Electrical concepts

[panthripu]

Hi guys
I need some information from experts here once again.My questions are
1)In an inductive load where currents lags the voltage , When the voltage is zero what makes the current flow with zero voltage ?

2)During arcing between the conductor or in breaker contacts ,why the arc does not extinguish when voltages passes through zero towards negative cycle ?This question is again related to question 1 , what makes the arcing continue ?

3) Reactive power is useful to keep the machine running. What makes voltage drop when reactive power increase ?

Thanks to all in advance.

 

[Smart $]

1)In an inductive load where currents lags the voltage , When the voltage is zero what makes the current flow with zero voltage ?

The energy stored by the inductor during the last quarter cycle.

2)During arcing between the conductor or in breaker contacts ,why the arc does not extinguish when voltages passes through zero towards negative cycle ?This question is again related to question 1 , what makes the arcing continue ?

I'm no expert on arcing, but I believe once an arc occurs, the resistance across the conductor gap decreases due to the creation of plasma. Even in non-reactive circuits, the arc can restrike because the plasma does not extinguish completely in the instance the voltage is zero.

3) Reactive power is useful to keep the machine running. What makes voltage drop when reactive power increase ?

I don't understand your question completely, but as reactive power increases there is greater I?R loss in the conductor feeding the load (more VA being transmitted), and thus greater resistance and voltage drop.

 

[gar]

110623-1731 EDT

Your question 2.

When the source voltage goes to 0 in an L-R circuit (inductive-resistive) the current is not zero and the voltage across the arc is not zero. When the current goes to zero then the arc voltage is zero and the arc extinguishes. The arc extinguishes because the current goes to zero not because the voltage is zero, and it just happens that when this occurs the arc voltage is also zero. But it might re-strike if enough residual ions are present.

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[wirenut1980]

For question 3, reactive power has reactive current, which is the same as real current, except the phase angle is different. With current, you get voltage drop.

 

[panthripu]

reactive current

reactive current

1)In an inductive load where currents lags the voltage , When the voltage is zero what makes the current flow with zero voltage ?
............................................................................................................
The energy stored by the inductor during the last quarter cycle.

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So as mentioned above , the current flow in the circuit when the voltage is zero is because of stored energy by inductor during the last quarter cycle.
What about the voltage ,when current is max. how come the voltage is zero.
I understand voltage is the driving force in electrical system which results in current based on circuit resistance.
Now if the voltage itself is zero, what makes current to flow?

 

[gar]

110626-0911 EDT

panthripu:

I would never think to ask the question you are asking.

The answer is the inductor voltage.

Consider a series circuit of a ideal sine wave voltage source (no internal impedance), an ideal resistor, and an ideal inductor.

Instantaneously the sum of the voltages around this circuit is ZERO. So then you write the equation for the sum of each of the voltages in the circuit.
vsource + vresistance + vinductance = 0
Obtain a solution for the voltage across the resistance and now you can determine the instantaneous current. The current will be what it will be based on the results of the solution of the equation vs time.

Fundamentally you have two voltage sources -- the driving voltage source and the inductor. Both are sources of energy although originally the energy in the inductor comes from the vsource.

Since the current in this series loop at any instant has to be the same at every point in the loop, and since in a pure inductor current lags inductor voltage by 90 deg, this means the inductor voltage has to lead its current by 90 deg.

In the real world the inductors you encounter have internal resistance so you can not directly measure the inductor voltage.

Since you have two voltage sources in series in a loop you have to combine them to get the voltage across the resistance.

If the resistance is zero, then the inductor voltage is equal to the source voltage, but of opposite polarity, so they cancel.

As you increase the resistance from 0 the phase angle of the inductor voltage shifts from 180 deg relative to the source voltage to lesser values. At some point this shift is 90 deg, and that is where the inductive reactance at the excitation frequency is equal to the resistance. Here the loop current is 45 degrees from the vsource reference voltage.

From a steady-state analysis this is also called the half-power point. or 3 db point. At this point the voltage across the resistor is 0.707 times the source voltage. Since 0.707 is the (sq-root of 2) / 2 this means that 0.707 * 0.707 = 0.5 and that is why this is the half power point.

If you are familiar with differential equations, then you can write the equation for the series voltage loop and develop solutions for voltages or current and see mathematically what happens, but this may not provide a means for you to intuitively understand the circuit.

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[panthripu]

electrical concept

electrical concept

Hi
Thanks for your explaination which is no doubt very good. I always try to imagine the electrical concepts and phenomenon rather than writting them on a paper & calculating them on paper , that is what made me ask this stupid question.

 

[Besoeker]

I don't understand your question completely, but as reactive power increases there is greater I?R loss in the conductor feeding the load (more VA being transmitted), and thus greater resistance and voltage drop.

There is also voltage drop for the reactive component of the supply.

 

[gar]

110627-0903 EDT

panthripu:

It is not a stupid question.

The question now is can you visualize that under steady-state conditions that the series circuit consists of two voltage sources, the primary voltage source and the inductor voltage source, in series to produce the voltage across the resistance? In turn the current thru the series circuit and its phase relationship to the primary voltage source is directly proportional to the resistor voltage.

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