问题描述
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 (见图. 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.
The electrician returned to the transformer to examine the current waveforms. As he was making measurements, the chattering noise suddenly stopped. The electrician rushed back to the occupied classroom and asked the teacher, “What did you just now turn off?” The teacher pointed to two overhead projectors, which were plugged into the same receptacle.
测量
The electrician connected his current clamp and Fluke 43B to one of the conductors of a split extension cord. 然后, he displayed the waveform of current drawn by one of the overhead projectors. The waveform showed half-wave rectification by the projector (见图. 2). 无花果. 3 is the harmonic spectrum. 对学校使用的其他高射投影仪的测试表明，所有旧设备都产生了相同的效果, 但较新的单位没有.
理论与分析
一些较旧的电器设备使用半波整流器来降低功耗. 一个例子是带有“高/低”开关的早期吹风机. 低速时, 串联二极管允许电路仅在电压周期的一半上汲取电流. 高速时, 开关使二极管短路——以允许电流在整个周期内流动. 这些设备对交流配电系统造成严重破坏, 因为它们在半波配置中产生直流电流. 直流电流会使变压器中的磁通量不平衡，并在半个电流周期内使变压器铁芯进入饱和状态. 进入和脱离饱和的过程中，变压器铁芯会产生奇怪的噪音. 学校变压器负荷过重, 这降低了其对直流电流的耐受性. 使问题更加复杂化, 两台投影仪连接到同一个分支电路，从而使直流电流量加倍.
解
电工考虑将投影仪连接到相反的相位，以便消除直流电流. 但, 这可能会让老师们感到太困惑. 真正的解决方法是仅使用没有半波整流电路的新型高射投影仪.
源:
福禄克公司
PO Box 9090, 埃弗雷特, 美国华盛顿州 98206
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