I am working on a 35 kVA circuit breaker test set that was originally a=20 Multi-Amp (now Megger) CB7140. In 1994 I performed a retrofit on it for = a=20 customer in NJ, in which I replaced the manual tap switch with relays, = and I=20 motorized the Powerstat, so that it could be operated under the control = of=20 an MSDOS computer. The test set was then sold and resold to several = others=20 before the present customer, and he asked me to fix it. He said that the =
computer would not work, but when I tested it everything seemed OK. But=20 because of the age of the components and other reliability issues I = agreed=20 to retrofit the unit with a standard package I had designed for=20
He agreed to have the work done, and I completed the work, but when I = tested=20 it on higher output currents (above 4000 amps or so), it became very = noisy,=20 although the output waveform and all functions seemed OK. The = transformers=20 also draw more current than seems reasonable under no load conditions = (about=20
6 amps at 480 VAC for a 35 kVA transformer), and there is a fairly loud = hum.I think the noise is coming from the output transformer, which has a = very=20 unusual construction. The secondary consists of three turns of 4" x 3/8" =
copper bus, wound in a spiral pattern, and butt welded, and the = primaries=20 are sandwiched between the bus. My guess is that the secondary is not=20 properly braced and it is vibrating because of the magnetic forces at = high=20 currents. I regularly work with and design high current transformers = that=20 are relatively quiet at 12,000 amps and not this loud even at 40,000 = amps.
Fortunately there is another similar test set at the shop that is being=20 retrofitted and I can compare its noise level to this one. Then I may=20 determine if it is a design flaw or deterioration of laminations or=20 something else. Then I may be able to add bracing to the transformer or=20 otherwise reduce the noise to acceptable levels. Here are some videos = that=20 demonstrate the noise and show the construction of the transformer, as = well=20 as other test sets.
I'll know more tomorrow, but some ideas from magnetics gurus would be=20 appreciated. Not my strong point.
Thanks,
Paul