Inductors - Am I missing something?

[lauraj]

I was teaching a lesson the other day on inductors, using a transformer as an example. I talked about how a high current flows for a short time until the field is built up. I've always thought that this is due to the absence of cemf, but as the field builds up, the current drops. Is this correct?

A little bit later, we were looking at the time constant for inductors, t=L/R and how inductors resist a change in current.

Then a question comes up - if inductors oppose a change in current, why do we have that initial high current in transformers?

I believe the time constant would still be there, but because we have bigger R and smaller L at this point, it is so fast that we do not notice. Is this correct?

 

[Smart $]

Inductors oppose a change in current. I believe you have inductors confused with capacitors... slightly. Capacitors have high initial current when a volatage is applied. As the "plates" build up a charge the current decreases. It is said for capacitors the current is ahead of the voltage by 90 degrees. Inductors are exactly the opposite, in that the current lags the voltage by 90 degrees. When a voltage is applied to an inductor, it resists current flowing initially then eases up. Of, course all this happens in a fraction of the time it takes an eye to blink.

The current inrush on transformers is a result of magnetic coupling. Current on the primary side flows until the transformer realizes there's no where for the current to go on the coupled secondary side. The fields balance out and the current drops. This is the non-technical explanation

 

[cpal]

lauraj

you seem to be confusing intensity of current flow and phase relationships of voltage and current in resistive , inductive, and capacitive circuits.

you are correct AC inductors have a low ohmic resistance when at rest. (the DC resistance of the coil conductor R=LXK\CMA). when an alternating voltage is applies it causes current to flow (inrush) the current may be thought of as pulsating, (think of the sine wave), This applied volatge generates a second EMF (in all inductor) which is opposed to that applied (lenzes law of self induced EMF)This induced EMF cancels the applied EMF reducing the pressure on the coil( from the source) and reducing the applied current flow.

The nature of a coil causes the current to lag the voltage by some degree (think sine wave ) this lag is amplitude and polarity. rember E-L-I current lags the volatge (Voltage does not lead ).

The nature of a capacitor causes the current to lead the voltage again refer to the sine wave. ICE, current leads the voltage.

The helper phrase is "ELI the ICE man"

As a disclaimer, I'm not an engineer, so the articulate specifics are not present here, but these explanations are serviceable and generally produce positive results for the every day electrician. If you need to dig deeper the internet and this forum can provide a treasure chest of info
Hope it helps
Charlie

 

[lauraj]

I believe the time constant would still be there, but because we have bigger R and smaller L at this point, it is so fast that we do not notice.

I think I see a flaw in my original statement. R and L are fixed quantities and cannot vary, and the time constant is just the ratio of those two values. My question still is, does anyone know what an average time constant would be for a transformer? Is it fast enough that it appears as though the current rises instantaneously on startup, although I know that it really can't.

 

[winnie]

lauraj

Take a look at this link:
http://www.allaboutcircuits.com/vol_2/chpt_9/12.html

As you note, inductors act against a change in current, and have a current versus voltage time constant. In fact the inductance is simply the number of volts which must be applied to get a rate of current change of 1A per second.

When you have positive voltage applied to the transformer coil, the current will become more positive in a steady fashion; similarly when you have negative voltage applied to the coil the current will become more negative. The rate of change of current flow will depend upon the inductance of the coil and the applied voltage. The issue is that when a transformer is operating in steady state, at the '0' point in the voltage cycle (when the _positive_ half of the voltage cycle begins), the current in the transformer is _negative_. About half of the positive voltage is used up simply getting back to _zero_ current, and the rest used to get up to maximum positive current. At this point the negative half of the voltage cycle is beginning, but the current flow is _positive_.

But what happens when you first turn on the transformer? Freshly turned on the current flow is zero, and if you just happen to be at a zero point in the voltage cycle, then the entire hump of the sine wave is used to push the current in one direction or the other. If this were a perfect inductor, then the peak current would be roughly twice the steady state value.

But transformers are not perfect inductors; they saturate. Rather than getting 2x normal peak current, the transformer tries for 2x normal peak magnetic flux. With saturation this can mean much more than 2x peak current.

So transformer inrush is not so much the lack of 'back emf', but instead the lack of 'reverse current flow' from previous cycles, which limits the peak current that flows in the present cycle.

-Jon