Data Sheet
Di erential pressure switch
Type MP 54, 55 and 55A
MP 54 and MP 55 are used in refrigerating systems using HCFC and non-flammable HFC refrigerants.
MP 54 and MP 55 oil differential pressure switches are used as safety switches to protect refrigeration compressors against low lubricating oil pressure.
If the oil pressure fails, the oil differential pressure switch stops the compressor after a certain time period.
MP 54 and MP 55 are used in refrigerating systems using HCFC and non-flammable HFC refrigerants.
MP 55A is used in refrigerating systems with
R717. MP 55A can also be used in systems with
HCFC and non-flammable HFC refrigerants.
MP 54 has a fixed differential pressure setting. It also incorporates a thermal time relay with a fixed release time setting.
MP 55 and MP 55A have adjustable differential 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-flammable 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 differential
AI211986438834en-000901
Differential pressure switch, Type MP 54, 55 and 55A
Functions
Figure 1: Electrical diagram
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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 effective lubricating oil pressure, the oil differential pressure (the difference between the oil pump pressure and suction pressure), is lower than the set value.
The timer is deactivated when the oil differential pressure is more than the set value plus the contact differential.
Figure 2: Functions for During start
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On start-up
The two diagrams below explain the terms "oil differential pressure" and "contact differential". Both have to be considered when using oil differential pressure switches.
The first diagram shows the function of the differential 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/fixed differential plus the contact differential, 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/fixed differential plus the contact differential 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.
© Danfoss | Climate Solutions | 2021.02 |
AI211986438834en-000901 | 2 |
Differential 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/fixed differential. At point C, safety circuit T1-T2 cuts in and the timer is activated.
Pos. D: The lubricating oil pressure reaches the set/fixed differential plus the contact differential 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/fixed differential 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/fixed differential. 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 differential 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.
© Danfoss | Climate Solutions | 2021.02 |
AI211986438834en-000901 | 3 |
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