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18A1. Description. A megger is an ohmmeter-type instrument by means of which the value of a resistance can be measured and directly indicated by the position of a pointer on a scale. The resistance indicated in an ohmmeter-type instrument is independent of the voltage applied for a test. The megger consists of two principal elements: a hand-driven magneto type direct current generator, which supplies the current for making the measurement; and the moving element with pointer, by means of which the value of the resistance under measurement is indicated.

Figures 18-1 and 18-2 illustrate the construction of the moving element and the magnetic circuit and electrical connections in the instrument. The permanent magnets serve for both the ohmmeter and the generator. The armature of the generator is hand-driven. The rotational speed is stepped up through gears and maintained at a constant rate, if a certain cranking speed is exceeded, by means of a clutching mechanism. The type III instrument generates 500 volts and has a scale of 0 to 100 megohms.

18A2. Principle of operation. The instrument system consists primarily of two coils, A and B (Figure 18-1), mounted on the same moving element, with pointer attached, in a permanent magnet field, Coil A is connected in series with a resistance between the negative side of the generator and the line terminal, and is called the current coil. Coil B, in series with another resistance, is connected across the generator terminals, and is called the potential coil.

The moving element is mounted in spring-supported jewel bearings and is free to rotate about its axis, since there are no restraining or controlling springs such as there are in an ammeter or voltmeter. Current is led to the coils by flexible conducting ligaments having the least possible torsion, so that the pointer floats

  over the scale. Hence, when the generator is not being operated, the pointer may stand in any position over the scale.

When current flows in coils A and B, they tend to turn the moving element in opposite directions. The pointer then takes a position over the scale where the two forces are equal.

When the instrument is operated, either with perfect insulation, or with nothing at all connected across the earth and line terminals, no current flows in coil A. The potential coil B alone controls the movement and takes a position opposite the gap in the C-shaped core, and the pointer indicates infinity.

When, however, a resistance is connected across the terminals, a current flows in coil A and the corresponding torque draws the potential coil B away from the infinity position into a field of gradually increasing magnetic strength until a balance is obtained between the forces acting on the respective coils. Hence, by introducing resistances of different known values across the terminals and marking the corresponding position of the pointer in each case, a scale calibrated in resistance can be obtained.

Since changes in voltage affect both coils A and B in the same proportion, the position of the moving element is independent of the voltage. In the event that the instrument is short circuited, the ballast resistance is sufficient to protect the current coil.

The resistance range of meggers is very great. For insulation resistance measurements, their range is in thousands of megohms. They are also designed to measure a resistance of only a few ohms, such as the resistance to ground of tower footings or ground wires. In service, the megger is used for measuring insulation resistance of cables, insulators, and windings of motors and generators.

To prevent the demagnetization of the


Figure 18-1. Megger magnetic circuit and electrical connections.
Figure 18-1. Megger magnetic circuit and electrical connections.
permanent magnets, a megger should never be connected to a circuit in which current is flowing and should not be placed on a pole piece or the bedplate of a motor or generator.

18A3. Maintenance. The megger should be given the same care and consideration as any delicate instrument, as it contains a moving coil with steel pivots turning in jewels and can be injured by rough handling. There is an insulating guard ring around each terminal post which is wired to an internal circuit. This serves to bypass around the moving coil element any leakage current which may pass across the moist or dirty surfaces of the box and which would

  otherwise give an incorrect reading of the circuit under test. The guard ring should be maintained intact.

Care should be taken to keep the terminals and terminal posts clean and the leads from being partly broken, as such conditions would add resistance to the circuit and give incorrect readings.

There are no provisions for oiling any of the bearings in the megger from the outside of the case. The original assembly provides sufficient lubrication for several years of use.

The megger has no external adjustments. It can be checked for accuracy by shorting the


Figure 18-2. Megger moving element.
Figure 18-2. Megger moving element.

terminals, when it should read zero. With terminals open, the pointer should stand at infinity when the handle is turned at the usual speed. Intermediate points of the scale can be checked by measuring a known resistance, such as a voltmeter of high range. Weston model No. 24 voltmeter averages a resistance of about 100 ohms per volt. The voltmeter should indicate about 160 volts at 120 rpm of the handle. A falling off of the voltage generated does not affect the accuracy of the megger, as the results are independent of the test electromotive force. This means that even though the permanent magnets should change, or the speed of the turning vary, the accuracy remains unaffected. However, if the pointer stands at zero or infinity, as stated above, the megger can be considered as being fairly accurate. The pointer may stand anywhere on the scale when the instrument is idle.

Repairs of any kind should be undertaken only by an instrument maker who understands the theory of operation, as the circuit resistances have a certain relationship which must be maintained.

  Figure 18-3. Operating principle of direct current
Figure 18-3. Operating principle of direct current instruments.
18B1. Description. The ammeters and voltmeters supplied as part of the ship's measuring instruments are direct current instruments. Direct current instruments are fundamentally current measuring devices and their indications or calibration depend upon the characteristics of the meter.

Ammeters and voltmeters are alike in construction except for the fact that the coil of the ammeter is wound with fewer turns of coarser wire than the coil of the voltmeter. Thus the coil of the ammeter is of lower resistance than the coil of the voltmeter.

A coil with steel pivots and turning in jewel bearings is mounted in a magnetic field which is produced by permanent magnets. Motion of the coil is restrained by two small flat coiled springs which also serve to conduct the current to the coil. The deflections of the coil are read with a lightweight pointer which is attached to the coil and moves over a graduated scale.

