Schering Bridge, Circuit Diagram, Relative Permeability, Applications, Advantages & Disadvantages
Table of Contents
An electrical circuit called the Schering Bridge is used to gauge how well electrical cables and other equipment insulate. Harald Ernst Malmsten Schering (25 November 1880 – 10 April 1959) created an AC bridge circuit. The balanced equation’s independence from frequency is its greatest benefit. So The most widely used, practical, and precise instruments for measuring AC resistance, capacitance, and inductance are AC bridges, which are the originating current bridges. So The only distinction between alternating current bridges and direct current bridges is the power supply. The Ac bridges are identical to the DC bridges.
What is Schering Bridge?
One type of AC bridge used to determine a capacitor’s unknown capacitance, relative permeability, dissipation factor, and dielectric loss is the Schering bridge. The step-up transformer is used to create the high voltage in this bridge. This bridge’s primary goal is to determine capacitance value. So The trainer kit, decade capacitance box, multimeter, CRO, and patch chords are the main tools needed for connection. The capacitance value is calculated using the formula CX=C2(R4/R3).
Basic AC Bridge Circuit
In AC bridges, oscillators are used as a source during high-frequency measurements, while power lines are used as an excitation source during low frequency measurements. An oscillator’s frequency range is 40 Hz to 125 Hz. The Wheatstone bridge serves as the foundation for all AC bridges, which all measure resistance, capacitance, inductance, and power factor in addition to storage factor. The figure below displays an alternating current bridge’s basic circuit diagram.
So And Z1, Z2, Z3, and Z4 are the four impedances that make up the basic components of an AC bridge circuit, along with a detector and an AC voltage source. The detector, which is used to balance the bridge, is situated between the points “b” and “d.” The bridge network is powered by an AC voltage source that is positioned between points ‘a’ and ‘c’. Points ‘b’ and ‘d’ both have the same potential. Both potential points, such as b & d, are equal in amplitude and phase. The voltage drop from point ‘a’ to ‘b’ is equal to the voltage drop from point ‘a’ to ‘d’ in both magnitude and phase.
When using AC bridges to measure at low frequencies, power lines are used as a source of supply, and when using electronic oscillators to measure at high frequencies, power lines are used as a source of supply. An electronic oscillator is used as a source of power supply; the oscillator’s output waveforms are sinusoidal in nature, and the frequencies it offers are fixed. So In AC bridges, three different types of detectors are employed: vibrating galvanometers, headphones, and tunable amplifier circuits.
There are various frequency ranges, and for each of those, a specific detector will be used. The low-frequency range for headphones is 250Hz, and the high-frequency range is above 3 to 4KHz. So The frequency range of a vibrational galvanometer is 5Hz to 1000Hz, and it is more sensitive below 200Hz. The frequency range of tunable amplifier circuits is 10Hz to 100KHz.
High Voltage Schering Bridge Circuit Diagram
The diagram of a high voltage Schering bridge is displayed in the figure below. The bridge has four arms; the first arm has two unknown capacitances (C1 and C2) that we must determine, as well as a resistor R1, and the second arm has a variable capacitance (C4) as well as resistors R3 and R4. ‘D’ detector is connected in the bridge’s center.
So The capacitor whose capacitance must be increased is labeled “C1” in the diagram; the loss in the capacitor is represented by “R1” in series; “C2” is a standard capacitor; “R3” is a non-inductive resistance; “C4” is a variable capacitor; and “R4” is a variable non-inductive resistance in parallel with “C4”.
So 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 Z1 = R1 + 1/jwC1 ; Z2 = 1/jwC2 ; Z3 = R3 ; Z4 = (R4 + 1/jwC4R4)/( R4 – 1/jwC4R4)
So 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
Z4 = (R4 + 1/jwC4R4)/( R4 – 1/jwC4R4)
Z4 = 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 Schering Bridge
A versatile amount of charge storage is supported by an effective electrical material with little energy loss in the form of heat. This heat loss, also known as dielectric loss, is the energy dissipation that occurs naturally in dielectrics. It can be parameterized securely using loss tangent delta or loss angle delta. Conduction loss and dielectric loss are the two main types of loss that can cause energy to be lost inside an insulator. In conduction loss, energy is lost as a result of the flow of charge through the material. For instance, the leakage current that passes through the insulator. Materials with a high dielectric constant tend to have a higher dielectric loss.
