AC Drift Velocity: Understand Reactive Power & Movement of Electrons

[Mark_Djorn]

TL;DR Summary

Drift velociry for Alternating Current
Concept of power transmission.
Reactive Power and reverse power

Hi All

I hope this topic makes sense for this forum and somebody can help me to understand.

This is what I understand (please allow numerical approximations):

1. Electrons move in a conductor at a speed anywhere near the speed of light.

2. Electrons gain actual velocity (drift) only when supply is ON. Propagation of EM field is about 270.000 km/s but actual drift in the order of mm/ or ųm/s according to current and conductor section

3. For DC, drift velocity can be calculated.

Now, am I right to say that, in AC, actual average velocity is zero? That's because in a PERIOD, electrons move back and forth. I could calculate the drift in half a period, but in a period would be zero. Correct? This means there is no actual transmissions of electrons from source to load, right?

Under the assumption above, why we keep talking about "power transmission"? As a matter of fact, there is nothing moving from source to load. Electrons simply move back and forth while performing work and the actual drift is in the order of mm/s in obe direction, and same drift in opposit direction. Hence leaving their velocity to zero. Correct?

Under the considerations above, can anyone explain the concept of "reactive power going back and forth from source to load" in cases where power factor is lesser than 1?
I cannot figure out the concept of power "moving" or simply the concept of "reverse power". If in average electrons don't actually travel, what's truly causing reactive power to go back to the source?

Thank you very much in advance!

 

[anorlunda]

P=V*I. If voltage and current are in phase, then when V is negative I is also negative. That makes V*I positive for the whole cycle, not half of the cycle.

 

[phinds]

Now, am I right to say that, in AC, actual average velocity is zero?

Correct, but as @anorlunda explained, that does not make the power zero.

 

[Borek]

There is a system of levers and cogwheels that when I push a rod here 1 kg weight goes up by one meter at your place.

I push the rod, weight goes up.

You pull a lever to switch a gear. Now the weight goes up when I pull the rod.

I pull the rod back.

Rod is in the same position it was, yet the weight on your side is 2 meters higher than it was.

 

[anorlunda]

Here's a mechanical analogy. Fire starting with reciprocal motion.



@Mark_Djorn, might you have the mental model that electrons are little capsules of energy? When they reach the light bulb at the end of the wire, it lights?

 

[Baluncore]

Welcome to PF.

Under the considerations above, can anyone explain the concept of "reactive power going back and forth from source to load" in cases where power factor is lesser than 1?

A sinewave voltage will cause a sinewave current in a resistor. The product of the current and voltage will be a sinewave squared which is always positive.

A sinewave voltage will cause a cosine wave current in an inductor or capacitor, due to the 90° phase shift. Over a full cycle, the product of a sine and a cosine sums to zero real power. During the full cycle, the sign of the product alternates every quarter cycle so energy travels backwards and then forwards twice during the cycle. That constitutes circulating energy.

If in average electrons don't actually travel, what's truly causing reactive power to go back to the source?

Reactive power is not real power which is the real rate of energy flow that you must pay for. Reactive energy is "circulated" by temporary storage in the electric or magnetic fields of capacitors or inductors. It does not flow into the resistive component of a load.

 

[Mark_Djorn]

P=V*I. If voltage and current are in phase, then when V is negative I is also negative. That makes V*I positive for the whole cycle, not half of the cycle.

Understood the concept for reverse power. If I draw cartesian diagrams of the waveforms help. So basically nothing to do with current drift

 

[Mark_Djorn]

Correct, but as @anorlunda explained, that does not make the power zero.

So let's compartimentalise the two topics:
1. drift velocity
2. Power

Power only takes in consideration phases for V and I and it disregard the concept of zero drift velocity. Right?

 

[Mark_Djorn]

There is a system of levers and cogwheels that when I push a rod here 1 kg weight goes up by one meter at your place.

I push the rod, weight goes up.

You pull a lever to switch a gear. Now the weight goes up when I pull the rod.

I pull the rod back.

Rod is in the same position it was, yet the weight on your side is 2 meters higher than it was.

Really liked the analogy. Let me get this right...
Rod=electrons
Weight=work done (power)
Right?

 

[Mark_Djorn]

Here's a mechanical analogy. Fire starting with reciprocal motion.



@Mark_Djorn, might you have the mental model that electrons are little capsules of energy? When they reach the light bulb at the end of the wire, it lights?

Perfect... understood! So definitelly the averafe drift veoocity is zero. Correct? Although work is always there, whether positive or negative

 

[Mark_Djorn]

Welcome to PF.

A sinewave voltage will cause a sinewave current in a resistor. The product of the current and voltage will be a sinewave squared which is always positive.

A sinewave voltage will cause a cosine wave current in an inductor or capacitor, due to the 90° phase shift. Over a full cycle, the product of a sine and a cosine sums to zero real power. During the full cycle, the sign of the product alternates every quarter cycle so energy travels backwards and then forwards twice during the cycle. That constitutes circulating energy.Reactive power is not real power which is the real rate of energy flow that you must pay for. Reactive energy is "circulated" by temporary storage in the electric or magnetic fields of capacitors or inductors. It does not flow into the resistive component of a load.

Perfect, well understood thank you!

 

[anorlunda]

Perfect... understood! So definitelly the averafe drift veoocity is zero. Correct?

Better still, forget that you ever heard of drift velocity or electrons. Almost all attempts to think about electricity one electron at a time result in misconceptions.