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Noisy high current transformer

Discussion in 'Electronic Design' started by P E Schoen, Jun 29, 2011.

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  1. P E Schoen

    P E Schoen Guest

    I am working on a 35 kVA circuit breaker test set that was originally a
    Multi-Amp (now Megger) CB7140. In 1994 I performed a retrofit on it for a
    customer in NJ, in which I replaced the manual tap switch with relays, and I
    motorized the Powerstat, so that it could be operated under the control of
    an MSDOS computer. The test set was then sold and resold to several others
    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
    because of the age of the components and other reliability issues I agreed
    to retrofit the unit with a standard package I had designed for

    He agreed to have the work done, and I completed the work, but when I tested
    it on higher output currents (above 4000 amps or so), it became very noisy,
    although the output waveform and all functions seemed OK. The transformers
    also draw more current than seems reasonable under no load conditions (about
    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
    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
    are sandwiched between the bus. My guess is that the secondary is not
    properly braced and it is vibrating because of the magnetic forces at high
    currents. I regularly work with and design high current transformers that
    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
    retrofitted and I can compare its noise level to this one. Then I may
    determine if it is a design flaw or deterioration of laminations or
    something else. Then I may be able to add bracing to the transformer or
    otherwise reduce the noise to acceptable levels. Here are some videos that
    demonstrate the noise and show the construction of the transformer, as well
    as other test sets.

    (construction of breaker test sets)

    (calibration and testing, noise)

    (4000A DC Test Set, 3 phase)

    I'll know more tomorrow, but some ideas from magnetics gurus would be
    appreciated. Not my strong point.


  2. P E Schoen

    P E Schoen Guest

    "Artemus" wrote in message
    I just finished putting bracing spacers on the output bus, which was rather
    loose and seems to have been coated with epoxy which had cracked. But it
    still had a lot of noise. It was so loud that I don't think the mechanic's
    trick would have worked, and I don't like the idea of poking a metal rod
    between my ear and a device with 480 VAC exposed in many places.

    So, I checked the input current to the transformer under load. It has a 510
    VAC primary and 8 VAC secondary, so a 64:1 ratio. At 2.84 kA output it was
    drawing 70 A, while it should be only about 45. And at 5.19 kA output, it
    drew 114 A while it should be only 81. I suspect a shorted primary turn, but
    I think I will pull the tranny out so I can inspect it thoroughly and maybe
    see something that could be repaired, or signs of heat where the shorted
    turn may be. But I think the transformer will need to be replaced, and I
    don't think it can be rewound.

    Maybe I can find a similar transformer surplus, but it's rather special
    having an output of 8V at 4500A. That's way above most welding and plating
    transformers. I may see if I can use some big toroidal transformers which
    were originally designed to use bus bar secondaries at a similar output

    Otherwise I probably just have a thousand pound boat anchor...


  3. P E Schoen

    P E Schoen Guest

    "Robert Baer" wrote in message
    I contacted the customer and he asked me to get quotes on rewinding. I found
    a company in PA that can do it, for a ballpark cost of $2000.

    But I did some more testing, and I found that the transformer seems to be
    working normally except for the noise. Here are some test data:

    TAP Vernier Output Amps Input Amps Ratio
    1 20% 1071 20.20 53.0
    1 30% 1451 27.69 52.4
    1 50% 1982 38.63 51.3
    1 60% 2424 47.30 51.2
    1 65% 2600 51.00 51.0
    1 80% 3270 63.90 51.2
    1 100% 4545 88.30 51.5
    2 50% 5680 109.60 51.8
    3 20% 8680 166.00 52.3

    TAP Vernier Input Volts Output Volts Ratio 8V
    1 50% 40.72 0.803 50.7
    1 100% 84.60 1.650 51.3
    2 0% 75.30 1.469 51.3
    2 50% 118.50 2.319 51.1
    2 100% 160.10 3.132 51.1
    3 0% 154.00 3.011 51.1
    3 50% 195.00 3.817 51.1
    3 100% 238.20 4.670 51.0
    4 0% 231.60 4.540 51.0
    4 50% 274.80 5.390 51.0
    4 100% 316.70 6.210 51.0
    5 0% 304.90 5.980 51.0
    5 50% 345.40 6.780 50.9
    5 100% 388.20 7.620 50.9
    6 0% 376.50 7.400 50.9
    6 50% 422.00 8.240 51.2
    6 100% 464.00 9.070 51.2
    7 0% 469.00 9.160 51.2
    7 50% 510.00 9.950 51.3
    7 100% 553.00 10.790 51.3
    8 0% 541.00 10.560 51.2
    8 50% 584.00 11.390 51.3
    8 100% 625.00 12.200 51.2

    There does seem to be a problem, but it does not appear to be a shorted turn
    as I had suspected. I also checked input current on the transformer with no
    load and it is only about 0.25 amps up to tap 5 and maximum current (tap 8)
    is 3.5 amps with 589 VAC into the 510 VAC tap. That is as expected above
    saturation, with a 20% overvoltage.

