【正文】 = 1500=552A 式（55）().它比實際流過晶閘管的要大。A 的變壓器即可。對晶閘管 VT1 來講，應(yīng)先選滯后 Uu180176。依次類推，可以卻確定六個器件相應(yīng)的觸發(fā)單元電路的雙脈沖環(huán)節(jié)間的接線。的脈沖的電路。在一個正弦波周期內(nèi)，V2 包括截止與導(dǎo)通兩個狀態(tài)，對應(yīng)鋸齒波波形恰好是一個周期，與主回路電源頻率完全一致，達(dá)到同步的目的。這時，如 uco 為正值，M 點(diǎn)就向前移，控制?角 90176。當(dāng) uco 為正值時，b 4 點(diǎn)的波形由 uh+u’p+u’co 確定。電路由 VV V 3 和 C2 等元件組成，其中 VV S、R P2 和 R3為一恒流源電路。此外，電路中還有強(qiáng)觸發(fā)和雙窄脈沖形成環(huán)節(jié)。設(shè)計要求為恒轉(zhuǎn)矩調(diào)速，且軋鋼機(jī)他勵直流電動機(jī)具有優(yōu)良的調(diào)速性能，能在很寬的范圍內(nèi)實現(xiàn)平滑的無級調(diào)速，所以我們選擇調(diào)節(jié)電樞供電電壓的方法來調(diào)速。抑制過電壓的方法不外乎有三種：用非線性元件限制過電壓的幅度；用電阻消除產(chǎn)生過電壓的能量；用儲能元件吸收產(chǎn)生過電壓的能量。這時，啟動電流成方波形，而轉(zhuǎn)速是線性增長的。第 1 章 緒論 任務(wù)背景概述 自 70 年代以來，國內(nèi)外在電氣傳動領(lǐng)域內(nèi)，大量地采用了“晶閘管直流電動機(jī)調(diào)速”技術(shù)(簡稱 KZ—D 調(diào)速系統(tǒng))。這是在最大電流（轉(zhuǎn)矩）首相的條件下調(diào)速系統(tǒng)所能得到的最快的起動過程。本設(shè)計用的方法是用 RC 抑制過電壓。當(dāng)保持他勵直流電動機(jī)的磁通為額定值，電樞回路不串電阻，若將電源電壓降低為 UU U 3 等不同數(shù)時，則可得到與固有機(jī)械特性互相平行的人為機(jī)械特性，如圖 31 所示圖 31 降低電源電壓調(diào)速時的機(jī)械特性 調(diào)速系統(tǒng)設(shè)計的整體思想 +15VUCGT~VUdLM并 勵圖 32 基本設(shè)計思想原理圖降低電源電壓調(diào)速需要獨(dú)立可調(diào)的直流電源，可采用單獨(dú)的并勵直流發(fā)電機(jī)或晶閘管可控整流器，而本設(shè)計采用的是后者。圖 35 同步信號為鋸齒波的觸發(fā)電路 脈沖形成與放大環(huán)節(jié) 如圖 36 所示，脈沖形成環(huán)節(jié)是由晶體管 VV 5 組成；放大環(huán)節(jié)由 VV 8 組成。當(dāng) V2 截止時，恒流源電流 I1c 對電容 C2 充電，所以 C2 兩端電壓 為cu1cuidt??因為 i=I 1c所以 式 (34)1cCItuc 按線性增長，即 V3 的基極電位 ub3 按線性增長。由于 V4 的存在，上述電壓波形與實際波形有點(diǎn)出入，當(dāng)電壓等于 后，V 4 導(dǎo)通。晶閘管電路處于整流工作狀態(tài)；如 uco 為負(fù)值，則 M 點(diǎn)向后移，?90176?？梢钥闯觯琎 點(diǎn)電位從同步電壓負(fù)半周上升段開始時刻到達(dá) 的時間越長，V 2 截止時間就越長，鋸齒波就越寬。圖 33 中，V V 6 兩個晶體管構(gòu)成一個“或”門。 觸發(fā)電路的定相在三相晶閘管變流裝置中，選擇觸發(fā)電路的同步信號是個很重要的問題。的Usu 作為vU?sw?s同步電壓，按此原則，再選其余晶閘管的同步電壓，見表 31。變壓器的型號可選 S9200/10（）。再考慮到安全裕量，取 2 倍為 1104A，可選通態(tài)平均電流參數(shù)為 1000A 的晶閘管。fidnKI?? 則 39。為使系統(tǒng)的穩(wěn)態(tài)性能更好，該系統(tǒng)采用無靜差調(diào)節(jié)，即轉(zhuǎn)速調(diào)節(jié)器采用比例積分調(diào)節(jié)器（PI 調(diào)節(jié)器） ，使系統(tǒng)保證恒速運(yùn)行，以保證滿足更嚴(yán)格的生產(chǎn)要求。 that is, the area above the axis will equal the area below.3. Then switch to DC (to permit both the dc and the ac ponents of the waveform to enter the oscilloscope), and note the shift in the chosen level of part 2, as shown in Fig. (b). Equation() can then be used to determine the dc or average value of the waveform. For the waveform of Fig. (b), the average value is aboutThe procedure outlined above can be applied to any alternating waveform such as the one in Fig. . In some cases the average value may require moving the starting position of the waveform under the AC option to a different region of the screen or choosing a higher voltage scale. DMMs can read the average or dc level of any waveform by simply choosing the appropriate scale. EFFECTIVE (rms) VALUESThis section will begin to relate dc and ac quantities with respect to the power delivered to a load. It will help us determine the amplitude of a sinusoidal ac current required to deliver the same power as a particular dc current. The question frequently arises, How is it possible for a sinusoidal ac quantity to deliver a power if, over a full cycle, the current in any one direction is zero (average value