Equivalent Circuit of Dielectric
Assume for the moment that any dielectric material used in an electrical circuit to separate conductors serves as a functional capacitor. A typical lumped element model can be used to create the electrical equivalent of such a system. This model includes a lossless ideal capacitor in series with resistance, which is referred to as an equivalent series resistance, or ESR. The ESR, which specifically represents capacitor losses, has a very small value in a good capacitor and a fairly large value in a bad capacitor.
Due to the oscillation in the dielectric material caused by the applied AC voltage, it is a measurement of the energy loss rate in the dielectric. The dissipation factor, which is defined as Q=1/D, is the inverse of the quality factor. The dissipation factor can be used to determine a capacitor’s quality. The formula for the dissipation factor is
The phasor diagram, which shows the relationship between the capacitance reactance and the ESR, can be used to interpret mathematics. Additionally called a tangent of loss angle, it is frequently expressed as
Tan delta= ESR/XC
Tan Delta Testing
The insulation of windings and cables is tested using the Tan Delta method. The cable’s deterioration is measured through this testing.
Performing Tan Delta Testing
The insulation of the cables or windings that will be subjected to the tan delta testing is first isolated and disconnected. The test voltage is applied from the low-frequency power source, the required measurements are taken by the tan delta controller, and the test voltage is gradually increased up to the cable’s rated voltage. We can determine the value of tan delta, also known as D (Dissipation Factor), from the Schering bridge phasor diagram shown above. Tan delta is written as
Tan delta = WC1R1= W *(C2R4 / R3)* (R3C4/ C2) = WC4R4
Measurement of Relative Permeability with Schering Bridge
Using the Schering bridge, the low permeability of dielectric materials is determined. The relative permeability parallel plate arrangement is mathematically represented as
εr = Cs d / ε0 A
Where ‘Cs’ stands for the capacitance measured value when the specimen is treated as a dielectric or specimen capacitance, ‘d’ for the distance between electrodes, ‘A’ for the effective area of the electrodes, ‘d’ for the thickness of the specimen, ‘t’ for the gap between the electrode and specimen, ‘x’ for the reduction in separation between the electrode and specimen, and ‘0’ for the permittivity of free space.
The mathematical formula for the capacitance between the electrode and the sample is
C=CS C0 / CS+C0 ……… eq(a)
Where CS = εr ε0 A / d ; C0 = ε0 A / t
Substitute CS and C0 values in the equation (a) will get
C = (εr ε0 A / d)( ε0 A / t) / (εr ε0 A / d)+( ε0 A / t)
So The mathematical expression to reduce the specimen is shown below
εr= d/d – x
So This explains how relative permeability was measured using the Schering bridge.
The features of the Schering bridge are
- Obtaining a high voltage supply from the potential amplifier.
- The galvanometer serves as a detector for the bridge vibration.
- The high voltage capacitors are positioned in the arms ab and ad.
- Arms bc and cd have low impedances, while arms ab and ad have high impedances.
- The ‘c’ point in the figure is earthed.
- The arm ‘ab’ and ‘ad’ impedance is kept high.
- Because of the arms’ high impedance, the power loss in arms “ab” and “ad” is very minimal.
The Schering bridge circuit kit received the connections in the manner described below.
- Join the positive terminal of the input to the circuit’s positive terminal.
- Join the negative terminal of the input to the circuit’s negative terminal.
- So Set the capacitance value C3 and the resistance value R3 to their respective zero positions.
- Set the resistance R2 to 1000 ohms
- Switch on the power supply
- So After making all of these connections, the null detector should display a reading. So Next, change decade resistance R1 to obtain the digital null detector’s minimum reading.
- Write down the values for the resistances R1, R2, and capacitance C2, then use the formula to determine the value of the unknown capacitor.
- So Repeat the above steps by adjusting the resistance R2 value
- Finally, use the formula to calculate the capacitance and resistance. So Here is a description of how the Schering Bridge operates and connects.
Among the safety measures we should employ when connecting to the bridge are
- Make sure that the voltage is not greater than 5 volts.
- Before turning on the power supply, ensure that the connections are secure.
The following are some uses for the Schering bridge:
- Used by power engines
- Schering bridges used by generators
- Used in house industrial networks, etc
Advantages of Schering Bridge
The advantages of the Schering bridge are
- This bridge is less expensive than other bridges.
- The balance equations are free from frequency.
- It can assess small capacitors at low voltages.
Disadvantages of Schering Bridge
Low voltage Schering bridge has a number of drawbacks, and as a result, high frequency and voltage Schering bridge are needed to measure small capacitance.