Can the Work Done by an Inductor Exceed the Electrical Power Used to Make It?

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Discussion Overview

The discussion centers around the relationship between the work done by an inductor and the electrical power supplied to it. Participants explore concepts related to energy storage in inductors, the timing of current changes, and the implications of inductance and resistance on power calculations. The scope includes theoretical considerations and technical reasoning regarding inductive devices.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant states that the energy stored in an inductor is given by the formula \(\frac{1}{2}L\mathbf{I}^2\) and emphasizes that this energy does not depend on the rate at which current is attained.
  • Another participant suggests that the timing of the magnetic field's on/off cycles affects the work done by the inductor and proposes a relationship involving frequency and resistance.
  • A different participant presents a formula for the power of an inductor, relating it to inductance, resistance, and the time to change current.
  • One participant challenges the mixing of concepts, arguing that inductive devices should not be analyzed using Ohm's law due to their low resistance.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between inductance, resistance, and power calculations. There is no consensus on the correctness of the proposed formulas or the implications of the concepts discussed.

Contextual Notes

Participants reference various assumptions about power factors, time constants, and the behavior of inductive devices, but these assumptions remain unresolved and depend on specific definitions and contexts.

kmarinas86
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Energy stored in an inductor is equal to:

\frac{1}{2}L\mathbf{I}^2

Where L is the inductance in henries and I is the current in amps.

The energy stored in the inductor doesn't depend on how fast the current is attained, just the fact that it has some inductance and some current.

However, how fast the current can be attained can very well depend on what is supplying the inductor with current. The timing of turning on and off of the inductor magnetic field affects how much work could done by it. So what if we take the energy of the inductor and multiply by the frequency of the pulses? This should not exceed overall electrical power R{I}^2 if current electromagnetic theory is correct.

Assuming power factor is 1 (or not assuming power factor is one), \frac{1}{2}Lf, where f is frequency of on/off periods the inductor handles (to influence a magnetic rotor), cannot be greater than the resistance R of the coil. Do I have this right?

For example it should be impossible that a coil of 1100 henries with 770 Ohms of resistance can switch its whole magnetic field (i.e. with the current throughout the whole length of the coil) on and off 1.4 times every second. Isn't this well understood in engineering literature (I hope it is)? So \frac{1}{2}Lf \le R?

The time constant of any motor is simply inductance divided by resistance. This would mean that the time constant of the circuit times the frequency of the pulses cannot be greater than 2. Right?
 
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Hello?
 
1/2*Inductance*(change of current from zero)^2/(time to change current) = power of an inductor

1/2*(Inductance/Resistance)*voltage*(change of current from zero)/(time to change current) = power of an inductor

(1/2*(time constant of circuit)/(time to change current))*voltage*(change of current from zero) = power of an inductor

power of inductor / electric power sent to coil = 1/2*(time constant of circuit)/(time to change current)
 
Last edited:
It just looks to me like you are mixing unrelated concepts there: Inductive devices don't have a high resistance, it would defeat their purpose. So you don't use ohm's law on them.
 

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