Noisy high current transformer

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

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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.

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(construction of breaker test sets)
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qEc=20 (calibration and testing, noise)
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Jws=20 (4000A DC Test Set, 3 phase)

I'll know more tomorrow, but some ideas from magnetics gurus would be=20 appreciated. Not my strong point.

Thanks,

Paul

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Reply to
P E Schoen
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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.

formatting link
(construction of breaker test sets)
formatting link
(calibration and testing, noise)
formatting link
(4000A DC Test Set, 3 phase)

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

Thanks,

Paul

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Have you tried the old automotive tech's method of using a rod to find the noise source? Put one end of the rod to your ear and the other is the probe end which is moved into contact at various places on the equipment to locate the loudest noise location. In addition to the windings & laminations you mentioned, it could also be any ferrous metal adjacent to the core that is being vibrated by the mag field. Art

Reply to
Artemus

I just finished putting bracing spacers on the output bus, which was = rather=20 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=20 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=20 VAC primary and 8 VAC secondary, so a 64:1 ratio. At 2.84 kA output it = was=20 drawing 70 A, while it should be only about 45. And at 5.19 kA output, = it=20 drew 114 A while it should be only 81. I suspect a shorted primary turn, = but=20 I think I will pull the tranny out so I can inspect it thoroughly and = maybe=20 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=20 having an output of 8V at 4500A. That's way above most welding and = plating=20 transformers. I may see if I can use some big toroidal transformers = which=20 were originally designed to use bus bar secondaries at a similar output=20 current.

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

Thanks,

Paul=20

Reply to
P E Schoen

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.

********** It doesn't have to be metal. Wood, fiberglass, or plastic works just fine too. -Art **********

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 current.

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

Thanks,

Paul

Reply to
Artemus

Furthermore, re-winding is always an option when all else fails.

Reply to
Robert Baer

I contacted the customer and he asked me to get quotes on rewinding. I = found=20 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=20 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=20 as I had suspected. I also checked input current on the transformer with = no=20 load and it is only about 0.25 amps up to tap 5 and maximum current (tap =

8)=20 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=20 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=20 set, and powered it from a Variac. At a primary current of about 46 amps = it=20 starts to buzz as if there is something like a loose piece of steel in = the=20 magnetic field.

Given the 51:1 ratio, it will draw 100 amps for each 5100 amps output. = The=20 vernier powerstat sees about 35 amps (which is its rated continuous=20 capacity) at that output. I had originally computed the ratio by the = rated=20 output voltage (8 and 12 VAC), but it is actually about 12 and 18 VAC). = The=20 internal impedance of the 4500A tap is about 0.8/2000 =3D 400 uOhms.

This means that it may be able to provide 12/0.4 =3D 30 kA into a solid = short,=20 although in practical terms it will probably only be good for 20-25 kA = into=20 a breaker. And for that much current it may draw as much as 400 amps = from=20 the 480 VAC line.

I will need to remove the transformer to examine it thoroughly and = hopefully=20 I=E2=80=99ll just find something that can be fixed easily. I=E2=80=99ll = get back on it=20 Tuesday when I=E2=80=99ll know more.

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

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Paul=20

Reply to
P E Schoen

Paul,

The noise problem appears to be that the cracked epoxy is permitting slight physical movement between the primary and secondary windings. Under short-circuit, repulsion forces between the two windings is sufficient to allow relative movement or flexing the secondary windings during current peaks. The magnetic forces scale as the square of secondary current, so doubling the output current quadruples these forces.

Minimizing noise involves preventing any differential movement between windings or flexing of the secondary winding. Since the secondary winding protrudes significantly beyond the primary windings in your transformer, you are introducing a significant amount of leakage flux. The free (outermost) portions of the secondary are attracted to neighboring turns while the inner portions (that overlap the primary) are repelled by the closer-proximity primary windings. Flexural forces, particularly at the top- and bottom-most turns, may have contributed to the destruction of the original epoxy. And, if you are using an E-I core, winding separation forces may be increasing the air gap in the core magnetic path. If so, this may also contribute to transformer noise and unexpectedly higher primary current under heavy loading.

If you do decide to rewind, you may wish to consider changing the primary windings so that their outer diameter extends to the outermost edges of the secondary winding to reduce secondary flexural forces. The entire primary and secondary assembly needs to be epoxied together. Also, the winding assembly needs to be clamped/braced to place it under heavy compression to handle axial repulsion forces. Interleaving of E-core lamination should be used instead of E-I laminations to better handle axial spreading forces from the braced winding stack.

Since you're dealing with similar short-circuit forces, you may also wish to locate a book on power transformer design to see how these forces are dealt with in large transformers. Some suggested books include "Transformer Engineering Design and Practice" by Kulkarni and Khaparde, "Power Transformers, Principles and Applications" by Winders, or Waters, "The Short-Circuit Strength of Power Transformers".

Bert

--
Bert Hickman
Stoneridge Engineering
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Reply to
Bert Hickman

the=20

I appreciate the ideas. This transformer is used in a test set that = dates=20 back to 1971, and AFAIK it is one of the first commercially available = high=20 current primary injection test sets. By 1975, when I first became = involved=20 with them, the output transformers were redesigned with a more efficient =

configuration, with the high current output winding closely wrapped = around=20 the core, and the additional turn wound next to the higher current loop = for=20 twice the voltage and half the current. EIL's version, the BTS-50, which = had=20 an output rated at 7500 amperes at 7.5 volts. It was fairly quiet up to=20 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=20 transformer with dual windings rated at 10,000 amps at 11.5 volts. EIL = had a=20 similar design called the BTS-1000. Both test sets were rated at 100 kVA = and=20 supposedly could produce 100,000 amps into a short circuit. But the=20 Multi-Amp design was quieter and more efficient, with a lower internal=20 impedance due to its shorter and wider winding. At ETI, we designed a=20 PI-4000 which was based on an old EIL PS-600, which unfortunately was = not as=20 well designed as the Multi-Amp, and it was noisy and had a much higher=20 internal impedance. Finally we emulated the CB8160 and used C-cores = instead=20 of EI and had much better results.

Meanwhile, I had become convinced that a high current transformer could = be=20 ideally constructed with toroidal primary cores, and around 1995 I = designed=20 and built a prototype from four 1.4 kVA toroids and a four-turn = secondary of=20

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=20 efficient, and could produce up to about 12,000 amps into a short.

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

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We tried to design a much larger toroid test set but ran into some = problems=20 and chose to use our tried and true PI4000B as a two-piece unit. This = now=20 ETI's most popular design but we have had some problems with = reliability. It=20 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=20 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=20 it would cost at least $4000 additional and I think this transformer can = be=20 repaired, even though it is not a good design. I think it will be much = more=20 apparent what is causing the noise once I remove it from the enclosure. = It=20 has to be a mechanical problem such as a loose lamination or foreign=20 material that I can't see. Once it's out I may be able to pour epoxy = into=20 the gaps and solidly brace the output bus. It is also possible that the=20 magnetic flux is leaking into nearby steel panels and causing them to=20 vibrate. I'll know more next week.

Stay tunes, and thanks for the information and discussion.

Paul

Reply to
P E Schoen

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