We agree James: Liquid cooling (not just water) is superior to air-cooling generally.
Water isn’t necessarily the best liquid, but it is available and easy on a boat, so why not use it… J
You noted:
“The combination of forcing more air in and around the motor, plus the water cooling is what finally gave us the results we were looking for.”
I’m confused. With your 7kw system, I don’t see any forced air arrangement with that motor. Sure, there’s a fan attached to its enclosure, but nothing close to the forced air shroud solution implemented on the ETEK on my outboard:
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of James Lambden
Sent: Sunday, March 20, 2011 6:22 PM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Water Cooled Electric Drive
Myles, Tom, Mark and Qsk,
Knowing the motor temperature is the most important information that an electric boat owner can have when using their motor for continuous operation. This allows an electric boat operator to avoid motor failure due to overheating, or motor thermal cutback in the case of more sophisticated controllers like the Sevcon Gen 4.
The windings of an electric motor are electrically isolated from the case of the motor - there is no direct path between the two. The case heats up from being in close proximity to the windings. However if we are able to cool the case, then the windings will benefit. The double stator motor from Motenergy (formerly Mars) has a temperature sensor in the windings and the Clearview displays the temperature.
The combination of forcing more air in and around the motor, plus the water cooling is what finally gave us the results we were looking for.
These motors are capable of winding temperatures up to 150 degrees C, though my preference is to run them at 100 to 120 max.
The amount of cooling is a function of the temperature differential, the cross sectional area of the cooling surface, and the thermal transferability of the metal being used.
The cooling coil is difficult to make. The machinist didn't like working with copper at all because it is gummy. However copper has twice the heat transfer ability of the next best metal - aluminum.
I also used heat transfer paste and noticed that it worked far better when I used less rather than more of this paste between the surfaces. We were able to get almost complete contact with the backside of the motor with the cooling plate.
We machined a positive register in one side and a negative register on the other side of the cooling plate.
Getting rid of heat is the best way of increasing the power that a system develops. If a motor is 85 - 90 percent efficient, running at 5 KW, that means 500 to 750 watts are given off in heat. That is a lot of heat. Imagine putting a hair dryer on the low setting into a 1/2 a cubic foot aluminum box and leaving it alone for a while.
Some controllers do need cooling. The Sevcon Gen 4 doesn't have a heat build up problem - an indication of the efficiency of this controller.
Perm Motors are built from the get go with water cooling channels enabling sustained high outputs of 10 KW at 48 volts and up to 20 KW at 84 volts. These motors will be used in our next generation of high output electric drives.
Polar Power generators have a water cooling option for their high power generators.
Water cooling remains the best way of transferring heat due to the heat transferability of heat through a liquid being so much higher than through a gas (air).
James
Water cooling would work better if the path of the water was in contact with the windings.
On Mar 20, 2011, at 1:31 PM, Tom wrote:
I've never seen the innards of a PMAC motor, and this discussion about cooling is interesting to me since I may want to add cooling to my installation this summer.
Since the windings are generating the heat, what is the thermal path from the windings to the motor case? Is there a sizable contact area between the two which will allow the heat to transfer effectively?
If not, then perhaps blowing air through the motor is the best bet...
I had considered (and still might build) a hollow band that would be fitted around the case of the motor to circulate water. I was planning to use aluminum to minimize galvanic issues. The band would tighten over the portion of the motor that has no cooling vents. The water could be closed-loop to a small heater core with a fan for cabin heat. A small pump could be driven by an eccentric on the prop shaft.
The controller doesn't seem to need much cooling, so I thought about simply putting a fan blade on the prop shaft to move air around a bit in the vicinity of the motor and controller.
I suppose the best way to move air through the motor is with a suitably ducted blower aimed at the motor.
Still thinking this one through.
-Tom
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