PROBLEM WITH TRANSFORMER NOISE

[fgillitzer]

An electrical contractor install a 650kva 480v delta to 208/120v wye transformer feeding and existing distribution system about 8 yrs ago at a school. The distribution system feeds the auditorium, which consist of dimming system for the stage lighting which is approx 30yrs old. When the dimming system is below the max, any amplifiers use on a typical outlet, or the main sound system for the auditorium, the 60 cycle noise from the transformer is picked up in the sound system. The transformer is noisy. How can I elminate the noise. HELP! Frank

 

[Skogsgurra]

This is a classical problem. You probably understand that it isn't the audible noise as such that is being picked up by the amplifiers but the electrical pollution that the dimmers cause.

Those dimmers work with delayed firin of triacs or thyristors and at around 50 % output, the waveforms are at their peak steepness and are intense interference generators.

So, there is nothing wrong with the transformer. The noise you hear from it is caused by the dimmers. It is sometimes possible to reduce the electrical noise by putting reactors in series with the dimmers. Use ordinary commutating reactors and, if that doesn't help 100 %, add PFC capacitirs (2 uF or so) from line to neutral. Make sure that the return from lamps go to neutral and not to GND (if they are separated, which I hope they are).

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[WoodrowJWeen]

And if that doesn't work you may want to put the audio amplifiers on a separate step-down transformer from the dimmers, possibly a shielded transformer. Many A/V designers request this setup on commercial projects these days.