What is Schering Bridge : Circuit, Working & Its Applications

Schering Bridge is an electrical circuit used for measuring the insulating properties of an electrical cable and equipment. It is an AC bridge circuit developed by Harald Ernst Malmsten Schering (25th November 1880 – 10th April 1959). It has the greatest advantage that the balanced equation is independent of frequency. The originating current bridges are the AC bridges, they are the most popular, convenient and prominent or accurate instruments, used for the measurement of AC resistance, capacitance, and the inductance. The Ac bridges are just like the DC bridges but the difference between the alternating current bridges and the direct current bridges is the power supply.

What is Schering Bridge?

Definition: The Schering bridge is one type of AC bridge, which is used to measure the unknown capacitance, relative permeability, dissipation factor, and dielectric loss of a capacitor. The high voltage in this bridge is obtained by using the step-up transformer. The main objective of this bridge is to find capacitance value. The main apparatus required for connection are trainer kit, decade capacitance box, multimeter, CRO, and patch chords. The formula used to get the capacitance value is the CX=C₂(R₄/R₃).

Basic AC Bridge Circuit

In AC bridges, the power lines are used as a source of excitation at low frequencies, oscillators are used as a source at high-frequency measurements. The frequency range of an oscillator is 40 Hz to 125 Hz. The AC bridges not only measure the resistance, capacitance, and inductance but also measure the power factor, and storage factor and all AC bridges are based on the Wheatstone bridge. The basic circuit diagram of an alternating current bridge is shown in the below figure.

The basic circuit diagram of an AC bridge circuit consists of Z1, Z2, Z3, and Z4 four impedances, a detector and an AC voltage source. The detector is placed between the point ‘b’ and, ‘d’ and this detector is used to balance the bridge. An AC voltage source is placed between the point ‘a’ and ‘c’ and it supplies power to the bridge network. The potential of point ‘b’ is the same as the potential point ‘d’. In terms of amplitude and phase, both the potential points like b & d are equal. In both magnitude and phase, the point ‘a’ to ‘b’ the voltage drop is equal to the voltage drop point a to d.

When the AC bridges used for the measurement at low frequencies then the power line is used as a source of supply and when the measurements are done at the high frequencies then the electronic oscillators are used for the power supply. An electronic oscillator is used as a source of power supply, the frequencies provided by the oscillator is fixed and the output waveforms of an electronic oscillator is sinusoidal in nature. There are three types of detectors used in AC bridges they are headphones, vibrational galvanometers, and tunable amplifier circuits.

There are different frequency ranges and in that, a particular detector will be used. The headphone lower frequency range is 250Hz and the high-frequency range is above up to 3 to 4KHz. The vibrational galvanometer frequency range is from 5Hz to 1000Hz and it is more sensitive below 200Hz. The tunable amplifier circuits frequency range is from 10Hz to 100KHz.

High Voltage Schering Bridge Circuit Diagram

The high voltage Schering bridge circuit diagram is shown in the below figure. The bridge consists of four-arms, in the first arm, there are two unknown capacitances C1 and C2 which we have to find and resistor R1 is connected and in the second arm, the variable capacitance C4 and the resistors R3 and R4 are connected. In the center of the bridge ‘D’ detector is connected.

In the figure, ‘C1’ is the capacitor whose capacitance has to be developed, ‘R1’ is a series resistance representing the loss in the capacitor C1, C2 is s standard capacitor, ‘R3’ is a non-inductive resistance, ‘C4’ is a variable capacitor, and ‘R4’ is a variable non-inductive resistance in parallel with the variable capacitor ‘C4’.

By using the balance condition of the bridge, the ratio of impedance ‘Z1 & Z2’ are equal to the impedance ‘Z3 & Z4’, it is expressed as

Z1/ Z2 = Z3/ Z4

Z1* Z4 = Z3*Z2………………… eq(1)

Where Z_{1 =} R₁ + 1/jwC₁ ; Z_{2 =} 1/jwC₂ ; Z_{3 =} R₃ ; Z_{4 =} (R₄ + 1/jwC₄R₄)/( R₄ – 1/jwC₄R₄)

Now substitute the values of impedances Z1, Z2, Z3, and Z4 in equation 1, will get the values of C1 and R1.

