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principle and commissioning of MF power

Principle and commissioning of MF power supply

The whole control circuit is divided into several parts: rectifier section, inverter section, DC reactor and control panel. Our company’s power source control board is a single integrated control board whose features are as follows:
It adopts digital trigger and it is characterized by its high reliability, high precision, easy adjustment and few relay components.
 Input line in main loop can be arranged without ordering the phase sequence. (Phase sequence can be adjusted automatically)
 Advanced swept reset technology is a successful reset that can be fully achieved without choosing reset voltage and frequency.
 This power source adopts advanced controlling technology that has the perfect protection.
 It auto-follows the variation of load, during the running, its feature is the skill of auto-resetting non-exceptionally and the skill of auto-adjustment of power.
 It has the perfect current & voltage cutting, the accurate cut time or inverting angle control, which assure the equipment to perform unfailingly.
 It also has the perfect multilevel-protecting system (including hydraulic pressure, lack of phase and voltage, over current and voltage, cut time, chain operation, etc) and the higher efficiency of frequency conversion, which is bigger than 94% when 1000H or below.
 It is suitable for controlling medium-frequency power source that is between 100KW and 3000KW or between 200HZ and 8000HZ.
ich The whole control electric circuit is produced as a print circuit board except inverting end-level trigger circuit board. And it can be classified as rectifying trigger section, adjustor section, inverter section, reset mathematical calculations section. Details can be found in “ Chart of the Principle of Control Electric Circuit”.
1.1 The performing principle of rectifying trigger.
The electric circuits in this section include three-phase synchronous digital trigger, and end-level driving electric circuits, etc. The trigger section adopts digital trigger with the feature of high reliability, high precision, easy adjustment, etc. The feature of digital trigger is to achieve trans-phase by counting (clock pulse). Its clock pulse oscillator is a kind of voltage controlling oscillator, the trans-phase controlling voltage Uk controls the output pulse frequency. When Uk falls, the oscillatory frequency increases, while the data of counter is permanent (256), if the pulse frequency of counter is high, it means that it will take shorter time to count a certain time of pulse, namely the extended time is short. Angle α is small, whereas angle α is big. The moment when counter begins to count is also controlled by synchronous signal and starts to count when α=0. Now supposed at a certain value of Uk, we can get that the oscillatory frequency is 25KHZ according to the relation between the voltage of the v-f oscillator and frequency, then the time needed to count 256 pulse is (1/25000)×256 = 10.2 (ms) which is equal to 180° electric angle, the reset pulse of this trigger is at 30° of the synchronous voltage( line voltage), which equals to the three-phase full controlling bridge rectifying circuit, the place where β = 30°. From the reset pulse, the output trigger pulse was produced during the extended time, 10.2ms, which also closes to the place of a certain phase α=150° of SCR in three phases bridge rectifier circuit. If you need a accurate trigger pulse ofα=150°,you can adjust the pot w4 tinily. Obviously, there are three set of same trigger circuit, the v-f oscillator and Uk controlled voltage are shared, in this way, during one period, six trigger pulses wh discrepancy of phase is 60° were produced.
The advantage of digital trigger is its steady performance; especially you use the digital IC of HTL and CMOS that have the feature of strong anti-jamming power.
The constitutive voltage of IC16A and its surrounding circuit—frequency switch, which period of output single linear changes with the variation of output voltage Uk of adjustor,W4 pot is the adjustment of the lowest output frequency that is equal to the adjustment of zigzag amplitude of analog circuit.
Three-phrase synchronous signal is directly got through three phases input wire of main loop by gate down-lead K4, K6, K2, then its wave is filtered trans-phased by R23, C1, R63, C40, R102, C63. After that, it is potential insulated through six photoelectric couplers then get six rectangular synchronous signals, the phase of each of them has a gap of 60° and the space occupied rate is a little less than 50%.
IC3, IC8 and IC12 (4536 counter) form a three-route digital delay circuit. After three-phase synchronous signal repositioned the counter, the output pulse of voltage-frequency transformer outputs a delay pulse when counted 256 pulses. Because the frequency of counting pulse is controlled by Uk, in another word, Uk controls the delay pulse.
