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The sheets making up the steel cores of transformers, motors and generators must be insulated from each other to restrain the flow of eddy currents. The interlaminar insulation is achieved by various means. For transformer steel a combined silicate glass-phosphate inorganic coating is used. It performs other duties besides insulation.

Many large machines employ a fully organic coating which cannot be exposed to annealing temperatures. Laminations which do have to be annealed can carry a mixed organic/inorganic coating. Small machines may use no formal coating, or only natural oxide or steam-induced blueing.

10.25 Test methods

There is a demand for quantitative information about the degree of interlaminar insu-lation afforded by various coatings. Consequently a wide range of test methods have evolved. Some are included in international standards.

A quite separate problem exists in setting levels of insulation deemed to be ade-quate for particular duties. Because exact models of the translation of interlaminar leakage currents into machine losses are lacking there is a tendency to specify higher and higher levels of insulation so as to ‘be safe’. The provision of coatings represents a considerable expense so that there is a strong motivation to understand the matter as fully as possible.

The desire for a rapid result has often led to ad hoc tests being made, such as applying the probes of a commercial multimeter (ohms range) to a steel sheet surface (Figure 10.37). This is a quite uncontrolled procedure, the probe area and pressure are undetermined and the operating voltage hugely unrepresentative of the service conditions of the steel.

Attempts to formalise testing somewhat have led to the hand-held gripper (Figure 10.38). Here the area of pressure and the force applied are better controlled but the applied emf is likely to be some 1.5 volts – still much too high.

Recognising that steel is used in aggregated stacks, tests have been devised in which stacks of sheets are measured from top to bottom (Figure 10.39) sometimes with copper interleaves, sometimes without. Precautions to avoid layer-to-layer burr contact may or may not be taken and various applied voltages have been used. However present-day thinking and practice make use of three main devices.

Ohm-meter

Sheet

Probes pressed onto sheet by hand Ohm-meter and probes

Figure 10.37 Ad hoc insulation ‘test’

Meter and battery

Handles

Spring

Electrodes

Simple hand-held gripper

Figure 10.38 Gripper test

Applied pressure

Pile of cut sheets

Connections to electrodes

Pile of plates method

Figure 10.39 Interleave technique

10.25.1 The British Standard test

This was featured in the now obsolete BS 601 and is fully described in BS 6404 pt.20 1996. Figure 10.40 outlines the system. Contact probes of 645 mm²area were pressed together with a force of 450 N. A standard AC voltage was used for excitation, the

Testing and measurement 157

V A Applied

pressure

Electrode

Test sheet Electrode (Resilient + mounting)

250 mV

±10 mV

Basic British Standard System BS 6404 insulation tester

Pressure of up to 3.5 N/mm²

Movable electrode of area 645 mm²

Static electrode of area 645 mm²

Resilient base Coating Base steel

Microprocessor Display

250 mV RMS power supply

Figure 10.40 Details of the BSI type test

value varying over the years, 250 mV being the most recent. The current drawn gave an indication of insulation quality. Clearly two insulated surfaces are under test in series, though a subsidiary contact applied to the substrate would give access to one surface only for test.

To get a useful assessment of surface insulation a statistically significant number of readings must be taken, spread over the sheet being examined. There is a severe temptation to make too few tests and fail to comply with the statistical treatment set out in the relevant standard. In response to this, multi-electrode tester systems

Figure 10.41 Pneumatically operated BSI type insulation tester able to interrogate five areas at one time

have been devised by European Electrical Steels so that many readings may be taken rapidly.

Operation and pressurisation of the electrode is done pneumatically. Figure 10.41 shows single and multiple electrode pair testers.

Testing and measurement 159 10.25.2 The Franklin test

In contrast to the British method, the Franklin test, originating in the USA [10.12], used ten button electrodes pressed on to one side of a sheet of steel, each electrode having an area of 64.5 mm²and pressed with a force of 129 N.

Each electrode was fed via a 5 ohm resistor from a 0.5 V source of DC.

Figure 10.42(a) outlines the method. While the Franklin test was widely used 0.5 volts is a very high test voltage and depending on the surface insulation and current drawn, the emf at the electrodes will vary from zero to 0.5 volts. For perfect insulation the aggregated current drawn by the ten electrodes is nil, and for total short-circuit it is 1.0 amp. A twist drill cuts into the substrate and forms the return current path so that only one surface is evaluated at a time.

