وصف المشكلة
This case history involves an electrician who works for a large suburban school district. One winter morning, the electrician received a call from a local school. The caller said a transformer supplying power to three portable classrooms was making a “chattering” noise, as if something were loose inside (انظر الشكل. 1).
Load measurements showed the transformer was heavily loaded, but not overloaded. The loads included lights, electric heat, and a few computers. Most of the load was electric heat, due to cold winter weather.
Because the electrician had arrived near lunchtime and the school had a staggered lunch
schedule, two of the classrooms were unoccupied. A third classroom was occupied and the
teacher was making a presentation with overhead slides.
عاد الكهربائي إلى المحول لفحص أشكال الموجات الحالية. كما كان يقوم بالقياسات, توقف ضجيج الثرثرة فجأة. أسرع الكهربائي إلى الفصل المحتل وسأل المعلم, "ماذا قمت بإيقاف تشغيله الآن?"أشار المعلم إلى جهازي عرض علويين, والتي تم توصيلها بنفس الوعاء.
قياسات
قام الكهربائي بتوصيل المشبك الحالي وFluke 43B بأحد موصلات سلك التمديد المنفصل. ثم, قام بعرض الشكل الموجي للتيار الذي يرسمه أحد أجهزة العرض العلوية. أظهر الشكل الموجي تصحيح نصف الموجة بواسطة جهاز العرض (انظر الشكل. 2). تين. 3 هو الطيف التوافقي. Testing of other overhead projectors used at the school showed that all of the older units produced the same effect, but the newer units did not.
Theory and Analysis
Some older electrical devices use half wave rectifiers to reduce power consumption. An example would be an early hair dryer with a “high/low” switch. At low speed, a series diode allows the circuit to draw current on only half of the voltage cycle. At high speed, the switch shorts out the diode — to allow current to flow during the full cycle. These devices wreak havoc on ac power distribution systems, because they generate dc current in the half wave configuration. The dc current will unbalance the magnetic flux in the transformer and push the transformer core into saturation on one half of the current cycle. The process of going in and out of saturation will produce strange noises from the transformer core. The school’s transformer was heavily loaded, which reduced its tolerance for dc current. Compounding the problem, two projectors were connected to the same branch circuit — thereby doubling the amount of dc current.
حل
The electrician considered connecting the projectors on opposite phases so that the dc currents would cancel. لكن, this would likely prove to be too confusing for the teachers. The real cure was to use only the new overhead projectors that did not have the half wave rectifier circuits.
مصدر:
شركة فلوك
صندوق البريد 9090, ايفرت, WA الولايات المتحدة الأمريكية 98206
لمزيد من المعلومات اتصل:
في U.S.A. (800) 443-5853
في أوروبا / M-الأوسط / أفريقيا (31 40) 2 675 200
في كندا (800)-36-FLUKE
من بلدان أخرى +1 (425) 446-5500 أو
الوصول إلى شبكة الإنترنت: HTTP://www.fluke.com/