ESC Heat Exchangers

Here is another DIY from our own Charlie Fite!

Splash

By Charlie Fite

I live in the South, right on the border of the MissArk delta. In the Spring, Summer, and Fall, it can be hot and humid. Temperatures frequently exceed 90 degrees, and the relative humidity can exceed 65%....Heat indexes are often near 110 F. Consequently, trying to keep ESC’s from overheating can be all but futile……until the magic blue smoke escapes!

Let’s talk a bit about heat transfer.

Q(dot) = m(dot)*Cp*dT

Q(dot) = heat transfer in BTU’s/hr M(dot) = mass flow rate in lbs/hr Cp = the specific heat of air = 0.24 BTU/lbm-F dT= Temperature difference

The efficiency of cooling fins depends on three things: How much air is passing over the fins; the difference between the temperature of the air and the fins; and the surface area of the fins. If the ESC is at 100 F and the “cooling” air is at 96 F, not much cooling will happen. But if there’s at least a 10-20 degree gradient, there’s enough of a cooling effect to usually keep the ESC from overheating.

I built this little tool because trying to find a heat sinks that fit my particular ESCs could cost anywhere between $5.00 and $20.00……..never mind the actual dimensions and the delivery charges…..

So here’s what I came up with……. A piece of 1/4” ply and several slats of 1/4x1/2………..1/8x1/2 might be better…

Now take a piece of heavy duty aluminum foil and fold it 6 to 8 times to form a strip.

Then place the strip over the “die” and gently tease the strip down between the slats. I’m using a little plastic squeegee …but almost any old credit card or hotel room key will do.

Continue to make successive fins.

Trim to suit. A little CA on the inside of the fins will hold them together.

A few drops of CA will secure the fins to the top of the ESC. Heat transfer mastic is also available, but it’s expensive. I made some using aluminum/brass filings and epoxy…CA is easier.

I look at it this way. If I can save one 85 Amp ESC (about $60) with a bit of aluminum foil for almost zero cost, it’s worth the effort…..all ten minutes of it.

Keep your nose up in the turns.

Note from Jim T. - Any off-topic or negative posts will be redacted.

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Aug 18, 2021, 12:56 PM
Registered User
SwampFlier's Avatar
Very interesting... have you checked how much heat the foil dissipates?
Cheers,
Patrick
Aug 18, 2021, 01:12 PM
Ad eos qui nesciunt crepitus
Old_Pilot's Avatar

Heat dissipation


It's like I said in the article.......

The efficiency of cooling fins depends on three things: How much air is passing over the fins; the difference between the temperature of the air and the fins; and the surface area of the fins. If the ESC is at 100 F and the “cooling” air is at 96 F, not much cooling will happen. But if there’s at least a 10-20 degree gradient, there’s enough of a cooling effect to usually keep the ESC from overheating.

In this particular case, the surface area of the ESC is 2.25 in2....the surface area of the fins is 9 in2......(3) 2 x 1/2" fins plus a 2 x 1 1/2" base....

To calculate the dissipation you'd have to know the mass flow rate of the air and the temperature gradient
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Aug 18, 2021, 02:08 PM
An itch?. Scratch build.
eflightray's Avatar
I think quite a few people make the mistake(s) of wrapping ESC's in foam to protect them, forgetting they are also insulating them, and stopping any cooling air doing its job.

There are also people who don't like the idea of cutting air exit holes in fuselages, or ensuring a clear air flow path all the way through from intake to the exit.

It can be a tricky one to calculate, so the easier method is to ensure the components, ESC, motor, wire gage and cooling, are suitably rated.

If things get seriously hot, something must be seriously wrong.

.
Aug 18, 2021, 03:03 PM
Ad eos qui nesciunt crepitus
Old_Pilot's Avatar
Quote:
Originally Posted by eflightray
I think quite a few people make the mistake(s) of wrapping ESC's in foam to protect them, forgetting they are also insulating them, and stopping any cooling air doing its job.

There are also people who don't like the idea of cutting air exit holes in fuselages, or ensuring a clear air flow path all the way through from intake to the exit.

It can be a tricky one to calculate, so the easier method is to ensure the components, ESC, motor, wire gage and cooling, are suitably rated.

If things get seriously hot, something must be seriously wrong.

.
You and I could be good friends.........You are exactly correct......heat transfer basically depends on air flow and temperature difference...I just hesitated to get into basic thermodynamics.......
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Aug 19, 2021, 04:34 AM
Registered User
Aluminium foil is quite shiny so it won't be super efficient in transferring heat to the air. A surface that's smooth has way less effective surface area than a surface that's not smooth. I'd be tempted to scuff it up a bit with some abrasive pad.

Ensure any thermal paste you use is non-conductive just to be on the safe side of things.

