pretty confused...

[mblaquière]

Hi guys before I start I wanted to say that I'm training to become an electrician but I've come across some thing in my studies that's completely confused me.

My understanding of magnetic flux is that it rotates around the conductor when a current passes through it and the direction it rotates depends on what end your looking at it from. Hopefully that makes sense for a start? So clockwise if you were looking from the - end with current flowing away from you.

Now I've come across a sentence in my theory books that is: "a single phase a.c supply produces a pulsating magnetic field, not the rotating field produced by a three phase supply". This has confused me in two ways:
1. Why is it pulsating for single and rotating for three phase, and
Something I hadn't thought about until I saw this sentence and I apologise if it's a ridiculous question but
2. Wouldn't the magnetic flux be changing it's rotation with the change of direction of electrons due to the a.c?
Hopefully that makes sense.
I'm completely baffled so if anyone could help I'd be very very grateful!

Thanks

 

[telectrix]

too difficult to explian on a forum. suggest you google "basic a.c. theory" then go through some of the results.

 

[Lucien Nunes]

Two separate concepts are being confused here.

Magnetic flux does not 'rotate around a conductor'. It has direction, and we show this by 'lines' with direction arrows on, but nothing is actually rotating just as there aren't really any 'lines', it's just a visualisation of the effect the current produces.

When we speak of a rotating flux vector in the context of a three phase machine, we mean that the vector representing the net effect of the three phases rotates relative to some reference. This is a real effect not just a visualisation.

In a single phase machine, the magnitude of the net flux varies (approx sinusoidally) with time passing through zero twice per cycle, so that is described as 'pulsating' i.e. on and off. In a three phase machine, all three phases behave in this way but their equal mutual angles and magnitudes cause their vector sum to have constant magnitude but continuously changing direction. This is described as 'rotation'

 

[Knobhead]

Fleming's left-hand rule for motors - Wikipedia, the free encyclopedia

 

[telectrix]

Fleming's left-hand rule for motors - Wikipedia, the free encyclopedia

is that something to do with james bond?

 

[mblaquière]

Haha ok cool I think I made sense of that, the thing that is still confusing me though is the whole a.c changing current direction. Wouldn't this change the flux's behaviour? I understand that it does t actually rotate around the cable hit theoretically would it change it's rotation every time the current goes back and forth?

 

[Lucien Nunes]

Yes, the direction of the flux reverses with the current. In a medium such as air where the flux is accurately proportional to the magnetomotive force produced by the current, their waveforms will be identical. In ferromagnetic materials such as steel the current and flux might not change in proportion or exactly in phase but the flux will still reverse every time the current reverses. Even at high frequencies: Wifi operates at 2.4GHz, therefore the magnetic field associated with the transmitter and receiver currents reverses 2,400,000,000 times per second.

The principle that changing current produces changing flux was one of the pivotal discoveries of the 1830s, enabling Michael Faraday and Joseph Henry to demonstrate induction and make the first working transformer.

 

[44ALLAN]

I doff my cap...............and that is meant as the biggest compliment I can give

I'm probably after 40 year in this trade still on the slippery 2nd rung of the ladder..........brilliant stuff it really is thanks Lucien/tony/eng and many many more...........you know who you are