Harmonic suppression circuit, frequency conversion system and fan

Abstract

The utility model relates to the technical field of fans, in particular to a harmonic suppression circuit, a frequency conversion system and a fan. The utility model aims to solve the problem that when the harmonic current is too large, the filter in the related technology may have an overload phenomenon. Therefore, the harmonic suppression circuit comprises a harmonic detector used for detecting the intensity of harmonic current generated by the frequency converter; the main circuit contactor is arranged between the output end of the frequency converter and the electric equipment; the input end of the bypass contactor is connected with the input end of the frequency converter, and the output end of the bypass contactor is connected with the output end of the main contactor; under the condition that the intensity of the harmonic current exceeds a harmonic current threshold value, the bypass contactor is connected, and the main circuit contactor is disconnected; and when the intensity of the harmonic current is located at the harmonic current threshold value, the main circuit contactor is connected, and the bypass contactor is disconnected. Harmonic waves generated by the frequency converter can be prevented from influencing a power grid and electric equipment.

Description

TECHNICAL FIELD

[0001] The present application relates to the fan technical field, specifically relates to a harmonic suppression circuit, frequency conversion system and fan. BACKGROUND

[0002] Frequency converters are widely used in industrial and commercial applications, for example, fans use frequency converters for speed regulation operation. Harmonic current can be generated during the frequency conversion operation of the frequency converter, which can have an adverse effect on the power grid and other electrical equipment.

[0003] The related technology uses a filter to govern the harmonic current generated by the frequency converter, and the filter includes multiple passive filter branches and single-tuned filters and high-pass filters arranged at different passive filter branches. The single-tuned filter is used to filter out harmonics of a specific order, and the high-pass filter is used to filter out harmonic currents with frequencies higher than the cutoff frequency.

[0004] However, when the harmonic current is too large, the filter in the related technology can overload, and because the range of high-order harmonics is large, the filter requires more passive filter branches, resulting in a large overall device size and increased losses.

[0005] Accordingly, there is a need in the art for a new technical solution to solve the above problems. UTILITY MODEL CONTENT

[0006] In order to solve at least one of the above problems in the prior art, that is, to solve the problem that when the harmonic current is too large, the filter in the related technology can overload, and because the range of high-order harmonics is large, the filter requires more passive filter branches, resulting in a large overall device size and increased losses.

[0007] In a first aspect, the present application provides a harmonic suppression circuit applied to a frequency converter, wherein an input end of the frequency converter is connected with a power supply, an output end of the frequency converter is connected with an electrical equipment, the harmonic suppression circuit comprises a harmonic detector, a main circuit contactor and a bypass contactor; the harmonic detector is used to detect the intensity of the harmonic current generated by the frequency converter; the main circuit contactor is arranged between the output end of the frequency converter and the electrical equipment; an input end of the bypass contactor is connected with the input end of the frequency converter, and an output end of the bypass contactor is connected with an output end of the main circuit contactor; wherein in the case that the intensity of the harmonic current exceeds a harmonic current threshold value, the bypass contactor is connected, and the main circuit contactor is disconnected; in the case that the intensity of the harmonic current is within the harmonic current threshold value, the main circuit contactor is connected, and the bypass contactor is disconnected.

[0008] In some embodiments, the harmonic detector is arranged between the output end of the frequency converter and the input end of the main circuit contactor.

[0009] In some embodiments, the harmonic detector is disposed between an input of the frequency converter and the power supply, and an input of the bypass contactor is connected to the harmonic detector.

[0010] In some embodiments, the harmonic detector is further configured to control the bypass contactor to be connected or disconnected, and the harmonic suppression circuit further comprises a first control device, a signal receiving end of the first control device being connected to the harmonic detector for obtaining the intensity of the harmonic current, and a control output end of the first control device being connected to the main contactor for controlling the main contactor to be connected or disconnected.

[0011] In some embodiments, the harmonic suppression circuit further comprises a second control device, a signal receiving end of the second control device being connected to the harmonic detector for obtaining the intensity of the harmonic current, and a control output end of the second control device being connected to the main contactor and the bypass contactor for controlling the main contactor and the bypass contactor to be connected or disconnected.

[0012] In some embodiments, the bypass contactor comprises an electromagnetic switch or a mechanical switch.

