COURSE NO.: EE-211DEPARTMENT OF ELECTRICAL ENGINEERING

EXPERIMENT NO: 4

AIM: Measurement of self inductance using Anderson Bridge.

APPARATUS: Anderson Bridge, Unknown inductor Lx, non inductive resistive arm R, Q and P = 1000 ohms,

Connecting wires, Detector (galvanometer for DC and head phone for AC), audio oscillator. CIRCUIT DIAGRAM:

FIGURE 4.1

THEORY:The Anderson bridge is most common bridge for precise measurement of self inductance of an inductor range from 10mH to 3H. The self inductance is determined in terms of known capacitance and resistance. This method is quite similar to Maxwell L-C bridge except that it is modified by inserting another resistive arm (r) in series with C’ in “P” arm. As shown in figure 4.1, the three arms of bridge are P,Q,R where the other has unknown inductance Lx with a series resistance “S” to fulfill

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COURSE NO.: EE-211DEPARTMENT OF ELECTRICAL ENGINEERING

EXPERIMENT NO: 4

the resistive balance. The unusual connection for detector has series resistance “r” which does not take part in it during the resistive balance.

Figure 4.2

In figure 4.2 an equivalent circuit is shown where P,Q,R,S are substituted for resistor R and their respective currents. The condition of balance is that F and B should have same potentials. The current shown at balance, where the potential difference along ABC and ADC is equal. In view of theses currents it can be written as

……………………………………(1) Where jωL is the reactance of L at source frequency “ω”

For close loop circuit CDFC which has no source of E.M.F it is represented as

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COURSE NO.: EE-211DEPARTMENT OF ELECTRICAL ENGINEERING

EXPERIMENT NO: 4

………………………………………………..(2)

The potential difference across C and F is equal to that between C and B, therefore

………………………………………………………………..(3)

Substituting I3 in equation (1)

Rearranging it with expression (2), eliminate I1 and Ic leaving

Equating the real and imaginary terms,

Now,

……………………………………………………...(4)

Where , and Arms respectively

Now, ………………………………………………….(5)

Now if P = Q = 1 then equation (4) reduce toArm S = R ………………………………………………………………...(6)

where S arm consist S dial resistance + dc resistance of Lx

Equation (5) reduce to

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COURSE NO.: EE-211DEPARTMENT OF ELECTRICAL ENGINEERING

EXPERIMENT NO: 4

PRECAUTIONS:

1. Make the connections neat and tight.2. Adjust the resistance S starting from high order dial.3. Balance the bridge properly.4. Galvanometer should show zero reading when dc balanced.5. Head phone should give minimum sound at ac balance condition.

PROCEDURE:

1. For D.C balance(a) Connect the given audio oscillator with “OSC” marked set of binding posts. The

polarity does not matter.(b) Connect dc galvanometer with “DET” marked binding posts. Again polarity does not

matter.(c) Connect unknown inductance with Lx marked binding post.(d) Select DC mode from panel switch given at audio oscillator.(e) Keep “r” dial value to 500 ohms value, keeping x100 ohm dial at 5, as the “r” does not

take part in dc balance but it is sufficient to protect galvanometer.(f) Switch on audio oscillator. The given galvanometer deflect either side.(g) Now adjust the resistance “S” by mean of given “S” dials to bring the null position.(h) When dc balance is obtained, the 1st arm resistance is

2. For A.C balance(a) Adjust C’ dial to 0 position.(b) After obtaining the dc balance switch off audio oscillator.(c) Connect the given head phone in place of galvanometer with binding post marked DET.(d) Select AC mode from given switch upon audio oscillator.(e) Switch on power at audio oscillator. Keep the amplitude control for maximum. If head

phone sound louder then it may bring to low volume by mean of amplitude control.(f) Now adjust the capacitor dial, starting from the 0, x0.01 μF range. As at any range the

sound from headphone is minimized leave it here.(g) Now adjust “r” dial as usual to minimized the sound.(h) When “r” and “C” are adjusted for minimum sound in headphone adjust “S” arm dial to

further minimize the sound.(i) Now when ac balance is obtained the self inductance of “Lx” is given as

OBSERVATIONS & CALCULATIONS:

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COURSE NO.: EE-211DEPARTMENT OF ELECTRICAL ENGINEERING

EXPERIMENT NO: 4

P = R = Q = 1000 ohms

SR. NO.By D.C balancing By A.C balancing Unknown inductance

LX = C’ R (Q +2r) (henrys)r (ohms) C’ (Μ farads)

1

2

3

4

QUESTIONS FOR VIVA:1. What is the purpose of using Anderson bridge?2. Define inductance.3. What are the precautions to be taken while performing this experiment?4. How dc balancing is done in Anderson bridge?5. How ac balancing is done in Anderson bridge?6. What type of detectors are used in this bridge?7. What is the value of resistance P,Q and R ?8. Give the formula for calculating the self inductance ?9. What is the purpose of using the audio oscillator in Anderson bridge ?10. Which other type of bridge can be used for measuring inductance ?

Prepared by

Reviewed by

Approved by

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