Hi,
BL is the force produced per unit current ie F (in newtons) when 1A current passes thru the coil. In other words BL is nothing but the amount of flux that the coil can integrate. Now, if motor is not changed and only the coil is changed from 4 ohm to 8 ohm then BL must not change.
So why does BL of 8 ohm driver differ from 4 ohm driver in the same product line up.
I checked 3 drivers types 12"/10" and 8" listed at parts express
and noted that BL of 8 ohm drivers is generally 1.3 times BL of 4 ohm driver. Why is that so?
Thanks in advance,
WA
BL is the force produced per unit current ie F (in newtons) when 1A current passes thru the coil. In other words BL is nothing but the amount of flux that the coil can integrate. Now, if motor is not changed and only the coil is changed from 4 ohm to 8 ohm then BL must not change.
So why does BL of 8 ohm driver differ from 4 ohm driver in the same product line up.
I checked 3 drivers types 12"/10" and 8" listed at parts express
and noted that BL of 8 ohm drivers is generally 1.3 times BL of 4 ohm driver. Why is that so?
Thanks in advance,
WA
BLs of few drivers, first 4 ohm, then 8 ohm
RSS210HO:-12, 15.6
RSS315HF:-13.9, 18
SW312WA03:- 14.5, 18.4
RSS210HO:-12, 15.6
RSS315HF:-13.9, 18
SW312WA03:- 14.5, 18.4
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BL is the Magnetic flux(B) times the length of the wire(L) in the flux. Usually a driver with a higher DC resistance and a higher Le has more wire in the voice coil. A longer wire in the same magnet structure has a higher BL.
If you want to see the math take a look at this post
Bl conversion factor
If you want to see the math take a look at this post
Bl conversion factor
But isnt wire in the voice coil a function of the space in the gap. Space is constant so the amount of copper is also constant.
That fixed space could be filled by a fewer turns of thicker wire in case of 4 ohms and more turns of thinner wire in case of 8 ohms . Does that make all the difference?
Model a dual voice coil driver in series and parallel configuration. BL stays the same in the parallel config because the overall length of the conductor is the same, it just has twice the cross section. BL doubles in the series config because the overall length of the conductor has doubled.
When I said "same gap" I meant same volume of wire.
A gap can be filled (fully) by 1,2,4,or 8 ohm wire and I thought BL stays same irrespective of how the Re is arrived at.
So, if thinner wire is used to arrive at a higher Re (say 100ohms, for discussion sake), the BL will be very high?
In essence to have the same essential T/S-parameters, 4 and 8 Ohm drivers are very different below the surface. You want the 4 Ohm version and 8 Ohm driver to behave similar to each other (other wise they would be two different drivers). That means you want to have at least the Fs, Qts and Vas to be very similar for both the 8 Ohm and 4 Ohm version.
If you look at this formula:
For Qes (plus Fs and Mms) to remain the same, if Re halves (BL)^2 should also halve. That means BL should be 1.414 times lower, as 1.414^2 = 2, so that is close to that 1.3 value that you found.
Usually the Re of a 4 Ohm driver isn't exactly half that of an 8 Ohm driver and because of the wire change (thickness/ length, etc.) Qes, Fs and Mms (amongst others) can also change a bit, giving a deviation from the theoretical 1.414 factor.
If you look at this formula:
An externally hosted image should be here but it was not working when we last tested it.
For Qes (plus Fs and Mms) to remain the same, if Re halves (BL)^2 should also halve. That means BL should be 1.414 times lower, as 1.414^2 = 2, so that is close to that 1.3 value that you found.
Usually the Re of a 4 Ohm driver isn't exactly half that of an 8 Ohm driver and because of the wire change (thickness/ length, etc.) Qes, Fs and Mms (amongst others) can also change a bit, giving a deviation from the theoretical 1.414 factor.
The way I look at it, the driving force is ampere-turns. Current flowing in the coil x number of turns in the coil. How a change in Re causes various parameters to change is a rather more complex issue. Thickness of wire, thickness of the enamel coating on the wire, mass and volume of the adhesive used to bond the wire to the former ...
So, If one wants to know how to extract the max force for a given current (say 1 A) out of a given motor what should one do? Pls note the 1 A current, for discussion sake (lets ignore what voltage gets that). A super low Re of 0.1 ohm, or super high Re of 100 ohm?
Assume, Fs (since mass & volume of coil will remain same, ignoring minor differences of winding tolerances, glues, etc) and Mms, Cms remain same.
Assume, Fs (since mass & volume of coil will remain same, ignoring minor differences of winding tolerances, glues, etc) and Mms, Cms remain same.
A large impedance, combined with a constant-current source, means the voltages are high, so there must be a large input power.
Sam's right - the driving force is not current. It's current multiplied by the number of turns in the gap.
More turns = higher BL = longer bit of thinner wire = higher Re.
Chris
Sam's right - the driving force is not current. It's current multiplied by the number of turns in the gap.
More turns = higher BL = longer bit of thinner wire = higher Re.
Chris
May this little thougt will help? If you look at a short bit of a wire in a magnetic field (the letter "o" below as a cross section of the wire), there will be a force acting on the wire. The arrow represents the force.
o ->
If you make another turn of the same wire it will be conducting the same amount of current, but the force (and resistance, hence voltage needed) will be doubled:
o ->
o ->
If you instead use two wires in parallel the force is also doubled, but you need twice the current.
This is not exactly the same as BL, but you get the nature of a coil.
o ->
If you make another turn of the same wire it will be conducting the same amount of current, but the force (and resistance, hence voltage needed) will be doubled:
o ->
o ->
If you instead use two wires in parallel the force is also doubled, but you need twice the current.
This is not exactly the same as BL, but you get the nature of a coil.