(R₁ + 1/jw C₁) [(R₄ + 1/jwC₄R₄)/( R₄ – 1/jwC₄R₄)] = R₃ (1/jwC₂) ……….. eq(2)

By simplifying the impedance Z4 will get

Z_{4 =} (R₄ + 1/jwC₄R₄)/( R₄ – 1/jwC₄R₄)

Z_{4 =} R₄ /jwC₄R₄…………….eq(3)

Substitute eq (3) in eq (2) will get

(R₁ + 1/jw C₁) (R₄ /jwC₄R₄) = R₃ (1/jwC₂)

(R₁ R₄) + (R₄/jw C₁) = (R₃ /jwC₂)(1+ jwC₄R₄)

By simplifying the above equation will get

(R₁ R₄) + (R₄/jw C₁) = (R₃ /jwC₂) + (R₃*R₄C₄/C₂)…………eq(4)

Compare real parts R1 R4 and R3*R4C4/2 in eq (4) will get unknown resistance R1 value

R1 R4 = R3*R4C4/ C2

R1 = R3*C4/ C2…………eq(5)

Similarly compare imaginary parts R₄/jw C₁ and R₃ /jwC₂ will get unknown capacitance C₁ value

R₄/jw C₁ = R₃ /jwC₂

R₄/ C₁ = R₃ / C₂

C₁ = (R₄ / R3)C₂ …………eq(6)

An equation (5) and (6) are the unknown resistance and unknown capacitance

Tan Delta Measurement using ScheringBridge

Dielectric Loss

An efficient electrical material supports a varying amount of charge storage with minimal dissipation of energy in the form of heat. This heat loss, effectively termed as dielectric loss, is the dielectric inherent dissipation of energy. It is parameterized safely in terms of loss angle delta or loss tangent tan delta. There are essentially two main forms of loss that may dissipate energy within an insulator, they are conduction loss and dielectric loss. In conduction loss, the flow of charge through the material causes energy dissipation. For example, the flow of leakage current through the insulator. The dielectric loss tends to be higher in materials having high dielectric constant

Equivalent Circuit of Dielectric

Let us assume that, any dielectric material connected in an electric circuit as a dielectric between conductors acts as a practical capacitor. The electrical equivalent of such a system can be designed as a typical lumped element model, which includes a lossless ideal capacitor in series with resistance is known as an equivalent series resistance or ESR. The ESR especially represents losses in the capacitor, the ESR value is very small in a good capacitor, and the value of ESR is quite large in a bad capacitor.

Dissipation Factor

It is a measure of loss rate of the energy in the dielectric, because of the oscillation in dielectric material due to applied AC voltage. The reciprocal of quality factor is known as the dissipation factor which is expressed as Q=1/D. The quality of the capacitor is known by the dissipation factor. The dissipation factor formula is

D=wR₄ C₄

For mathematical interpretation, look at the phasor diagram, it is the ratio of the ESR and the capacitance reactance. It is also known as a tangent of loss angle and commonly expressed as

Tan delta=ESR/X_C

Tan Delta Testing

The tan delta testing conducts on the insulation of windings and cables. This testing is used to measure the deterioration in the cable.

