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Reactors - Example

Assume a stack of silicon-steel laminations having a cross section 7/8 in. by 7/8 in., and with iron filling 92 per cent of the space. The length of the flux path lc in this core is 7 1/2 in. It is desired to know how many turns of wire and what air gap are necessary to produce 70 henrys when 20 ma direct current are flowing in the winding.

This problem is solved as follows:

In Fig. 66 the abscissa corresponding to LI2/V = 53 · 10-4 is NI/lc = 25 for silicon steel. The ratio of air gap to core length lg/lc is between 0.0005 and 0.001.

The total air gap is nearly 0.001 · 7 1/2 or 7.5 mils; the gap at each joint is half of this value, or 3.75 mils.

The conditions underlying Hanna's method of design are met in most applications. In receivers and amplifiers working at low audio levels, the alternating voltage is small and hence the alternating flux is small compared to the steady flux. Even if the alternating voltage is of the same order as the direct voltage, the alternating flux may be small, especially if a large number of turns is necessary to produce the required inductance; for a given core the alternating flux is inversely proportional to the number of turns. D-c resistance of the coil is usually fixed by the regulation or size requirements. Heating seldom affects size.



Last Update: 2011-02-17