Danfoss MP 54, MP 55, MP 55A Data sheet

Data Sheet

Diﬀerential pressure switch Type MP 54, 55 and 55A

MP 54 and MP 55 are used in refrigerating systems using HCFC and non-ammable HFC refrigerants.

MP 54 and MP 55 oil dierential pressure switches are used as safety switches to protect refrigeration compressors against low lubricating oil pressure.

If the oil pressure fails, the oil dierential pressure switch stops the compressor after a certain time period.

MP 54 and MP 55 are used in refrigerating systems using HCFC and non-ammable HFC refrigerants.

MP 55A is used in refrigerating systems with R717. MP 55A can also be used in systems with HCFC and non-ammable HFC refrigerants.

MP 54 has a xed dierential pressure setting. It also incorporates a thermal time relay with a xed release time setting.

MP 55 and MP 55A have adjustable dierential pressure and are available both with and without thermal time relay.

Features

• Wide regulating range

• Can be used for deep freeze, refrigeration and air conditioning plant

• Can be used for HCFC and non-ammable HFC refrigerants

• Electrical connection at the front of the unit

• Suitable for both alternating and direct current (AC and DC)

• Screwed cable entry for cables from 6 – 14 mm diameter

• Small contact dierential

AI211986438834en-000901

Danfoss

60B55.13

Danfoss

60B59.13

On start-up

Dierential pressure switch, Type MP 54, 55 and 55A

Functions

Figure 1: Electrical diagram

If there is no oil pressure on starting, or if the oil pressure falls below the set pressure during operation, the compressor will stop after the release time has elapsed.

The electrical circuit is divided into two completely separate circuits: a safety circuit and an operational circuit.

The timer (e) in the safety circuit is activated when the eective lubricating oil pressure, the oil dierential pressure (the dierence between the oil pump pressure and suction pressure), is lower than the set value.

The timer is deactivated when the oil dierential pressure is more than the set value plus the contact dierential.

Figure 2: Functions for During start

The two diagrams below explain the terms "oil dierential pressure" and "contact dierential". Both have to be considered when using oil dierential pressure switches.

The rst diagram shows the function of the dierential switch during start; the second shows the function of the switch during operation.

Pos. A: Normal start-up

The lubricating oil pressure is built up during start to the set/xed dierential plus the contact dierential, before the timer cuts out (in this example, after 45 seconds).

At point A contacts T1-T2 open and timer (e) is stopped, i.e. normal lubricating oil conditions for the compressor have been established.

Pos. B: The lubricating oil pressure does not reach the set/xed dierential plus the contact dierential before the timer period elapses. At point B the timer cuts out operational circuit L-M and the compressor stops. If a signal source is connected to terminal S, it will be activated. Restart can only be performed after about 2 minutes by activation of the reset button, provided the cause of the fault has been determined.

Dierential pressure switch, Type MP 54, 55 and 55A

Figure 3: Functions for During operation

Pos. C: The lubricating oil pressure falls during operation to a value lower than the set/xed dierential. At point C, safety circuit T1-T2 cuts in and the timer is activated.

Pos. D: The lubricating oil pressure reaches the set/xed dierential plus the contact dierential before the timer period elapses. At point D, safety circuit T1-T2 cuts out and the timer is stopped, i.e. normal lubricating oil conditions for the compressor have been established.

Pos. E: The lubricating oil pressure falls to a value lower than the set/xed dierential during operation. At point E, safety circuit T1-T2 cuts in and the timer is activated.

Pos. F: The lubricating oil pressure remains lower than the set/xed dierential. At point F the timer cuts out operational circuit L-M and the compressor stops. If a signal source is connected to terminal S, it will be activated. Restart can only be performed after about 2 minutes by activation of the reset button, provided the cause of the fault has been determined.

After start-up

It is important that a function check should be made to ensure that the dierential pressure control is operating as it should.

This check can be made by pressing the test device (inside the unit on the left hand side).

When the test device is pressed down and held in this position, the compressor motor should stop after the release time determined by the time relay has elapsed.

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