SCHERING BRIDGE

 

SCHERING BRIDGE

Introduction

A Schering Bridge is an AC bridge used to measure:

• Unknown capacitance
• Dielectric loss of a capacitor
• Dissipation factor (tan δ)
• Quality of insulating materials

It is especially useful for measuring high-voltage capacitors and insulation capacitance.The bridge works on the principle of null detection, similar to other AC bridges

Basic Principle

The Schering Bridge consists of four arms:

1. Unknown capacitor CxC_x with its resistance RxR_x
2. Standard capacitor CsC_s
3. Non-inductive resistor R3R_3
4. Variable resistor R4R_4 and capacitor C4C_4

An AC supply is connected across one diagonal of the bridge, and a null detector is connected across the other diagonal.

At balance:

Current through the detector is zero.

Therefore, the impedance ratio of the two arms is equal

Schering Bridge Circuit

Where:

• CxC_x = unknown capacitance
• RxR_x = equivalent series loss resistance of unknown capacitor
• CsC_s = standard capacitor
• R3R_3 = non-inductive resistor
• R4R_4 = variable non-inductive resistor
• C4C_4 = variable capacitor

Why is RxR_x present?

An ideal capacitor has zero resistance.

But a practical capacitor has some dielectric loss. Therefore, it can be represented by:

Zx=Rx−jωCx\boxed{Z_x=R_x-\frac{j}{\omega C_x}}

where:

• RxR_x = dielectric loss resistance
• CxC_x = unknown capacitance
• ω=2πf\omega=2\pi f

The Schering bridge allows us to determine both CxC_x and its dielectric loss.

Balance Equation

At bridge balance,

Z1Z2=Z3Z4\frac{Z_1}{Z_2}=\frac{Z_3}{Z_4}

For the Schering bridge, the balance equations are:

Cx=CsR4R3\boxed{C_x=C_s\frac{R_4}{R_3}}

and

Rx=R3C4Cs\boxed{R_x=R_3\frac{C_4}{C_s}}

These are the important equations used for measurement.

Measurement of Unknown Capacitance

The unknown capacitance is calculated using:

Cx=CsR4R3\boxed{C_x=C_s\frac{R_4}{R_3}}

Example

Suppose:

Cs=0.1 μFC_s=0.1\,\mu F R4=500 ΩR_4=500\,\Omega R3=1000 ΩR_3=1000\,\Omega

Then,

Cx=0.1×5001000C_x=0.1\times\frac{500}{1000} Cx=0.05 μF\boxed{C_x=0.05\,\mu F}

Therefore, the unknown capacitance is 0.05 μF.

Measurement of Dielectric Loss

The loss resistance is:

Rx=R3C4Cs\boxed{R_x=R_3\frac{C_4}{C_s}}

The dielectric loss is related to this resistance.

The dissipation factor is:

tan⁡δ=ωCxRx\boxed{\tan\delta=\omega C_xR_x}

where:

ω=2πf\omega=2\pi f

Therefore,

tan⁡δ=2πfCxRx\boxed{\tan\delta=2\pi f C_xR_x}

A smaller value of tan⁡δ\tan\delta indicates a better dielectric/insulating material.

Working Procedure

Step 1 – Connect the circuit

Connect the Schering bridge according to the circuit diagram.

Step 2 – Connect the unknown capacitor

Connect the capacitor whose capacitance is to be measured in the unknown arm.

Step 3 – Apply AC supply

Apply a suitable AC voltage to the bridge.

Step 4 – Adjust the controls

Adjust:

• R4R_4
• C4C_4

until the null detector shows zero deflection.

Step 5 – Note the readings

Record:

• R3R_3
• R4R_4
• CsC_s
• C4C_4
• Supply frequency

Step 6 – Calculate capacitance

Use:

Cx=CsR4R3C_x=C_s\frac{R_4}{R_3}

Step 7 – Calculate loss resistance

Rx=R3C4CsR_x=R_3\frac{C_4}{C_s}

Step 8 – Calculate dissipation factor

tan⁡δ=2πfCxRx\tan\delta=2\pi f C_xR_x

 Advantages

• Simple and accurate measurement of capacitance.
• Measures dielectric loss.
• Suitable for high-voltage capacitors.
• Can be used to test insulating materials.
• Balance equation is independent of the supply voltage.
• Gives information about the quality of the dielectric.

Applications

Schering Bridge is commonly used for:

• Measurement of capacitor capacitance
• Testing high-voltage capacitors
• Testing cable insulation
• Measuring dielectric loss
• Testing transformer insulation
• Testing insulating materials