[0013] In some embodiments, the main contactor comprises an electromagnetic switch or a mechanical switch.

[0014] In some embodiments, the harmonic current threshold is set to one or more.

[0015] In some embodiments, the bypass contactor is connected between an input of the frequency converter and an output of the main contactor through a bypass wire, and the harmonic suppression circuit further comprises a fuse disposed on the bypass wire.

[0016] In some embodiments, the harmonic suppression circuit further comprises an alarm device, and the alarm device sends an alarm when the intensity of the harmonic current exceeds an alarm threshold.

[0017] In some embodiments, the power supply is a three-phase power supply, and the bypass contactor comprises a first terminal, a second terminal and a third terminal for connecting to the three-phase power supply.

[0018] In some embodiments, the bypass contactor comprises a fourth terminal, a fifth terminal and a sixth terminal for connecting to the electrical equipment.

[0019] In a second aspect, the application provides a variable frequency system, which comprises a frequency converter and the harmonic suppression circuit.

[0020] In a third aspect, the application provides a fan, which comprises the variable frequency system.

[0021] With the above technical solutions, the application detects the intensity of the harmonic current generated by the frequency converter through the harmonic detector. Once the intensity of the harmonic current exceeds the set harmonic current threshold, the bypass contactor of the harmonic suppression circuit is connected and the main contactor is disconnected, so that the frequency converter is separated from the current loop, the current enters the electrical equipment through the bypass contactor without flowing through the frequency converter, and the electrical equipment is switched to the power grid to operate at a power frequency. In this way, the influence of the harmonic generated by the frequency converter on the power grid and the electrical equipment can be avoided. Meanwhile, when the intensity of the harmonic current is within the set harmonic current threshold, the main contactor of the harmonic suppression circuit is connected and the bypass contactor is disconnected, the frequency converter is located in the power supply loop, the current enters the electrical equipment through the frequency converter, and the electrical equipment operates at a variable frequency. BRIEF DESCRIPTION OF DRAWINGS

[0022] The motor, speed regulation system and air conditioner of the application will be described below with reference to the accompanying drawings. In the drawings:

[0023] Figure 1 Fig. 4 is a circuit connection schematic diagram of the variable frequency system in the application, in which the main contactor is connected and the bypass contactor is disconnected;

[0024] Figure 2 Fig. 5 is a circuit connection schematic diagram of the variable frequency system in the application in another state, in which the bypass contactor is connected and the main contactor is disconnected; Figure 1

[0025] Figure 3 Fig. 6 is another circuit connection schematic diagram of the variable frequency system in the application.

[0026] List of reference signs

[0027] 100, harmonic detector;

[0028] 200, main contactor;

[0029] 300, bypass contactor;

[0030] 400, frequency converter;

[0031] 500, fuse. DETAILED DESCRIPTION

[0032] ​The preferred embodiments of the present application are described below with reference to the accompanying drawings. Those skilled in the art will understand that the embodiments are only used to explain the technical principles of the present application and are not intended to limit the protection scope of the present application. For example, although the following embodiments of the present application are described in combination with a fan, this is not intended to limit the protection scope of the present application. The vibration isolation device of the present application can also be applied to the vibration isolation of other devices without departing from the principles of the present application.

[0033] It should be noted that in the description of the present application, the terms indicating the direction or positional relationship such as "middle", "upper", "lower", "vertical", "inner", "outer" and the like are based on the direction or positional relationship shown in the drawings, which is only for the convenience of description and does not indicate or imply that the device or element must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation of the present application. In addition, the terms "first", "second" are only for the purpose of description and cannot be understood as indicating or implying relative importance.

[0034] In addition, it should be noted that in the description of the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connecting" should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the communication inside two elements. Those skilled in the art can understand the specific meaning of the above terms in the present application according to the specific circumstances.

[0035] The present application provides a harmonic suppression circuit, a frequency conversion system and a fan, which can avoid the influence of harmonic generated by the frequency converter on the power grid and the electrical equipment.

[0036] In a first aspect, the present application provides a harmonic suppression circuit.

[0037] The harmonic suppression circuit is applied to a frequency converter 400. The input end of the frequency converter 400 is connected with a power supply, and the output end of the frequency converter 400 is connected with electrical equipment. At present, household appliances or industrial appliances usually obtain electrical energy through a power grid, and the power supply is the power grid. The harmonic current generated by the frequency converter 400 will pollute the power grid.

