# current transformer, high-impedance relay question

Discussion in 'Homework Help' started by bizuputyi, Mar 10, 2015.

1. ### bizuputyi Thread Starter New Member

Aug 3, 2014
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What value of resistance must be added to the relay circuit if the relay current has not to exceed 0.02A? Neglect relay resistance.

My idea:
0.02A is the normal threshold for the relay to operate, so under normal operation relay current must not exceed 0.02A. To achieve that balance I would say there is a need for R_S1 and R_L1 (that are identical to R_L2 and R_S2) on the left hand side of the upper line. If that's the case how do I calculate them?

Thank you anyone's effort to look into that.

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2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
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What is CT1? is it a voltage source? AC or DC?

Think of the problem as limiting the voltage that appears across the relay coil rather than the current through it. For that, we need to know the nominal voltage at which the relay pulls-in, or at least its coil impedance.

3. ### bizuputyi Thread Starter New Member

Aug 3, 2014
21
0
Both CTs are a current transformer for fault protection.
These are all the information I've been given. It looks insufficient to me, too. Or they might just want a theoretical answer.

4. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
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So it is an AC circuit. A CT requires a "burden" resistor. The relay coil is part (or all) of the CT burden.
Presumably, the relay is supposed to pull-in only when a current threshold is exceeded, so I would look at shunting the relay coil with some sort of network that conducts only after the relay has pulled in...

5. ### bizuputyi Thread Starter New Member

Aug 3, 2014
21
0
I see, so it's actually the value of Rr that is required?

6. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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772
You cant work anything out if there are no voltage or resistance values.

7. ### bizuputyi Thread Starter New Member

Aug 3, 2014
21
0
It must be the Merz-Price principle, I believe Rr is the one that needs to be calculated somehow.
What I know for sure: CT1=CT2, however they are not identical in real life. o.02A is 2% of the rated current of 1A.