It is the force set up in the moving element

  by the reaction between the permanent magnet field and the field resulting from the current flowing through the moving coil that causes deflection and gives an indication of the current or voltage being measured. An instrument of this kind measures direct currents only.

18B2. Operating principle of direct current instruments. If the moving coil of an ammeter carries a current, a magnetic field results with a north and a south pole at opposite ends of the coil. If the coil carrying the current were placed in a magnetic field, the coil would tend to turn in such a direction that the resulting magnetic field due to both the main field and that of the coil would be at a maximum. Also the north pole of the coil would be attracted toward the south pole of the magnet, and the south pole of the coil would be attracted to the north pole of the magnet.

The moving coil of a direct current instrument is made of several turns of wire carefully insulated and wound upon a rectangular


aluminum frame. This coil is supported at the top and bottom by hardened steel pivots turning in cup-shaped jewels, usually sapphires. This method of supporting the moving coil is almost frictionless. The current is led in and out of the coil by two flat spiral springs, one at the top of the coil and the other at the bottom. These springs also serve as the means of measuring the force exerted by the current through the moving coil and cause the pointer to return to zero when current ceases to flow.

When current flows through the moving coil, it rotates to a position where the force due to the field of the coil is just equal to the returning force of the springs. The top and the bottom springs are coiled in opposite directions so that the effect of change of temperature, which causes a spiral spring to coil or uncoil, does not cause the needle to leave its zero position. A light, delicate, aluminum pointer is attached to the moving element to indicate the deflection of the coil. This is carefully balanced by small counterweights so that the whole moving element holds its zero position very closely, even if the instrument is not level. The pointer moves over a graduated scale, marked in volts or amperes as the case may be. Because of the uniform radial field, the deflection of the moving coil in this type of instrument is practically proportional to the current in the moving coil with the result that the scale of the instrument has substantially uniform graduations.

If the moving coil, which is mounted on jeweled bearings, starts to swing, it continues swinging back and forth for some time, unless it is in some way retarded or damped. One method of damping is to attach an air vane to the coil. This air vane is enclosed so that it swings in a restricted space and damps any swinging movement of the coil. The most satisfactory method is electrical damping. If the coil is wound on an aluminum bobbin, the motion of the bobbin through the magnetic field induces magnetic currents within itself in such a direction as to put an electric load on the moving coil. This opposes the motion of the coil and thus brings the pointer to rest at the value to be read. The pointer of a properly damped instrument moves quickly and comes to rest with only about two or three overswings. Not only

  does proper damping give faster readings, but these slight oscillating swings serve to assure the user of the instrument that there is no frictional lag present.

From the foregoing we have learned that the deflection of a direct current instrument is a measure of the current passing through it. The field of the moving coil tends to rotate the coil to include as much of the flux from the permanent magnet as possible. This motion is opposed by the phosphor-bronze springs.

18B3. Operation of ammeters and voltmeters. An ammeter, or the external ammeter shunt, if there is one, is always placed in series with the line, while voltmeters are placed in shunt across the line. If the ammeter is used with an external shunt, the shunt should have the same serial number as the instrument, and the calibrated leads, considered a part of the instrument and furnished with it, should always be used to connect the instrument to the shunt. Ammeters ranging up to 50 amperes have self-contained shunts, while ammeters for over 50 amperes usually have separate or external shunts. Special care should be taken to see that all contacts are clean, well-made and tight.

CAUTION. An ammeter should never be connected across the line. Such a connection would destroy the instrument.

18B4. Maintenance. Instruments should always be carefully handled and any shock or vibration avoided. In use, they should not be placed in close proximity to any current-carrying conductor or magnetic field. If more than one instrument is used, they should be placed at least 6 inches apart, to avoid mutual magnetic effects. If the pointer does not read zero when the current is off, use the zero adjuster to bring the pointer to zero. By a quick side shift of the instrument, it can readily be determined whether the pointer or moving element is free from unusual friction, and by turning about an axis of rotation, whether it is out of balance.

The instruments require no oiling at any time. The covers should always be kept free from dust and dirt and the screws tightened down to prevent dust from getting inside to the


working parts. The instruments at all times should be carefully handled and kept in a dry, clean locker under the charge of a responsible man. Note that the instruments are sealed when received. When the instruments have been repaired, the seals should be renewed so that tampering with the instrument can be detected.

Repairs and adjustments can readily be made by a competent instrument man, but owing to the fact that the instruments them selves are used as the working or secondary standards on shipboard, there is usually no instrument of similar range available for checking or calibrating them. For this reason, when an

  instrument needs repairing or when there is any doubt as to accuracy, it should be calibrated by a tender that is equipped for this work, and if that is impracticable it should be sent to a navy yard for necessary repairs and calibration. Likewise, after repairs or replacement of any parts, a check must be made with a secondary standard instrument and any adjustments necessary to bring the meter within its guaranteed accuracy should be made It should then be sealed by the expert and returned to the vessel from which it was received. Important secondary standard instruments should be checked as a regular routine at frequent intervals whenever primary standards are available.
18C1. Description. Ammeters and voltmeters that are actuated by a few thousandths of a volt are called millivoltmeters.

Millivoltmeters can be used as ammeters by using a shunt across the coil. This shunt makes it possible for the millivoltmeter to carry and indicate a moderately large current. Only a small

  fraction of the main current flows through the moving coil.

Millivoltmeters can be used for measuring voltage by placing a high resistance in series with the moving coil. A high resistance connected thus in series is generally known as a multiplier.


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