Magnet Question

[IDSkoT 5,000+ posts]

MOAR KNOWLEDGE!!!!!!!!!

Srsly. I have no idea about the inner workings of subs/speakers. I just get their basic principle.

 

[Haunz 10+ year member]

The motor assembly will sink some heat from the coil but the former and the air flow within the magnetic gap make a much bigger difference than motor size in cooling because the transfer of heat from the coil to the motor is horrible inefficient since they don't even touch. The magnet itself is a terrible heatsink material anyway, so the size of the actual magnet has no bearing on cooling.

Are you really going to argue that a motor is a poor heat sink because there is a thin layer of air between it and the coil.. then state in the next sentance that the same air is the main method of cooling ?

Clearly, the material in a motor structure has a much greater heat capacity and thermal coefficient then air.. and it also provides a great deal of surface area for heat to disapait from..

No doubt the air moving in and out of the gap is the main source of cooling to a voice coil; but the thermal mass and surface area of the motor does effect power handeling of a woofer...

Anyone who has handled a large driver after it has recieved a healthy amount of power for any significan't period can attest to the amount of heat that is given off by the motor.. //content.invisioncic.com/y282845/emoticons/fyi.gif.9f1f679348da7204ce960cfc74bca8e0.gif

 

[audioholic 5,000+ posts]

I wouldn't consider the size of the motor as any realisitic difference in heat dissipation, unless a method is used like on the GTi subs that actually have cooling fins built into the motor structure.

Yes the motor gets hot. But will the relatively small difference in surface over a 'medium' motor and a 'large' motor make a significant difference in cooling the voice coil? No.

 

[1ndatrunk 10+ year member]

That being said, what is the reason for these monster motors on subs such as Nightshades and BTLs, if the motor size really plays no significant roll?

 

[audioholic 5,000+ posts]

That being said, what is the reason for these monster motors on subs such as Nightshades and BTLs, if the motor size really plays no significant roll?

Nobody said they play no significant role, we said they play no significant role in heat dissipation (ie: thermal power handling). The magnet size certainly affects the motor force of the assembly (BL), along with the gap tolerance etc. But to add to the complxity, there is certainly such a thing as too much motor force as well. Bigger is certainly not always better.

Many manufacturers have already publically admitted they use the large ferrite based motors on their subs, rather than the smaller/lighter neo motor simply due to people's impression that bigger must be better.

 

[Haunz 10+ year member]

I wouldn't consider the size of the motor as any realisitic difference in heat dissipation, unless a method is used like on the GTi subs that actually have cooling fins built into the motor structure.
Yes the motor gets hot. But will the relatively small difference in surface over a 'medium' motor and a 'large' motor make a significant difference in cooling the voice coil? No.

If there is a significant difference in the size of the motor then there will be a significant difference in it's performance as a heat sink.. (I didn't know that motors came in medium and large.. //content.invisioncic.com/y282845/emoticons/confused.gif.e820e0216602db4765798ac39d28caa9.gif)

If you had the same coil & same power in a 20lb motor and moved to a larger, heavier 40lb motor... you wouldn't double the surface area, which is the main key to the whole equation... performance as a heatsink would not be 'doubled'... but I'm also quite sure that the coil would definatly run a bit cooler.. (I am also quite sure these types of differences have been modeled out by different manufacturers/lab guys before)

There is a reason why the GTI has cooling fins and DDs and some others have cooling vents machined into the motor... it increases the surface area of the heatsink.. (aka the motor). The simple fact that some companies try to increase the surface area of their motors, should tell you that it's a heatsink to begin with... but I digress

 

[Haunz 10+ year member]

Many manufacturers have already publically admitted they use the large ferrite based motors on their subs, rather than the smaller/lighter neo motor simply due to people's impression that bigger must be better.

I will agree that bigger isn't necassarilly better.. but I've got some 3hp neodymium motors in the mail, and they arn't exactly small or light weight either.. //content.invisioncic.com/y282845/emoticons/crazy.gif.c13912c32de98515d3142759a824dae7.gif

 

[audioholic 5,000+ posts]

If there is a significant difference in the size of the motor then there will be a significant difference in it's performance as a heat sink.. (I didn't know that motors came in medium and large.. //content.invisioncic.com/y282845/emoticons/confused.gif.e820e0216602db4765798ac39d28caa9.gif)
If you had the same coil & same power in a 20lb motor and moved to a larger, heavier 40lb motor... you wouldn't double the surface area, which is the main key to the whole equation... performance as a heatsink would not be 'doubled'... but I'm also quite sure that the coil would definatly run a bit cooler.. (I am also quite sure these types of differences have been modeled out by different manufacturers/lab guys before)

There is a reason why the GTI has cooling fins and DDs and some others have cooling vents machined into the motor... it increases the surface area of the heatsink.. (aka the motor). The simple fact that some companies try to increase the surface area of their motors, should tell you that it's a heatsink to begin with... but I digress

Its amazing, you just conceded every point I made, while sounding as if you disagree with me, on all of it.
The 'medium' versus 'large' comment was to avoid someone (you) from trying to compare a Nightshade motor to a Funky Pup. I think this was obvious. Your sarcastic "I didnt know subs came in medium" comment only shows you are angry, for some reason. Care to share the reason why with the whole class?