Obviously one electrode short-circuiting to substrate will swamp the fact that nine others may have perfect insulation. Again stringent use of statistical methods of operation is called for in standards before reporting a result. Human nature tempts the user to make one measurement.

A later development of the Franklin test currently incorporated into IEC standards alters the test conditions so that 250 mV (stabilised) is maintained at each of the ten buttons and the current drawn by each separately examined. Computer control of such a system in which each button is energised and monitored and its result recorded is included in an automated version of this device made and sold by European Electrical Steels (Newport).

Figure 10.42(b) shows one variety of Franklin tester for factory use. Analysis of the source of interlaminar emf shows that this is a constant (stabilised) amount linked to the induction attained in the steel, so that a low stabilised emf is appropriate for incorporation in an insulation test system. For an example of interlaminar emf see Figure 10.43.

Notional cross-section of region generating emf: t× w. From

˜V = 4.44 ˆBnf A

˜V = 4.44 ˆBnf tw for ˆB = 1.5 T, n = 1, f = 50, t = 0.3 mm, w = 1 m,

˜V = 4.44 × 1.5 × 50 × 0.3 × 10⁻³

˜V  100 mV

The size of the worst case electric stress varies with the position and number of other leakages and short-circuits. Papers currently in preparation will address this matter in detail.

10.25.3 The Schmidt tester

It has long been recognised that the real situation in a machine involves two insulated surfaces pressed against each other rather than a single insulated surface in contact

Display

Power supply

A total of 10 contact electrodes

(four shown)

Coating Base steel Electrodes have an area of 64.5 mm² Twist drill for contact

Supply

Drill contact

10 Electrodes (5 shown here)

Steel sheet Low value resistor (eg 0.2Ω) used to sense current drawn

When electrode actuated volts here stabilised at 250 mV during test Control computer excites each electrode in turn and records current drawn for stabilised electrode voltage

Improvements to Franklin test Franklin insulation tester (a)

microprocessor

Figure 10.42 Continued

Testing and measurement 161 (b)

Figure 10.42 a Details of the Franklin test circuitry and electrodes along with twist drill

b An every-day factory model of the Franklin tester

W Short

circuit

t Interlaminar emf

Figure 10.43 It is convenient to consider emf arising from the metal enclosed by the dotted line, assuming a short-circuit on one side

with a test electrode. With this in mind Dr Schmidt devised a test in which two sheets of steel are placed one on top of each other and separated by a paper mask, Figure 10.44, so that any burrs are kept away from each other. Contact is made to each substrate via clamp electrodes. A stabilised AC or DC emf may be applied to the two sheets via the clamp electrodes.

The assembly as described is now interrogated by applying a pair of insulated pressors in a pattern of positions within the mask area. This procedure simulates the circumstances of steel in an actual machine. The applied emf, shape and area of pressors and the treatment of data produced can be varied widely.

Because two coated surfaces appear ‘in series’ the measured insulation figures are a lot higher than for tests in which a single surface is interrogated. This is so, not only due to two coatings being in series, but due to the lower statistical probability of failed areas in two sheets coming into line at the same time.

Shape of mask Size of steel sheet

Pressing force pressing area

Leads one to each sheet Pressing areas

laid out in grid pattern

Paper mask has square hole.

Mask extends beyond edges of sheets to separate burrs.

Outer part of paper masks not shown here for clarity.

V A Schmidt method

Pressure of up to 3.5 N/mm²

Movable electrode of area 645 mm²

Resilient base Clamps

Base steel

Microprocessor Display

100 mV RMS power supply

Coating Base steel Paper mask

Figure 10.44 Arrangement of the Schmidt tester

The principal use of the Schmidt test is as a type test for insulation to get a view of its probable performance, whereas the Franklin-based tests are more used for quality control purposes.

It is possible to conduct surface insulation tests on steel held at temperatures representative of service conditions in a machine. Clearly this test is much more

Testing and measurement 163 complex to perform and tends to be reserved for type testing only. Full details of the tests and their associated statistical management of data may be found in the relevant standards [IEC/BS].