[0038] In combination with Figure 1 and Figure 2 It is shown that the harmonic suppression circuit provided by the present application includes a harmonic detector 100, a main circuit contactor 200 and a bypass contactor 300.

[0039] The harmonic detector 100 is used to detect the intensity of the harmonic current generated by the frequency converter 400. The harmonic detector is a device used to detect the harmonic content in the power system. In the power system, the harmonic is a periodic waveform with a frequency higher than the 50Hz fundamental frequency. The existence of the harmonic will cause many problems, such as electromagnetic interference, energy loss, etc. Therefore, the use of the harmonic detector is very important for protecting the stable operation of the power system and improving the power quality.

[0040] The main circuit contactor 200 is arranged between the output end of the frequency converter 400 and the electrical equipment. The main circuit contactor 200 can be controlled to be connected or cut off.

[0041] The input end of the bypass contactor 300 is connected with the input end of the frequency converter 400, and the output end of the bypass contactor 300 is connected with the output end of the main circuit contactor 200. The bypass contactor 300 can be controlled to be connected or cut off.

[0042] In the case that the intensity of the harmonic current exceeds the harmonic current threshold value, the bypass contactor 300 is connected, and the main circuit contactor 200 is cut off. In the case that the intensity of the harmonic current is within the harmonic current threshold value, the main circuit contactor 200 is connected, and the bypass contactor 300 is cut off.

[0043] Under the premise of adopting the above technical solutions, the harmonic suppression circuit provided by the present application detects the intensity of the harmonic current generated by the frequency converter 400 through the harmonic detector 100. Once it is detected that the intensity of the harmonic current exceeds the set harmonic current threshold value, the bypass contactor 300 of the harmonic suppression circuit is connected, and the main circuit contactor 200 is cut off, so that the frequency converter 400 is disconnected from the current loop, the current enters the electrical equipment through the bypass contactor 300 without flowing through the frequency converter 400, and the electrical equipment is switched to the power grid to operate at the power frequency. In this way, the influence of the harmonic generated by the frequency converter 400 on the power grid and the electrical equipment can be avoided. At the same time, when the intensity of the harmonic current is within the set harmonic current threshold value, the main circuit contactor 200 of the harmonic suppression circuit is connected, and the bypass contactor 300 is cut off, so that the frequency converter 400 is located in the power supply loop, and the current enters the electrical equipment through the frequency converter 400, and the electrical equipment operates at the frequency.

[0044] Among them, the power frequency is the abbreviation of the power grid working frequency, which refers to the frequency of the alternating current in the power grid. All generators, power transmission and distribution equipment and users in the same power grid use alternating current of this frequency. At present, most countries in the world use 50Hz as the power grid working frequency.

[0045] Optionally, the harmonic current threshold value is provided with one or more. In this way, multiple frequency harmonics can be shielded without setting multiple passive filtering branches as in the related art.

[0046] Optionally, the harmonic current threshold value is preset in the control device.

[0047] Optionally, the harmonic current threshold is determined according to the system requirements of the variable frequency system and real-time harmonic data. The harmonic current threshold for triggering the bypass contactor is set according to the real-time harmonic data and the system requirements. In this way, the harmonic current threshold can be dynamically adjusted according to the actual operating conditions and load characteristics to ensure timely and accurate response to harmonic over-standard situations.

[0048] For the harmonic detector 100, there are two possible settings.

[0049] In the first possible implementation, as shown in Figure 1 and Figure 2 , the harmonic detector 100 is arranged between the output of the frequency converter 400 and the input of the main contactor 200.

[0050] In this way, the strength of the harmonic current generated by the frequency converter 400 can be accurately detected. In the case where the strength of the harmonic current exceeds the set harmonic current threshold, the bypass contactor is quickly connected and the main contactor is quickly disconnected, the frequency converter is disconnected from the current loop, and the harmonic current generated by the frequency converter is prevented from affecting the electrical device and the power grid.

