For procedures, see your electrical laboratory manual.

Title: Measurement of Inductance

By —
1. Using Maxwell's Bridge
2. Measuring transient voltage and time of RL circuit

First Method - Maxwell's Bridge

Apparatus

1. AF Generator.
2. Three decade Resistance Boxes.
3. One decade Capacitance Box.
4. Headphones.
5. Inductor of unknown value.

Aim

To find the value of the unknown inductor Lx and its resistance Rx by balancing the bridge.

Theory




The Maxwell Bridge is a modified version of a Wheatstone bridge which is used to measure the self-inductance of a circuit. A Maxwell bridge uses the null deflection method (also known as the “bridge method”) to calculate an unknown inductance in a circuit. When the calibrated components are a parallel capacitor and resistor, the bridge is known as a Maxwell-Wien bridge.

The working principle is that the positive phase angle of an inductive impedance can be compensated by the negative phase angle of a capacitive impedance when put in the opposite arm and the circuit is at resonance (i.e., no potential difference across the detector and hence no current flowing through it). The unknown inductance then becomes known in terms of this capacitance.

An inductor also called a coil, choke or reactor is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. It is basically a current-carrying conductor in the form of a coil that stores energy in a magnetic field. An inductor typically consists of an insulated wire wound into a coil around a core.

Inductance describes the tendency of an electrical conductor, such as a coil to oppose a change in electric current through it. The change in current induces a reverse electromotive force (voltage). The bigger the coil or the greater the number of turns, the greater the induced field.

Inductive reactance is the name given to the opposition to a charging current flows. It is measured in Ohms. In inductors, voltage leads by 90 degrees.

Inductive reactance is expressed as
Xₗ = 2πfL
Xₗ = Inductive reactance
π = pi
f = The AC frequency of the electrical supply
L = The inductive value of the coil

Second Method - Measuring transient voltage and time of RL circuit

Apparatus

1. AF Signal Generator.
2. Inductor of unknown value.
3. A Resistor of known value.
4. Oscilloscope

Theory

A transient voltage is a spike, dip, or fluctuating voltage that occurs along with the intended signal.

Transient voltage is a temporary voltage imposed on power lines during other than normal operating conditions. It can be caused by switching on or off capacitors, starting a large customer motor, a fault on the power line, or a superimposed lightning stroke. In a well-designed distribution system, there are line devices that will operate automatically that will allow the transient voltage to settle off of the system.

Transient can reach an amplitude of tens of thousands of volts that can damage, degrade, or destroy electrical/electronic equipment within any home, commercial building, industrial, or manufacturing facility. Transient surges are generally measured in microseconds.

The waves can be viewed with an oscilloscope.



An oscilloscope is a type of electronic test instrument that graphically displays varying signal voltages, usually as a two-dimensional plot of one or more signals as a function of time.

An oscillator is an electronic circuit that produces a periodic oscillating electronic signal often a dime wave or a square wave. Oscillators convert direct current (DC) from a power supply to an alternating current (AC) signal.