LR circuit current calculation

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SUMMARY

The discussion focuses on calculating the current in an LR circuit, specifically analyzing a circuit with a 100 Ohm resistor and an inductor. The initial current through the circuit is calculated as 1.2 Amps using Ohm's Law (I = V/R). After the circuit breaker is removed, the inductor behaves as a current source, and the current through the series combination of a 100 Ohm and 400 Ohm resistor is determined to be 441 mA after 100 ms. The confusion arises from miscalculating the exponential decay factor, which should be e^-1.

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  • Understanding of Ohm's Law and circuit analysis
  • Familiarity with inductors and their behavior in circuits
  • Knowledge of exponential decay in electrical circuits
  • Experience with analyzing transient responses in LR circuits
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  • Study the behavior of inductors in RL circuits during transient analysis
  • Learn about the mathematical derivation of exponential decay in electrical circuits
  • Explore the use of oscilloscopes for observing current and voltage waveforms
  • Review circuit schematic design principles for clarity in problem statements
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Electrical engineering students, circuit designers, and anyone involved in analyzing transient responses in LR circuits will benefit from this discussion.

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Homework Statement
A 120-V dc generator energizes a motor whose coil has an inductance of 50 H and a resistance of
A field discharge resistor of is connected in parallel with the motor to avoid damage to the
motor, as shown in Fig. 7.147. The system is at steady state. Find the current through the discharge resistor 100 ms after the breaker is tripped.
Relevant Equations
I=I0(1 - e^-t/tau)
1762575029851.webp


In the steady state the current through 100 Ohm path and inductor acts as a short circuit, inductor stores the current equal to as below
I = 120/100 = 1.2Amps
once the circuit breaker is removed the inductor works as current source the 100 Ohm and 400 Ohm are in series and the current after 100ms is
1762575124328.webp

But somehow the answer is not matching which is 441mA. I am not sure why.
 
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Yes i think i have made wrong calculations it is e^-1 and it comes correctly as 441
 
Because 500*0.1/50 ≠ 0.1. Your method was correct.

Also, yet another example of a sloppy/bad/confusing HW question. Motors have mechanical loads, maybe just the rotor inertia. This shows up on the oscilloscope. Why do they have to be cute? Why can't they just draw a schematic with an inductor if they want you to just use the inductor model?
 

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