inductive or capacitive?

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electrics

Senior Member
which one is more difficult to break? if the same x/r ratio in both of the circuits they present the same difficulty for a circuit breaker generally spoken? İ also want to ask you of light inductive currents? there are many essays of this LİGHT İNDUCTİVES. So here light means cos phi >= 0,9 or it means cos phi=< 0,1 It is very important to know to what "light" word refer .
active power or reactive power? So as for my real question why most of the analysis are about light inductive currents? if it means almost pure inductive current so why the capacitive ones are not so important?
 
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electrics

Senior Member
okey just tell me about which one of these two current is harder to interrupt if the magnitudes are all equal. İs there any difference for circuit breakers to break inductive or capacitive currents?
 
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jghrist

Senior Member
There are different concerns and you can't make a general statement about which is more difficult to interrupt. Changing current in an inductor causes a high voltage across the inductor. Interrupting a capacitive current causes high voltages across the interrupting contacts (recovery voltage).

See http://www.eng-tips.com/viewthread.cfm?qid=260396&page=1 for a discussion of the two.
 
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electrics

Senior Member
so u all keep watchinh engtips:) anyway we need to find the answer all the same. so i think there is no such a satisfactory answer. You say in general both of them have the same impact on the c.b ?
 
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__dan

Banned
Breaking reactive components

Breaking reactive components

The reactive components that are harder to break are the ones that are present.

In power systems switching under load, you are probably more concerned with 'extinguishing the arc'. ie, The equipment will be built to handle arc conditions but the circuit may not be specially modified to reduce reactive power components solely for easier switching. Filters added to the power circuit are probably for loaded conditions.

For switching lights, buy switching duty rated breakers.

In control applications where sensitive control electronics pull in solenoid coils, objectionable inductive kick voltage is present at switching. E = L dI/dT. As coil current I collapses instantaneously at the moment of opening, the coil magnetic field collapses and generates a very fast time voltage kick. Into an open circuit, this inductive kick voltage can be high enough to blow out semiconductor electronics or cause wear on small pilot duty contacts. Large starter coils can take out 'fast response time' fuses, you need time delay 'motor' fuses for large solenoid coils.

In this application, where the manufacturer cares enough, it is common to see small transient snubber circuits, added to the coil circuits in parallel, to damp the coil switching transient. A wire wound resistor or resistor and cap is present near the coil.
 
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rcwilson

Senior Member
Location
Redmond, WA
The ability of a set of contacts to successfully interrupt a current depends on more than just the RMS magnitude of the current. An AC circuit breaker or contactor depends on the current going through zero to help extinguish the arc. In a highly inductive circuit, a fault current's AC waveform may be offset so much that a current zero may not occur for 3-4 cycles. The breaker has to be derated to be able to interrupt currents like that. (High X/R's don't usually occur in low or medium voltage circuits unless a lot of generation is on line.)

As the contacts open, the increasing distance between them builds up insulation. The objective is to make that contact distance bigger than the voltage can jump as quickly as possible. (Remember, voltage across the contacts = 0 V when closed and full circuit voltage when open.)

When the distance is greater than the voltage can jump, current interruption can occur at the next current zero. If there is no current zero, the arc between the contacts can provide a conductive path of ionized air for current to continue to flow. If the distance is too small, the arc jumps the gap and current continues flowing.

A capacitive circuit can increase the voltage across the contacts to more than line voltage. For example, if the capacitor is de-energized when the voltage wave is maximum positive, that voltage is trapped in the capacitor and impressed on the one contact while the other contact's AC voltage is going to maximum negative. The result is higher voltage across the contacts, making it more difficult to extinguish the current.

Capacitive circuits also have other phenomena that need to be considered when applying circuit breakers and contactors. That is why data sheets will list a reduced amp rating for capacitor switching.

This is not a rigourous technical explanation, but I hope it gets the idea across.

Current magnitude is the biggest governing factor in circuit breaker design in each voltage class, but other parameters are also very important.
 
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electrics

Senior Member
I am asking a simple question friends.İf they have the same X/R ratio, a circuit breaker will break these currents with same capability ? what i am asking is just this.
 
StephenSDH

StephenSDH

Senior Member
Location
Allentown, PA
electrics said:
I am asking a simple question friends.İf they have the same X/R ratio, a circuit breaker will break these currents with same capability ? what i am asking is just this.
Click to expand...

If it is a simple question then find a simple book. Your question has be answered many times over here. You can't expect a concrete answer when you ask such a broad question.
 
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skeshesh

Senior Member
Location
Los Angeles, Ca
electrics said:
I am asking a simple question friends.İf they have the same X/R ratio, a circuit breaker will break these currents with same capability ? what i am asking is just this.
Click to expand...

You should read some of the responses. Simple answer is no. Different combinations of inductive/capacitive loads can end up giving you the same X/R at the breaker, while their transient response may vary. I think you really don't have to worry about this issue unless you've been specifically asked to or are designing in a specific case where you know that there are special circumstances. If you do have a situation in a project that you're dealing with, posting more information would allow a more percise answer.
 
steve66

steve66

Senior Member
Location
Illinois
Occupation
Engineer
From a purely theroetical standpoint, and starting with DC current; inductive currents are much harder to break.

When you break an inductive load, the magnetic field starts to collapse, and the energy from the magnetic field tends to keep keep the current flowing in the same direction, with the same magnitude.

Capacitive circuits don't have this problem. When you break the charging (or discharging) current of a capacitor, the remaining energy just remains in the capacitor.

Now considering more pratical circuits, there will always be some inductance in the connecting wiring, so there will always be some small inductive kick across the contacts. But it generally isn't nearly as much as having an inductive load.

Considering AC circuits, transient response, and resonance and other things come into play. But I still believe that in most cases inductive loads would be much harder to open.

So my general answer is that inductive loads are much harder to break.

That's why realys sometimes have separate ratings for inductive loads. I've even see relays with different ratings for different power factors.

Steve
 
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jghrist

Senior Member
electrics said:
I am asking a simple question friends.İf they have the same X/R ratio, a circuit breaker will break these currents with same capability ? what i am asking is just this.
Click to expand...
Bob Wilson's response covered the situation well and illustrates that even though the question was simple, the answer is not.
 
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