newusername
10+ year member
CarAudio.com Veteran
This is a very misunderstood concept. I posted this on SSA earlier today...hopefully it makes sense and/or someone can add to it. If it doesn't make sense, make sure you say so. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
First, let's just think about how a pole vent would help cool the driver. It would have to either draw cool air up or push hot air down. This will work in either of two ways:I see the BL has vent holes at the sides all around. But how do other subs do their cooling since they do not have side vents but only pole vent? i believe that is the conventional way of cooling the voice coilPole vents are useless..
1) The heat from the coil is absorbed by the pole, then air traveling over the inner wall of the pole piece carries that heat out of the pole.
2) The heat from the coil must manage to get off the coil, through the former, and down into the pole piece, at which time you count on convection to take the air out of the pole.
If you think about this logically, I think it should be very obvious that either approach is not going to do a whole lot. In fact, if #1 is the preferred method, then a smaller pole vent is often preferable because you can achieve higher air velocity and there is more steel to absorb the heat (steel absorbs heat a lot better than air does).
So what is a better approach? Firstly, I think there are a few things that we should target.
1) Get the heat off the coil - It should be easier to move heat off the coil and into another region with increased air velocity. What you really need to find is the balance between air mass and air velocity, at which the most heat can be removed from the coil. The pole vent is generally a low velocity, high mass method of moving air and is not particularly effective. The method we will review is more of a high velocity, medium mass approach.
2) A short path to move the heat along - Obviously the shortest path will be the most effective.
3) A place to put the heat - Once we have moved the heat off the coil, we want to move it to a place that will be absorbed.
The approach for many is to try and move this into the surrounding atmosphere (ie. the air in the enclosure), but as I already mentioned, the air is not very thermally conductive. Some clever engineers use deposits of aluminum or copper in the motor to absorb some of the heat; for the sake of comparison, the aluminum is about 10,000x more thermally conductive than air, while copper is about 16,000x more thermally conductive than air. How thermally conductive the steel is in the motor will depend a lot on the grade, but it is always much more thermally conductive than the air around it.
Anyways....
The pole vent approach is just dumb. First, you remove steel from the pole: the pole is just a short jump from the coil, and is also a good place to put heat. Not only that, but removing steel means you have less steel to carry flux and, potentially, lower flux density, lower BL, lower efficiency, lower SPL, etc. Second, you have to move a large mass of air quite a distance (from the top of the pole out the vent). Ask yourself: if the goal is to move the heat into the surrounding atmosphere, isn't there a shorter path we can do this with?
Hopefully the answer is obvious: yes! Hell, the coil (which is where the heat is coming from) sits just a few cm from the surrounding atmosphere if we go up through the top plate instead. We can also avoid taking steel out of the motor (which is good for all of the above reasons), and the steel top plate and basket (of whatever material) are right there to absorb heat as well. So we've found narrowed down a more effective path and an equally (or more) effective place to put the heat.
In order to get the heat off the coil, we are going to want to draw air at a high velocity across the top plate and down into the gap (where the coil resides). Via convection, we can now pass heat into the steel pole piece, the surrounding magnets, the steel top plate, or draw it back out of the gap and into the basket (or, to a lesser extent, the surrounding atmosphere). We can also add aluminum or copper inside the motor to provide even more mass to absorb the heat (a thick copper or aluminum ring on the ID of the magnets might be quite effective). To make the drawing of air into the gap most effective, we can add a spacer between the basket and the top plate; we can also machine channels through which air is drawn down into the motor. The size and shape of the channels will vary for the application, but this is a very effective method.
What I have described is, in essence, exactly what Fi is doing. There are a number of small tweaks you can make (adjusting the geometry of the pole or the top plate), but this is way more effective than the traditional pole vent method. In fact, if moving heat out of the rear of the motor is your goal, it is often better to utilize a series of holes drilled into the back plate that allow you to vent heat out of rear of the gap (rather than out of the pole).
Other companies are starting to figure this out (very slowly), but the vast majority do not do this (or even understand it). Part of the problem is that consumers don't really understand it either, so we have the blind asking for the blind to give them something neither of them can see (metaphorically speaking). That's why it is so important to share what we know with others. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