Your 20lb versus 40lb motor not displaying double the surface area would seem to agree with my comment "...the relatively small difference in surface over a 'medium' motor and a 'large' motor..." Even in a doubling of weight (approx going from a single stack to a double stack... not quite because f top and bottom plate) you concede you will not receive a doubling of surface area. Now how about a less extreme example, like going form a double stack to a triple stack? Significantly heavier motor, not very much increase in surface area however (especially considering the shape of the magnets and their stacked position, not at all optimized for surface area cooling). Very little extra cooling will be received from the third slug, but it does increase the mass of the motor significantly.

The DD example you cited has holes drilled for 'direct cooling' of the coils, iirc. This allows cool air to be more easily dispersed across the coil and through the motor, not simply to increase surface area for convection purposes. In either case, its certainly not an example of a motor cooling better because its physically larger.

The GTi example you use, was exactly my point. Some companies, like JBL, add hinksinks to make the motor surface are a more substantial part of the cooling process with their drivers. And those designs lend themselves to the size of the motor making an significant difference in cooling, again by design. But to use your own logic against you, maybe the fact that so many companies DONT use heat sink fins on their motors should tell you the limited role it plays in cooling versus more traditional methods.

If surface area plays such a significant role in motor cooling, how do neo motors still have high power ratings? Their surface area is modest, even compared to the smallest of ferrite motors.

 

[squeak9798 5,000+ posts]

Its an older Treo csx motor off an 18. Will that work or will I have to go another route?It has a 3in coil also if that helps

It will be a completely different subwoofer than a BTL even if you recone it with BTL parts as the design of the motor (gap height in relation to coil length, gap width and motor strength, cooling features etc) is the most significant difference between drivers.

 

[squeak9798 5,000+ posts]

Its amazing, you just conceded every point I made, while sounding as if you disagree with me, on all of it.

I'd have to agree. So far he comes off as if he's arguing with a post, but then simply restates what was originally said in argument form.

For example;

The motor assembly will sink some heat from the coil but the former and the air flow within the magnetic gap make a much bigger difference than motor size in cooling

No doubt the air moving in and out of the gap is the main source of cooling to a voice coil; but the thermal mass and surface area of the motor does effect power handeling of a woofer...

He just restated what helotaxi said, but said it as if he was disagreeing with helotaxi //content.invisioncic.com/y282845/emoticons/confused.gif.e820e0216602db4765798ac39d28caa9.gif

 

[squeak9798 5,000+ posts]

That being said, what is the reason for these monster motors on subs such as Nightshades and BTLs, if the motor size really plays no significant roll?

There are several reasons for large motor structures, it's going to be product-specific on why they chose the motor they did.

As was mentioned, Adire openly stated that the motor on the Brahma was oversized simply for cosmetic, and hence sales, reasons. They could have gotten equal performance out of the driver with a smaller motor, but the smaller motor didn't have the "eye candy" effect that can help sell drivers.

Another reason some companies use large magnet structures is that it's typically cheaper to increase rearward coil clearance with ferrite than it is steel.

Another reason might be that it was the most cost effective means of achieving their desired motor strength for the design.

Another reason might be, as was mentioned, a slight increase or benefit in coil cooling.

One thing to keep in mind is though is that stacking magnets is not an overly effective way of increasing motor strength. I don't recall the exact figures, but IIRC stacking a 2nd magnet only increases motor strength by ~20% (feel free to correct me if I'm wrong on the %, I know it's a less than stellar increase). Transferring magnetic force is all about surface area. It's more effective to widen the diameter of the magnet than it is to stack a magnet in order to increase the force of the motor. But again, this is where cost/benefit is going to come into play. Maybe it's more cost effect or otherwise beneficial (rearward clearance, cosmetics, etc) to the design of a particular sub to stack as opposed to widen the structure.

 

[John_E_Janowitz 10+ year member]

There are many things to look at within the motor for a woofer that determines its strength. The type of magnet (ceramic, alnico, neodymium, etc), grade of magnet (ceramic has 3 main grades), size of magnet, the type of steel used and the carbon content, the width of the airgap, type and material of the VC wire, etc etc. In reality you need to define the goal for your woofer and that will determine all of these aspects.

