specific value. As shown in figure 21, one L,/R time constant

is the time required for the current in an inductor to increase

to 63% (actually 63.2%) of the maximum current. Each time

constant is equal to the time required for the current to

increase by 63.2% of the difference in value between the current

flowing in the inductor and the maximum current. Maximum

current flows in the inductor after five L/R time constants are

completed. The following example should clear up any confusion

about time constants. Assume that the maximum current in an LR

circuit is 10 amperes. As you know, when the circuit is

energized, it takes time for the current to go from zero to 10

amperes.

FIGURE 21. L/R TIME CONSTANT.

(a) When the first time constant is completed, the current in

the circuit is equal to 63.2% (.632) of 10 amperes. Thus the

amplitude of current at the end of 1 time constant is 6.32

amperes.

(b) During the second time constant, current again increases

by 63.2% (.632) of the difference in value between the current

flowing in the inductor and the maximum current. This

difference is 10 amperes minus 6.32 amperes, and equals 3.68

amperes; 63.2% of 3.68 amperes is 2.32 amperes. This increase

in current during the second time constant is added to that of

the first time constant. Thus, upon completion of the second

time constant, the amount of current in the LR circuit is 6.32

amperes + 2.32 amperes = 8.64 amperes.

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