SOLUTIONS

1.AC Bridge 

Figure 1 AC Bridge Principle

AC bridges consist of 4 pieces of AC electrical components,Z1,Z2,Z3,and Z4. They may be resistor, inductor and capacitor. One zero indicator G, it is used to check if the bridge in balance condition ,ie, I0=0. AC voltage is applied between terminals a and b.

 2.The balance condition of AC Bridges  

While one sinusoidal AC voltage is applied between terminals a and b, then the balance condition of AC Bridge as below:

Both conditions above formulas must be met at the same time. i.e.

Z1Z3 amplitude product= the product of Z2 and Z4;

φ1 is the phase angle of Z1, So do φ3, φ2,and φ4. The sum ofφ1 and φ3, must be equal to the sum ofφ2,and φ4. 

3.Commonly used AC Bridges

3.1 Schering Bridge, also called Capacitance Bridge 

Actual capacitor doesn’t pure capacitance, It has dielectric loss.

One actual capacitor could be equal to one pure capacitor and one resistor in serial or in parallel.

3.1.1 In serial for low dielectric loss bridge 

Figure 2. Schering Bridge-Rx in serial 

The balance condition of AC Bridge as below:

Both conditions above formulas must be met at the same time

tgδ=ωCxRx=ωCnRn

3.1.2 In parallel for big dielectric loss bridge

Figure 3. Schering Bridge-R’x in parallel

The balance condition of AC Bridge as below:

3.2 Inductance Bridge 

Inductance Bridge is used to measure unknown inductance. Usually, standard capacitor is used as the criterion to measure unknown inductor.

Actual inductor doesn’t pure inductance, It has resistance.

XL/R.is called Q vale of one inductor. XL=ωL

3.2.1 Maxwell Bridge which is used to measure low Q value inductor

Figure 4. Maxwell Bridge

For Maxwell Bridge, its balance is independent of its AC power frequency.

3.2.2 Hay Bridge which is used to measure big Q value inductor

Figure 5. Hay Bridge

3.3 Wheatstone bridge to measure unknown resistances.

Figure 6. Wheatstone Bridge

To measure unknown resistance, Usually AC bridge is replaced with DC bridge.

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