When looking at cooling in a driver there are many things to consider. The first thing is preventing the coil from heating up i the first place. More efficient drivers with more motor strength require less power to reach the same levels. Less power means less heat in the VC to begin with.
Once the coil begins to heat up though, that heat needs to go somewhere or the coil will burn up. So the first step is to get the heat from the coil itself. As mentioned, air is not a good material for heat transfer. The wider the air gap adjacent to the coil, the worse the heat transfer is going to be. Also the thinner the top plate, the less steel you have next to the coil and again the worse the transfer is going to be. For effective transfer you need a tight air gap, and lots of steel around the coil. This is the same as any heat transfer equation where the greater the surface area, the more heat is transferred.
Increasing the thermal conductivity around the coil then also greatly helps in the heat transfer. As I mentioned before, steel can absorb a lot of heat, but is not very quick at doing it. The copper sleeve on the pole, adjacent to the entire ID of the coil at all points throughout it's travel, greatly increases the ability of the coil to pass heat to the pole. It is typically this first point where drivers fail. They simply can't dump enough heat quick enough.
It is very rare that a driver will fail because the entire motor heats up greatly. If you were to look at a VC which may have 50-100 grams of copper/aluminum in it, the amount of heat required to warm and entire 10lb-40lb motor structure to a point that would cause thermal failure is just enormous. There has to be an extreme amount of power applied for long continuous periods of time to cause this.
We experimented with one 12" underhung driver here awhile back. We applied approximately 500W of continuous filtered pink noise, 20-200hz, 6dB crest factor, for a period of 2 hours. After this period of time the entire motor structure was quite warm to the touch and measured nearly 180F at the perimeter, and over 200degrees at the hottest point in the pole vent. Our VC's however are able to withstand temperatures of 550-600F. As the temperature of the pole is still 350-400F less than the failure temperature of the coil, the coil will still be able to transfer much more heat to the motor structure. However, if we had widened the airgap in the driver, the coil would not have been able to transfer the heat to the steel, and it would have likely failed well before 2 hours.
Of course cooling the motor itself does have benefits as well. The more heat you pull from the motor, the easier it is for the coil to transfer heat away. A properly flared pole vent can help cool the pole, venting under the spider can help cool the top plate, etc. Keep in mind though that direct conduction is much more efficient at transferring heat than is air. The cooling device that Wayne from Pi Speakers created for use in his horn loaded subs definitely helped transfer heat from the motor. Heat sinks and items like the phase plug on our Lambda drivers also help remove heat from the pole. Any time you increase the thermal mass, you will decrease the ambient temperature. Any time you increase the surface area you will increase the ability to radiate heat away from the object.
John
Once the coil begins to heat up though, that heat needs to go somewhere or the coil will burn up. So the first step is to get the heat from the coil itself. As mentioned, air is not a good material for heat transfer. The wider the air gap adjacent to the coil, the worse the heat transfer is going to be. Also the thinner the top plate, the less steel you have next to the coil and again the worse the transfer is going to be. For effective transfer you need a tight air gap, and lots of steel around the coil. This is the same as any heat transfer equation where the greater the surface area, the more heat is transferred.
Increasing the thermal conductivity around the coil then also greatly helps in the heat transfer. As I mentioned before, steel can absorb a lot of heat, but is not very quick at doing it. The copper sleeve on the pole, adjacent to the entire ID of the coil at all points throughout it's travel, greatly increases the ability of the coil to pass heat to the pole. It is typically this first point where drivers fail. They simply can't dump enough heat quick enough.
It is very rare that a driver will fail because the entire motor heats up greatly. If you were to look at a VC which may have 50-100 grams of copper/aluminum in it, the amount of heat required to warm and entire 10lb-40lb motor structure to a point that would cause thermal failure is just enormous. There has to be an extreme amount of power applied for long continuous periods of time to cause this.
We experimented with one 12" underhung driver here awhile back. We applied approximately 500W of continuous filtered pink noise, 20-200hz, 6dB crest factor, for a period of 2 hours. After this period of time the entire motor structure was quite warm to the touch and measured nearly 180F at the perimeter, and over 200degrees at the hottest point in the pole vent. Our VC's however are able to withstand temperatures of 550-600F. As the temperature of the pole is still 350-400F less than the failure temperature of the coil, the coil will still be able to transfer much more heat to the motor structure. However, if we had widened the airgap in the driver, the coil would not have been able to transfer the heat to the steel, and it would have likely failed well before 2 hours.
Of course cooling the motor itself does have benefits as well. The more heat you pull from the motor, the easier it is for the coil to transfer heat away. A properly flared pole vent can help cool the pole, venting under the spider can help cool the top plate, etc. Keep in mind though that direct conduction is much more efficient at transferring heat than is air. The cooling device that Wayne from Pi Speakers created for use in his horn loaded subs definitely helped transfer heat from the motor. Heat sinks and items like the phase plug on our Lambda drivers also help remove heat from the pole. Any time you increase the thermal mass, you will decrease the ambient temperature. Any time you increase the surface area you will increase the ability to radiate heat away from the object.
John