    I also checked hipot primary to frame and primary to secondary, and it is OK
    up to 2500 VDC. Output bus to frame is OK at 1500 VDC.

    Finally, I isolated the transformer from the other components of the test
    set, and powered it from a Variac. At a primary current of about 46 amps it
    starts to buzz as if there is something like a loose piece of steel in the
    magnetic field.

    Given the 51:1 ratio, it will draw 100 amps for each 5100 amps output. The
    vernier powerstat sees about 35 amps (which is its rated continuous
    capacity) at that output. I had originally computed the ratio by the rated
    output voltage (8 and 12 VAC), but it is actually about 12 and 18 VAC). The
    internal impedance of the 4500A tap is about 0.8/2000 = 400 uOhms.

    This means that it may be able to provide 12/0.4 = 30 kA into a solid short,
    although in practical terms it will probably only be good for 20-25 kA into
    a breaker. And for that much current it may draw as much as 400 amps from
    the 480 VAC line.

    I will need to remove the transformer to examine it thoroughly and hopefully
    I’ll just find something that can be fixed easily. I’ll get back on it
    Tuesday when I’ll know more.

    BTW, here is another video of the transformer and testing:

  4. P E Schoen

    P E Schoen Guest

    "Bert Hickman" wrote in message
    I appreciate the ideas. This transformer is used in a test set that dates
    back to 1971, and AFAIK it is one of the first commercially available high
    current primary injection test sets. By 1975, when I first became involved
    with them, the output transformers were redesigned with a more efficient
    configuration, with the high current output winding closely wrapped around
    the core, and the additional turn wound next to the higher current loop for
    twice the voltage and half the current. EIL's version, the BTS-50, which had
    an output rated at 7500 amperes at 7.5 volts. It was fairly quiet up to
    about 10 kA, and not too bad at 50-75 kA which was its maximum capability.

    In 1981 Multi-Amp introduced the CB8160, which had a 5-stage output
    transformer with dual windings rated at 10,000 amps at 11.5 volts. EIL had a
    similar design called the BTS-1000. Both test sets were rated at 100 kVA and
    supposedly could produce 100,000 amps into a short circuit. But the
    Multi-Amp design was quieter and more efficient, with a lower internal
    impedance due to its shorter and wider winding. At ETI, we designed a
    PI-4000 which was based on an old EIL PS-600, which unfortunately was not as
    well designed as the Multi-Amp, and it was noisy and had a much higher
    internal impedance. Finally we emulated the CB8160 and used C-cores instead
    of EI and had much better results.

    Meanwhile, I had become convinced that a high current transformer could be
    ideally constructed with toroidal primary cores, and around 1995 I designed
    and built a prototype from four 1.4 kVA toroids and a four-turn secondary of
    1" x 1/4" bus, with outputs of 2.8, 5.6, and 11.2 VAC at true continuous
    current of 2000, 1000, and 500 amps. It proved to be very quiet and
    efficient, and could produce up to about 12,000 amps into a short.

    Finally I was able to design and build a toroid-based larger test set, the
    PI-800 and the PI-1600, which has an output of 6 VAC at 1600 amps, and can
    produce up to 20,000 amps quietly and efficiently: 003.jpg 007.jpg

    We tried to design a much larger toroid test set but ran into some problems
    and chose to use our tried and true PI4000B as a two-piece unit. This now
    ETI's most popular design but we have had some problems with reliability. It
    can be seen in one of the videos.

    I'll have a look at some of the books you have recommended. I think I have a
    good understanding of high current transformer design and I would really
    like to build a toroidal replacement for this customer's old test set. But
    it would cost at least $4000 additional and I think this transformer can be
    repaired, even though it is not a good design. I think it will be much more
    apparent what is causing the noise once I remove it from the enclosure. It
    has to be a mechanical problem such as a loose lamination or foreign
    material that I can't see. Once it's out I may be able to pour epoxy into
    the gaps and solidly brace the output bus. It is also possible that the
    magnetic flux is leaking into nearby steel panels and causing them to
    vibrate. I'll know more next week.

    Stay tunes, and thanks for the information and discussion.

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