1. frequency changers overtones produce the mechanism

The frequency changer main circuit generally for hands over - is straight - hands over the composition, exterior inputs 380V/50Hz the labor frequency power source to become the DC voltage signal after the three-phase bridge connection not controllable rectification, inverts after the filter electric capacity filter and the high efficiency transistor on-off element for the frequency invariable exchange signal. In the rectification return route, the input current profile for the irregular rectangular wave, the profile decomposes according to the Fourier progression into the fundamental wave and each 次谐�? higher harmonic will disturb the input power supply system. In the inversion output circuit, the output current signal is the PWM intelligence signal modulation pulse profile, regarding the GTR high efficiency inversion part, its PWM carrier frequency is 2 ~ 3kHz, but IGBT the high efficiency inversion part PWM highest carrier frequency may reach 15kHz. Similarly, the output circuit electric current signal also decomposable for only contains the sine wave the fundamental wave and other each 次谐�? but higher harmonic current to load direct disturbance. Moreover higher harmonic current also through electric cable to space radiation, disturbance neighbor electrical equipment.

2. suppressions harmonic interference commonly used method

The overtone dissemination way is the conduction and the radiation, the solution conducted interference mainly is filters out in the electric circuit the conduction high-frequency current or the isolation; The solution radiated interference is or the line which disturbs is carried on to the radiant the shield. Specifically commonly used method: (1) the frequency conversion system electric power supply is mutually independent with other equipment electric power supplies, or in the frequency changer and other current collector 输入�?the installment insulating transformer, shuts off the harmonic current. (2) in the frequency changer 输入�?AND output side series connection appropriate reactor, or installs the harmonic filter, the filter composition must be LC, the absorption overtone and increases the power source or the load impedance, achieves the suppression overtone the goal. (3) between the electric motor and the frequency changer the electric cable should put on the steel pipe to lay or with the armored cable, and separately lays with other weak electrical signals in the different cable trench, avoids the radiated interference. (4) the holding wire uses the shielded wire, also when wiring staggers the certain distance with the frequency changer host return route pilot wire (at least above 20cm), shuts off the radiated interference. (5) the frequency changer use special-purpose meets the grounding, also with the thick short line earth, is close to other electric appliance equipment the grounding to have to match the line with the frequency changer to separate, use short line. This can effectively suppress the current harmonics to be close to the equipment the radiated interference.

3. suppressions harmonic interference example

The example 1, some frequency conversion hand-off control system, the frequency changer starts the movement normally, but is close to the liquidometer reading to be high, when a table inputs 4mA, the fluid position demonstration is not under the limiting value; When fluid position not to hypothesis in limiting value, the liquidometer actually demonstrates the upper limit, causes the frequency changer receive engine off instruction, forces the frequency changer to stop moving.

This obviously is the frequency changer higher harmonic disturbance liquidometer, the disturbance dissemination way is the liquidometer power source return route or the holding wire. Solution: Takes the liquidometer electric power supply from another power supply transformer, the harmonic interference weakens, again penetrates the holding wire the steel pipe to lay, and returns to the route with the frequency changer host to separate the certain distance, after processes like this, the harmonic interference basic suppression, the liquidometer work restores normally.

The example 2, some frequency conversion control fluid position display system, the liquidometer and the frequency changer in the identical cabinet body installment, the frequency changer work are normal, but the liquidometer demonstrated does not permit also is not steady, at first we cherish congeal a time of table, two tables, the holding wire and the fluid medium have the question, replacement all these measuring appliances, the signal electric cable, and the improvement fluid characteristic, the breakdown still exists, but this breakdown is the frequency changer higher harmonic current through the output circuit electric cable to the external radiation, transmits the signal electric cable, causes the disturbance.

Solution: The liquidometer holding wire and its the pilot wire and the frequency changer pilot wire and the host return route line separates the certain distance, also the cabinet in vitro holding wire penetrates the steel pipe to lay, outer covering good earth, trouble shooting. The example 3, some frequency conversion control system, is composed by two frequency changers, also when identical cabinet in vivo, the frequency changer frequency modulation way for the potentiometer 手调 way, moves some frequency changer, work normal, when two simultaneously move, the frequency mutually disturbs, namely adjusts a frequency changer the potentiometer to be influential to another frequency changer frequency, in turn is also same. Starts us to think is the potentiometer and the pilot wire breakdown, after rules out this kind of possibility, concluded is the harmonic interference causes.

Solution: Transfers to other cabinet bodies potentiometer to be fixed, also the lead wire with shields the holding wire, the result disturbance weakens. In order to thoroughly suppresses the disturbance, reprocesses an electrically controlled cabinet, and with the original cabinet body certain distance laying aside, transfers to a frequency changer this electrically controlled cabinet, the corresponding wiring and the lead wire make the essential modification, after like this processes, disturbance basic elimination, trouble shooting. The example 4, some frequency conversion control system, cuts two sets machine to pump, original machine pumps is depends on falls from the pair presses the start labor frequency movement to be normal, presently changes the frequency conversion movement, although can realize the frequency modulation to decelerate the function, but seriously gives off heat the frequency changer out-port to electric motor between output line, the electric motor outer covering 温升 aggravates, frequently appears the protection to trip. This is because in the frequency changer output voltage and the electric current signal contains the PWM higher harmonic, but the harmonic current 绕线on forms the attachment power loss in the output wire and the electric motor.

Solution: Separate the frequency changer input line and the output line, separately walks respective cable trench, selects the big section the electric cable to trade the original electric cable, between the out-port and the electric motor electric cable length is as far as possible short. After like this processes, gives off heat trouble shooting. Each kind of frequency changer higher harmonic disturbance appears which to the scene, basically all can the method smooth suppression which introduced according to above, but to the harmonic components and the scope request very strict equipment, the thorough suppression higher harmonic disturbance is extremely difficult, waits for further attacks the solution.