[0051] In the first possible implementation, the bypass contactor 300 and the main contactor 200 are controlled by the second control device. Specifically, the harmonic suppression circuit further comprises a second control device, the signal receiving end of the second control device is connected with the harmonic detector 100 for obtaining the strength of the harmonic current, and the control output end of the second control device is connected with the main contactor 200 and the bypass contactor 300 for controlling the main contactor 200 and the bypass contactor 300 to be connected or disconnected. After receiving the strength signal of the harmonic current, the second control device judges whether the strength of the harmonic current is within the harmonic current threshold. If the strength of the harmonic current is within the harmonic current threshold, the second control device controls the main contactor of the harmonic suppression circuit to be connected and the bypass contactor to be disconnected, the frequency converter is in the power supply loop, the current enters the electrical equipment through the frequency converter, and the electrical equipment operates at variable frequency. If the strength of the harmonic current is not within the harmonic current threshold, the second control device controls the bypass contactor to be connected and the main contactor to be disconnected, so that the frequency converter is disconnected from the current loop, the current enters the electrical equipment through the bypass contactor without flowing through the frequency converter, and the electrical equipment is switched to the power grid to operate at power frequency.

[0052] In the second possible implementation, as shown in Figure 3 , the harmonic detector is arranged between the input of the frequency converter and the power supply, and the input of the bypass contactor is connected with the harmonic detector. In this way, the strength of the harmonic current fed back to the power grid by the frequency converter 400 can be accurately detected, and the harmonic current generated by the frequency converter is prevented from polluting the power grid.

[0053] In the second possible implementation, the bypass contactor 300 and the main contactor 200 can be controlled uniformly by the second control device, or can be controlled separately.

[0054] The bypass contactor 300 and the main contactor 200 are controlled uniformly by the second control device, and specifically, a signal receiving end of the second control device is connected with the harmonic detector for obtaining the intensity of the harmonic current, and a control output end of the second control device is connected with the main contactor and the bypass contactor for controlling the main contactor and the bypass contactor to be connected or disconnected.

[0055] The bypass contactor 300 and the main contactor 200 are controlled separately, and specifically, the harmonic detector is further used for controlling the bypass contactor to be connected or disconnected, and the harmonic suppression circuit further comprises a first control device, a signal receiving end of the first control device is connected with the harmonic detector for obtaining the intensity of the harmonic current, and a control output end of the first control device is connected with the main contactor for controlling the main contactor to be connected or disconnected. In this embodiment, since the bypass contactor 300 is directly connected with the harmonic detector, the response speed of the bypass contactor controlled by the harmonic detector 100 is relatively fast.

[0056] In some embodiments, the bypass contactor 300 comprises an electromagnetic switch or a mechanical switch. The bypass contactor 300 is used for switching the load to the power grid when the harmonic exceeds the standard, so as to avoid harmonic pollution and ensure uninterrupted power supply and normal use of the user. The response speed of the electromagnetic switch and the mechanical switch is relatively fast and stable and reliable, and by setting the switch of the bypass contactor 300 as the electromagnetic switch or the mechanical switch, the response speed and the reliability are improved, and the harmonic suppression circuit responds quickly when the harmonic exceeds the standard, so as to ensure the stability and the reliability of the power supply system.

[0057] In some embodiments, the main contactor 200 comprises an electromagnetic switch or a mechanical switch. The main contactor 200 is used for cooperating with the bypass contactor 300 to switch the load to the power grid when the harmonic exceeds the standard, so as to avoid harmonic pollution and ensure uninterrupted power supply and normal use of the user. The response speed of the electromagnetic switch and the mechanical switch is relatively fast and stable and reliable, and by setting the switch of the main contactor 200 as the electromagnetic switch or the mechanical switch, the response speed and the reliability are improved, and the harmonic suppression circuit responds quickly when the harmonic exceeds the standard, so as to ensure the stability and the reliability of the power supply system.

[0058] In some embodiments, the bypass contactor 300 and the main contactor 200 are controlled by the first control device and the second control device. Figure 1 and Figure 2As shown, the bypass contactor is connected between the input of the frequency converter and the output of the main contactor through a bypass wire; wherein the harmonic suppression circuit further comprises a fuse 500, which is arranged on the bypass wire. In the harmonic suppression circuit, if the bypass contactor 300 has an abnormality during switching, causing the current to directly impact the electrical equipment, the fuse 500 can serve as an additional protective barrier to prevent excessive current from damaging the electrical equipment, thereby improving the reliability of the entire harmonic suppression circuit. In addition, by arranging the fuse 500 on the bypass wire, when the harmonic detector 100, the main contactor 200, the bypass contactor 300, and other electrical components fail, the fuse 500 can play a protective role to ensure the safety of the entire circuit and improve system reliability.