After the insulation and differential for the pulse outputted by counter, it changes to narrow pulse, then it is sent to back-stage NE556, which has the function of synchronous frequency divider as well as the function of rated output pulse duration. The narrow pulse output is synthesized into double-narrow pulse by resistance, then magnified by transistor, at last output through driving pulse transformer. Detailed temporal order can be found in attached drawing.


3.5.1 Working principle of adjustor
There are four adjustors in the section of adjustor: medium-frequency voltage adjustor, current adjustor, impedance adjustor and inverting angle adjustor.
The voltage and current adjustor form the regular voltage and current double-close loop system. During the whole period of resetting and running, the current adjustor is always working, but the voltage loop only works in the period of running. Seen from the input, the relation between another impedance adjustor and current adjustor LT2 is parallel. The only difference between them is the negative reactive factor of impedance adjustor is a little bigger than that of current adjustor. In addition, the output of current adjustor controls the output volts D.C. of rectifying bridge, but the output of impedance adjustor controls the scale between medium-frequency voltage and volts D.C., namely inverting power factor angle.
The performing procedure of adjustor circuit can be divided into two statuses: one is the moment when the voltage D.C. doesn’t reach the maximum. Due to the bigger reactive factor of impedance adjustor, the rated of impedance adjustor is smaller than reactive factor. At this moment, the impedance adjustor is working in the status of range fixing which opposites the minimum inverting angle θ. Now we can deem that the impedance adjustor works with no efficiency. The system is actually a typical voltage and current double close loop system. Another is the moment when the voltage D.C. has reached the maximum, the range fixing of current adjustor stops to work, the output of voltage adjustor begins to increase, while the reactive current is immovable. To impedance adjustor, when the reactive current signal is a little less than the rated current, it will quit the range fixing then begin to work. It will adjust the rated value of θ angle of inverting angle adjustor in order to increase the output medium-frequency voltage; the current D.C. also increases along with it and reaches a new balance. At this time, only the voltage adjustor and impedance adjustor work, if laden equivalent resistance continues to increase, the inverting angle θ will increase with it until reaches the maximum inverting θ angle.
The inverting angle adjustor is used for making inverting bridge work steadily in a certain θ angle.
The medium-frequency signal comes from the medium-frequency transformer is output through CON2-1 and CON2-2 and divided into two routes: one route is sent to inverting section; another route is rectified by D7-D10 and divided into three routes: one is sent to voltage adjustor; one is sent to over voltage protection; and the last one is used for auto-join of voltage closed loop.
Voltage PI adjustor is consisted of IC13A which output signal is clamp limited by IC13D. IC13D and IC21C form voltage loop and join into circuit automatically. DIP-3 is used for adjustment of voltage open loop. The loop adopts the PI current adjustor to adjust current automatically that has the precision of above 1%. The current signal got from alternating mutual inductor of main loop is input through CON2-3, CIB2-4, CON2-5 and rectified by diode three-phase rectifying bridge then divided into three routes: one is used as current protecting signal; one is used as the reactive signal of current adjusting signal; and the last one is used as the reactive signal of impedance adjustor. PI current adjustor is consisted of IC17B, then insulated by IC17A and controls the voltage-frequency switch of trigger circuit.
IC17C forms the impedance adjustor that has the parallel relation with current adjustor. It is used to control the lead angle of inverting circuit. Its action can reach constant frequency output indirectly or promote the input power factor of rectifying bridge. DIP-1 can turn off this adjustor.
IC19B forms the inverting angle adjustor, which output is clamp limited by IC19C.
3.5.2 Working principle of inverting section
The inverting trigger section of this circuit adopts swept zero voltage soft reset. Due to the requirement of auto-swept, although inverting circuit adopts the self-driven work style, control signal is also got from load, the main loop doesn’t need additive start circuit and the start procedure of charge electricity and magnetism in advance, the main loop is simplified. But it leads a problem which is the controlling circuit is more complicated.
