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Old 09-03-2019, 13:11   #1
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A Danfoss BD35 refrigeration system control box

Here is a diagram of a box I put together to add to my Danfoss BD35 refrigeration system. Mine was set up for fixed low speed, no LED error light, and since the condenser was a "keel cooler" it did not use a fan for anything. This circuit adds a way to control a fan (I have it blowing on the compressor itself now), adds the diagnostic LED, and provides a way to manually vary compressor speed from low/max efficiency to high/max cooling with a potentiometer. I'm providing a public Dropbox link to an image of the schematic if anyone might find it useful. You will probably want to look at the BD35 datasheet at the same time.

Schematic Diagram

BD35 Datasheet
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Old 09-03-2019, 16:32   #2
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Re: A Danfoss BD35 refrigeration system control box

Your idea for the compressor and module fan is good But I would add a $10 snap action closed at 120 degrees F thermo switch attached to compressor.
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Old 09-03-2019, 16:49   #3
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Re: A Danfoss BD35 refrigeration system control box

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Your idea for the compressor and module fan is good But I would add a $10 snap action closed at 120 degrees F thermo switch attached to compressor.


Yes, that would be the most elegant. The fan is only pulling 120 mA anyway so I just let it run with the compressor.
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Old 09-03-2019, 17:01   #4
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Re: A Danfoss BD35 refrigeration system control box

Richard,
That's what I did with my BD35 keel cooled as well.
The fan runs whenever the compressor exceeds 120f. and it's independent of compressor operation, so it only runs when the compressor actually needs it.
Further, the fan may run a bit after the compressor shuts off if it's been working hard.
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Old 09-03-2019, 17:43   #5
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Re: A Danfoss BD35 refrigeration system control box

I have a fan on mine, but also put some heat sinks on the compressor
They are designed for stepper motors I believe. I figure they can’t hurt cause even with the fan it was warmer than I like.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
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Old 10-03-2019, 11:50   #6
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Re: A Danfoss BD35 refrigeration system control box

Hermetically sealed compressors used on water cooled systems need external air cooling especially when operating at low evaporator temperatures in order to prevent extended over heat damage of oil and field coil insulation.
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Old 10-03-2019, 12:42   #7
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Re: A Danfoss BD35 refrigeration system control box

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I have a fan on mine, but also put some heat sinks on the compressor
They are designed for stepper motors I believe. I figure they can’t hurt cause even with the fan it was warmer than I like.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Thanks for the idea and link. Compressor on my (air cooled) system also gets quite hot on initial cool down. After that it seems ok but figure it can't hurt to add at little more cooling.
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Old 10-03-2019, 12:43   #8
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Re: A Danfoss BD35 refrigeration system control box

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Your idea for the compressor and module fan is good But I would add a $10 snap action closed at 120 degrees F thermo switch attached to compressor.
Would this be for power saving or is it better to not over cool the compressor?
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Old 10-03-2019, 13:39   #9
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Re: A Danfoss BD35 refrigeration system control box

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Would this be for power saving or is it better to not over cool the compressor?
Compressor sustained dome temperatures above 130 degree F alone would not be good for compressor's extended life. If current draw is higher than normal due to hot compressor energy performance would be poor. Another problem with overheated remote condensers from compressor is line connectors with final sealing O rings eventually leak if they run hotter than normal.
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Old 10-03-2019, 14:22   #10
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Re: A Danfoss BD35 refrigeration system control box

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Compressor sustained dome temperatures above 130 degree F alone would not be good for compressor's extended life. If current draw is higher than normal due to hot compressor energy performance would be poor. Another problem with overheated remote condensers from compressor is line connectors with final sealing O rings eventually leak if they run hotter than normal.
Thanks Richard. So can a compressor be too cool? Any downside to extra cooling?
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Old 10-03-2019, 16:49   #11
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Re: A Danfoss BD35 refrigeration system control box

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Thanks Richard. So can a compressor be too cool? Any downside to extra cooling?
Yes if the liquid situated refrigerant reaches compressor instead of vapor.
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Old 10-03-2019, 16:55   #12
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Re: A Danfoss BD35 refrigeration system control box

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Yes if the liquid situated refrigerant reaches compressor instead of vapor.
Again, thanks. Can I assume that adding an extra fan over the compressor that is always on when the compressor is running (and maybe a couple of heat sinks) would not cool the compressor enough to incur a risk of this happening or is a thermostat control over the extra fan essential?
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Old 11-03-2019, 06:11   #13
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Re: A Danfoss BD35 refrigeration system control box

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Again, thanks. Can I assume that adding an extra fan over the compressor that is always on when the compressor is running (and maybe a couple of heat sinks) would not cool the compressor enough to incur a risk of this happening or is a thermostat control over the extra fan essential?
All compressors can and most do run warm the Danfoss BD compressor was designed for air cooling and applications where its design limits are not exceeded. Safety protections used on older refrigerant system to prevent compressor damage like high and low pressure switches and thermo overload trip disc do not exist on BD compressors. The BD compressor service life is protected by electronic monitoring and the fact that system designer and person installing unit followed compressor manufacturer’s advice.

The only protection for a Danfoss BD compressor over heating now is a safety circuit in control module that measures current to fan between module terminals small + and F. This means Danfoss engineers feel a high refrigerant pressure switch is not needed. The problem with water cooled units is there is no fan cooling compressor protection.

If I suspected my compressor was running to hot I would first check for restrictions in condenser’s cooling medium air or water. Next compare amperage draw of system to determine if liquid refrigerant pressure is higher than normal. Another signal that compressor is working hard is temperature of high pressure line after condensing unit This lines temperature should be less than 115 degrees F.

I had three 80 cfm fans on my 2.5 BD compressor for more than 15 years that ran when ever compressor ran keeping frozen three plates containing a total of eight gallons of eutectic solution during the night.
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Old 11-03-2019, 07:58   #14
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Re: A Danfoss BD35 refrigeration system control box

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Originally Posted by a64pilot View Post
I have a fan on mine, but also put some heat sinks on the compressor
They are designed for stepper motors I believe. I figure they can’t hurt cause even with the fan it was warmer than I like.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1



Hi A64, How did you attach the heatsinks?
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Old 11-03-2019, 13:08   #15
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Re: A Danfoss BD35 refrigeration system control box

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Hi A64, How did you attach the heatsinks?


They come with double sided sticky silicone pad, it’s like slightly gooey double sided tape, assumption is the silicone transmits heat.
It’s not real sticky though, but seems to work I guess.
Cheap, easy to do. I originally had a second fan blowing down directly on the compressor, that I have since removed/

It was all part of a failed attempt to optimize the aspects of a Cold plate by increasing compressor speed and lowering the thermostat set point when there was excess Solar.

It failed due to my lack of understanding of exactly how my system functioned and exactly how the system I purchased to control it functioned.

Expensive lesson.
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