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News and Events from Academy of Production Technology
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Ask APT
A reader writes:
"I worked on a movie set last week with an arena
set up of theater lights using a ETC Sensor dimmer (96 channel). There were 4 racks
at various location all with their own 200- or 400-amp feeder transformers. We ran
feeder with double neutral. They said it was to carry excess voltage back. Can
you explain this to me? This is not the norm in theaters in my experience."
A: Dimmers like Sensor dimmers and the vast majority of dimmers we work with in theatre and live event production take a sinewave voltage input and switch the current to the load on and off several times every second. When it does this it changes the waveform from a sinewave to what is known as a complex waveform (anything other than a sinewave). In the process, it generates harmonics (whole number multiples of the supply frequency), which causes current to flow in the neutral of the feeder (the conductor with the white Camlock connector or white markings). Under certain conditions the current flowing in the neutral due to harmonics can exceed the phase current, and since the neutral is the only conductor that is not protected by a circuit breaker, it could burn or melt. Therefore, we sometimes run two neutral feeders to handle the extra current.
If you have worked in the theatre before you may or may not noticed whether or not the neutral is oversized. Rather than run two neutrals in a permanent installation like a theatre or performing arts center, often the neutral is sized larger than the phase conductors for the same reason that we run double neutrals in a portable power distribution system feeding a large number of dimmers.
By the same token, the feeder transformers also have to be able to handle more current in the neutral; therefore we sometimes see K-rated feeder transformers or harmonic mitigating transformers (HMTs). Either way, the idea is to make sure that not only the feeder cable is protected from excess current in the neutral but also the feeder transformer as well.
Do you have a question? Email info@APTxl.com.
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Entertainment
Electricity, Power Distribution, and Controls - Sept 1-3, Nashville, TN:
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