RL series circuit and TIME CONSTANT

I believe the answer to this question is counter intuitive, but we shall see.

I am testing a DC circuit breaker. My circuit consists of a variable inductor and a variable resistor. THAT'S IT !

Now, here's what I know. As Resistance increases with a set Inductance my time constant will decrease, and vice versa for decreases in R. Also, as my Inductance increases, obviously my Time Constant will increase, and decrease if Inductance is lowered, with a set R.

Question ? Why would a time constant of lets say 1 mS be an easier "shot", than a circuit designed with a 10mS time constant ?

The reason I ask is because I'm imagining that if I apply for example 10,000 Amps to a circuit breaker, and my time constant is 1 mS, there's more Current available almost immediately. If I apply the same circuit with a time constant of 10mS the Current available will be substantially less at 1mS.

Another thing I'm sure of is that a longer time constant is a "tougher" shot on a breaker and shorter time constants are for some reason "easier" shot's. I just can't seem to imagine why ? Why ?

What is a "shot"?

A "shot" is the actual application of current to the breaker. Therefore either closing the breaker to complete your circuit, OR, closing a switch to complete the circuit while the breaker is already closed.

To put it in perspective, remember that a circuit breaker is essentially a resettable fuse. When the Current reaches a certain level, the circuit breaker should OPEN, preventing current from flowing.

A "shot" is just slang.

Is that a magnetic breaker, or a thermal one?

Also please remember that all this is happening in a very short amount of time. We are talking about 8-12 mSec. for the entire event. Meaning from the time the circuit is closed to the time it is open. There's not a stable current draw, as it would be if I were applying current for minutes or hours. This is happening FAST. That's why the Time Constant is so critical.

circuit #1 - 600VDC 10,000A Time Constant = 3mS (meaning at 3mS, the first time constant in the charging of an Inductor is 63%, therefore Current = 6300 Amps at 3mS.

circuit #2 - 600VDC 10,000A Time Constant = 10mS (therefore at 10mS, Current = 6300A)

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Comparing these two circuits, I would imagine that the shorter the Time Constant, the harsher it would be on the Circuit Breaker. Anyhow for some reason it's not. I MUST FIND OUT WHY, IT'S BUGGING ME. It must have something to do with the way it heats up contacts on the Circuit breaker. The circuit breaker is the "WEAKEST LINK", and will impede Current flow to the point it opens up.

I'm starting to think the same thing, but the specification doesn't discriminate between magnetic or thermal.

I'll check back later... going to sleep for a few hours... Thanks in advance for any responses. Thanks Mike for your help. Nite.

bioroboto said:

circuit #1 - 600VDC 10,000A Time Constant = 3mS (meaning at 3mS, the first time constant in the charging of an Inductor is 63%, therefore Current = 6300 Amps at 3mS.

circuit #2 - 600VDC 10,000A Time Constant = 10mS (therefore at 10mS, Current = 6300A)

Click to expand...

The current will be about 63% of the steady state maximum (the final value), not 65% of the capability of your source. In other words, you have to take the series resistance into account.

circuit #1 - 600VDC 10,000A Time Constant = 3mS :

If your R is 3.3 ohms and L is 10mH, then the time constant is 3ms. Maximum steady state current with 600 volts is 181 A. Assuming you apply a constant (600V) voltage across the LR circuit.
So, after 3 ms the current is roughly 115 A

With circuit #2 the current after 10ms will also depend on the resistance, but assuming 3.3 ohms the current will be the same 115 A. But if you change the the time constant by changing the value of the resistor, then the current after 10ms will be different than in the first circuit after 3ms.

Or, did I understand something wrong and you are using 0.06 ohms resistor in your LR circuit? In that case your calculations are correct.
Is your problem that if you increase the current more slowly, the circuit breaker becomes more unreliable? How do you know that shorter time constant is easier for the breaker? Try to repeat your experiment, but only vary the inductor and keep the resistance constant.

You are correct in terms of the values of the components being used. They are all in the micro and milli range. Very small resistances and imedance values to allow large amounts of current and large amounts of wattage.

As for Time Constant : Changing the L is the main method used to make a circuit. R does have an effect on TC but not as much as L. Therefore R is usually left alone when it's in range, and the circuit is fine tuned for the desired TC using L.

I am sure that a lower TC is easier because the specification calls for Minimum 3mS TC, therefore anything over 3mS is tougher. Basically it's been pre-determined that anything under 3mS, lets say 1mS, is easier for the Circuit Breaker.

I'll ask around the Laboratory, but I wanted to find out the answer myself. Thanks anyhow for your input, it's well appreciated.