# Does an inductor always create a back EMF?

• I
• Jaccobtw
Jaccobtw
TL;DR Summary
Does an inductor always create a back emf?
Let’s use a solenoid for example. Let’s say you have a current that passes through a solenoid clockwise. Due to Faraday’s law of induction, the induced current from the magnetic field from an increasing current opposes the initial direction of current causing resistance. If the initial current were to decrease then the induced current would flow in the same direction as the current slowing the decrease. So we can say the inductor in this orientation opposes a change in current.

However if a current were to pass through an inductor that is wound counterclockwise, wouldn’t the magnetic field produced by the changing initial current assist the initial increasing of current instead of impede it? Or if it was decreasing wouldn’t it “speed up” the decrease in current?

Basically what I’m asking is doesn’t the direction the inductor is wound matter for how it will affect the change in current?

alan123hk
it's out of the question. When the direction of the current is reversed, the direction of the magnetic field generated by the current is also reversed, so the direction of the back EMF also becomes reversed. In fact, according to Faraday's law, the direction of the back EMF is always opposite to the direction in which the current increases, or in the same direction as the current decreases.

• vanhees71, Delta2 and Jaccobtw
Homework Helper
Gold Member
It is simply ##V=-L\frac{dI}{dt}## where in this equation does enter the factor if the current is clockwise or counterclockwise? All that it matters is if the current is increasing (dI/dt positive) or decreasing (dI/dt negative).

• Jaccobtw
Basically what I’m asking is doesn’t the direction the inductor is wound matter for how it will affect the change in current?
What would happen, if it had only one turn? Would one turn be clockwise or counterclockwise? If there was a core, how would the core know which way it faced?

It does not matter which way an inductor is wound, unless it is being used as a transformer. An inductor is not polarised, and can operate symmetrically on AC.

• • DaveE and Delta2
Homework Helper
Gold Member
We will deviate from the main subject but anyway why in transformers the direction of wounding plays a role?

• vanhees71
Gold Member
We will deviate from the main subject but anyway why in transformers the direction of wounding plays a role?
The winding direction referred to the other winding direction(s) matters. The OP is referring to switching a winding from CW to CCW. In a transformer, you would need to reverse all of the windings to have no effect. i.e. transformers have polarity, inductors don't.

Last edited:
• vanhees71
Homework Helper
Gold Member
In a transformer, you would need to reverse all of the windings to have no effect.
Sorry I lost you here, do you mean that if we simultaneously reverse the wounding of primary and secondary then nothing happens, but what if we only reverse the wounding of the primary?

Gold Member
Sorry I lost you here, do you mean that if we simultaneously reverse the wounding of primary and secondary then nothing happens, but what if we only reverse the wounding of the primary?
Then the polarity of the secondary voltage will be reversed because the direction of the flux generated by the primary has been reversed.

Homework Helper
Gold Member
Then the polarity of the secondary voltage will be reversed because the direction of the flux generated by the primary has been reversed.
Ok I think I can understand that, but what the OP claims is that the direction of wounding can affect the way the back EMF affects current. The back EMF is always such as to oppose the change of the current through the inductor, this is Lenz's law (or the minus sign in Faraday's law), this CANNOT change by the direction of the wounding since it is a law of physics, a law of the universe. That's all I can say.

Gold Member
Ok I think I can understand that, but what the OP claims is that the direction of wounding can affect the way the back EMF affects current. The back EMF is always such as to oppose the change of the current through the inductor, this is Lenz's law (or the minus sign in Faraday's law), this CANNOT change by the direction of the wounding since it is a law of physics, a law of the universe. That's all I can say.
OK, so we've switched back to inductors then? Yes, that's what everyone else here is saying too.

Jaccobtw
OK I think I understand. The direction the inductor is wound will affect the direction of the magnetic field inside the inductor (or at least this is what I've concluded using the right hand rule), but it doesn't change the back emf.

So what about placing a magnet inside of the inductor? How does this increase inductance? If inductance depends on a changing magnetic field, how does placing a constant magnetic field inside an inductor increase inductance?

• Delta2
Gold Member
OK I think I understand. The direction the inductor is wound will affect the direction of the magnetic field inside the inductor (or at least this is what I've concluded using the right hand rule), but it doesn't change the back emf.

So what about placing a magnet inside of the inductor? How does this increase inductance? If inductance depends on a changing magnetic field, how does placing a constant magnetic field inside an inductor increase inductance?
It doesn't (not counting that the materials may be different, permeability wise). It just adds a constant (DC) flux bias into the core. Inductance only relates to dynamic changes in the parameters, which you can see readily in the equation ## v = L \frac{ di}{dt} ##. This is essentially the same as putting some DC current into the winding in addition to the current that changes. The derivative of the constant current is zero and has no effect on the induced voltages, you only get those when things change.

BTW, I've seen this done in some switching power supplies to increase the amount of dynamic energy that can be stored without saturating the core. But that's a pretty advanced concept for this thread.

Last edited:
Gold Member
So what about placing a magnet inside of the inductor?
That is done with some small signal relay coils to reduce their current requirements, but those relays are polarised and operate from logic signals.
A magnet does not change the inductance, it only affects the saturation. A magnet near an AC inductor or transformer is a liability because it makes the saturation asymmetric, and so generates harmonics.

Homework Helper
Gold Member
So what about placing a magnet inside of the inductor? How does this increase inductance?
Inductance would probably increase, not because it's a magnet but because it's probably a high-permeability material like iron.

• • vanhees71 and Delta2