(a) Draw the diagram of a device which is used to decrease high ac voltage into a low ac voltage and state its working principle. Write four sources of energy loss in this device.
(b) A small town with a demand of 1200 kW of electric power at 220 V is situated 20 km away from an electric plant generating power at 440 V. The resistance of the two-wire line carrying power is 0.5 Ω per km. The town gets the power from the line through a 4000-220 V step-down transformer at a sub-station in the town. Estimate the line power loss in the form of heat.
Answers
(a) Working Principle : A transformer is based on the principle of mutual Induction. An e.m.f. is induced in a coil when a changing current flows through its nearby coil
(a) Working Principle : A transformer is based on the principle of mutual Induction. An e.m.f. is induced in a coil when a changing current flows through its nearby coil
Four sources of energy losses :
(i) Copper losses.
(ii) Flux leakage losses.
(iii) Iron losses (Eddy current losses).
(iv) Magnetostriction effect (Losses due to vibration of core due to humming noise).
(b) Current in lines.
I=PV=1200−10004000=30Amp
Power loss in the form of heat :
R=0.5×20×2=20Ω
P=I2R
=300×300×20=1800000W=18000kW.
tage is 4000 V = Vin. Output voltage is 220 V = Vout. Therefore, the rms current is
Irms=P/Vrms=1200*10^3/4000=300A
Power loss in the form of heat is
I^2 cc800*10^3
Therefore, the power loss in form of heat is 1800 kW.