(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\; \Omega\; 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 (1)
S safeer

(a)    A device which is used to decrease high ac voltage is known as a step- down transformer,

It works on the principle of mutual induction, It states that, if the current is changing through the primary coil, then there is a change in magnetic flux in the secondary coil. This change in flux induces an emf in the secondary coil.

   

The four sources of energy loss in the device are : 

Flux leakage, eddy currents, Resistance of the windings and the hysteresis loss.

(b) Given,

       The  town has a power demand of 12000\; kw at 220 \; V 

        Resistance per unit length =0.5\; \Omega /km

        Length of each wire =20 \; km

        Total resistance, r=2\times 0.5\times 20\; \Omega =20\; \Omega

The transformers step down from 4000 V (V_1)  to  220\; V(V_{2}) .

If there is no power loss in the transformers, then we can calculate the RMS current in the lines by :

I=\frac{P}{V_{1}}=\frac{1200\times 10^{3}w}{4000v}=300A

The power loss in the lines is 

    P_{loss}=I^{2}r

              =(300\; A)^{2}\times 20\; \Omega

              =1800000\; w

                =1800\; kw

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