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ozepete 09-12-2019 07:44 PM

Fridge wont run!
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Why a marine fridge fails to start or cuts out prematurely:

By far the most common problem affecting DC refrigerators is the 12VDC power supply, and not the actual fridge, compressor 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 or Soft Start. (These compressor motors are 3 phase AC, NOT 'brush-less DC' )

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 excessive inrush current..

The MDM has a low voltage cut out to prevent the module driving the motor if the voltage drops away at a start attempt or when running.

This shut down is because of a poor power supply and 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.

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. (See sketch below)

Any voltage reading with no load before the compressor starts or tries to start 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 confirms that most likely the power supply is the problem and that the compressor and MDM are fine.
Again with multi-meter, check again 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.

Below is the easiest and industry recognised way to identify if voltage drop is the problem or if the MDM is faulty.. (Click on sketch to expand)

Attachment 124692

I hope this helps..

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