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Electrical Tester April 2010 from Megger Print
Published by Megger
April 2010
The industry's recognised information tool
ELECTRICAL
TESTER
In this issue
Who do you rely on for battery backup testing?
Magnetic core technologies
Redundant multimeters?
Who do you rely on for battery backup testing?
Andrew Sagl
Product Marketing Engineer
Think you’ve got battery backup issues? You’ve got a big system that depends on those batteries in case something knocks out the main power?
No doubt you do. Any system that relies on battery backups to keep it going when the main power fails is vitally important.
So what do you rely on to make sure your batteries are always ready at an instant’s notice to kick in and take over?
Con Edison relies on the BITE® 2/2P battery impedance test equipment from Megger. That’s no small task. Con Edison operates one of the world’s largest and most complex - yet most reliable - energy delivery systems, in the world’s most dynamic marketplace; New York City, pulsing with energy. It is the global centre of finance, communications, information technology, and other industries dependent on reliable energy. For more than 100 years, Con Edison has been supplying the energy that powers New York.
To provide back up power to this vast expansive system should something fail, Con Edison relies on their 254 banks of vented batteries.
The batteries provide back-up DC power to Con Edison’s substation DC systems. The DC systems provide a reliable source of power to operate, protect and control transmission and distribution supply systems during normal operation and contingencies that affect off-site AC power. All substations have two DC power systems to provide power for the control and operation of the circuit breakers, circuit switchers, alarms, power transformers, protective relaying systems, supervisory controls, fire protection controls, etc.
Battery maintenance and testing at Con Edison is an ongoing operation. Requirements include a monthly visual inspection, a quarterly BITE2/ BITE2P cell impedance test and specific gravity readings. Con Edison transfers their data results to the PowerDB database software, for easy analysis.
In the past, battery testing was simple voltage testing. If a battery registered enough voltage, it was deemed to pass. Records were paper based and subject to error and loss.
The methods used for recording battery test data coupled with such a large system led to problems with lost data records, battery condition information that was not readily available or known, and problem resolution that was not timely.
The solution was to implement new electronic battery test equipment, include electronic data capture and improved diagnostic capability, as well as integrated data alerts for quick problem resolution.
And the tool chosen to accomplish this was Megger’s BITE 2/2P battery impedance test equipment. To the standard transmitter/receiver setup, Con Edison added the following
accessories: 20 ft. fused current source leads, current probe with a 2 in. opening and a 5 ft. lead, 6 ft. CT extension cable and the PowerDB Software. Currently, Con Edison employs eight BITE 2/2P battery impedance testers with plans to add several more.
A battery impedance test is a method of measuring the condition of a battery. Unlike other internal ohmic measurement methods, this method measures the vector sum of the internal cell resistance and the capacitive reactance of the cell plate.
A cell in poor condition will typically have a high internal impedance. A cell in poor condition can degrade performance of other cells or the entire battery, reducing its life and leading to early failure.
In addition to the BITE 2/2P, Con Edison uses the PowerDB database software. This enables them to store their test results in their database and easily create custom reports. Power DB improves their diagnostic capabilities and decreases response time to real and potential problems, heading off battery bank failures.
The BITE 2 and BITE 2P performs testing while the battery bank is on-line. Impedance testing requires no downtime and does not discharge the battery cell. The BITE 2 and BITE 2P reduce test time to less than 3 seconds for each cell to determine the condition of lead-acid and NiCd cells up to 7000 Ah quickly and easily.
By using the BITE 2 and BITE 2P for their impedance testing, Con Edison has been able to make certain that their battery backups are ready each and every time they are called upon.
Magnetic core technologies
Dr Stan Zurek
Magnetics Technical Specialist
Depending on the size, type of magnetic material, operating frequency and mechanical constraints, magnetic cores are made using many different technologies. Read on to learn about the techniques that are currently dominating in the market.
Bulk cores
Fig. 1. An example of a bulk core
When magnetism was discovered it also became apparent that iron was magnetic. So the first cores of inductors, generators and motors were made of solid or bulk iron (Fig. 1). The core was made either by casting or by machining from a larger piece.
However, it was soon discovered that for larger cores, eddy currents induced in the bulk of the conductive iron substantially reduced efficiency. This forced engineers to make the cores from thin laminations, as will be explained below.
Nevertheless, bulk cores are still widely used today. This is possible because new materials have been invented, like the so-called soft ferrites, which have such high resistivity that the eddy currents are limited to negligible levels, even at frequencies up into the GHz range.
Ferrite cores are made by sintering powder-like raw ferrite material under high pressure at elevated temperatures. In a single operation extremely complex shapes can be created, as for instance the RM-type cores for high frequency electronic transformers. Another class of materials based on iron powder (Somaloy, Kool-mu, Sendust, etc.) is also in use and is formed in a similar way.
Bulk iron cores are also still sometimes used for inexpensive applications, working under DC conditions where the eddy currents can be neglected.
Stacked cores
Fig. 2. Laminated core (dimensions not to scale)
Stacked cores are made up of a stack of laminations, the thickness of laminations depending on operating frequency. For example, at 50/60 Hz the thickness most commonly used is between 0.23–0.5 mm.
The simplest way in which the core can be made is to cut straight pieces of electrical steel sheet and stack them up to form each limb of a magnetic core, as shown in Fig. 2. In larger cores for power transformers the width of subsequent laminations is varied in steps. This “stepped” crosssection can approximate a circle, which allows the amount of copper and insulating material to be minimised. To improve the performance of stacked cores, the neighbouring laminations are often overlapped by a few millimetres at the point of contact. (This is not shown in Fig. 2, however, for reasons of clarity.) The “thickness” of the core is only limited by the number of laminations used in the stack.
