Magnetic flux

[John Rudd]

Ok, I'm really out on a limb with this........

We all know that if a current passes through a coil a mag field is generated which is dependant on the number of turns and the applied voltage......

But is that through a winding of copper wire or what?
 
Does the wire characteristics  influence the field?

Reason I ask is.... I recently rewound a rotor for a generator, fortunately it had copper windings, so I cut them away and counted the ends. Multiplied that by the average length and arrived at what I needed to rewind it...As it happens it turned out I need around 284 metres which is roughly 500 gramms ( in actual fact it is exact!!!)...

Now I have another rotor but it has been wound with Aluminium enamelled wire!!!!.
This really threw me, when I tried to calc the length/mass and it wasnt until today I revisited weighing the wire I'd cut off....

So now I need to decide on how much copper wire I need..........

Any offers of advice? ( apart from call the Samaritans or take up knitting....)

[Bluechip]

John
 
Just had a shufti at a couple of leccy text books ... all they ever seem to say '(Ampere * Turns ) / Metre of Flux Path'.
 
 
None mention conductor material. All I can think is the Al will be higher R for a given CSA, so you would either have to drop the current or use a larger gauge for Al.  ( Assuming you want the I²R loss to be the same )
 
Nothing new in Al. conductors. There was once a use for it in Domestic Wiring. It had much bigger conductors. IIRC The 2.5mm ring main T&E was about the same size as 4mm T&E copper. Not popular, broke too often.
Tighten a screw terminal on the stuff and it just went ping !! OK with 'rising clamp' terminations, but it really was dire stuff to use. Did teach me a lot of variations on standard profanities though ...
 
http://en.wikipedia.org/wiki/Aluminum_wire
 
 
 
Dave BC

[Noitoen]

For motor windings, the number of turns is related to the voltage and the thickness of the wire, the current. So you have to count the number of turns to wind the same amount. Since on a motor, higher resistance of wire will result in efficiency loss due to heat, the thicker the wire, the better. When they design a motor's slot to take aluminium to save on cost, they have to make more room for the wire in relation to copper. If you want, you can use the same diameter copper wire.

[John Rudd]

If you want, you can use the same diameter copper wire.

Yeah, I had bargained on using the same diameter in Copper...But what threw me was the difference in weight per unit length...I couldnt work out why the mass of copper wire was different compared to what I got for a weight of the original windings......

Copper is 3.2 more dense than Aluminium

Looks like I need around 1100g of copper to get me where I need to be....I need a turn counter now!

Dave, Yup thanks for the techie bit...I could'nt be ar*ed  to look it all up...Its all behind me from years back when I was at college...

[awemawson]

A lot of data cables these days are "CCA" which is copper coated aluminium. I bought some 12 core stranded (so flexible) alarm wire the other day to make up some RS232 cables. Turns out that all that was available was CCA. Takes a Sub D crimp and solders ok but I'd much rather have 'proper copper' !

[Bluechip]

For motor windings, the number of turns is related to the voltage and the thickness of the wire, the current. So you have to count the number of turns to wind the same amount. Since on a motor, higher resistance of wire will result in efficiency loss due to heat, the thicker the wire, the better. When they design a motor's slot to take aluminium to save on cost, they have to make more room for the wire in relation to copper. If you want, you can use the same diameter copper wire.

Not sure about that. If the Al. is replaced by the same number of turns and the same gauge in copper, the resistance will be much lower. Current will increase. ??
 
Dave BC

[Noitoen]

No, the current will not increase. The current is "regulated" by the core and the back EMF generated by the moving rotor. If you measure the resistance of an ac circuit, being it a motor or transformer and try to calculate current with ohm's law, you will see that it doesn't ad up. What works here is the Z in ohms which is related to the inductance in Henry and frequency in Hz. The inductance is related to the size of the magnetic core and number of turns.
If you power up a transformer coil without the core, it will burn in seconds due to over current.

[Bluechip]

Still not convinced. That winding is a series L/R circuit. If you change R Then the phase angle will change as will the Z vector. If the impedance changes, so will the current.
It can do no other.
 
Too late in the day for me to fool about with vector diagrams though ... 
 
Early start tomorrow .. 
 
 
Dvae BC

[awemawson]

It's the overall impedance of the windings that counts regarding current flow. ie the R plus the Z. In the case of an aluminium winding compared to a copper one the difference in R will be so small as to be insignificant compared to the Z component unless it's a very odd generator.