[0059] In some embodiments, the power supply is a three-phase power supply, and the bypass contactor 300 comprises a first wiring terminal, a second wiring terminal, and a third wiring terminal for connecting with the three-phase power supply. The first wiring terminal, the second wiring terminal, and the third wiring terminal are used to connect with the three-phase lines of the three-phase power supply. The three-phase power supply is widely used in industrial and commercial fields due to its high efficiency and stability, and can provide balanced and stable power supply.

[0060] In some embodiments, the bypass contactor 300 comprises a fourth wiring terminal, a fifth wiring terminal, and a sixth wiring terminal for connecting with the electrical equipment. The electrical equipment requires a three-phase power supply to work normally, especially in large mechanical equipment, electric motor, and other application scenarios. The fourth wiring terminal, the fifth wiring terminal, and the sixth wiring terminal can ensure that the power supply can be stably transmitted to the electrical equipment, and also provide a path for the current to return to the power supply.

[0061] Optionally, when the strength of the harmonic current generated by the frequency converter 400 is reduced to within the harmonic current threshold range, the main contactor 200 is connected, the bypass contactor 300 is disconnected, the current enters the electrical equipment through the frequency converter 400, and the frequency converter 400 control is restored.

[0062] The electrical equipment includes a fan, a compressor, a motor, a water pump, and a generator set, etc.

[0063] Optionally, the harmonic suppression circuit further comprises an alarm device, which sends an alarm to the user to remind the user of the abnormality of the frequency converter when the strength of the harmonic current exceeds the alarm threshold.

[0064] Optionally, the alarm device further comprises an indicator light, which is controlled by the alarm device to flash when the strength of the harmonic current exceeds the alarm threshold.

[0065] In a second aspect, the present application provides a frequency conversion system.

[0066] The variable frequency system provided by the application comprises a frequency converter 400 and the harmonic suppression circuit. The harmonic detector 100 of the harmonic suppression circuit is used for detecting the intensity of the harmonic current generated by the frequency converter 400. The main path contactor 200 is arranged between the output end of the frequency converter 400 and the electrical equipment. The input end of the bypass contactor 300 is connected with the input end of the frequency converter 400, and the output end of the bypass contactor 300 is connected with the output end of the main path contactor 200. In the case that the intensity of the harmonic current exceeds the harmonic current threshold value, the bypass contactor 300 is connected, and the main path contactor 200 is disconnected. In the case that the intensity of the harmonic current is within the harmonic current threshold value, the main path contactor 200 is connected, and the bypass contactor 300 is disconnected.

[0067] Under the premise of adopting the above technical solutions, the variable frequency system provided by the application detects the intensity of the harmonic current generated by the frequency converter 400 through the harmonic detector 100. Once it is detected that the intensity of the harmonic current exceeds the set harmonic current threshold value, the bypass contactor 300 of the harmonic suppression circuit is connected, and the main path contactor 200 is disconnected, so that the frequency converter 400 is separated from the current loop, the current enters the electrical equipment through the bypass contactor 300 without flowing through the frequency converter 400, and the electrical equipment is switched to the power grid to operate at the power frequency. In this way, the harm of the harmonic generated by the frequency converter 400 to the power grid and the electrical equipment can be reduced. At the same time, when the intensity of the harmonic current is within the set harmonic current threshold value, the main path contactor 200 of the harmonic suppression circuit is connected, and the bypass contactor 300 is disconnected, so that the frequency converter 400 is located in the power supply loop, the current enters the electrical equipment through the frequency converter 400, and the electrical equipment operates at the variable frequency.

[0068] In a third aspect, the application provides a fan.

[0069] The fan provided by the application comprises the variable frequency system.

