Inductor

Wendy

Joined Mar 24, 2008
22,145
Counter EMF only gets involved if there is another mechanism, such as in a motor. When you power up a motor it also becomes a generator simultaniously, which generates a nearly equal but opposite current, which resists the current you are feeding it. If you force a motor to stall the counter EMF goes away, and causes the motor to draw a lot more current (sometimes huge amounts).

Inductors are exactly opposite of capacitors, their antiparticle if you will. They also store electricity, as current. You stop current flow and the inductor tries to keep it flowing by generating large voltage surges, much like a capacitor can generate huge current surges.
 

Thread Starter

James4553

Joined Jun 7, 2008
35
Bill; Sorry for being such a newbie but is it capacitors that store voltage and inductors that store current?

Also, is it correct that inductor's impedence to the circuit is determined by the frequency of the signal?

Also (sorry!), why would you use an inductor to oppose current instead of just using a resistor?
 
Last edited:

bertus

Joined Apr 5, 2008
20,566
Hello,

There is a piece written in the AAC eBook.
http://www.allaboutcircuits.com/vol_1/chpt_15/index.html

More links on the subject Coils and inductors can be found in the EDUCYPEDIA.
http://www.educypedia.be/electronics/coilsrelay.htm

There are more pages on other subjects over there too.

Passive semiconductors technology: Batteries Diodes Photovoltaic systems Switches Buzzers Diode types Quartz crystals Thermoelectric Capacitors General overview Relays Transformers Capacitor types Heat Sinks Resistors Transformers-RF Coils Photo diode Resistors-Nonlinear Transformer types

Greetings,
Bertus
 

scubasteve_911

Joined Dec 27, 2007
1,203
Inductors and Capacitors both store energy. It's their relationship to voltage and current that are opposite.

V = L*dI/dt
I = C*dV/dt

Energy

Inductor: 0.5*L*I^2
Capacitor: 0.5*C*V^2

Steve
 

Ratch

Joined Mar 20, 2007
1,070
James4553,

I'm just wondering what an inductor is useful for?
1) As a dynamic energy storage device.
2) As part of a resonant circuit.
3) As an AC current limiter.
4) As an AC current/voltage phase shifter.

And what about the counter EMF?
Counter voltage is always present when the coil current is changing. During that time it is storing or releasing energy in a magnetic field. The value of the counter voltage is directly proportional to the rate of current change. During the time that no current change is present, the coil sustains a steady magnetic field where energy is stored. This field always present when current exists.

Bill; Sorry for being such a newbie but is it capacitors that store voltage and inductors that store current?
No, both store energy. Capacitors store energy in a electric field, and inductors store energy in a magnetic field. Voltage and current by themselves are not energy. Voltage sustains a electric field, and current sustains a magnetic field.

Also, is it correct that inductor's impedence to the circuit is determined by the frequency of the signal?
Yes, inductive impedance is proportional to the frequency and also to the value of the inductance.

Also (sorry!), why would you use an inductor to oppose current instead of just using a resistor?
Inductors only oppose current in AC and transient circuits. Inductors and resistors oppose current by two different mechanisms. Inductors generate a back voltage which opposes current changes. During that time, energy is stored during one-half cycle and released during the next half-cycle. No energy is expended. Resistors reduce voltage by dissipating energy. Since voltage is the energy density of the charge, pushing a charge through a resistor will reduce the amount of energy the charges contain, and thereby the energy density of the charge. So the voltage will decrease because the energy is reduced and dissipated as heat. Of course this works with resistors for DC too. In summary, inductors and capacitors reduce current by back-voltage, and resistors reduce current by dissipating energy.

Ratch
 

Wendy

Joined Mar 24, 2008
22,145
Also (sorry!), why would you use an inductor to oppose current instead of just using a resistor?
A common application for this is florescent ballasts. A resistor generates heat (sometimes lots of heat) resisting current flow. This is pure waste. A coil does it differently, no heat involved, which is why florescent bulbs are generally so efficient.

WWII subs used florescent bulbs in subs, but had DC power, so they were stuck with the resistors. The bolted the resistors to the nearest bulkhead to dissipate the heat.

TMI? :D
 

Thread Starter

James4553

Joined Jun 7, 2008
35
Quote:

Yes, inductive impedance is proportional to the frequency and also to the value of the inductance.
So the inductive impedance is proportional to the frequency and the value of the inductor, but what about the magnitude of the signal? I would think a larger signal would mean more impedance since the change of current in the postive and negative cycles of an AC signal would be greater. Or does the magnitude not make a difference, only the frequency and value of the inductor?

And (last question), is inductive reactance just another name for the resistance a inductor provides to the circuit?
 

Ratch

Joined Mar 20, 2007
1,070
James4553,

.…..Or does the magnitude not make a difference, Or does the magnitude not make a difference, only the frequency and value of the inductor?
Only the frequency and value of the inductor. You should be able to discern that easily by examinining the formula for inductive reactance.

And (last question), is inductive reactance just another name for the resistance a inductor provides to the circuit?
No, inductive reactance is part of the impedance of the coil. The resistance of the coil from its wire is usually minimal compared to the inductive reactance. Read up on impedance and reactance relate to each other, and review what I posted before in this thread.

Ratch
 

scubasteve_911

Joined Dec 27, 2007
1,203
Or maybe you should familiarize yourself with what a differential equation means. I gave you the information above, which is pretty much all the theory you need to analyze a circuit with an inductor.
 
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