System Monitors and Speed Controls

Danfoss BD35 and BD50 compressors have variable speed capability and can run at speeds between 2,000 and 3,500 RPM. The longer and slower the compressor runs, the more efficient it is; the faster the compressor runs, the more cooling power it has.

Ideally, we initially want to run the compressor at the fastest speed to cool the box down and then run it as slowly as possible to maintain temperatures. There should then be a provision for the speed to be adjusted manually or automatically when warm goods are put into the box and/or when external conditions change.

The highest loading on the electronics, and therefore the highest risk of failure, occurs during the first few minutes of start-up when the box is warm and the compressor is running at maximum speed.

As a safeguard against electronic failure from overloading, the compressor should be started at a low speed where there is less strain on the electronics, and then gradually increase it to maximum.

By fulling utilizing the variable speed feature, your system will be far more efficient and you will provide much-needed protection for the electronics. Note: It is not recommended that pumped-water cooled systems and those with holding plates be operated at maximum speed.


  • Fixed speed: Speed is set either at the lowest speed (2000 RPM), where thermostat wires are connected directly to the thermostat terminals on the compressor controller, or at a fixed speed between 2000 and 3500 RPM, accomplished by the addition of a resistor in the thermostat circuit.

  • Manual Speed Adjustment – Speed Board: This gives user-adjustable speed selection from 2000 to 3500 RPM, by adjustment of a potentiometer with a small screwdriver. The Speed Board is a printed circuit board (PCB) that plugs onto the compressor controller.

  • Smart Speed – Merlin: Speed is automatically adjusted to achieve the best compressor run time for increased efficiency. Cold-start ramp-up routine from medium speed protects electronics. Merlin is a printed circuit board that plugs onto the compressor controller.

  • System Control – Guardian: Remote panel allows speed selection with a rotary knob. The Guardian also gives precise temperature control using an integrated Coastal MKII digital thermostat/thermometer, and fault diagnostics provided by troubleshooting LED.

  • The Ultimate Combo – Guardian Plus Merlin: This combination gives full system temperature control and diagnostics, plus the choice of using either the automatic speed routine of Merlin, or manual speed override.

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