The procedure of start is generally like this: Before inverting circuit starting, first use a start signal which has a higher frequency than synchronic frequency of trough route to trigger inverting SCR. When circuit finds the direct current of main loop, it will control frequency of start signal to scan from high to low. When frequency of signal reduces to a place closes to the synchronic frequency of trough route, the medium-frequency voltage was built and feedback to the auto-frequency modulation circuit. Once the auto-frequency modulation circuit begins to work, it will stop frequency scanning of start signal and turns to auto-frequency modulation circuit control inverting lead angle, and let equipment go into steady performance.
If the first start is failing, namely auto-frequency modulation circuit hasn’t got the reactive signal of medium-frequency voltage. At this time, start signal will scan to the lowest frequency. Once reset circuit finds start signal go into the lowest frequency, it will reset and promote the start signal to the highest frequency to scan again until the successful start. The period of reset is about 0.5 second. It will take not more than 1 second to finish once start to running in full frequency.
The medium-frequency voltage signal which is input through CON2-1 and CON2-2 is sent to the sixth and seventh foot of IC22 through insulation by transformer, then it is sent to IC23. The signal which is output by the third and fourth foot of IC23 is differentiated and then turned to narrow pulse through IC18B and IC20B and output. And it is used to drive inverting end-level MOS pipe. The frequency-voltage switch consisted of IC20A is used for driving frequency meter. W7 is used for adjusting the reading of frequency meter. IC20A is used for forming over voltage protecting oscillator. When inverting bridge produces over voltage, the oscillator begins to oscillate which makes four SCR of inverting bridge open in order to release the energy of impedance.
IC19D is the failing start detector which output controls start circuit repeatedly. IC19A is the successful start detector which output controls the output limit voltage of medium-frequency voltage adjustor, namely D.C. in main loop.
W6 is used as the pot of the highest frequency of inverting start signal.
3.5.3 Working principle of start circuit
After over current protecting signal has been inverted by IC13B and is sent to over current cut-off trigger consisted of IC5A to close off the trigger pulse or perform inverting, and at the same time, it drives over current indicator light and alarm relay. When over current trigger works, only through reset signal or turning off then turning on again to start again. The limit current can be adjusted through W2 jiggle adjusting pot.
When the A.C. output of three-phase is lacking phase, this control board can protect and indicate the power source. Its principle is to get A, B, C three-phase voltage signal from the cathode of K4, K6, K2 SCR, then are insulated by photoelectric coupler and sent to IC14 and IC18B to detect and distinguish. Once there is the lack of phase, it will no only close off the trigger pulse, but also drive phase lacking indicator light and alarm relay.
In order to make the control circuit perform more reliably, there also set a starting timer and voltage-lacking detecting protect of control circuit. At the moment of start, the work of control circuit is not steady. Set a timer of about 3 second, after timed, it is just allowed to output trigger pulse. The circuit in this section is consisted of C11, R20, etc. If there is a certain reason that causes the voltage of D.C power supply to become too low, it also can make controlling mistake. Set a voltage lacking detecting circuit consisted of DW4, IC9B, when voltage of VCC is lower than 12.5v, it will blank off trigger pulse in order to prevent false trigger.
Auto-repeat starting circuit is consisted of IC9A. DIP-2 switch is used to turn off auto-repeat starting circuit.
IC5B forms voltage cut-off trigger. It works to blank off rectifying bridge trigger pulse or perform inverting and drive over voltage indicator light and alarm relay. In addition, over voltage protecting oscillator IC18A begins to oscillate through Q9. After over voltage trigger works, it just likes over current trigger and it also only through reset signal or turning off and then turning on to start again. Over voltage is adjusted to balance through adjusting jiggle-adjusting pot W1.
Q7 and surrounding circuit form delay-protecting circuit when water pressure is too low. The delay time is about 8 seconds.
Reset switch signal is output through CON2-6 and CON2-7, it resets when closed.

 
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