replace 1000w metal halide ballast and 1000w lamps

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my question is they hum very loudly when energized but when they reach full brightness it stops. is this normal? voltage is 277 and checks out fine.
 

hurk27

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Not un-common for any transformer to hum and a ballast is just that a transformer, but I have learned to stay away from aluminum wired windings in ballast because it seems to have a high failure rate, but any ballast with loose parts and or windings will make noise.

With that said, if you changed this ballast, make sure you didn't leave any ferrous metal parts loose, the most common parts I have found to make noise is the retainer straps that hold igniter's and caps (Does anyone ever tighten these?) the magnetic field will cause these to vibrate, in one I found just a screw left in the bottom of the fixture that would vibrate against the housing when the lamp was in start up mode (time it pulls the most current) so these are some areas to look at, but I have also found some aluminum winding ballast that just hum on their own.
 
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