Insulation resistance (IR) should accordingly be measured and recorded at regular intervals, the recording being preferably on a separate log-sheet for each important machine or circuit, so that each fresh reading can be compared with previous values and any downward trend immediately observed. It cannot be too strongly emphasised that, subject of course to reasonable minimum values being maintained, trends are more important than actual values; a single value is of comparatively little significance. The intelligent interpretation of results is therefore of great importance and a general appreciation of the factors involved is essential.
Measuring Insulation Resistance
There are several methods of measuring insulation resistance, but the instrument universally used for ship's installations comprises a combined hand-driven d.c. generator and an ohmmeter giving direct readings in ohms and megohms. The principles of design are shown in Fig. 22.1. The details of con-struction of an Evershed and Vignoles Megger Tester, Series 3, is shown in Fig. 22.2.
In principle this instrument consists of a permanent magnet in the field of which is a voltage and a current coil fixed at an angle to one another with a pointer pivoted at the centre of rotation of the coils. The needle deflection is a function of the ratio of the current in the coils and provides a direct reading of insulation resistance. The hand-driven generator has a permanent magnet and provides the test voltage. For general use a 500-V. set is recommended. The open-circuit voltage (i.e. when the IR is infinity) is generally about 5 per cent above the rated voltage and with zero resistance the voltage will be practically zero. The voltage characteristic curve of this generator rises steeply and reaches its rated value at about the mid point of the scale. When using the miniature types of instrument where approximate readings are sufficient, the question of which terminal to connect to earth is relatively unimportant but for more accurate results using the larger instruments it is important that the terminal marked "earth", which is the positive pole, should be the one connected to earth. It is also important in all cases to turn the handle at a steady constant speed of about 160 rev./min.
Factors Affecting Insulation Resistance
Insulation materials are not perfect and when a voltage is applied a leakage current will flow, although with a good insulator the current will be extremely small. It is the passage of this current which enables the insulation resistance to be measured.
Dust, moisture, oily or saline deposits and other foreign matter deposited on end windings and connections, of machines or on cable terminations will result in low readings, and periodical cleaning is therefore important. Solvents left in the varnishes on coils and windings also affect the results, and as the solvents disperse the insulation resistance may increase with age if the machine is kept clean and dry.
Temperature has a considerable effect as the resistance varies inversely with temperatures. At 75-80° C. (the normal working temperature of a machine in the tropics) the resistance may be one-tenth of the value when cold, at say 20° C. Humidity and conditions which lead to the deposit of condensation on the insulation will also produce low readings.
The type of insulation also affects the results. For example, a machine insulated solely with Class A insulation will generally give lower readings than one insulated with mica, and if it is not vacuum-dried and impregnated it will have a lower IR than one so treated. The current flow through the insulation combined with the leakage current along the surface determines the insulation resistance, and the use of mica between the windings and earth as required by Lloyd's Register limits the current flow and thereby reduces the risk of failure.
A machine which runs continuously will have a better IR than one which runs only intermittently; not only will it dry itself out but the machine which runs intermittently will breathe because of the alternate heating and cooling.
It will be apparent from what has been said that the IR is very much dependent on climatic conditions and the temperature and conditions of the machine or apparatus, e.g. whether it has recently been cleaned or re-varnished. It is therefore important to include a reference to these circumstances in the log. A typical log-sheet is given below.
The date and place are an indication of the time of year and geographical location and therefore give some idea of the climatic conditions. Under the heading of "Weather" such entries as fine and bright, cold, very cold, rainy, misty, dry, humid, etc., are appropriate.
Where the number of items is too large to warrant a separate log-sheet for each an alternative log as indicated below might be used.
