Because exchanges the asynchronous motor 主磁�? magnetic flux is defers to the sine rule to design and the movement. In order to cause the electric motor the performance characteristic to be fine, the electric motor frequency conversion velocity modulation technology usually uses the sine wave pulse width modulation the method, is called the SPWM method.
As a result of in the SPWM voltage pulse sequence, each pulse scope is equal but the pulse width not to be equal. The width change is decided by two comparisons voltages Ura (sine wave reference voltage) and Ut (the triangle wave voltage. Namely so-called carrier frequency voltage) point of intersection and point of intersection time distance. In this pulse sequence, occupies the spatial ratio to defer to the sine rule change, therefore the pulse sequence transient voltage mean value is according to the sine rule change. So-called SPWM is equal but the width different rectangle pulse sequence with the peak-to-peak value to approach the sine exchange signal which needs with equivalent us.
Must want to obtain the good SPWM profile, the sine wave reference signal Ura size and the intelligence signal Ut size existence certain connection, the reference signal frequency and the intelligence signal frequency relations is deciding in for half cycle the SPWM pulse number.
In order to attribute this kind of relations, moudulation percentage M and carrier compared to N definition as follows:
M=Urm/Utm
N=ft/fr
In the formula Urm is the reference signal maximum value.
Utm is the intelligence signal maximum value.
Ft is the intelligence signal frequency.
Fr is the reference signal frequency.
Usually, the M value quite is appropriate between 0.1~0.9.
The N value in theoretically is Viet Nam is greatly better, but actual receives the high efficiency switch component the turn-on frequency limit. Therefore, how the switch component performance does have a more tremendous influence to the frequency changer velocity modulation performance.
When practical application, the frequency changer uses 双极�?the pulse-duration modulation method. The characteristic is the reference signal and the intelligence signal for has is having the negative 双极�?signal.
The frequency changer outputs the alternating current voltage size adjustment is, the output alternating current frequency control which realizes by the change reference signal voltage size is, and these two frequencies similar size which realizes by the change reference control wave frequency.
Because under SPWM way, when needs to adjust the frequency carries on the frequency conversion velocity modulation, if only adjusts the reference control signal the frequency, can bring the overtone to increase the influence system normal work the question; When low frequency low speed, in half cyclical pulse number or the carrier compare the speech which does not increase, can bring question and so on torque pulsation.
Above in order to solve the problem, the convertor circuit has selected the different modulation method:
1, synchronous modulation way.
2, asynchronous modulation way.
3, partition synchronous modulation way.
Three ways, respectively have the advantages:
Synchronous modulation way: When modulation, the maintenance carrier is more invariable than N=ft/fr, namely when reference control signal frequency Fr change, synchronized change triangle wave carrier frequency Ft. This way in the frequency changer output voltage each cycle triangle wave number is fixed, therefore produces the SPWM pulse number also is fixed invariable.
The merit is: In in the frequency changer output frequency shift entire scope, may maintain the output wave shape the positive and negative half period to be symmetrical, in the half period the profile the bilateral symmetry, is advantageous to the overtone elimination. And can strictly achieve the frequency changer to output between the three-phase profile to have the difference 120° 电角�?symmetric relation.
The shortcoming is: When frequency changer audio-output, because in a cyclical pulse number (carrier compared to N) too few, 低次the harmonic component quite is big, the electricity opportunity has the torque pulsation and the noise. Frequency lower, the torque pulsation and the rotational speed pulsation are more serious.
Asynchronous modulation way: In order to solve under the synchronous modulation way the low frequency torque pulsation question, should consider low frequency when increases the triangle wave carrier the number, causes the SPWM pulse number increase. Namely the frequency is lower but the SPWM pulse number more modulations way.
This time, uses the fixed invariable triangle wave frequency, when velocity modulation can cause the carrier to compare N the change.
The merit is: When low speed movement, in the frequency changer output voltage each cycle SPWM pulse number corresponding increase, may reduce the electric motor the torque pulsation and the noise, enable the frequency conversion velocity modulation system to have a better low-frequency response.