As far as thermal power handling goes, the magnet itself has little to do with this, but the motor strength will. A more efficient driver will require less power to reach the same output levels. It is always beneficial to minimize the amount of power needed for a given output. How tight the gap is also has an affect on power handling. The closer the coil is to the pole, the better it can sink heat to it. As mentioned, air does not transfer heat very well, so you want the airgap as tight as practical.

In the case of our new AV woofers we have tight gaps but also add a highly conductive thick copper shorting ring to the whole pole. Among other benefits, this greatly increases the ability of the coil to sink heat into the pole. The steel pole can hold lots of heat but it doesn't absorb it very quickly. The copper absorbs heat much more quickly where it can then slowly transfer it via direct contact to the steel pole. This is the same reason many pots and pans have a copper layer on the bottom, it heats the whole pan quicker. By doing this, our 2.5" VC drivers will withstand higher continuous power levels than most 3" and 4" VC's.

Lower carbon grades of steel have higher magnetic saturation points, meaning they can have more flux going through and into the gap where it is needed. The more flux that can go through the steel, the less steel you need and the lighter your motor can be for the same motor strength. The Y35 or Ceramic 5 magnet is about 10% stronger than the Y30 magnet used in most all chinese made drivers. Many companies simply go bigger on the Y30 magnet vs going with the better grade Y35. This then adds more weight to the driver and requires larger top and back plates as well. In the end, the cost comes out about equal but with a lighter driver in the case of the higher grade magnet. Also as stated above, the biggest issue that affects the strength of a ceramic magnet is it's surface area in contact with the top and back plate. Going with thicker magnets does not have as much affect as going larger in diameter. In our TD drivers, the second magnet adds only about 10% BL to the driver, but it is a needed 10% and as stated above gives the physical clearance to not bottom the driver.

One of the other big factors in terms of power handling is the VC itself. Nearly all chinese made VC's have wire that is wound into an adhesive layer that then dries naturally. These adhesives melt at about 375 degrees F. A high temperature US made coil like the ones we use are wound, dipped, and baked at 550 degrees F. When fully cured they will handle temperatures up to 600 degrees F. In a case like this where you have a high temperature coil and good ability to transfer heat from the coil to the pole, thermal failures are very uncommon.

John

 

[1ndatrunk 10+ year member]

Ok now I'm getting the drift. What other steps can I do then to build this woofer to my goals? As stated its an older CSX motor from an 18 and I'm either going with a 12" basket and recone or back to an 18". I want this for daily and to make it as loud as possible on about 2000watts. Sq is not manditory, but I would like to be able to keep up with music. I'll have about 6.5 cuft after displacement to fire the sub up/port back in my trunk for an 18", or I can do a 12" firing back. I need and appreciate all useful info you gentlemen can provide.

 

[Haunz 10+ year member]

Its amazing, you just conceded every point I made, while sounding as if you disagree with me, on all of it.
The DD example you cited has holes drilled for 'direct cooling' of the coils, iirc. This allows cool air to be more easily dispersed across the coil and through the motor, not simply to increase surface area for convection purposes. In either case, its certainly not an example of a motor cooling better because its physically larger.

The GTi example you use, was exactly my point. Some companies, like JBL, add hinksinks to make the motor surface are a more substantial part of the cooling process with their drivers. And those designs lend themselves to the size of the motor making an significant difference in cooling, again by design. But to use your own logic against you, maybe the fact that so many companies DONT use heat sink fins on their motors should tell you the limited role it plays in cooling versus more traditional methods.

My first post stated that it's worth noting that a motor acts as a heat sink... you and helotaxi seem to think otherwise or you wouldn't have posted...

let me say that again... a motor clearly acts as a heatsink...

I disagree about the channels in a motor.. they are there primarilly to increase the cooling of the motor itself.. not the coil.. changing the volume of air around the coil will not impact the ability of the air that is immediatly around and in physical contact with the coil...

As far as the JBL.. a motor dosen't have to have fins per say to act as a heatsink.. I've got amps that don't have fins on their heatsink... JBL just capitilized and improved on the fact that a motor acts as heatsink...

I really don't want to get into a clown match with anyone over something I already know about.. To state the obvious, again.. a motor does infact act as a heatsink... and like any heatsink increasing it's surface area increases it's ability to disapate heat.. and increasing it's mass increases it's heat capacity..

 

[Haunz 10+ year member]

I'd have to agree. So far he comes off as if he's arguing with a post, but then simply restates what was originally said in argument form.

Read my first post.. then go from there..

And for the record... a motor dosen't just sink 'some' heat... it sinks a lot...