# Can there be an resonance in LR circuit?

Discussion in 'Homework Help' started by Tera-Scale, Jan 2, 2011.

1. ### Tera-Scale Thread Starter Active Member

Jan 1, 2011
164
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Why am I asked to find the frequancy at which an LR circuit is resonant? Every formula that I saw always included a negative reactance for a capacitor in series.

2. ### edgetrigger Member

Dec 19, 2010
133
20
There is no resonance without C. There shoul be something missing in the question. LR can be used as a filter at the most.

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3. ### Tera-Scale Thread Starter Active Member

Jan 1, 2011
164
5
Thanks, I would assume it's a mistake in my assignment.. :/

4. ### Papabravo Expert

Feb 24, 2006
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Resonance can only occur in a second order system. The series RL circuit is a first order system.

5. ### hobbyist AAC Fanatic!

Aug 10, 2008
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"at what frequency, does (XL = R), such as used in filter branches.

6. ### mik3 Senior Member

Feb 4, 2008
4,846
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Maybe the question refers to parasitic capacitance which resonates with the inductance.

7. ### edgetrigger Member

Dec 19, 2010
133
20
if we were to consider stray capacitance then it will be an LC and still not an LR resonance.

8. ### Papabravo Expert

Feb 24, 2006
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A series LR circuit is an idealization, without capacitance, and thus incapable of resonance. Now it is true that in a real circuit as you increase the applied frequency you reach a point where there is a small capacitance between the individual windings of the inductor whose reactance becomes significant. Now it is no longer an RL circuit and we can get resonance.

9. ### edgetrigger Member

Dec 19, 2010
133
20
Even then it is a LC resonace and not a RL resonance.

10. ### Papabravo Expert

Feb 24, 2006
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Of course. That is what we have been trying to say. There is no such thing as either an RL resonance or an RC resonance. You need BOTH an L and a C for rsonance to occur.