[0070] Under the premise of adopting the above technical solutions, the fan provided by the application detects the intensity of the harmonic current generated by the frequency converter 400 through the harmonic detector 100. Once it is detected that the intensity of the harmonic current exceeds the set harmonic current threshold value, the bypass contactor 300 of the harmonic suppression circuit is connected, and the main path contactor 200 is disconnected, so that the frequency converter 400 is separated from the current loop, the current enters the fan through the bypass contactor 300 without flowing through the frequency converter 400, and the fan is switched to the power grid to operate at the power frequency. In this way, the harm of the harmonic generated by the frequency converter 400 to the power grid and the fan can be reduced. At the same time, when the intensity of the harmonic current is within the set harmonic current threshold value, the main path contactor 200 of the harmonic suppression circuit is connected, and the bypass contactor 300 is disconnected, so that the frequency converter 400 is located in the power supply loop, the current enters the fan through the frequency converter 400, and the fan operates at the variable frequency.

[0071] It should be noted that the above preferred embodiments are only used to illustrate the principles of the present application, and are not intended to limit the protection scope of the present application. Those skilled in the art can adjust the above setting mode without departing from the principles of the present application, so that the present application can be applied to more specific application scenarios.

[0072] Those skilled in the art can understand that the combination of features of different embodiments means to be within the scope of the present application and form different embodiments, although some embodiments herein include certain features rather than other features included in other embodiments. For example, in the claims of the present application, any one of the claimed embodiments can be used in any combination.

[0073] So far, the technical solutions of the present application have been described in combination with the preferred embodiments shown in the drawings, but those skilled in the art can easily understand that the protection scope of the present application is obviously not limited to these specific embodiments. Those skilled in the art can make equivalent changes or replacements to the related technical features without departing from the principles of the present application, and the technical solutions after the changes or replacements will fall within the protection scope of the present application.

Claims

1. A harmonic suppression circuit applied to a frequency converter, an input end of the frequency converter being connected with a power supply, an output end of the frequency converter being connected with an electrical equipment, characterized in that, The harmonic suppression circuit comprises a harmonic detector, a main path contactor and a bypass contactor. The harmonic detector is configured to detect the intensity of the harmonic current generated by the frequency converter. The main path contactor is arranged between the output of the frequency converter and the electrical equipment. The input of the bypass contactor is connected to the input of the frequency converter, and the output of the bypass contactor is connected to the output of the main path contactor. When the intensity of the harmonic current exceeds a harmonic current threshold, the bypass contactor is connected, and the main path contactor is disconnected. When the intensity of the harmonic current is within the harmonic current threshold, the main path contactor is connected, and the bypass contactor is disconnected.

2. The harmonic rejection circuit of claim 1, wherein, The harmonic detector is arranged between the output of the frequency converter and the input of the main path contactor.

3. The harmonic rejection circuit of claim 1, wherein, The harmonic detector is arranged between the input of the frequency converter and the power supply, and the input of the bypass contactor is connected to the harmonic detector.

4. The harmonic rejection circuit of claim 3, wherein, The harmonic detector is further configured to control the connection or disconnection of the bypass contactor. The harmonic suppression circuit further comprises a first control device, a signal receiving end of the first control device is connected to the harmonic detector, configured to obtain the intensity of the harmonic current, and a control output end of the first control device is connected to the main path contactor, configured to control the connection or disconnection of the main path contactor.

5. A harmonic rejection circuit as claimed in claim 2 or 3, characterised in that, The harmonic suppression circuit further comprises a second control device, a signal receiving end of the second control device is connected to the harmonic detector, configured to obtain the intensity of the harmonic current, and a control output end of the second control device is connected to the main path contactor and the bypass contactor, configured to control the connection or disconnection of the main path contactor and the bypass contactor.

6. The harmonic rejection circuit of any one of claims 1 to 4, wherein, The bypass contactor comprises an electromagnetic switch or a mechanical switch, and / or the main path contactor comprises an electromagnetic switch or a mechanical switch; and / or the harmonic current threshold is set to one or more.

7. The harmonic rejection circuit of any one of claims 1 to 4, wherein, The bypass contactor is connected between the input of the frequency converter and the output of the main path contactor through a bypass wire; wherein the harmonic suppression circuit further comprises a fuse, which is arranged on the bypass wire.

8. The harmonic rejection circuit of claim 7, wherein, The harmonic suppression circuit further comprises an alarm device, which issues an alarm when the intensity of the harmonic current exceeds an alarm threshold.

9. A variable frequency system characterized by, The frequency conversion system comprises a frequency converter and the harmonic suppression circuit according to any one of claims 1 to 8.

10. A fan, characterized by The fan comprises the frequency conversion system according to claim 9.

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