The shortcoming is: Because triangle wave carrier frequency Ft maintains invariablely, when reference control voltage frequency Fr continuously adjusts, is impossible in the entire frequency conversion scope to guarantee the carrier compared to N is an integer, specially can by 3 contains numbers, thus cannot guarantee the frequency changer output voltage positive and negative half period, in the half period about between as well as the three-phase between symmetric relation, causes the electric motor moves when the harmonic components greatly is the increase.
Partition synchronous modulation way: In order to the comprehensive utilization synchronous modulation and the asynchronous modulation merit, overcomes two insufficiencies, may select the partition synchronous modulation method in the technical processing stratification plane, this way has more practical application. Puts briefly, the partition synchronous modulation way is between Duan Yuduan the asynchronous way, in the section the synchronous mode.
Selects this method the merit is: May eliminate not good affects, and improvement frequency conversion velocity modulation system low frequency movement characteristic which the frequency changer output voltage profile not assymetrical creates.
The shortcoming is: Compares the N cut when the carrier, possibly appears the voltage the sudden change even to vibrate.
Compares the cut when the carrier should pay attention the question is: When cut does not appear the voltage the sudden change; Should keep a lag area in the cut critical point place, avoids the different carrier compares when appearing the vibration.
But in the practical application, has often neglected these questions.
Mainly displays in following several aspects:
1, the user are less to the frequency changer application understanding, all tins of designs department's opinion.
2, the design department's scene practical experience quite are few, to actual operating mode estimate insufficiency.
3, the equipment necessary department only is according to designs the department the design to carry on the assembly, the scene debugging only is restricted in the idling test run.
4, when load test run cannot carry on the related parameter according to the actual operating mode the again hypothesis.
Down to creates the user in view of the question which appears to start without knowing where to begin in the actual use.
Main performance question:
1, when low speed cannot lead the load, the electric motor pulsation revolution, the electric motor and the transformer has the noise.
2, in the velocity modulation process has the pulsation as well as the electric motor and the transformer has the noise.
3, fast goes past the high electric motor and the transformer noise is bigger.
Above the question mainly occurs in has not added the reactor on the equipment, selects and matches the inappropriate equipment regarding the reactor also to have this kind of problem. Some equipment have the improvement after the change carrier frequency, but actually cannot thorough settlement the problem.
The reactor question solution is basic.
About frequency changer reactor choice question
1, fixed alternating current choice
The fixed alternating current is from gives off heat the aspect to design the reactor the long lasting work electric current, simultaneously should consider the enough higher harmonic component. Namely outputs the reactor actually has flowed the electric current is the frequency changer electrical machinery load output current.
2, loss of voltage
The loss of voltage is when refers to 50HZ, when correspondence actual nominal current reactor coil beginnings and ends actual loss of voltage. Usual choice loss of voltage about 4V~8V.
3, inductance quantity choice
The reactor fixed inductance quantity also is an important parameter! If the inductance quantity choice is inappropriate, can directly affect under the nominal current the loss of voltage change, thus causes the breakdown. But the inductance quantity size is decided by the reactor ferrite core cross-sectional area and the coil number of windings and the air gap adjustment.
The output reactor inductance quantity choice is the basis determined in the nominal frequency scope electric cable length that, then chooses under the corresponding inductance quantity request again according to the electric motor actual nominal current the ferrite core cross-sectional area and the wire cross-sectional area, can determine the actual loss of voltage.
4, correspondence nominal current inductance quantity and electric cable length:
Electric cable length fixed output current inductance quantity
300 meter 100A 46 mu H
200A 23 mu H
250A 16 mu H
300A 13 mu H
600 meter 100A 92 mu H
200A 46 mu H
250A 34 mu H
300A 27 mu H
The ideal reactor in the fixed alternating current and below, the inductance quantity should maintain invariablely, along with electric current increasing, but the inductance quantity gradually reduces.
When the nominal current is bigger than 2 times, the inductance quantity reduces to fixed inductance quantity 0.6 time.
When the nominal current is bigger than 2.5 times, the inductance quantity reduces to fixed inductance quantity 0.5 time.
When the nominal current is bigger than 4 times, the inductance quantity reduces to fixed inductance quantity 0.35 time.

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