Module 21 - Test Methods and Practices
Navy Electricity and Electronics Training Series (NEETS)
Chapter 1:  Pages 1-21 through 1-26

MAXWELL BRIDGE

The Maxwell bridge, shown in view C of figure 1-14, measures inductance by comparing it with a capacitance and (effectively) two resistances.] This bridge circuit is employed for measuring inductances having losses greater than 0.05 (expressed by the D dial reading). For such inductors it is necessary to introduce, in place of the series control (D dial), a new loss control (Q dial), which shunts the standard capacitor. This control, which becomes effective when the Function switch is turned to the L(Q) position, is conveniently calibrated in values of Q, the storage factor of the inductor under measurement. The balance for inductance is the same for either bridge circuit. This permits the use of the same markings on the RANGE switch for both the L(D) and L(Q) positions of the Function switch.

REACTANCE MEASURING Equipment

The reactance type of inductance measuring equipment makes use of the following principle: If an ac voltage of fixed frequency is applied across an inductor (and a resistor in series), the voltage drop produced across the reactance of the inductor by the resulting current flow is directly proportional to the value of the inductance. An inductance measurement using the reactance method is identical to capacitance measurements using the same method, except that current flow is directly proportional to the value of inductance, rather than inversely proportional as in the case of capacitance. It follows then that if a reactance-type capacitance measuring equipment is provided with a chart that converts the capacitance readings to equivalent inductance values and a proper range multiplying factor, the same test setup can be used to measure both capacitance and inductance. In practice, test equipment using the reactance method for capacitance measurements usually provides an inductance conversion chart. Because the current flowing through the inductance under test is directly proportional to the value of inductance, the reciprocals of the capacitance range multipliers must be used; for example, a multiplier of 0.1 becomes

and a multiplier of 100 becomes

The reactance-type equipment gives approximate values only. Like the analog multimeter, it is used only when portability and speed are more important than precision. If the ohmic resistance of the inductor is low, the inductance value obtained from the conversion chart can be used directly. If the ohmic value (as measured with an ohmmeter) is appreciable, a more accurate value of inductance can be obtained by use of the following formula:

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Q-16.    Is the current flow through an inductor directly proportional or inversely proportional to its inductance value?

MEASUREMENT of Inductance USING The VTVM

If you do not have a 250DE+1325 at your disposal, the inductance of a coil can be determined by using a VTVM and a decade resistance box, as shown in figure 1-15. In the following example the inductance of an unknown coil in the secondary winding of a 6.3-volt filament transformer will be determined with a VTVM and decade resistance box. The unknown coil must be connected in series with the decade resistance box. The voltage across the decade box and across the coil must be monitored as the decade box is adjusted. When equal voltages are reached, read the resistance of the decade box. Since the

voltage across the inductor equals the voltage across the decade box, the X_L of the coil must be equal to the resistance read on the decade box. For example, assume that the resistance reading on the decade box is 4 kilohms and the frequency is 60 hertz. This must mean that the X_L of the coil is also equal to 4,000 ohms. The inductance formula L = X_L/2πf

Figure 1-15. - Determining inductance with a VTVM and decade resistance box.

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Summary

This chapter has presented information on basic measurements. The information that follows summarizes the important points of this chapter.

The five basic measurements are Voltage, CURRENT, Resistance, Capacitance, and Inductance. The accuracy of all measurements depends upon YOUR SKILL as a technician and the accuracy of your Test Equipment.

Accuracy of different types of test equipment varies greatly and depends on design characteristics, tolerances of individual components, and YOUR KNOWLEDGE of test equipment applications. The METCAL program ensures that your calibrated test equipment meets established specifications. Most equipment technical manuals contain Voltage CHARTS which list correct voltages that should be obtained at various test points.

It is important to remember that the Input Impedance of your test equipment must be high enough to prevent circuit loading.

When you are performing ac voltage measurements, an additional consideration that greatly affects the accuracy of your measurements is the Frequency Limitations of your test equipment.

Ac and dc CURRENT MEASUREMENTS can be performed using a wide variety of test equipment. Most current measurements require you to break the current path by unsoldering components and wires and inserting an ammeter in series with the current path. One alternative method is to compute (using OHM'S LAW) the current through a circuit by measuring the voltage drop across a known resistance. Another alternative is to use a CURRENT PROBE that requires no unsoldering.

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When performing resistance measurements, your primary concerns are the RANGE and DEGREE of ACCURACY of your test equipment. In most instances, an analog multimeter is accurate enough to perform basic troubleshooting. When measuring extremely large resistances, you are sometimes required to use a MEGGER or a Differential VOLTMETER.

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When testing current-sensitive devices, you must be certain that the current produced by your test equipment does not exceed the current limitations of the device being tested.

Capacitance and inductance measurements are seldom required in The Course of troubleshooting. These measurements are usually performed with various types of BRIDGES or with a reactance type of measuring device. The bridge -measuring techniques are more commonly used and are more accurate than reactance types of measurements.

REFERENCES

8000A Digital Multimeter, NAVSEA 0969-LP-279-9010, Naval Sea Systems Command, Washington, D.C., undated.

EIMB, Test Methods and Practices Handbook, NAVSEA 0967-LP-000-0130, Naval Sea Systems Command, Washington, D.C., 1980.

EIBM, General, NAVSEA 0967-000-0100, Naval Sea Systems Command, Washington, D.C., 1983.

Instruction Manual for Universal Impedance Bridge, Model 250DE, 13202, Electro Scientific Industries, 13900 N. W. Science Park Drive, Portland, Oregon 97229, March 1971.

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Instruction Manual, Model 893A/AR AC-DC Differential Voltmeter, NAVSEA 0969-LP-279-7010, Naval Sea Systems Command, Washington, D.C., 1969.

Operation and Maintenance Instruction, Current Tracer 547A, NAVAIR 16-45-3103, Naval Air Systems Command, Washington, D.C., 1979.

Operation and Maintenance Instructions, Volt-Ohm-Milliammeter, 260 Series 6P, NAVSEA 0969-LP-286-1010, Naval Sea Systems Command, Washington, D.C., 1974.

Answers to Questions Q1. Through Q16.

A-1.     Its calibration.

A-2.     10 to 1

A-3.     Increased input impedance, greater accuracy, and increased voltage range.

A-4.     Midscale.

A-5.     Accuracy and high input impedance.

A-6.     The range of frequencies that can accurately be measured.

A-7.     At least 60% of the vertical trace.

A-8.     Decreased internal meter resistance, greater accuracy, and greater current range.

A-9.     Current probes enable you to perform current measurements without disconnecting wires. Current probes are clamped around the insulated wire.

A-10.    By zeroing the meter with the test leads shorted.

A-11.    The current flow through the component is limited to 1 milliamp.

A-12.    True.

A-13.    Bridge type.

A-14.    Magnetic-metal core.

A-15.    a capacitor.

A-16.    Directly proportional.

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