问题描述
这个案例是一个典型的例子，说明了系统方法解决问题的重要性. 它涉及与多家高科技制造厂合作的承包商.
该案件始于对一位对其电气系统感到失望的大楼经理的现场拜访。. 据经理介绍, 他的大楼里所有电气设备似乎都无法正常工作，而且没有人能够确定原因或提供解决方案. 经理接着说，他的三名电工已经辞职了，他现在遇到了真正的麻烦.
承包商提出了很多问题——试图更详细地了解问题. 这一努力被证明是徒劳的, 因此承包商要求前往建筑物中问题最严重的部分. He was following the axiom, “When in doubt, start at the victim load.”
In a far corner, a large medical machine was going through a critical test procedure. The machine was equipped with a large display screen, a keyboard and a control panel with several cables and hoses leading to other pieces of equipment. The operator display screen showed the test procedure was in progress.
Next to the machine was a workbench set up for circuit board repair. The workbench had a soldering iron, a lighted magnifying lens and a fan. The workbench power strip plugged into the same receptacle as the large medical machine. As the contractor watched, the person at the workbench reached over and turned on the fan. At that moment, the operator display screen momentarily went blank and then came back on with the words “Program Reset” displayed in large letters.
测量
The contractor measured the voltage at the receptacle feeding both loads. His Fluke 87 true-rms DMM measured 115 在. The building manager repeated the measurement with his Fluke 27 average-responding DMM, which displayed 118 在. Why did the true-rms unit read lower?
True-rms instruments will give correct, but lower readings than average-responding instruments on square waves or waveforms that look like square waves or waveforms that look like square waves. The contractor connected his Fluke 43B and displayed the voltage waveform. The display showed that the waveform (simulated in Fig. 1) was severely clipped at the top — making it more like a square wave than a sine wave. 仅测量峰值 135 在, 而不是预期的 162 在.
然后承包商绘制了系统的单线图. 单线显示为测试区域供电的变压器位于建筑物的另一端 - 几乎 500 英尺远 (见图. 2). 该变压器上的大部分负载都是非线性的, 他们在电压峰值时吸收高峰值电流. 高峰值电流和长期的高阻抗相结合，在电路末端（测试区域所在的位置）产生严重的电压削波.
理论与分析
由于医疗机器的内部电路在低压直流电下运行, 内部电源将具有二极管/电容器输入电路，需要一定的最小峰值电压才能正常运行. 医疗机器上的铭牌显示该机器需要的电源电压介于 100 和 135 交流有效值. 设计机器并指定铭牌的工程师假设电源电压为正弦波, 所以最小峰值是 141 峰值电压 (100 x 1.41). 由于峰值输入电压的测量值仅为 135 在, 机器正在以峰值电压运行，该电压已经是 6 V 低于所需的绝对最小值. 当风扇打开时, 风扇电机吸收的浪涌电流进一步降低了电压，导致机器电源失控. 这就是导致机器重置的原因.
解
电压削峰问题 (平顶) 在高科技建筑中很常见. 现在使用的许多建筑物的设计并不是为了处理当今常见的大量计算机和非线性负载. 在这种情况下, 为了减少变压器和负载之间的电压降，需要进行大量的重新接线. 另一种方法是将最敏感的负载移近变压器.
源:
福禄克公司
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