AML 883 Properties and selection of engineering materials LECTURE 18. Electrical properties and design for their exploitation M P Gururajan Email: guru.courses@gmail.com Room No. MS 207/A 3 Phone: 1340
Breakdown potential or dielectric strength Units V/m (Typically, MV/m) The electrical gradient at which an insulator breaks down and a damaging surge of current flows through it Measured by increasing, at a uniform rate, a 60 Hz alternating potential applied across the faces of a plate of the material until breakdown occurs 2
Dielectric properties Dielectric loss tangent: tan Voltage, V and Current, i i V Oscillating Charge, + Q h f 3 Time
Loss tangent or loss factor Polarization involves the small displacement of charge (either of electrons or of ions) or molecules that carry a dipole moment when an electric field is applied to the material Alternating field drives the charge between two alternating configurations Charge motion like an electric current which is out of phase with the voltage by 90 degrees if there were no losses 4
Loss tangent In real dielectric, current dissipates energy giving it a small phase shift The loss tangent, tan (or the dissipation factor, D) is the tangent of the loss angle The power factor is the sine of the loss angle When the angle is small, all three, namely, dissipation factor, power factor, and loss angle are the same Loss factor loss tangent times the dielectric constant 5
Loss factor Loss factor loss tangent times the dielectric constant Measure of energy dissipated in a dielectric when in an oscillating field Selection of materials that extremise dielectric loss L = tan is the measure r 6
Power dissipation Place a dielectric in a cycle of electric field of amplitude E and frequency f A power P is dissipated and the field is correspondingly attenuated Power dissipated per unit volume (P) is f E L This power appears as heat and is generated uniformly throughout the material Higher the frequency or field strength, higher the heating Radio frequency welding of polymers 2 0 7
Electrostriction and piezo electricity All dielectrics change shape in an electric field; consequence of the small shift in charge that allows them to polarize Electro striction one sided relationship between electric field and deformation Piezo electric materials two sided: electric field causes deformation and deformation induces charge differences between its surfaces, thus creating a field Piezo electric a true, linear effect 8
Pyro electric materials Contain molecules with permanent dipole moments that, in a single crystal, are aligned, giving the crystal permanent polarization With temperature, polarization changes creating surface charges, or, if surfaces are connected, a pyor electric current Principle of intruder detection systems and of thermal imaging Name one common pyro electric material that we all have and use! 9
Pyro electric materials Contain molecules with permanent dipole moments that, in a single crystal, are aligned, giving the crystal permanent polarization With temperature, polarization changes creating surface charges, or, if surfaces are connected, a pyor electric current Principle of intruder detection systems and of thermal imaging Name one common pyro electric material that we all have and use! Bones and tendon 10
Ferro electric materials Materials with natural dipole moment Dipole moment align like magnetic moments in a magnet Direction of polarisation can be changed with the application of electric fields This change leads to a change of shape! 11
Property charts Strength electrical resistivity Strength dielectric loss Electrical resistivity Thermal conductivity (Metals since both the current and thermal carriers are free electrons, there exists a correlation between the two: the constant that connects the ratio of these two quantities with temperature is called Lorenz number) K / = 2 /3 k B /e 2 T 12
Resistivity versus cost Image courtesy: Cambridge University Homepage 13
Transmitting electrical power Objective: Minimize electrical loss Loss: i 2 R For a given cross section, the material with the lowest resistivity is then the criterion The only other constraint is material cost! 14
Resistivity versus cost metals Image courtesy: Cambridge University Homepage 15
Beyond cost and resistivity Cost and resistivity copper or aluminium! Is that true? Can you think of any other consideration? 16
Transmission lines! Image courtesy: wiki 17
Another era transmisson lines! Transmission lines in New York city in 1890 Why so many lines (apart from the telegraph line, of course)? Image courtesy: wiki 18
Nikola Tesla and Thomas A Edison Images courtesy: Wiki 19
Other constraints Buried are lines fully supported their entire length cost and resistivity alone are the constraints copper and aluminium Lines above ground strength and density are important High strength to support self weight and wind loads Low density to minimize self weight 20
Conflict! Materials with lowest resistivity lowest in strength High strength high resistivity (why?) Precipitation or work hardening optimization for strength you pay in resistivity increase Hybrid wires 21
Hybrid wires Copper g O sin a c r ute Steel 50:50 mix of pure copper or aluminium core surrounded by high strength carbon steel Half the strength of steel and twice the resistivity of copper! 22