Re: controller cooling

[Lee Hart <leeahart@earthlink.net>]

Rich Rudman wrote:
> A few comments on controller cooling. Alot of you are forgetting the
> KISS principle. Water is very good and very cheap.  For the 1-5 Kw heat
> loads, a simple drilled passage is over Kill. Phase change, and
> multipath serpitine routing, is a expensive waste of time.

It's a matter of finesse. There are lots of ways to cool things:

1. A big finned heatsink with no fan.

Very simple and easy for small amounts of heat. But at 1 kilowatt, the
heatsink gets enormous! It would take in excess of 1 cubic foot of
heatsink to dissipate 1kw of heat without a fan.

2. A smaller finned heatsink with a fan.

Heatsink size goes down drastically as airflow increases. But the
airflow has to be well directed to be effective. Many fans are
carelessly mounted, so the air flows over only part of the fins. Also,
propeller type fans are poor at moving air against even a slight back
pressure, so the fins have to be widely spaced (like 1/4" or more).

3. A very small heatsink, with fine fins and a high pressure blower.

A centrifugal or squirrel-cage blower can move significant amounts of
air even through tightly packed fins. This allows the heatsink to be
smaller yet. The fins are thin and densely packed, like a car radiator
or heater core. Heatsink size and weight shrinks accordingly. But fan
power and noise go up significantly, too.

4. Liquid cooling; heatsink replaced by a "cold plate".

The heatsink has tubes or passages for a liquid, which is either pumped
or circulates by convection. The cold plate itself is drastically
smaller than any air-cooled heatsink; however, you also need a radiator
to cool the liquid; this will be just as big as the heatsink in the
above cases. The liquid can be water, water/antifreeze, oil, freon, etc.
You usually need a pump to circulate enough liquid.

5. Phase change cooling; use a boiling liquid to carry away the heat.

This method uses a cold plate as above, but the liquid chosen and the
temperature and pressure inside the tubing causes it to boil in the hot
places, and condense elsewhere in the cold places in the plumbing
system. The phase change means less than 10% of the amount of liquid is
needed. The temperature of the things being cooled are "pegged" at the
boiling piont of the liquid.

6. Thermal inertia.

Connect the things to be cooled to a massive chunk of something that can
absorb the heat with little change in temperature. This will impose a
definite power-time-temperature limit. For example, 1 KWH = 3410 BTU,
which will raise the temperature of 100 pounds of water by 32.1 deg.F.
While you wouldn't put 100 pounds of something in the car just for this,
you can use an *existing* 100 pounds of something that incidentally gets
heated while you drive, and can cool back down when parked. For example,
the batteries, transmission, etc.
 
7. Vapor phase cooling.

Like a steam engine, have some disposable liquid that gets boiled to
carry away the heat. Refill the tank every so often. Probably only
practical for racing or when cooling is very rarely needed.

8. There are other ways to cool things (thermoacoustics, radiation,
etc.) but they are not practical for EV controllers.

> Remember I have to build stuff you folks can pay for. Not just the
> Racers that have deep pockets. Simple, practical and cheap.

The usual problem with cooling is that it is "let go" until the last
minute. Then, some expensive awkward solution is used.

For EVs, I think it makes sense to integrate controller and motor
cooling. Both need significant amounts of cooling, and both need it at
the same times (high current, high temperature operation). For example,
use an external blower to cools both the controller and the motor. The
controller can now be much smaller and lighter, and the EV should have
the blower anyway so it is not an additional cost.
-- 
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
Sartell, MN 56377 USA      There is a crack in everything
leeahart_at_earthlink.net  That's how the light gets in - Leonard Cohen