Separation of Core Losses

Separation of Core Losses

AU May-10, 11, Dec.-13, 15, 18, 19

• It is seen that the core losses of transformer includes,

•1. Hysteresis loss 2. Eddy current loss bol

• For a given volume and thickeness of laminations, these losses depend on the operating frequency, maximum flux density in the core and the voltage.

• The hysteresis loss is given by Steinmet'z relation,

Where A = Constant assuming constant voltage.

 The eddy current loss is given by,

Where B = Constant for given thickness t of core

Thus the total core loss becomes,

P_i = P_h+P_e  = AB_m^1.67 f + BB_m²f² ..............(6.21.3)

• Practically conduct two tests on transformers at two different frequencies f₁ and f₂, keeping maximum flux density Bm in the core same. The results are to be used in the equations (6.21.1), (6.21.2) and (6.21.3) to obtain the constants A and B. Thus the e core losses i.e. iron losses can be separated into hysteresis and eddy current losses.

Ex 6.21.1 The total core loss of a specimen of silicon steel is found to be 1500 W at 50 Hz. Keeping the flux density constant the loss becomes 3000 W when the frequency is raised to 75 Hz. Calculate separately the hysteresis and eddy current loss at each of these frequencies. AU: May-10, Marks 5; May-11, Dec.-15,19, Marks 8

Sol. :

Ex. 6.21.2 A transformer with normal voltage impressed has a flux density of 1.2 T and core loss comprising 1200 W eddy current loss and 3500 W hysteresis loss. What at dot these losses become under the following conditions:

i) Increasing the applied voltage by 5 % at rated frequency;

ii) Reducing the frequency by 5% with normal voltage impressed; and

iii) Increasing both impressed voltage and frequency by 5%. AU: Dec.-13, 18, Marks 15

Sol. :

Review Questions

1. Explain how core losses are seperated.

2. A single phase transformer shows 63 W core losses at 40 Hz while 110 W at 60 Hz. Both the tests are performed at same value of maximum flux density in the core. Find hystersis and eddy current losses at 50 Hz frequency.

(Ans. : 52.92 W, 32.25 W)