Inductance and resistance

Chapter Inductance and resistance

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • Inductance and resistanceWhen the resistance in a resistance-inductance circuit is significant compared with theinductive reactance, the current lags the voltage by something less than 90 degrees(Fig. 13-9). If R is small compared with XL, the current lag is almost 90 degrees; as Rgets larger, the lag decreases. A circuit with resistance and inductance is called an RLcircuit.The value of R in an RL circuit might increase relative to XL because resistance isdeliberately placed in series with the inductance. Or, it might happen because the acfrequency gets so low that XL decreases until it is in the same ball park with the loss re-sistance R in the coil winding. In either case, the situation can be schematically repre-sented by a coil in series with a resistor (Fig. 13-10).If you know the values of XL and R, you can find the angle of lag, also called the RLphase angle, by plotting the point RjXL on the RL plane, drawing the vector fromthe origin 0j0 out to that point, and then measuring the angle of the vector,counterclockwise from the resistance axis. You can use a protractor to measure this an-gle, or you can compute its value using trigonometry.In fact, you don’t need to know the actual values of XL and R in order to find the an-gle of lag. All you need to know is their ratio. For example, if L5Ω and R3Ω, youwill get the same angle as you would get if XL50Ω and R30Ω, or if XL20Ω andR 12Ω. The angle of lag will be the same for any values of XL and R in the ratio of 5:3.It’s easy to find the angle of lag whenever you know the ratio of R to XL. You’ll seesome examples shortly.238 Inductive reactance13-8In a pure inductance, the current lags the voltage by 90 degrees.