0)? It would almost appear that the power delivered during the positive portion of the sinusoidal waveform is withdrawn during the negative portion, and since the two are equal in magnitude, the power delivered is zero. However, understand that irrespective of direction, current of any magnitude through a resistor will deliver power to that resistor. In other words, during the positive or negative portions of a sinusoidal ac current, power is being delivered at eachinstant of time to the resistor. The power delivered at each instant will, of course, vary with the magnitude of the sinusoidal ac current, but there will be a flow during either the positive or the negative pulses with a flow over the full cycle. The power flow will equal twice that delivered by either the positive or the negative regions of sinusoidal quantity. A fixed relationship between ac and dc voltages and currents can be derived from the experimental setup shown in Fig. . A resistor in a water bath is connected by switches to a dc and an ac supply. If switch 1 is closed, a dc current I, determined by the resistance R and battery voltage E, will be established through the resistor R. The temperature reached by the water is determined by the dc power dissipated in the form of heat by the resistor.If switch 2 is closed and switch 1 left open, the ac current through the resistor will have a peak value of Im. The temperature reached by the water is now determined by the ac power dissipated in the form of heat by the resistor. The ac input is varied until the temperature is the same as that reached with the dc input. When this is acplished, the average electrical power delivered to the resistor R by the ac source is the same as that delivered by the dc source. The power delivered by the ac supply at any instant of time isThe average power delivered by the ac source is just the first term, since the average value of a cosine wave is zero even though the wave may have twice the frequency of the original input current waveform. Equating the average power delivered by the ac generator to that delivered by the dc source,which, in words, states thatthe equivalent dc value of a sinusoidal current or voltage is 1/2
or of its maximum value.The equivalent dc value is called the effective value of the sinusoidal quantity.In summary,As a simple numerical example, it would require an ac current with a peak value of 2
(10)
A to deliver the same power to the resistor in Fig. as a dc current of 10 A. The effective value of any quantity plotted as a function of time can be found by using the following equation derived from the experiment just described: which, in words, states that to find the effective value, the function i(t) must first be squared. After i(t) is squared, the area under the curve isfound by integration. It is then divided by T, the length of the cycle or the period of the waveform, to obtain the average or mean value of thesquared waveform. The final step is to take the square root of the meanvalue. This procedure gives us another designation for the effectivevalue, the rootmeansquare (rms) value. In