Performing Tan Delta Testing

In order to perform the tan delta testing, insulation of the cables or windings is to be tested, is first isolated and disconnected. From the low-frequency power source, the test voltage is applied and the necessary measurements are taken by the tan delta controller, and up to cables rated voltage, the test voltage is increased in steps. From the above phasor diagram of Schering bridge, we can calculate the value of tan delta which is also called D (Dissipation Factor). The tan delta is expressed as

Tan delta = WC₁R₁= W *(C₂R₄ / R3)* (R₃C₄/ C₂) = WC₄R₄

Measurement of Relative Permeability with Schering Bridge

The dielectric material low permeability is measured by using the Schering bridge. The parallel plate arrangement of the relative permeability is mathematically expressed as

ε_{r =} C_s d / ε₀ A

Where ‘Cs’ is the capacitance measured value by considering specimen as dielectric or specimen capacitance, ‘d’ is the space between electrodes, ‘A’ is the electrodes effective area, ‘d’ is the specimen thickness, ‘t’ is the gap between the electrode and specimen, ‘x’ is the reduction in separation between electrode and specimen, and ε0 is the permittivity of free space.

The capacitance between the electrode and the specimen is mathematically expressed as

C=C_S C₀ / C_S+C₀ ……… eq(a)

Where C_S = ε_r ε₀ A / d ; C₀ = ε₀ A / t

Substitute C_S and C₀ values in the equation (a) will get

C = (ε_r ε₀ A / d)( ε₀ A / t) / (ε_r ε₀ A / d)+( ε₀ A / t)

The mathematical expression to reduce the specimen is shown below

ε_r= d/d – x

This is the explanation of the measurement of relative permeability with the Schering bridge.

Features

The features of the Schering bridge are

• From the potential amplifier, a high voltage supply is obtained.
• For the bridge vibration, the galvanometer is used as a detector
• In the arms ab and ad, the high voltage capacitors are placed.
• The impedance of the arm bc and cd are low and the impedances of an arm ab and ad are high.
• The ‘c’ point in the figure is earthed.
• The arm ‘ab’ and ‘ad’ impedance is kept high.
• In the arm ‘ab’ and ‘ad’, the power loss is very small because the impedance of arms ab and ad are high.

Connections

The connections were given to the Schering bridge circuit kit like the following.

• Connect the positive terminal of the input to the positive terminal of the circuit
• Connect the negative terminal of the input to the negative terminal of the circuit
• Set the resistance value R3 to zero position and set the capacitance value C3 to zero position
• Set the resistance R2 to 1000 ohms
• Switch on the power supply
• After all these connections you will see a reading in the null detector, now adjust the decade resistance R1 to get the minimum reading in the digital null detector
• Note down the readings of resistance R1, R2, and capacitance C2, and calculate the value of unknown capacitor using the formula
• Repeat the above steps by adjusting the resistance R2 value
• Finally, calculate the capacitance and resistance by using the formula. This is the explanation of working and connections of Schering bridge

Precautions

Some of the precautions we should take while giving connections to the bridge are

• Make sure that the voltage shouldn’t exceed 5 volts
• Check the connections properly before turning on the power supply

Applications

Some of the applications of using Schering bridge are

• Schering bridges used by generators
• Used by power engines
• Used in house industrial networks, etc

Advantages of Schering Bridge

The advantages of the Schering bridge are

• Compared to other bridges, the cost of this bridge is less
• From frequency the balance equations are free
• At low voltages, it can measure small capacitors

Disadvantages of Schering Bridge

There are several disadvantages in low voltage Schering bridge, because of these disadvantages the high frequency and voltage Schering bridge are required to measure the small capacitance.

FAQs

1). What is an inverted Schering bridge?

The Schering bridge is one type of an alternating current bridge which is used to measure the capacitance of the capacitors.

2). Which type of detector is used in AC bridges?

The type of detector used in AC bridges is a balanced detector.

3). What is meant by a bridge circuit?

The bridge circuit is one type of an electrical circuit which consists of two branches.

4). For what measurement Schering bridge is used?

The Schering bridge is used to measure the capacitance of the capacitors.

5). How do you balance a bridge circuit?

The bridge circuit should be balanced by following the two balance conditions they are magnitude and phase angle condition.

In this article, the overview of Schering bridge theory, advantages, applications, disadvantages, connections given to the bridge circuit, measurement of relative permeability, high voltage Schering bridge circuit, tan delta measurement, and basics of AC bridge circuit are discussed. Here is a question for you, what is the power factor of the Schering bridge?