Response Time

Author: Reuben Lee

Because of the inductance of the reactor coils, when a change is made in the control winding direct current, load current does not change immediately to its final value. An interval of time, called response time, elapses between the change in control current and the establishment of a new steady value of load current. If the inductance were constant during the change, the response time constant would be the time required for a load current increase to rise to 63 per cent of the final value after a sudden control current increase. Magnetic amplifier response time cannot be evaluated as an ordinary linear L/R time constant. Storm ¹ shows that the time of response of simple magnetic amplifiers is independent of core permeability. An average or equivalent control circuit inductance may be found from the relation

where

T_d = time for load current increment to reach 63 per cent of final value

L_c = equivalent total control coil inductance (henrys)

R_c = total control circuit resistance (ohms)

R_L = load resistance

f = line frequency

N_c = turns in control winding

N_L = turns in load winding.

An obvious method of decreasing magnetic amplifier response time is by increasing R_c, but this has the disadvantage of reducing overall power gain. Gain and response time are so related that the ratio of gain to time constant in a magnetic amplifier is usually given as a figure of merit.