Satisfactory IR Values
A formula previously quoted in various Regulations for the IR of motors and generators, based on rated voltage and output has now been discarded as being unrealistic. The formula did not take into account the type of machine, e.g. a d.c. motor, an induction motor, and an a.c. generator all of the same voltage and kVA rating, according to the formula would have equal IR, but in practice it will not be the same as due to their different construction the ratio of insulation thickness and insulation area will be radically different, and the construction and insulation will also differ. Also a slow-speed machine will have a lower IR than a high-speed machine of equal rating.
Lloyd's Register requires that for new installations the IR power and lighting circuits between all insulated poles and earth and, where practicable, between poles should be not less than one megohm. In practice much better results are attainable. At periodical surveys, i.e. after the installation has been in service, the minimum IR acceptable for any circuit is 100,000 ohms.
Whenever possible IR should be measured while machines are hot, i.e. immediately after shut-down. The insulation resistance is then probably at its lowest value and if satisfactory under this condition it will be even better when cold, provided it does not stand idle too long in a humid atmosphere. For machines it is recommended that periodical readings should be taken and the climatic and temperature conditions recorded. Trends in IR values under similar conditions are more important than the actual values.
A test which is applied in the factory to new machines and apparatus is a high voltage (a.c.) between separate windings and between all windings and earth. For machines above 3 h.p. or kVA per 1 ,000 rev./min. and, with certain exceptions, for apparatus generally, the test voltage is 1,000 + twice the rated voltage with a minimum of 2,000 V. But a word of warning is necessary. This type of test should not be applied to machines which have been in service or permanent damage may result. It can however be applied to machines or windings which have been completely renewed and reinsulated. Also, when such a test has been applied it is undesirable to repeat it. The duration of the test is usually one minute, and before applying the test the windings must if necessary be dried out.
If in special cases it should become necessary to repeat the works test after a new machine is installed, the test voltage should be not more than 8 j per cent of that quoted, bat such tests are rarely applied to installations in ships.
Drying-Out Electrical Machines
It frequently becomes necessary to dry out electrical machinery either because it has been exposed to the weather prior to commissioning or has been accidentally immersed. In the latter case if the immersion is in sea water all salt deposits should first be washed out with plain water. In many cases also it may be advisable to dismantle and rebuild the commutator as water de-posited inside the commutator cannot be got rid of by drying processes and even though an acceptable insulation resistance may be obtained subsequent failure may occur.
Machines may be dried out by either external or internal heating. If the former method is adopted the machine should be en-closed or covered up so as to retain the heat but in such a manner that moisture can escape either continuously or by periodically lifting the covers. Tarpaulins should not be used. Electric heaters are most suitable, taking care that the hottest part of the machine does not exceed 90° C. Coke, coal, gas or oil stoves should not be used. Internal heating is preferable if it can be arranged and before it is applied the insulation surfaces should be cleaned and surface moisture removed; for field coils not more than half the normal voltage or field current is suitable and if supplied from the busbars a suitable series resistance will be necessary* For load circuits about half full load current can be used. If the current is passed through brushes it is important to rotate the commutator or rotor at frequent intervals. It is much preferable in the case of d.c. machines to run them slowly at a low voltage and for a.c. rotors to clamp copper bands around the collector rings and attach the supply leads to these. Winding temperatures should be taken periodically and should not exceed 8o° C. Care must be taken not to melt the grease in bearings or to allow grease to run into the machine or along the shaft.
Whichever method is used regular readings or insulation resistance and temperature should be taken and recorded. If the insulation readings are plotted against time it will generally be found that initially there is a steady improvement but after a short while they will steadily drop and may remain low for some time. Eventually a steady recovery should occur until a constant value is reached, but drying-out should be continued, if possible, for several hours more.
Minimum insulation resistance values cannot be stated categorically and it may in some circumstances be necessary to put a machine back into service with a lower value than would normally be acceptable in which case it should be carefully watched until it is found to be operating satisfactorily. For normal ship's voltages a minimum of i-o megohms for machines up to 100 kW or B.H.P. and about 0-75 ohms for larger machines might be considered safe.
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