Soft Start compressor Motor Drive Modules
Most of us know that Motor Driver Modules have been the weak link in DC marine / mobile refrigeration systems. (The black box on the side of the compressor)
There are two main types of Motor Driver Modules.
Earlier models (and sadly some still!) were direct on line (DOL) start which means that they try to go from zero RPM to full speed virtually instantly. This causes a massive inrush of current putting extreme stress on the Motor Driver Module, the compressor and the power supply.
New solid state technology has seen the introduction of ‘Soft Start, Motor Driver Modules. These start the compressor slowly taking several seconds to reach full speed. The benefits of this are obvious as amperage never exceeds the run rate (Not a whack up to 7 to 10 times more than the run rate as with DOL starters!) This eliminates the start up stress on all components involved and greatly reduces the possibility of Motor Driver Module failure.
At Ozefridge we used DOL Modules for many years in the past as there were no options. We were embarrassed by the Module failure rate. A failure rate acknowledged by all in the industry that are ‘fair dinkum! For some years now we have been using Soft Start drivers and the failures are all but a thing of the past. (Still an occasional one gets ‘drowned’ etc!)
Our soft start Motor Driver Module has six stage start up (Only uses all six when configured for maximum speed other wise proportionate, four stages in the test models here.) This video shows the consumption of two similar 3.5cc Ozefridge compressors starting. Note that the current rate increases in steps and at no time exceeds the run rate for the selected maximum. Watch in the top RHS of the digital monitors for each soft start unit’s slow ramping up current draw.
We are currently trialling these six stage soft start modules on various earlier model compressors in the hope of being able to offer them as an alternative when replacement is next needed or to carry as a spare. This picture below is of tests being conducted on several fridges in our ‘torture room!
Left: Eight fridges with compressors equipped with soft start modules are put through practical testing subjecting each of them to extreme temperature, voltage drift, repeated starting so as to simulate actual use and environment.