In a variation of the stacked core, the laminations can have more complex shape – in fact almost any shape which is possible to stamp out from a sheet of metal. The parts can also be cut by other means like laser, wire erosion or water jet. Laminations with more complex shapes are commonly used in various small transformer core configurations like E-I, E-E, U-T, etc. Alternate layers are usually rotated or reversed in order to distribute the air gap more evenly (Fig. 3).
Fig. 3. Laminated core made of stamped out E-I parts with alternate layers rotated
Almost all magnetic cores in rotating machines (motors and generators) are made with this technique. The shapes can be extremely complex – single lamination can comprise hundreds of “teeth” around inner or other perimeter as shown in Fig. 4.
Fig. 4. A close-up of teeth in rotor and stator of induction motor
Wound cores
Another widely used technology is the wound core (Fig. 5). The total thickness of the core is defined by the width of a very long magnetic strip, from which the core is wound on a shaped mandrel. The thickness of the strip itself is the same as for the laminations used in stacked cores.
Fig. 5. Wound and cut core
This technology is slightly more expensive that the stacking, but it can offer benefits in terms of magnetic performance because the corners can be “rounded” which aids the magnetic flux distribution. Also, the flux always follows the easy magnetisation path of the rolling direction of the strip, whereas in the stacked cores this is achieved by using separate parts for each limb (compare with Fig. 2).
Wound cores are usually stabilised mechanically by impregnating the whole core with an epoxy resin. The cut is made after impregnation, and the mating faces are precisely polished to ensure that the gap is minimised.
Because of the limitations of the technology, only relatively simple shapes of cores can be made: round/toroidal, elliptical/oval and rectangular (Fig. 5). The rectangular cores can, however, be wound “on top” of each other to create five- and three-limb structures similar to that shown in Fig. 3. Larger wound cores are typically cut to aid insertion of the primary and secondary windings. The halves are kept together by a strong metal band and buckle.
Single- and three-phase power transformer cores can also be made from thin amorphous ribbon, which is only 0.02 mm thick. This mechanical flexibility of the amorphous ribbon allows the creation of cores with a distributed gap (Fig. 6). Each lamination begins and ends at different point thus creating a slanted gap. Because of the flexibility of the material one side can be “opened” to allow the windings to be inserted. The core is then closed and mechanically stabilised by an appropriate band or belt.
Bent cores
Cores made out of bent parts are a relatively new technology. There are two distinct variations, differing on how the laminations are bent.
The first technique is similar to the wound cores. Positioning of the laminations and the overall shape of the core is comparable in both cases. The subtle but very important difference is in the corners, which are created by computer-controlled bending of the laminations at two or more distinct lines (Fig. 7). As a result, the total volume of material exposed to the damaging bending (see the dashed areas) is much lower than in the wound cores (compare with Fig. 5). So the bent cores exhibit better magnetic performance than the wound cores. Bent cores can be either impregnated and cut, or made with the “openable” distributed gap (as in Fig. 6), which has recently become the preferred configuration.
Fig. 6. A wound “open” amorphous core
For some rotating machines, a different approach is taken to bending cores. As can be seen from Fig. 4, when conventional laminations are used, they can be very complex and cutting can be very expensive. There is also a lot of waste material.
To speed up and automate the production of rotating machines, some manufacturers have developed an easier and much quicker method for assembling the stator cores. The laminations are cut as straight, “rolled out” pieces, and then bent or “rolled in” to a circular shape (Fig. 8). This can be done for separate sheets as well as whole pre-assembled flat stacks.
In contrast to the bent transformer core described above, in the stators the sheets are bent in the plane rather than out of it. The process damages the magnetic properties of the material to a relatively large extent, but the savings made on the automatic processing justify the use of the method. This technology is currently being used for mass-produced cores for car alternators and washing machine motors.
It is also possible and actually much easier to insert the stator windings into the flat stack, and bend the whole structure into a circle. Then all that’s needed is to weld the joined ends and voilà! – the stator is complete. Ingenious indeed!
Fig. 7. Bent transformer core
Fig. 8. Bent stator core
Redundant multimeters?
Most professional electricians and electrical engineers use an array of test equipment in their daily lives. Testers in common use include insulation testers and multifunction testers, while some specialist applications will involve using relay test equipment, high voltage circuit breaker testers or cable fault equipment. Trouble-shooting faults using this equipment is routine, and most modern test equipment from Megger already includes the common multimeter
functionality of measurement of amps, volts and ohms. Consequently, for many professionals, a separate multimeter is an additional cost that is not needed and you should be wary of investing in redundant technology. It is certainly worth checking your existing test and measurement equipment before buying possibly unnecessary extra tools.
However it is sometimes useful to have a multimeter for those occasions when other test equipment is not available. One of Megger’s predecessor companies - AVO - innovated the first multimeter when Post Office engineer Donald Macadie was dissatisfied with having to carry many separate instruments with him as he went about his daily work. It occurred to him that he could ease his problem by integrating the functions of several instruments into one. Macadie took his idea to the Automatic Coil Winder and Electrical Equipment Company, where it was translated into reality. The first AVO multimeter - so named because it could measure Amps, Volts and Ohms - was put on sale in 1923. This was DC-only instrument, but it is a tribute to the fore-sight of its designers that many of its features remained unaltered right through to the last Model 8 which Megger finally discontinued in 2008.
Now, Megger offers a modern range of clamp meters and CAT IV multimeters, which together with other test tools have almost all the multimeter functions you will need.
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