JT> If I recall, the easiest way around it, is to gap the core (or,
JT> increase the gap if it already has one). It reduces the
JT> inductance, but that isn't the reason, it also increases the
JT> amount of current you can pass through the coil before the core
JT> saturates.

 I could teach this so you actually understood it. It's tricky,
because it runs backwards to what you think, after volts and amps

 The flux is set by the turns and the volts. It doesn't really change
much if you have an iron core in there or not. (only a second-order
effect)

 flux = e/(n.f)    e = volts, n = turns, f = frequency

 The *current* in the coil is what changes when you put a core in.
The flux is the same, but an iron core opposes any change, it has a
high permeability, it creates a high inductance, and a low current.

 When you put an airgap in its effect depends on it's length compared
to the iron core length divided by its permeability. Theiron and the
air are effectively in series. If iron has a mu of 1,000 then a
millimetre of airgap is equivalent to 1 metre of iron and the overall
effect is to halve total permability, halve inductance and double
current.

 mu/mo = 1/(lg + lc/m0)    mu is total permeability, mo iron
                           lc core, lg gap length

 The iron loss remains the same, because the flux remains the same.

 What you said is actually right. The same current in the coil with
the gap will actually halve the volts and halve the flux... but who
needs half-voltage? For a given *voltage like the mains, the only way
to reduce flux density is to increase turns or increase the area of
the core.

JT> I have no idea how one would go about doing this on a
JT> production run of a motor. With cores, it's easy, you just get
JT> the assemblers to add a sliver of paper of whatever inbetween
JT> the core to get a gap there, with a motor, it's all fixed, the
JT> only way around it is a redesign- which isn't an option. 

 Motors have no gaps.

 The only time you add gaps in iron is when a DC field exists with
the AC field, or if you want to make a linear inductor. Otherwise, you
make the gaps as small as possible and adjust the turns to set the
flux.

Regards,
Bob


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