FRIDGE ASSIST

Tips for simple problem resolution:  (Applies to all brands / types of DC compressor refrigerators / refrigeration)

Compressor fails to start or cuts out prematurely:

By far the most common problem DC refrigerators have is the 12VDC power supply, and not  the actual fridge or MDM  itself!

 

To explain:   All of these DC compressors have a ('black box') Motor Driver Module, (MDM) which inverts the supplied DC power into 3 phase AC power to drive the 3 phase AC compressor motor. These MDM's are of two very different types, DOL and Soft Start.

 

The DOL (direct on line) module causes a massive current inrush (can be up to 7-8 times run rate) as it tries to take the compressor from stationary to run speed virtually instantly while the Soft start MDM slowly starts the compressor without any inrush current..  

 

The MDM's low voltage cut out prevents the module driving the motor if the voltage drops away at a start attempt or when running.

This shut down because of a poor power supply, is usually due to either: a defective battery, or faulty battery to MDM wiring, or faulty devices along that circuit like bad spade terminal connection, switch or overload.     

 

If unit won't run, below is the easiest and industry recognised way to identify if voltage drop is the problem or if the MDM is faulty.. 

Voltage drop test: (Above)                                                                                                   Also see: https://www.ozefridge.com/news                                                                             

Using a good multi-meter connected to the negative and positive terminals (- & +) on the MDM, read the voltage before and when the compressor starts or attempts to start. 

The voltage reading with no load, before the compressor starts is meaningless but do note that no-load voltage.

Next, continue to monitor the voltage as the compressor starts or attempts to start. If this voltage drops away by say a volt or more or below 10 volts then that is confirmation that most likely the power supply is the problem and that the compressor and MDM are fine. Again with multi-meter, check again as above, from MDM back to the battery, testing at each / any terminal access etc.  

Thermostat test:

Terminals C & T are the thermostat switch connections. (Bottom and third from bottom terminals) If there is adequate DC power to the MDM but the compressor will not start, it may be because the thermostat (switch) is faulty. 

To check simply bridge the terminals C & T and if the compressor runs then the thermostat is faulty. If an older system with mechanical thermostat, consider replacing with a digital type for far more accurate control and temperature read out. (On Ozefridge systems simply switching to 'Manual' on controller panel to by pass the thermostat, will achieve the same)

Fan Corruption:

Most DC refrigeration units use small fans. If the fan draws greater than half an amp (6 watts) the MDM will shut the system down.  These fans are also driven via their own internal micro processor, a processor that if faulty can corrupt the MDM, therefore if fault persists try disconnecting the fan. (Neither of these issues concern Ozefridge systems as their fans are powered via a relay and this simply can't occur)

 

Voltage Spikes:

Voltage spikes or 'High voltage Transient Spikes' are usually very high voltage, (can be in the 1000's of volts, but negligible current) , very rapid / short lived and totally unpredictable. 

These high voltage spikes can damage electronic items like MDM's, Fan drivers and

digital thermostats due to arcing.  (Arcing like a spark plug!)

 

However while they are high voltage and can damage electronics, they are not the cause

of failure to start.

 

(The C / forums grandstanding guru's talk about 'Low voltage Spikes' preventing motor                                                                         start up is absolute nonsense!  Motor start failure is usually due to collapsing voltage as                                                               

the start load is applied, easily detected with a good multi-meter as described above) 

 

At Ozefridge we fit a Voltage Clamp, (AKA Zorb, AKA Spike arrestor), to all systems as

standard and highly recommend all users fit one of these very inexpensive devises to 

help protect all electronics coupled to the DC power supply.  We use a P6KE33CA - TVS

Diode, P6KE3 Series, Bidirectional, 28.2 V, 45.7 V, DO-204AC, 2 Pins. 

Refrigerant gas charge:

If a system is running but the evaporator or eutectic plate is not or only partly frosting, it is most likely that the system has lost refrigerant gas. Usually loss of gas causes low or high power consumption rate. (High if air contaminated. (evacuation and new filter essential) 

If a system needs gas it is because it has a gas leak, a leak that must be identified and repaired. After repair the filter dryer should be replaced and the system evacuated preferably for several hours before adding fresh refrigerant gas.

 

On the Ozefridge system simply press the arrow down button to monitor the condenser temperature (High side pressure).

When recharging, run the unit and only slowly allow refrigerant in sufficient to maintain a 12 - 14  PSIG suction. . Once this pressure is maintained without addition, close off and leave running.   Fine tune later once temperature lowers and settles.

Filter Dryer:

Unfortunately most small refrigeration systems are fitted with cheap 'spun copper dryers'. These items have no filter pad unlike the proper filter dryer type 032 (as used by Ozefridge).

Copper spun dryers only have a screen. The screen doesn't collect the fine gunk that rubs off the desiccant beads or sludge that can form as a result of contaminated oil, eventually to possibly cause capillary or TX valve restriction. 

If repairing a leaking system we suggest fitting a proper type 032 or 052 flared filter dryer mounted vertically, exit downwards. If this can't be fitted in place of the old copper type, then simply leave the copper one in place and fit the proper filter dryer as well. 

Compressor Oil: 

Most systems use R134a refrigerant with one of the Polyolester oils. If a gas leak occurs on the system low side (evaporator- compressor) the system is likely to have been running at negative pressure (vacuum) Moisture and other gunk can be drawn into the system specially if the leak is on the once frosty / wet evaporator. This contaminant will collect in the compressor oil with the likelihood of causing on going problems if not replaced. Oil is cheap compared to replacing a compressor later and the downtime.

 

NOTE: In some countries including Australia refrigerant gases can only be legally handled by licensed persons.