Measurement of inductance by Anderson bridge

Objective:

To determine the unknown inductance by Anderson Bridge in terms of known capacitance and resistance.

Theory:

Anderson bridge is an important modification of Maxwell method. This bridge is used to measure unknown frequency in terms of known capacitance and resistance.

Anderson method is capable of precise measurements of inductance over a wide range of values from a few micro henrys to several henrys. In this bridge double balance can obtain by fixing the value of capacitance and changing the electrical resistance only.

Figure gives the circuit diagram of Anderson bridge. In this circuit the unknown inductor is connected between the point a and b with electrical resistance s (which is pure resistive). The arms ac,cd and da consist of resistance p,q and r respectively which are purely resistive .

A standard capacitor is connected in series with variable electrical resistance r and this combination is connected in parallel with cd.

A supply is connected b and e.

At balance point, we have the following relations which are:

I1=I3 and I2=I4

Now equating the voltage drops we get,

L=c[RQ)+(R+S) m]H

Circuit diagram:

Advantage of Anderson’s bridge:

  1. It very easy to obtain the balance point in Anderson’s bridge as compare to Maxwell bridge in case of low frequency factor coils.
  2. There is no need variables standard capacitor is required instead of thin a fixed value capacitor is used.
  3. This bridge also gives accurate result for determination of capacitor in terms of inductance.

Disadvantage of Anderton’s bridge:

  1. The equation for inductor in this bridge is more complex as compared to Maxwell’s bridge.
  2. The addition of capacitor junction increases complexity as well as difficulty of shielding the bridge.

Considering above all the advantages and disadvantages, Maxwell’s bridge is preferred over Anderson’s bridge whenever use of variable capacitor is permissible.

Phasor diagram:

Procedure:

  1. Connect the bridge as per the circuit diagram given in the figure.
  2. Connect the bridge oscillator and headphones properly by the probes.
  3. Obtain the output from the bridge oscillator and feed it to the bridge circuit.
  4. Vary the capacitance (c) values with help of variac in the kit.
  5. For each constant value of c, change the resistance by variac.
  6. Now from the sound intensity of headphone obtain the null point.
  7. Minimum sound heard at the headphone indicates null point detected.
  8. Now measure the value of l from the form the formula

L=C×[(R×Q)+(R+S)×M] H

Calculation:

 

Conclusion:

 

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