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Frequency Converter: 40 FAQs You May Don’t Know

Table of Contents

01. What is frequency converter?

Frequency converter is an electric energy control device that converts the power frequency power supply into another frequency by using the on-off function of power semiconductor devices.

It can realize the functions of soft start, frequency conversion speed regulation, improving operation accuracy, changing power factor, overcurrent / overvoltage / overload protection and so on.

02. What are the differences between PWM and PAM?

PWM is the abbreviation of pulse width modulation, which changes the pulse width of the pulse train according to a certain law to adjust the output and waveform.

PAM is the abbreviation of pulse amplitude modulation in English.

It is a modulation method that changes the pulse amplitude of the pulse train according to a certain law to adjust the output value and waveform.

03. What is the difference between voltage mode and current mode?

The main circuit of frequency converter can be divided into two categories:

The voltage type is a frequency converter that converts the DC of the voltage source into AC, and the filter of the DC circuit is capacitance;

Current mode is a frequency converter that converts the DC of current source into AC, and its DC loop filter is inductance.

04. Why does the voltage of the frequency converter change in proportion to the frequency?

The electromagnetic torque of any motor is the result of the interaction between current and magnetic flux.

The current is not allowed to exceed the rated value, otherwise it will cause the heating of the motor.

Therefore, if the magnetic flux decreases, the electromagnetic torque will also decrease, resulting in the reduction of load capacity.

It can be seen from the formula E=4.44*K*F*N*Φ that during variable frequency speed regulation, the magnetic circuit of the motor changes in a considerable range with the operating frequency fX, which is very easy to seriously saturate the magnetic circuit of the motor, resulting in serious distortion of the waveform of the excitation current and high peak current.

Therefore, the frequency and voltage should be changed in proportion, that is, while changing the frequency, control the output voltage of the frequency converter to keep the magnetic flux of the motor at a certain level, so as to avoid the phenomenon of weak magnetic field and magnetic saturation.

This control mode is mostly used for energy-saving frequency converters of fans and pumps.

05. When the motor is driven by power frequency power supply, the current increases when the voltage decreases; For frequency converter drive, if the voltage also decreases when the frequency decreases, does the current increase?

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

06. What is the starting current and starting torque of the motor when the frequency converter is used?

The frequency converter is used for operation. With the acceleration of the motor, the frequency and voltage are increased accordingly, and the starting current is limited to less than 150% of the rated current (125% ~ 200% according to different models).

When starting directly with power frequency power supply, the starting current is 6 ~ 7 times of the rated current, so it will produce mechanical and electrical impact.

It can be started smoothly by using frequency converter drive (the 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 frequency converter with automatic torque enhancement function, the starting torque is more than 100% and can be started with full load.

07. What does V / f mode mean?

When the frequency decreases, the voltage V also decreases in proportion.

This question has been explained in answer 4.

The proportional relationship between V and f is determined in advance considering the characteristics of the motor.

Usually, there are several characteristics in the memory device (ROM) of the controller, which can be selected by switch or dial.

08. How does the torque of the motor change when changing V and f proportionally?

If the voltage is reduced completely in proportion when the frequency decreases, the ground torque generated at low speed will tend to decrease because the AC impedance becomes smaller and the DC resistance remains unchanged.

Therefore, given V / f at low frequency, the output voltage should be increased to obtain a certain starting torque.

This compensation is called enhanced starting.

It can be realized by various methods, including automatic method, selecting V / f mode or adjusting potentiometer.

09. The manual says that the speed range is 60 ~ 6Hz, i.e. 10:1, so there is no output power below 6Hz?

Power can still be output below 6Hz, but according to the motor temperature rise, starting torque and other conditions, the minimum 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 frequency converter is 0.5 ~ 3Hz according to the model.

10. For the combination of General Motors above 60Hz, a certain torque is also required. Is it OK?

Usually not. When the voltage is above 60Hz (there are also modes above 50Hz), it is basically constant power characteristic, and the same torque is required at high speed.

11. What does open loop mean?

The motor device used is equipped with a speed detector (PG), which feeds back the actual speed to the control device for control, which is called “closed loop”, and the motor device without PG operation is called “open loop”.

General frequency converters are mostly open-loop, and some machines can use options for PG feedback.

The speed sensorless closed-loop control mode is to calculate the actual speed of the motor according to the established mathematical model and the magnetic flux, which is equivalent to forming a closed-loop control with a virtual speed sensor.

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

When the loop is open, even if the frequency converter outputs a given frequency, when the motor is running with load, the speed of the motor changes within the range of rated slip rate (1% ~ 5%).

For the occasions requiring high speed regulation accuracy and requiring operation close to the given speed even if the load changes, the frequency converter with PG feedback function can be used (optional).

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

The accuracy of the frequency converter with PG feedback function is improved.

However, the value of speed accuracy depends on the accuracy of PG itself and the resolution of frequency converter output frequency.

14. What does stall prevention mean?

If the given acceleration time is too short and the output frequency of the frequency converter changes far more than the change of speed (electric angular frequency), the frequency converter will trip and stop operation due to overcurrent, which is called stall.

