Rotating Magnetic Field's and Inductors.

[verpies]

A degree wheel, a couple optocouplers, a divide by 90 and a 4046 PLL come to mind...   

I did that once but my strobe light was not  precise/fast enough.
I eventually settled for an optical shaft encoder with 3600 counts/rev.

[verpies]

If i change out the magnets for say laminated core pieces,would that have the same effect?.

In my opinion the effect of a soft ferromagnetic rotor on a stator coil is very different from a hard ferromagnetic rotor (a permanent magnet).

This is because domains in the soft one can freely rotate and in the absence of any external magnetic field, they can randomize their magnetic polarization down to almost zero net flux, while in a permanent magnet - they cannot because they are pinned. 
Yes, it takes energy to polarize these soft domains, but when these domains return to their randomized state, that energy can be recovered.

There are 20 different interactions in magnetic motors:
  1) A permanent magnet is approaching an attracting coil
  2) A permanent magnet is departing a repulsing coil
  3) A permanent magnet is departing an attracting coil
  4) A permanent magnet is approaching a repulsing coil
  5) A shorted coil is approaching an attracting permanent magnet
  6) A shorted coil is departing a repulsing permanent magnet
  7) A shorted coil is departing an attracting permanent magnet
  8) A shorted coil is approaching a repulsing permanent magnet
  9) A shorted coil is approaching an attracting coil
10) A shorted coil is departing a repulsing coil
11) A shorted coil is departing an attracting coil
12) A shorted coil is approaching a repulsing coil
13) A soft ferromagnetic is approaching an attracting coil
14) A soft ferromagnetic is departing an attracting coil
15) A soft ferromagnetic is  approaching an attracting permanent magnet
16) A soft ferromagnetic is departing an attracting permanent magnet
17) A permanent magnet is approaching an attracting permanent magnet
18) A permanent magnet is departing a repulsing permanent magnet
19) A permanent magnet is approaching a repulsing permanent magnet
20) A permanent magnet is departing an attracting permanent magnet


Note: Coils connected to constant voltage power supplies react like shorted coils.
I don't even consider unloaded coils (non-shorted) because such coils do not interact in motors.

So it is really easy to become confused among all these interactions.
For example: the current in the coil decreases only in cases 1,2,5,6,9,10,13.

[verpies]

If the magnets were increasing the effective inductance of the coil,why dose this increase not happen when the magnet(either pole) on the rotor is stationary in front of the coil?.

Because the permeability of permanent magnets is close to the permeability of air.  Strange but true...
By comparison, the permeability of soft-steel or ferrite is much much greater than air.

I think Picowatt was referring to the effective inductance which is the ratio of flux and current.
If the magnets are supplying   part of the total flux then there is more flux for the same current ...or the same flux for less current, which increases the ratio and hence the effective inductance.

I think the rotor magnets are inducing an EMF across the coil,and it is this EMF that is the cause of the reduce P/in,as the battery now dose not have to supply that bit of energy to create that EMF that already exist.

If we look at the scope shot,we can see that induced EMF from the rotor magnets has the correct polarity to that of what the battery will supply during switch on.

That is also a correct way to look at it.
The EMF of the battery and the induced EMF superimpose on each other.

I prefer a third way of looking at it, namely the superposition of currents ...because coils are current devices (they store energy proportionally to the square of current)

[verpies]

Thinking about that some more--that will not work out,as the field from the coil will want to pull on the rotor magnets in the opposite direction 50% of the time.

Yes, you have to energize these coils in such way that they interact with the rotor as described in case #1 or case #2 ...or both.

Im now thinking that in order for the rotor to spin at all,it must be synch'ed to every second set of magnets 

If you are limited to unipolar pulses then yes, but if you can do bipolar pulses then you can alternate the polarity of your pulses in synch with the polarity of your magnets.

UMmm-will have to do this strobe setup,so as i can see what is going on here.

The strobe is a really good idea, because it lets you see the mechanics and the scope lets you see electronics only.
The magnetics can be deduced.

P.S.
I was once using an xenon automotive strobelight but it was too slow    ...are these phosphorised LEDs any faster?


Brad

[verpies]

Yes i would, but [the rotor accelerates] also during the kickback cycle,as current still flows through the coil in the same direction during the kickback cycle.

I agree