TESTING OF MATERIALS Name of the Subject : Material Science N. R. Dagade Electrical Engineering Department NBNSSOE, Ambegaon, Pune

Measurement of tan delta by Schering Bridge–IS ) The circuit of the Schering bridge is shown in Figure. The unknown is represented by C and r in series. Thus Z1= r + 1/jωC, and the unknown has loss tangent, tan δ= ωCr.

Measurement of tan delta by Schering Bridge–IS ) Phasor relationship of charging and leakage currents in dielectric

Measurement of tan delta by Schering Bridge–IS ) C 2 is a standard capacitor. Often an air capacitor is used in this position, being loss free. To balance the bridge it is necessary to have two variable components. Normally one of these is C 4 and the other one is either R 3 or R 4. Balance occurs when (refer Figure of Schering Bridge) Z 1 Z 4 = Z 2 Z 3

Measurement of tan delta by Schering Bridge–IS ) Rearranging and separating into real and imaginary parts gives the balance condition, Substituting in tan δ= ωCr gives,

Measurement of Dielectric Strength (kV/mm) of Solids Insulating Materials

Observation table Sr. No.Solid dielectric Breakdown voltage (kV) Thickness (mm) Breakdown strength (kV/mm) 1 2 3

Measurement of Dielectric Strength (kV/mm) of Liquid Insulating Materials

Observation Table : Gap between electrodes = 2.5 mm Sr. No. Breakdown voltage (kv) Mean breakdown voltage (kv) Dielectric strength (kv/mm) Mean Dielectric strength (kv/mm)

Measurement of Dielectric Strength (kV/mm) of Gaseous Insulating Material using Sphere Gap Unit – IS 2584

Observation Table : Sr. No.Gap between spheres (mm) Dielectric strength (kv/mm) from meter reading

MEASUREMENT OF FLUX DENSITY BY GAUSS METER A device commonly used to measure flux density is the Hall generator or Gauss meter A Hall generator is a thin slice of a semiconductor material to which four leads are attached at the midpoint of each edge, as shown in Figure

MEASUREMENT OF FLUX DENSITY BY GAUSS METER A constant current (Ic) is forced through the material. In a zero magnetic field there is no voltage difference between the other two edges. When flux lines pass through the material the path of the current bends closer to one edge, creating a voltage difference known as the Hall voltage (Vh). In an ideal Hall generator there is a linear relationship between the number of flux lines passing through the material (flux density) and the Hall voltage.

MEASUREMENT OF FLUX DENSITY BY GAUSS METER The Hall voltage is also a function of the direction in which the flux lines pass through the material, producing a positive voltage in one direction and a negative voltage in the other. If the same number of flux lines pass through the material in either direction, the net result is zero volts. This sensitivity to flux direction makes it possible to measure both static (dc) and alternating (ac) magnetic fields.

MEASUREMENT OF FLUX DENSITY BY GAUSS METER The Hall voltage is also a function of the angle at which the flux lines pass through the material. The greatest Hall voltage occurs when the flux lines pass perpendicularly through the material. Otherwise the output is related to the cosine of the difference between 90° and the actual angle.

Questions 1. Presence of moisture in transformer oil______ its dielectric strength considerably (a) greatly reduces (b) increases (c) does not affect (d) partially reduces Answer: A 2. Which of the following thermal insulator is suitable above 180  C. (a)cotton (b)mica (c)silk(d)paper Answer: B

Questions 3. The drawback of paper as insulating material is (a)it is hygroscopic (b) it has high capacitance (c)it is an organic material(d) none of these Answer: A 4. The insulating materials used for cable should have (a) low cost(b) high dielectric strength (c) high mechanical strength (d) all the above Answer: D

Questions 5. Dielectric strength is expressed in ____ (a)kV/mm(b)kV/mm 2 (c)mV/mm 2 (d)V/cm 2 Answer: A 6. The electrical resistance of ceramics _______ with rise in temperature (a) decreases rapidly(b)increases rapidly (c) have no effect (d) none of these Answer: A

Thank You.