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Old 14-11-2021, 05:22   #1
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Adding resistor to Danfoss system

I replaced my mechanical thermostats with digital ones. This means my compressors now run at the low speed of 2000 rpm and my freezer takes forever to cool down.

So I added a 1000 ohm resistor to the thermostat loop and that worked great and was operating on about a 50% cycle. But 8 hours later the compressor seemed to just stop. The condenser fan would run so the unit knew it was being to run, but temp wouldn't drop. Took the resistor back out and it worked just fine again. I have now tried this twice and both times it worked great for about 8 hours.

I used different resistor both times and after I took them out each measured ok. But it just seems that the resistor is the problem. Has anyone else successfully added a resistor to their replacement digital thermostat, if so what type of resistor did you use?
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Old 14-11-2021, 06:42   #2
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Re: Adding resistor to Danfoss system

It is the resistance in the thermostat wiring loop between module terminals C and T that determines speed from 2000 to 3500 rpm. If you have an ohm meter put it in series with one of the thermostat wires zero milliamps will result in an rpm of 2000 and 5 milliamps 3500 rpm. Any compressor speed can be set in between by value of resistor. The thermostat when compressor runs and resistance in it and its wiring determines compressor speed within its design limits if it is a Danfoss BD35 or BD50.

It compressor is running too long adding resistance to circuit will help but not necessarily improve its efficiency.
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Old 14-11-2021, 08:19   #3
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Re: Adding resistor to Danfoss system

Yes i understand all that.

The issue is have added a 1000 ohm, 1/4W resistor in the loop twice. It worked great for 8 hours operating at 50% cycle. Then the compressor appears to stop operating.
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Old 14-11-2021, 08:54   #4
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Re: Adding resistor to Danfoss system

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Originally Posted by sailorboy1 View Post
Yes i understand all that.

The issue is have added a 1000 ohm, 1/4W resistor in the loop twice. It worked great for 8 hours operating at 50% cycle. Then the compressor appears to stop operating.
Were their any LED code flashes after compressor stopped?
Did you verify before adding the electronic thermostat what the resistance in
thermostat wiring was?
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Old 14-11-2021, 09:38   #5
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Re: Adding resistor to Danfoss system

FWIW our old Waeco unit expected a 680 ohm resistor.

What I did when adding the digital thermostat was to place it in series with the old manual controller, then set the dial to max cold so it stays on all the time and the digital switch runs the show. Has worked well.
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Old 14-11-2021, 11:22   #6
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Re: Adding resistor to Danfoss system

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FWIW our old Waeco unit expected a 680 ohm resistor.

What I did when adding the digital thermostat was to place it in series with the old manual controller, then set the dial to max cold so it stays on all the time and the digital switch runs the show. Has worked well.
OK but I do not know how that would work with only milliamps in that thermostat control circuit unless it was controlling 12 volt power to run refrigeration.
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Old 14-11-2021, 11:36   #7
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Re: Adding resistor to Danfoss system

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Originally Posted by Richard Kollmann View Post
Were their any LED code flashes after compressor stopped?
Did you verify before adding the electronic thermostat what the resistance in
thermostat wiring was?
I din’t notice any led on, but to really see would have taken more i was willing to do at the time. So all i did was notice was the condenser fan was running. If a 1000 ohm 1/4w resistor should work i can try it again. I installed the resistor at the digital thermostat back as that is easlier to get to. Like i said, it worked great twice with the resistor for 8 hours before cooling stopped.

The digital unit has been working for 3 years. I am sure it had the max resistor and ran at max speed on the mechanical as was originally a spillover system.
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Old 14-11-2021, 12:32   #8
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Re: Adding resistor to Danfoss system

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OK but I do not know how that would work with only milliamps in that thermostat control circuit unless it was controlling 12 volt power to run refrigeration.
Works great, has done so for a year. Placed in serial on the "C" line. The internal resistance of the digital relay is very low. Above the set point, the current is set by the resistor in the legacy thermostat so the control board is happy. Below the set point, the digital switch is open and the compressor shuts off.
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Old 14-11-2021, 14:24   #9
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Re: Adding resistor to Danfoss system

Is it a reasonable possible that at the higher speed the compressor would maybe stall at start? Would that trigger an LED? I will look up for the possible LED display.
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Old 14-11-2021, 14:28   #10
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Re: Adding resistor to Danfoss system

