reduced. Therefore, it cannot withstand the same heat as the high-speed operation. The load torque at low speed must be reduced, or use Combine a large-capacity inverter with a motor, or use a dedicated motor.

29. What should be paid attention to when using a motor with brake?

The brake excitation circuit power supply should be taken from the input side of the inverter. If the brake acts when the inverter is outputting power, it will cause the overcurrent to be cut off. Therefore, the brake must be activated after the inverter stops output.

30. I want to use a frequency converter to drive a motor with a capacitor for improving power factor, but the motor does not move. Please explain the reason.

The current of the inverter flows into the capacitor for improving the power factor, and the inverter cannot be started due to the overcurrent (OCT) of the inverter caused by its charging current. As a countermeasure, please remove the capacitor and operate, or even improve the power factor, on the input side of the inverter. It is effective to connect to AC reactor.

31. How long is the life of the inverter?

Although the inverter is a static device, it also has consumables such as filter capacitors and cooling fans. If they are regularly maintained, they are expected to have a life span of more than 10 years.

32. There is a cooling fan in the inverter, what's the direction of the wind? What happens if the fan is broken?

There are also models with or without cooling fans for small capacities. For models with fans, the direction of the wind is from bottom to top, so do not place mechanical equipment that hinders suction and exhaust in the place where the inverter is installed. Also, do not place heat-sensitive parts on the top of the inverter. When the fan fails, it is protected by the fan stop detection or the overheat detection on the cooling fan.

33. The filter capacitor is a consumable, so how to judge its life?

Capacitors used as filter capacitors have an electrostatic capacity that gradually decreases with the passage of time. The electrostatic capacity is measured regularly and the life is judged based on when it reaches 85% of the rated capacity of the product.

34. Is there any restriction on the installation direction when installing the inverter?

It should be basically stored in the tray. The problem is that the tray with a fully enclosed structure has a large shape, a large space, and a relatively high cost. The measures include:

(1) The design of the disk should be aimed at the heat dissipation required by the actual device;

(2) Use aluminum fins, fin coolant, etc. to increase the cooling area;

(3) Use heat pipes.

In addition, a type in which the back of the inverter can be exposed has been developed.

35. If I want to increase the speed of the original conveyor belt and run at 80Hz, how to choose the capacity of the frequency converter?

Suppose the reference speed is 50Hz, and above 50Hz is the constant power output characteristic. When a load with constant torque characteristics like a conveyor belt increases speed, the capacity needs to be increased to 80/50≈1.6 times. The motor capacity also increases like a frequency converter.