inductive reactance

[electricalperson]

in school we were talking about inductive reactance on circuits. well today i decided to do an experiment at work. i was installing a relay with a 24 volt coil to control a fan. i measured resistance with the power off at the coil and got 9 ohms. the measured voltage off the transformer located a couple inches away was 23.8 volts using ohms law i = e/r i get 2.64 amps. then i put my meter in series with the coil and measure .3 amps. using ohms law again R = E/I i get 79.3 ohms. was i measuring the effects of inductive reactance on ac inductive circuits?

 

[LarryFine]

Most inductive devices will exhibit a lower DC resistance than AC impedance.

 

[peter d]

Remember that a coil produces what's know as "counter EMF" which acts against the source voltage. This counter EMF will limit the current, which is why your calculations and your readings do not jive. This can be explained with lots of calculus but the last calculus class I took was 10 years ago.

 

[electricalperson]

im pretty sure i understand the theory of counter emf. im no expert on that by any means but in school the teacher used an example of measuring the ohm reading of a motor coil doing basic ohms law and getting a incredibly high ampere reading when the motor name plate says 10 amps for example.

 

[Gary Glaenzer]

" i measured resistance with the power off at the coil and got 9 ohms. the measured voltage off the transformer located a couple inches away was 23.8 volts using ohms law i = e/r i get 2.64 amps. then i put my meter in series with the coil and measure .3 amps. using ohms law again R = E/I i get 79.3 ohms. was i measuring the effects of inductive reactance on ac inductive circuits?"

in a word, YES

to elaborate:

the IMPEDANCE ( combination of resistance and reactance) is 79.3 ohms

the DC RESISTANCE is 9 ohms

if you diagram it as a right-triangle problem, with 79.3 as the hypotenuse, and 9 as one of the legs, then the remaining leg becomes the INDUCTIVE REACTANCE

(79.3 x 79.3) = ( 9 x 9) + X-squared

6288.49 = 81 + X-squared

6207.49 = X-squared

X = 78.79 ohms

we can go one step further, since Inductive Reactance (abbreviated X-sub L)
equals 2 x pi x (frequency in Hz) times INDUCTANCE, then

78.79 = 6.28 x 60 times inductance

so 78.79 / (6.28 x 60) = inductance

or 0.21 Henries ('Henry' is the unit of inductance)

Hope this helps !

Regards,

Gary

 

[electricalperson]

thanks guys you people are fantastic