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Works great, has done so for a year. Placed in serial on the "C" line. The internal resistance of the digital relay is very low. Above the set point, the current is set by the resistor in the legacy thermostat so the control board is happy. Below the set point, the digital switch is open and the compressor shuts off.
Understood digital is not in series with thermostat it is not grounded at module thermostat T terminal you must have found another way to ground digital thermometer. That is an unusual way to use wire C 12volt terminal and since module did not fail in a year no harm done with low current draw on C from plus 12 volt power to trouble LED, thermostat and digital thermometer. With the speed resistor connected to digital thermometer an it no being in searies with connected thermostat ground I wonder if compressor speed actually increased. Speed resistor should be in thermostat loop to T module terminal.
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Old 14-11-2021, 15:13   #11
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Re: Adding resistor to Danfoss system

Quote:
Originally Posted by sailorboy1 View Post
I replaced my mechanical thermostats with digital ones. This means my compressors now run at the low speed of 2000 rpm and my freezer takes forever to cool down.

So I added a 1000 ohm resistor to the thermostat loop and that worked great and was operating on about a 50% cycle. But 8 hours later the compressor seemed to just stop. The condenser fan would run so the unit knew it was being to run, but temp wouldn't drop. Took the resistor back out and it worked just fine again. I have now tried this twice and both times it worked great for about 8 hours.

I used different resistor both times and after I took them out each measured ok. But it just seems that the resistor is the problem. Has anyone else successfully added a resistor to their replacement digital thermostat, if so what type of resistor did you use?
Refrigeration!!! is a perfect example of Newton's Law (Every action has a reaction)
Each component of a refrigeration system has to match / complement others. If you excessively increase the compressor speed it correspondingly increases its pump rate, which increases compressor head pressure, which increases load on the condenser etc etc. the system is now way out of balance. This is why I shudder when someone advocates a 'remedy' without understanding or at least acknowledging the correct refrigeration industry procedures.

In this case you have increased the comp speed and increased its performance BUT you have gone a bit overboard! Try with adding say 250 ohms instead of 1k then increase but be aware that if the condenser (and air movement) are now marginal, consider at least increasing air through the condenser.

BTW your resistor type is not the issue just too many olms!
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Old 14-11-2021, 15:19   #12
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Re: Adding resistor to Danfoss system

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Originally Posted by OzeLouie View Post
Refrigeration!!! is a perfect example of Newton's Law (Every action has a reaction)
Each component of a refrigeration system has to match / complement others. If you excessively increase the compressor speed it correspondingly increases its pump rate, which increases compressor head pressure, which increases load on the condenser etc etc. the system is now way out of balance. This is why I shudder when someone advocates a 'remedy' without understanding or at least acknowledging the correct refrigeration industry procedures.

In this case you have increased the comp speed and increased its performance BUT you have gone a bit overboard! Try with adding say 250 ohms instead of 1k then increase but be aware that if the condenser (and air movement) are now marginal, consider at least increasing air through the condenser.

BTW your resistor type is not the issue just too many olms!
??? Danfoss instructions indicate 1000 ohms should yield a speed between 3000 to 3500 rpm?
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Old 14-11-2021, 15:22   #13
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Re: Adding resistor to Danfoss system

Maybe and have thinking of that possibly.
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Old 14-11-2021, 15:25   #14
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Re: Adding resistor to Danfoss system

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Originally Posted by Richard Kollmann View Post
Understood digital is not in series with thermostat it is not grounded at module thermostat T terminal you must have found another way to ground digital thermometer. That is an unusual way to use wire C 12volt terminal and since module did not fail in a year no harm done with low current draw on C from plus 12 volt power to trouble LED, thermostat and digital thermometer. With the speed resistor connected to digital thermometer an it no being in searies with connected thermostat ground I wonder if compressor speed actually increased. Speed resistor should be in thermostat loop to T module terminal.
So, the circuit is from C through thermostat back to T. Makes no difference if resistor is connected in series at either C or T. Electronic thermostat should have an open close output just like a mechanical thermostat.
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Old 14-11-2021, 15:41   #15
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Re: Adding resistor to Danfoss system

Also have a Danfoss compressor and have had a number of issues. My conclusion is that the electronic control unit is finicky. Can’t tolerate dc transients or spikes. And It has internal protective settings that prevent compressor from running either due to low voltage, compressor overload, or low compressor speed. I’ve attempted to get the actual set points programmed in unit without success (although a similar thread did ID low speed settings). Don’t know but suspect the electronic unit may drift over time? SAID All THAT to say, you may be encroaching into overload territory by increasing speed with resistor. Shouldn’t, because Danfoss instructions define 1000 ohm to get between 3000 to 3500 rpm speed which is allowable. BUT that speed for your electronic control unit may be tripping on overload.
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