NovaCool Speed Control Module - Variable Speed Model
Background
Most NovaCool refrigerators allow for variable speed of the compressor. This is documented in the
NovaCool web site on line manual:
This function is controlled by the "optional speed resistor" - no resistor is the default for units to be used
in RVs as that will have the lowest current draw. The resistor is connected to the BOTTOM of two
terminals which are both labeled "C" - it the lower "C" terminal that is used. See diagram:
The simplest way to effect speed control is to use the correct value resistor with a connector on each end.
While this will work, it creates a number of problems. The resistor must be changed to change the speed
of the compressor. This involves shutting off the refrigerator, then changing the resistor. Over time the
wear on the connector can lead to damage - not a good solution.
One step "up" from an individual resistor is switching between resistors. I built and used such a switch but
discovered that during the "switch" there is a momentary "open" circuit. When the circuit sees a resistance
over 1523 ohms (as in an open circuit) the refrigerator turns OFF. It's not good to "briefly" turn a
refrigerator off then on again - a few minutes should be allowed after OFF prior to ON.
I discussed this with Nova Cool and they suggested using a Rheostat or "variable resistor" and that
discussion resulted in my creation of the variable speed compressor control. I decided to use a 1500 ohm
variable resistor to eliminate any chance of the resistance going over 1523. I wanted one that was as
rugged as possible, and have a smooth variable adjustment. It had to "stay put" - unless I was turning it.
After much searching on the internet I found 5 pieces of NOS (new old stock) - military grade parts:
I constructed a total of 5 units (one is on my '07 Tiger CX - 4x4 Allison/Duramax Chevy). The box is 3.75
inches tall, 3" wide and 1.5 inches deep.
Inside, the Rheostat is securely mounted, with a strain relief and grommet at the top:
The back is ready to mount on the rear wall of the Nova Cool with peel and stick Velcro. Once mounted it
can be removed by pulling the Velcro apart.
The leads are prepared - the "female" end replaces the existing wire on the lower "C" terminal, and the
other end mates with the wire removed from the lower "C" terminal.
A small installation "kit" is provided of: 3 tie wraps, a razor blade to trim tie wraps, alcohol wipe to clean
mounting area, and lastly, a miniature flashlight - in case installation must be made at night.
OPERATIONS MANUAL.
First, it must be understood that this is NOT a digital accuracy device. All settings are approximate. By
that I mean it is about plus or minus 10 percent for all settings, except 2000 RPM which is actually Zero
Ohms. It is a practical approach and works well.
Installation is best on the rear left of the Nova Cool. Have the device "sitting" on the "floor" so the mount
does not support the full weight and is used to hold it to the wall. Note that this position requires your
hand to pass close to the fan, avoid touching the fan!
Starting with the yellow knob fully clockwise (at 2000) the device presents zero ohms and the fridge runs
exactly as if the device were not in the circuit. MOST RVs are delivered without any resistor so the motor
runs at its slowest speed. I spoke to the Nova Cool tech department about "straining the motor" with it set
to 3500 RPM. "Do not be at all concerned to leave it there" is what they told me. It is designed to run at
3500 RPM forever.
The additional compressor speed does the following: It makes the box "cool down" quicker - much
quicker. It also increases the "capacity" of the fridge to process a large amount of "not so cold" food
added at once. It does not affect the setting of the internal thermostat inside the fridge. What it does do is
draw more amps. Roughly the increase from 2000 RPM about 3 amps goes to 3500 RPM for about 5
amps. I have a solar panel and usually leave it at 3000 RPM, except for when just starting out on a trip;
then I crank it up to the full 3500 RPM as the trucks motor is running and I have amps to spare.
The knob will turn smoothly (Counterclockwise) from 2000 RPM to 3500 RPM then there is a little
"bump" - avoid turning past the "bump" (you will feel it clearly) - as past the bump is infinite resistance
(open circuit) which shuts off the compressor. It's a good idea when adjusting the setting to always put
you hand on the compressor to feel it running - it might be a bit hot - that's normal.
By analogy – the effect is similar to changing an 8000 BTU air conditioner for a 9000 BTU air conditioner. The
cooling effect increases at the expense of greater electrical consumption. I agree the Nova Kool is great “as is”.
However, it is possible to get still higher performance as documented in the manual. Their “higher capacity” – refers
to the ability to achieve the desired internal temp as requested by the thermostat. More compressor RPMs give the
“engine” greater power to pull heat out of the refrigerator. I had the 3way Dometic replaced at Provan – and am
delighted with its performance. However, I was intrigued by the manual’s reference to the speed control “resistor”
and decided to research it. When I called Nova they set me a trio of resistors each with the appropriate connectors
on the ends. I tried them and found the greater speed of the compressor “speeds up” the attaining of (in my
preference) an internal temperature of 36 degrees. Not “better cooling at high temps” but more rapid cooling.
Exactly the same effect as in the air conditioner mentioned above. If the 8000 BTU can maintain the room comfort –
the 9000 BTU can do so faster. With air conditioners too much capacity results in a damp room as it does not run
enough to extract moisture – that is not an issue with refrigerators.
Yes, the basic fridge works well, but at higher RPMs it would work faster – with no harm to the unit – just a
higher current draw. Think of their use of the word “capacity” as really meaning “cooling capability speed”. When we
“stock the fridge” for a trip – we unload the folding cloth “ice chest” into a “hot box” from sitting in the sun. I wanted it
to get to 36 degrees as quickly as possible. That was the main reason for my project. It (the highest speed) does
make a big difference in the “cooling down” phase. Capacity also relates to the time it takes to cool a “six pack” to
drinking temp. It will do so faster at higher RPM then lower RPM. You can cool more cans of soda in the same time
at higher RPMs. Is this an essential? Probably not – but it is more than a “technical exercise” – the speed control
allows varying the cooling capability to match the need at the moment.
The only way for you to see this is to start with the box at a known temp eg: 75 degrees and measure the
time it takes with some internal load – eg: 2 “six packs” also at 75 degrees. Wire up a single 1500 ohm resistor with
proper connectors. Turn off the fridge – at the internal thermostat. Wait for everything to get back to 75 degrees.
Install (with power off in the coach) the resistor – then measure time it takes to get back to desired temp. Be sure
resistor is not over 1523 ohms as that is a compressor off signal.
The analog changing value of the Rheostat allows for changing compressor RPM at any time without the
need to shut down – the compressor speed just changes to match the current flowing in the thermostat circuit. With
individual resistors, you should let the compressor “rest” for a few minutes prior to restarting; as it stopped as soon
as the in place resistor was removed creating an “open circuit”. I hope this helps.
On the driver’s side of the Tiger the (back of the) fridge access is here: (with the circle around it).
You need to access the BACK of the fridge – lower section – this is not done from inside the Tiger.
Mine is also 110/12 volt.
Just be sure to have the house power master switch RED by floor near fridge (inside Tiger) turned OFF during
installation. Once installed turn it back ON.
Also be sure when selecting 3500 that you don’t pass the little “bump” to the OFF position – it will NOT go to OFF
by itself.
Do not attempt to turn clockwise past 2000 setting – the 2000 setting is the “end of the dial” and it will turn no
further.
You verify fridge is running by putting your hand on the compressor – the large black “egg shaped” item to the right
of the fan.
It looks a bit like this:
The complete specifications for the data module can be found here:
http://www.novakool.com/support/ACDC%20module%20Data.pdf