In order to prevent stall and keep the motor running, it is necessary to detect the current and control the frequency.

When the acceleration current is too large, appropriately slow down the acceleration rate.

The same is true when decelerating.

The combination of the two is the stall function.

15. What is the significance of having models with acceleration time and deceleration time given separately and with acceleration and deceleration time given together?

The acceleration and deceleration can be given separately.

It is suitable for short-time acceleration and slow deceleration, or for small machine tools where the production beat time needs to be strictly given.

However, for fan transmission and other occasions, the acceleration and deceleration time is long, and the acceleration time and deceleration time can be given together.

16. What is regenerative braking?

If the command frequency of the motor is reduced during operation, the motor will become an asynchronous generator and work as a brake, which is called regenerative (Electrical) braking.

17. Can you get more braking force?

The energy regenerated from the motor is stored in the filter capacitor of the frequency converter.

Due to the relationship between the capacity of the capacitor and withstand voltage, the regenerative braking force of the general frequency converter is about 10% ~ 20% of the rated torque.

If the optional brake unit is used, it can reach 50% ~ 100%.

18. Please describe the protection function of the frequency converter?

Protection functions can be divided into the following two categories:

(1) After detecting the abnormal state, it will automatically carry out correction actions, such as overcurrent stall prevention and regeneration overvoltage stall prevention.

(2) After detecting the abnormality, block the PWM control signal of power semiconductor device to make the motor stop automatically.

Such as overcurrent cut-off, regeneration overvoltage cut-off, semiconductor cooling fan overheating and instantaneous power failure protection, etc.

19. Why does the protection function of the frequency converter act when the clutch is used to connect the load?

When the clutch is used to connect the load, at the moment of connection, the motor changes sharply from the no-load state to the area with large slip rate.

The large current flowing causes the over-current trip of the frequency converter and can not operate.

20. In the same factory, when the large motors move together, the frequency converter stops during operation. Why?

When the motor is started, the starting current corresponding to the capacity will flow, and the transformer on the stator side of the motor will produce a voltage drop.

When the motor capacity is large, the voltage drop will also have a great impact.

The frequency converter connected to the same transformer will make the judgment of undervoltage or instantaneous stop.

Therefore, sometimes the protection function (IPE) acts, resulting in stop of operation.

21. What is frequency conversion resolution? What’s the point?

For digitally controlled frequency converters, even if the frequency command is an analog signal, the output frequency is given in stages.

The smallest unit of this level difference is called frequency conversion resolution.

The frequency conversion resolution is usually 0.015 ~ 0.5Hz.

For example, if the resolution is 0.5Hz, 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 coiling control.

In this case, if the resolution is about 0.015hz, one stage difference of 4-stage motor is less than 1R / min, which 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 frequency converter?

The cooling effect is considered in the internal and back structure of the frequency converter, and the up-down relationship is also important for ventilation.

Therefore, the unit type in the panel and hanging on the wall shall be installed vertically as far as possible.

23. Is it possible to put the motor directly into a fixed frequency converter without soft start?

It is possible at very low frequency, but if the given frequency is high, it is similar to the condition of direct starting of power frequency power supply.

A large starting current (6 ~ 7 times the rated current) will flow.

The motor cannot be started because the frequency converter cuts off the overcurrent.

24. What should be paid attention to when the motor runs over 60Hz?

The following items shall be noted when operating above 60Hz:

(1) Machinery and devices shall be fully possible to operate at this speed (mechanical strength, noise, vibration, etc.).

(2) When the motor enters the constant power output range, its output torque should be able to maintain operation (the output power of fan, pump and other shafts increases in proportion to the cube of speed, so pay attention to when the speed increases a little).

(3) The problem of bearing life should be fully considered.

(4) For motors above medium capacity, especially 2-pole motors, it is necessary to carefully discuss with the manufacturer when operating above 60Hz.

According to the different structure and lubrication mode of reducer, several problems need to be paid attention to.

In the structure of gear, 70 ~ 80Hz can be considered as the maximum limit.

When oil lubrication is adopted, continuous operation at low speed is related to the damage of gear.

25. Can the frequency converter be used to drive single-phase motor? Can I use single-phase power supply?

Basically not. For the single-phase motor with governor switch starting type, the auxiliary winding will be burned out in the speed regulation range below the operating point;

For capacitor starting or capacitor operation mode, capacitor explosion will be induced.

The power supply of frequency converter is usually 3-phase, but for small capacity, single-phase power supply is also used.

26. How much power does the frequency converter itself consume?

It is related to the model, operation state and use frequency of the frequency converter, but it is difficult to answer.

However, the efficiency of the frequency converter below 60Hz is about 94% ~ 96%, which can be used to calculate the loss.

However, if the loss during braking is also taken into account, the power consumption will become larger.

Attention must be paid to the design of the operation panel.

27. Why can’t it operate continuously in the whole area of 6 ~ 60Hz?

Generally, the motor is cooled by the outer fan installed on the shaft or the blade on the rotor end ring.

