隔離變壓器作用原理

這篇 http://www.middleatlantic.com/pdf/PowerPaper.pdf
第19頁，Isolation Transformers (Separately Derived Systems): Benefits and Wiring Methods

Properly designed and installed shielded isolation transformers can significantly reduce AC line noise that affects AV systems. There are two types of AC line noise: differential-mode and common-mode.



Differential-mode noise on the power line is defined as voltage appearing between the hot and neutral conductors. Isolation transformers do not prevent differential-mode noise from passing between the primary and the secondary. The job of a power transformer is to magnetically couple 60Hz as efficiently as possible, but frequencies up to 1kHz can couple quite well. Depending upon the construction of the transformer, most frequencies above 1kHz are attenuated, more significantly as the frequency increases. Common-mode noise on the power line is defined as the voltage measured equally between hot and safety ground, and neutral and safety ground. This noise can couple into audio/video signal paths through poorly-designed safety ground systems, equipment and cabling. Common-mode noise is capacitively coupled through the transformer while differential-mode noise is magnetically coupled through the transformer.


Common-mode noise arising within a facility can be caused by many devices, including motors, lighting dimmers, etc. It can also be caused by high resistance ground connections. As the length of the circuit from the isolation transformer to the equipment increases, the chance for induced common-mode noise also increases.


When a voltage is provided by a transformer or derived from a generator or double conversion online uninterruptible power supply (UPS), it is termed “separately derived” (NEC Article 250.30). High frequency common-mode noise can capacitively couple between the primary and the secondary windings of a transformer. A separately derived system power isolation transformer eliminates common- mode noise between neutral and safety ground because these are bonded together immediately after the transformer. The use of an electrostatic (Faraday) shield, which reduces the capacitive coupling between the primary and secondary windings, provides additional isolation at high frequencies. The shield is also very effective at suppressing fast rise time voltage spikes.


Most AV installations benefit from a dedicated shielded isolation transformer with a single ground reference point. The transformer will be a buffer between the utility company and facility electrical system and the protected electronics systems such as AV equipment, control electronics, dimmers and data devices.