Khong thø hai m a 1 khong n μ y îc gäi l μ khong

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+ Kho¶ng thø hai m a > 1: kho¶ng n μ y ® îc gäi l μ kho¶ng ®iÒu khiÓn phi tuyÕn. Khi ®iÒu khiÓn trong kho¶ng n μ y th× ®iÖn ¸p ra tû lÖ phi tuyÕn víi ®iÖn ¸p ®iÒu khiÓn. Hai kho¶ng ®iÒu khiÓn trªn cã nh÷ng u v μ nh îc ®iÓm nhÊt ®Þnh. Trong kho¶ng ®iÒu khiÓn tuyÕn tÝnh th× ®iÖn ¸p ra gÇn ®iÖn ¸p h×nh sin h¬n th μ nh phÇn sãng h μ i ® îc läc tèt h¬n nh ng ®æi l¹i tæn hao trong bé nghÞch l u t¨ng do van b¸n dÉn ph¶i chuyÓn m¹ch nhiÒu lÇn trong mét chu k×. V μ khi ®iÒu khiÓn trong kho¶ng tuyÕn tÝnh th× ®iÖn ¸p v μ c«ng suÊt ®Çu ra kh«ng thÓ ®¹t gi¸ trÞ lín. Trong kho¶ng ®iÒu khiÓn phi tuyÕn th× cã thÓ cho ra ë ®Çu ra mét ®iÖn ¸p lín h¬n khi cïng mét gi¸ trÞ ®iÖn ¸p ®Çu v μ o nh chÕ ®é ®iÒu khiÓn tuyÕn tÝnh, nh ng bï l¹i th× trong th μ nh phÇn ®iÖn ¸p ra chøa nhiÒu th μ nh phÇn sãng h μ i do chuyÓn m¹ch ®ùoc thùc hiÖn phÇn lín ë chÕ ®é xung vu«ng. ChÕ ®é ®iÒu khiÓn phi tuyÕn chØ ® îc thùc hiÖn khi yªu cÇu c«ng suÊt ®Çu ra t ¬ng ®èi lín v μ th êng dïng cung cÊp cho ®éng c¬ ®ång bé. Trong bé nghÞch l u ta chän hÖ sè ®iÒu biÕn biªn ®é m a < 1. §iÒu ®ã cã nghÜa l μ ta ®iÒu khiÓn bé nghÞch l u theo ph ¬ng ph¸p tuyÕn tÝnh. §Ó thÊy ® îc qu¸ tr×nh ®iÒu khiÓn tuyÕn tÝnh ta tiÕn h μ nh ph©n tÝch ®iÖn ¸p ra tÇn sè c¬ b¶n v μ ®iÖn ¸p h μ i bËc cao. 2.3. Ph©n tÝch ®iÖn ¸p ®Çu ra khi m a < 1 Trong mét chu k× tÇn sè sãng mang, ®iÖn ¸p sãng ®iÒu biÕn u ®k biÕn ®æi rÊt chËm nªn ta coi nh ®iÖn ¸p ®ã kh«ng ®æi trong mét chu k× tÇn sè sãng mang (vÊn ®Ò n μ y ®· ® îc ®Ò cËp trong phÇn ®¹i c ¬ng vª PWM). TrÞ sè trung b×nh cña ®iÖn ¸p mét pha ®Çu ra cã thÓ tÝnh theo c«ng thøc: U AO = . 2 dk d xm u U U
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§å ¸n tèt nghiÖp ThiÕt kÕ bé nghÞch l u PWM 5kW Ho μ ng Ngäc Tu©n líp TB§1 - K46 §¹i häc B¸ch khoa H μ Néi 58 trong c«ng thøc trªn : + U AO : ®iÖn ¸p cña pha A so víi ®iÓm trung tÝnh (®iÓm trung tÝnh ® îc qui íc l μ ®iÓm gi÷a hai tô läc nguån) + u ®k : ®iÖn ¸p tøc thêi cña ®iÖn ¸p ®iÒu biÕn. + U xm : biªn ®é ®iÖn ¸p ®iÒu biÕn + U d : gi¸ trÞ ®iÖn ¸p mét chiÒu nguån cung cÊp cho m¹ch nghÞch l u. C«ng thøc trªn chØ ® îc sö dông trong tr êng hîp ®iÒu kiÓn trong phÇn tuyÕn tÝnh (m a <1), cßn khi kh«ng tuyÕn tÝnh (m a >1) th× c«ng thøc trªn kh«ng cßn chÝnh x¸c n÷a. §iÖn ¸p ®iÒu biÕn ® îc ® a v μ o m¹ch ®iÒu khiÓn l μ mét ®iÖn ¸p h×nh sin chuÈn cã tÇn sè l μ tÇn sè mong muèn cña tÇn sè c¬ b¶n ®iÖn ¸p ®Çu ra. TÇn sè ®ã cã gi¸ trÞ f 1 = 1 /2 ω π v μ ®iÖn ¸p sãng ®iÒu biÕn l μ : u ®k = U ®km sin 1 ω t ®iÒu kiÖn U ®km < U xm
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  • Spring '15
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