If the speed is reduced, the cooling effect will be reduced, so it can not bear the same heating as high-speed operation.

It is necessary to reduce the load torque at low speed, or use a high-capacity frequency converter to combine with the motor, or use a special motor.

28. What should be paid attention to when using the motor with brake?

The power supply of the brake excitation circuit shall be taken from the input side of the frequency converter.

If the brake acts when the frequency converter is outputting power, it will cause overcurrent cut-off.

Therefore, make the brake act after the frequency converter stops output.

29. If you want to use a frequency converter to drive a motor with a capacitor to improve the power factor, but the motor doesn’t move, please explain the reason.

As for the effective power factor caused by connecting the capacitor of the frequency converter to the OCT after the frequency converter is removed, please improve the power factor caused by the current flowing into the capacitor of the frequency converter.

30. How long is the service life of the frequency converter?

Although the frequency converter is a static device, it also has consumer devices such as filter capacitors and cooling fans.

If they are regularly maintained, they are expected to have a service life of more than 10 years.

31. There is a cooling fan hidden in the frequency converter. What is the direction of the wind? What happens if the fan breaks down?

For small capacity models with or without cooling fans.

For models with fans, the direction of wind is from bottom to top, so do not place mechanical equipment that hinders suction and exhaust at the upper and lower parts of the place where the frequency converter is installed.

Also, do not place parts that are afraid of heat above the frequency converter.

When the fan fails, it is protected by the stop detection of the electric fan or the overheat detection on the cooling fan.

32. Filter capacitor is a consumable, so how to judge its service life?

For the capacitor used as filter capacitor, its electrostatic capacity decreases slowly with the passage of time.

Measure the electrostatic capacity regularly to judge the service life based on 85% of the rated capacity of the product.

33. Is there any restriction on the installation direction when installing the frequency converter?

It should be basically stored in the disc.

The problem is that the disc with fully enclosed structure has large overall size, large occupied space and high cost.

The measures include:

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

(2) Use aluminum fins and fin coolant to increase the cooling area;

34. What is the function of frequency converter DC reactor?

Reduce the high-order harmonic interference of input current and improve the power factor of input power supply.

35. What is the function of sine filter attached to frequency converter?

The sinusoidal filter allows the frequency converter to operate with a long motor cable, and is also suitable for the circuit with intermediate transformer between the frequency converter and the motor.

36. What is the resistance value of the given potentiometer of the frequency converter?

The resistance value of the given potentiometer of the frequency converter is generally 1K Ω to 10K Ω.

37. What are the interference modes of frequency converter and how to deal with them?

1. Mode of communication:

(1) Radiation interference;

(2) Conducted interference.

2. Anti interference measures:

For the interference signal transmitted by radiation, it is mainly weakened by wiring and shielding the radiation source and the disturbed line.

The interference signal transmitted through the line is mainly processed by adding filter, reactor or magnetic ring on the input and output side of the frequency converter.

The specific methods and precautions are as follows:

(1) The signal line and power line shall be vertically crossed or trunked.

(2) Do not connect wires of different metals to each other.

(3) The shielding tube (layer) shall be reliably grounded and ensure continuous and reliable grounding on the whole length.

(4) Twisted pair shielded cable shall be used in signal circuit.

(5) The grounding point of the shielding layer shall be far away from the frequency converter as far as possible and separated from the grounding point of the frequency converter.

(6) The magnetic ring can be used on the input power line and output line of the frequency converter.

The specific method is as follows: the input line can be wound four times in the same direction, while the output line can be wound three times in the same direction.

When winding, pay attention to keep the magnetic ring close to the frequency converter as far as possible.

(7) Generally, shielding and other anti-interference measures can be taken for the disturbed equipment and instruments.

38. If you want to increase the speed of the original conveyor belt and operate at 80Hz, how to choose the capacity of the frequency converter?

The power consumed by the conveyor belt is directly proportional to the speed.

Therefore, if you want to operate at 80Hz, the power of the frequency converter and motor should be increased to 80Hz / 50Hz in proportion, that is, the capacity should be increased by 60%.

39. What is the difference between PWM and VVC +?

In VVC, the control circuit uses a mathematical model to calculate the optimal motor excitation when the motor load changes, and compensates the load.

In addition, the synchronous 60 ° PWM method integrated on ASIC circuit determines the optimal switching time of inverter semiconductor devices (IGBTS).

40. Why can’t frequency converter be used as frequency conversion power supply?

The whole circuit of variable frequency power supply is composed of AC constant current, AC filter and other parts, so its output voltage and current waveforms are pure sine waves, which is very close to the ideal AC power supply.

It can output the grid voltage and frequency of any country in the world.

The frequency converter is composed of AC constant current AC (modulation wave) and other circuits.

The standard name of the frequency converter should be variable frequency governor.

The waveform of its output voltage is pulse square wave, and there are many harmonic components.

The voltage and frequency change proportionally at the same time, which can not be adjusted separately, and does not meet the requirements of AC power supply.

In principle, it can not be used as power supply, and is generally only used for speed regulation of three-phase asynchronous motor.

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