Laminates, eddy currents, hysteresis, hysteria etc. 3

[bassnut]

I'm making bass guitar pickups of and old (expensive) design. It's acutally a recreation of a vintage pickup employed in the '50s and '60s.

The design utilises laminations with cylindrical steel pole pieces running through them as well as a wire coil underneath. Alnico bar magnets are situated on either side of the laminate stack. The mag. flux travels to the pole pieces via the laminates. The guitar strings vibrate over the charged pole pieces inducing voltage in the coil....you get the idea.

I'm hoping to get some insight into what's happening in this circuit. From reading and asking around I've learned that the laminations reduce the eddy currents which can effect the inductance of the unit, inhibiting the higher freq. Then there's capacitance, hysteresis not to mention resistance and a whole lot of other principals I barely have a grasp of.

I know this is something employed in electric motors.
Can somebody help me understand this better?
Do the laminations have to be completely insulated from each other? Would very small areas of contact cause the stack to act as a solid core?
What? Too many questions?

Thanks in advance.

Fred.

 

[zeitghost]

Hi.

The eddy currents are reduced by insulating the laminations from each other and making them as thin as practicable.

I don't know the effect of small areas of missing insulation, but I'd be surprised if it was particularly significant.

Hysteresis depends on the material the lamination is made of, with different kinds of steel/alloy/etc. having different effects.

The capacitance (of the coil I assume) depends on the number of turns and the method of winding. I'd suppose that the coil would normally be layer wound.

rgds
Zeit.

 

[electricpete]

I agree with zg.

The purpose of laminations in motors is to limit eddy currents for purposes of limiting the associated heating.

To accomplish this function the laminations must be insulated from each other.

If no laminations or no insulation, the effect is increased heating. There is no significnat change in inductance. The capacitance between laminations is usually not of interest.

 

[bassnut]

In this case heat loss is not a factor. I'm more concerned with anything that is limits frequency. Sound of varying frequencys are being produced by the strings. The idea is to reproduce the sound faithfully. I've noticed that checking the laminations with a VOM I find places where there is continuity, particularly in the holes provided for the pole pieces. I'm coating the laminations with spray insulating varnish before glueing them together.

Here is a view of the underside of the pickup. The top of the lamination stack can be seen between the bar magnets. The copper coil (#43 mag wire) going around the pole pieces are under the stack.

http://a1.cpimg.com/image/63/34/28460131-8677-00800060-.jpg

Here is a view of the top installed.

http://a5.cpimg.com/image/99/80/27994265-6986-00600080-.jpg.

So in this application is 100% insulation between the laminations as opposed to 95% a factor?
BTW the lams are .028 thick low carb steel I had made by a motor lamination company.
I'm doing all the assembly myself. Are there other methods of insulating and attaching them together? The four thru holes present a bit of a problem. Everything must be lined up perfectly.

Thanks everybody for your input.

Fred

 

[jbartos]

Suggestion: Beside laminations being mutually insulated to reduce Eddy Currents, the lamination material is more effective if it has the higher resistance. What are those laminations made of?

 

[bassnut]

I found this text on a website.

"The laminations are often oxidised to form a surface film of oxide that has a higher resistance than plain steel, thus isolating each layer to a certain extent and reducing eddy currents that may occur perpendicular to the plane of lamination."

If this is accurate then it isn't necessary to have complete isolation between the laminations. Agreed?

Another question. I'm using thin copper shielding tape on this pickup assembly to shield the coil from RF and EMF "hum". I've got the tape going over the top and down the sides of the lamination stack (this being under the coil) and between the magnets and the lamination stack. Will this adversly effect the function of the laminations? The stack is essentially wraped in copper tape on 3 sides.


Fred

 

[jbartos]

Suggestion: Copper shielding may impact magnetic circuits since it can create the magnetism if some induced current circulate in the copper shield tape.

 

[bassnut]

I was afraid of that. The tape would cause yet more eddy currents right?

 

[jclough]

Electric Pete,

We see a significant difference in the industance values of coils woune around solid vs laminated stator cores. We can even tell the difference between .018 and .014 thk lams. We use a higher frequency test to detect the difference (10Khz). As I recall there was a difference at 1khz but one would have to put on thier statistics hat to tell the lot to lot difference.

It was necessary to develop this test as the stator cores with the two different lams looked identical and would occasionally get mixed. Each of the two has a different speed torque curve and of course the customer wants to get what they're paying for.

 

[jbartos]

Suggestion to bassnut (Mechanical) Jan 13, 2004 marked ///\\I was afraid of that. The tape would cause yet more eddy currents right?
///Yes, potentially it may.\\\

 

[pwdi]

Regarding the matter of having the laminations insulated from each other it ocurred to me that having any two laminations in contact at one single point wouldn't matter much, whereas if they touched at two points then that would constitute a circuit, thus allowing a current to circulate. Does anyone have any thoughts on that?

 

[jbartos]

Suggestion: Yes, this may happen. However, considering many laminations stacked and relatively small losses by eddy current, the impact may be found tolerable up to a certain extent.

 

[bassnut]

Thanks.
The frequency range on the bass guitar is between 40Hz to around 3KHz. What happens to these currents at the higher frequencies? I read somewhere that the ability of the laminations to resist eddy currents lessens or ceasses completely with the insulation acting as a dialectric and the stack becomes like a capacitor. I don't exactly know what this means. If true, how would it affect the circuit?
The idea of the whole design is to reproduce the largest frequency range possible. I'm trying to get a grasp on how that happens...since I'm actually selling these things. I need to understand what's happening physicaly so I can be sure that they're all they can be.
BTW, the output on this pickup is immense compaired to other more common and simpler designs. It sounds like a pickup with about 2x the number or turns on the coil without the load limiting loss to high freq. There's a real "mojo" happening between the coil, magnets and steel.

Fred.

 

[Manfred12345]

I would suggest that your bass guitar has much highr frequency componets. The relative amplitudes of these harmonics to the fundamental and each other determine the "sound". This lets us humans distinguish a certain note as comming from a guitar or a piano. You might need most of the harmonics up to 15kHZ to maintain the full tone.

Manfred12345

 

[bassnut]

Right you are! I guess I was thinking of fundimentals. Even then I may be off...
I'm still trying to understand what's happening with the magnets,laminates and coil at these higher freq. There's a few different principals that come into play and they keep changing with freq. i.e. hysteresis, eddy currents, skin effect, distributed capacatance etc. I would like to get a mental picture of what happens. I'm more mechanically inclined and need some help with visualization.
My sincere thanks for all of your input so far.

Fred.