Why would a coil of wire in a DC circuit not produce inductive reactance?

Answer 1

From memory the formulae for inductive reactance is (2xpiexFxL)
Where pie = 3.14 cause I dont know how to get the pie symbol
F= the frequency of the current in the circuit
L=the inductance of the coil in henrys.
Because the F=0 in DC the whole equation comes out at 0 and that's why there is no inductive reactance.

Answer 2

Inductive reactance only happens because, in AC current, the voltage and current vary with time. Because an inductor's inductance slows down the rate at which the current changes, it causes an effect analagous to resistance in an AC circuit.

In a DC circuit, the same thing happens - the inductor slows down the rate at which the current changes - but since the voltage in a DC circuit does not vary, once the current reaches its highest level in the inductor, it stays there. Hence, no resistance-like effect due to the inductance, so, no inductive reactance.

Answer 3

Because a coil only produces inductive reactance in an AC circuit. In a DC circuit it's little more than a resistor.

Answer 4

rsdudm's answer is correct, the Frequency is 0 so no reactance.
However, there is a inductive reactance at the initial on and the off of the circuit. It is transitory, The coil will resist the current change of state from (off to on) or (on to off). But in steady state it is just another piece of wire with DC resistance.

Yours: Grumpy

Answer 5

You are referring to is the imaginery component of impedance. In a DC circuit, E = IR. However, in an AC circuit, E = IZ where Z is impedance. Impedance is known as a complex number, in other words, it has both a real component and an imaginery component. The real component is resistance, the imaginery component is reactance. There are two types of reactance. Inductive reactance (Xl) and capacitive reactance (Xc).

The equation for inductive reactance is Xl = j2(pie)fL

In a DC circuit the frequency is 0. As such, if you put a 0 into this equation then you get 0 for an answer. As such, DC circuits have no reactance.

At first, an inductor acts like a resistor. But its resistance decreases as its magnetic field reaches full inductance.

In order to understand inductive reactance you have to understand that the inductor provides resistance until its magnetic field is fully inductive. In a DC circuit you apply a DC voltage and the current starts off at zero. With the voltage applied, the current begins going through the inductor. The inductor requires energy to create the magnetic field. That energy requirement manifests itself in the form of resistance until the inductor's magnetic field is at full inductance, at which time it applies no resistance. This is why the current starts at zero and builds towards its full value when going through an inductor. Now in an AC circuit, you apply voltage and thus current in one direction, and the inductor's magnetic field increases and the resistance decreases, but then you reverse the voltage. The inductor's magnetic field tries to force the current to keep traveling in the original direction. However, eventually the voltage wins and the current reverses direction, then the inductor's magnetic field increases in that opposite direction. You then reverse the voltage and the same thing happens over and over. As such, it takes the current a specific amount of time to reverse and catch up to the already reversed voltage. In other words, the current's sine wave lags the voltage's sine wave. The opposite happens with capacitive reactance, the current leads the voltage.

Inductive reactance, just like capacitive reactance, is considered imaginery because it adds a phantom power load to the circuit. In other words, it wastes power without actually creating usable work, heat, light, etc.

You may have individual inductances and capacitances in individual branches of a circuit, However, once the entire circuit is completely designed, the electrical engineer will then determine the circuit's total reactance, and add a capacitor or inductor to cancel out all reactance so that the circuit doesn't waste power in an undue manner.

Inductive reactance and capacitive reactance can be added linearly when in series. The equation for capacitive reactance is:

Xc = -1/(j2(pie)fC)

So in analyzing these two equations, the larger the inductor, the larger the inductive reactance. However, the opposite is true with capacitive reactance. The smaller the capacitor the larger the capacitive reactance. This is because the value of C is in the demoninator in Xc, but the value of L is in the numerator in Xl. If you have a highly inductive circuit, such as an electric motor, it will require a small capacitor to cancel out the inductive reactance. An increase in inductance will require a decrease in the size of capacitor required to cancel out the Xl. The same applies for capacitive circuits. The smaller the capacitors used in the AC circuit, the larger the inductor will be required to cancel out the Xc.

So when analyzing a series RLC circuit:

1. Calculate the reactances of each individual capacitor and inductor.

2. Linearly add the resistances with each other

3. Linearly add the reactances, both inductive and capacitive, with each other. Capacitive reactances are negative and inductive reactances are positive, so when added, it may equal 0. In the real world, this is desirable.

4. You now know the total resistance and total reactance.

5. Ideally, you will then neutralize the reactance by adding an inductor or capacitor to make it equal to zero as mentioned in step 3. But if you are simply analyzing the circuit, see step 6.

6. The amplitude of the current is equal to the square root of the squares of the resistance and reactances, in other words:

current amplitude = square root of (R squared + X squared)

7. The phase shift resulting to the current sinusoid caused by the reactance is equal to:

d theta = arctan (X/R)

So now you can apply steps 6 and 7 to the voltage sinusoid to determine the current sinusoid.

For parallel RLC circuits, there are a couple of new terms. Admittance is equal to inverse impedance. In other words Y = 1/Z. Just like impedance, admittance is also a complex number, and has both a real component and an imaginery component. The real component is conductance, which is equal to inverse resistance, in other words G = 1/R and the imaginery component is suseptance, which is equal to inverse reactance, in other words B = 1/X.

Inductive suseptance is:

Bl = -1/(j2(pie)fL)

and capacitive suseptance is:

Bc = j2(pie)fC

Suseptances add linearly when in parallel. So do conductances (again, this is the inverse of resistance).

So when analyzing a parallel RLC circuit:

1. Determine circuit conductances (by taking the inverse of the resistances)

2. Determine the suseptances of each capacitor and inductor

3. Linearly add the conductances with each other

4. Linearly add the suseptances with each other. Just like reactance, they may equal zero because inductive suseptance is negative and capacitive suseptance is positive.

5. Change your conductances and suseptances into resistances and reactances by taking the inverse of each.

6. Start with step 4 in the first set of instructions (not the 4 immediately above this step 6, but the step 4 in the original instructions for series RLC circuits above).

This covers a good extent as to what your electrical engineering professor will want you to understand as you get closer to finishing your first course in electric circuit analysis.

Answer 6

because in a dc circuit current can only travel in one direction