Maxwell’s Inductance bridge - Phasor diagram- Advantages - Disadvantages

Ram Swaroop March 11, 2017
The Maxwell’s Inductance bridge is used to measure medium inductances or the ones having medium quality factor (1<Q<10).


                               Maxwell bridge circuit diagram 

Maxwell bridge

Here,
R2 ,R3 = pure (non-inductive) resistances
L1 = unknown inductor whose inductance is to be measured
R1 = resistance of unknown inductor L1
L4 = Variable inductor having constant resistance R4

since it is an ac bridge, therefore phase and magnitude both must be balanced.The balancing of the bridge can be carried out in two ways 
  1. by varying L4 and one of the resistances R2  or R3
  2.  by adding an additional variable resistance R5 in series with L4 and then keep R2  or R3 constant and vary  L4 and R5
at balance,
                        Z1 Z3 = Z2 Z4
             (R1+jwL1)R3 = (R4+jwL4)R2

 equating real and imaginary parts
R1 R3 = R2 R4
and  L1 = L4R2 /R3

So, the unknown inductance is measured in terms of known inductance and the two resistors.

If we combine the above two equations we get, (L1/ R1) = (L4/R4) i.e at balance condition the time constants of two inductors are equal

The Maxwell bridge phasor diagram as shown

Maxwell bridge phasor diagram


Advantages Of Maxwell bridge
  1. Maxwell’s bridge measures medium inductances with considerable precision.
  2. The frequency does not appear in the final expression of both equations, hence it is independent of frequency.


Disadvantages Of Maxwell bridge
  1. The bridge is limited to measurement of low quality inductors (1 < Q < 10) and not suitable for low value of Q i.e. (Q < 1) .
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