Also don't forget, those 5 thick wires that solder onto your ESC are a great way to conduct heat out of the PCB tracks which in turn conduct heat out of the semiconductors. Big, fat wires are good for more than just carrying current.
Aug 19, 2021, 05:30 AM
Wake up, feel pulse, be happy!
Piece's Avatar
What transfers the heat from the FET bodies to the foil? I see CA mentioned in the article, but I don't understand if this is just being used to tack the foil to the ESC (shrinkwrap?) or if it's actually being used as a thermal bridge? If the aluminum foil isn't solidly mated to the FETs with something that conducts heat (e.g. thermal paste or epoxy), then it's literally reflecting heat back into the ESC like a space blanket.

Big job of a heat sink is to provide a chunk of mass with excellent thermal conductivity (like aluminum) to suck waste energy out of a component. Dissipating that energy to the air is a secondary concern - it does no good to have a million square feet of surface area if the contact is a postage stamp.

If ESC heat is an issue, I would get a bigger ESC or a smaller prop long before I put pliable aluminum foil anywhere near high-power connections.
Aug 19, 2021, 06:20 AM
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Old_Pilot's Avatar

Heat dissipation


Well, I can't quantify the effectiveness of this little trick. I don't have a lab...I did however test the fins in flight, 6 times, 3 with, 3 without.
I fly a lot of scratch built EDF foamies, so to test the "exchanger" I flew the same jet 3 times with a plain ESC and 3 times with a "finned" ESC.
It was a typical hot, humid, summer day. The plain ESC shut down all three times because of thermal overload...the finned ESC didn't.

ESCs were new. Batteries were fully charged.......

Heat will always flow from a region of higher concentration to a region of lower concentration...a basic concept of thermodynamics.
If the addition of a few fins can keep the ESC below its thermal cut off threshold, then they've accomplished the goal..I look at it this way, If I can save one aircraft with a few pennies of aluminum foil, I'm way a head of the game.
Last edited by Old_Pilot; Aug 19, 2021 at 06:58 AM.
Aug 19, 2021, 06:36 AM
Registered User
My solution is to use IC heat sinks which is after all what an ESC is!
The ones with 'fingers' have 3 times the heat dissipation capability.
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In this case the heat sink has replaced the original along with a new thermal pad. It is held down with nylon line.
A neat solution particularly for an EDF is to poke just the fingers out into the air steam.
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I always touch the fingers after landing and they are barely warm.
Aug 19, 2021, 07:21 AM
Ad eos qui nesciunt crepitus
Old_Pilot's Avatar
Quote:
Originally Posted by Quorneng
A neat solution particularly for an EDF is to poke just the fingers out into the air stream.
Exactly, the more air that flows over the sink, the more heat it transfers....provided the air temp is less than the sink....
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Aug 19, 2021, 11:02 AM
I'd rather be Flying
davecee's Avatar
I did this shortly after my move to NC in 1989, so early 1990s. Robart, frame rate, brushed ESC, powering an Astro 40 Cobalt motor on 18 NiCds. Aluminum sheet shaped and then attached with brass screws and nuts to the exposed FETs. Aluminum plate then fit over the fuselage nose area and the ESC/plate assembly held in place with rubber bands. I would actually move the runner bands and lift the ESC/plate assembly to plug and unplug the batter/ESC connectors. Sorry for the picture quality, it's the only pic I have of it.
Aug 19, 2021, 11:05 AM
Registered User
cgoffa's Avatar

another option


one trick I have learned in dealing with heat over the years is to oversize the components that are suseptible to it, 60 amp esc's in 40 amp applications etc. internal heating goes up it seems exponentially as a function of used maximum working load so a 40 amp esc working at 38 amps gets way hot and will live a short glory filled life while an 60 amp or even 80 amp esc will not really notice the load much
Aug 19, 2021, 01:53 PM
homo ludens modellisticus
Ron van Sommeren's Avatar



Careful with metal-tabbed powerFETs. The tabs/housing of the high-side FETs (connected to battery positive) can internally be connected to a different voltage compared to the tabs of the low-side FETs (connected to battery ground). This connection too transports heat away from the die.
Connect those tabs with a single heat sink and you'll create a beautiful massive battery short.
Check beforehand, and use two separate heat sinks.


Brushless controller power stage principle, an H-bridge.
3 high-side FETs/switches, 3 low-side. Or multiples of 3.
No tabs in this diagram.
The diodes are flyback diodes, they prevent voltage spikes.
They are not relevant for this post.
Last edited by Ron van Sommeren; Aug 23, 2021 at 02:29 PM.
Aug 19, 2021, 02:03 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Don't use too much thermal compound, metal conducts heat better.
Apply to both surface and scrape excess away with credit card.
Compound should only fill the microscopic voids.

Make sure all FET surfaces are in the same plane. Sand them on a mirror.


Vriendelijke groeten Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
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