Oil differential pressure switch

Introduction Oil differential pressure switch MP 54, MP 55, MP 55A

MP 54 and MP 55 Oil differential pressure switches are used as safety switches to protect refrigeration compressors from too low a lubricating oil pressure in the refrigeration compressor. If the oil pressure is too low, the oil differential pressure switch shuts off the compressor after a certain time. MP 54 and MP 55 are used in refrigeration systems where HCFCs and non-flammable HFC refrigerants are used. MP 55A is used in refrigeration systems with R717. MP 55A can also be used in systems with HCFCs and non-flammable HFC refrigerants. MP 54 has a fixed differential pressure and a thermal time relay with a fixed trip time. MP 55 and 55A have adjustable differential pressure and are available with or without thermal time relay.

particularities

• large control range
• can be used in freezer, refrigeration and air conditioning systems
• can be used for HCFC and non-flammable HFC refrigerants
• electrical connections on the front of the unit
• suitable for AC and DC
• cable gland for cables with 6 - 14 mm diameter
• small contact difference

approvals

• CE mark according to LVD 2006/95 / EC EN 60947-1, EN 60947-4-1, EN 60947-5-1
• China Compulsory Certificate, CCC versions with UL and CSA approval are available on request

ship approvals

• Germanischer Lloyd, GL Registro Italiano Navale, RINA (MP55)

Construction of oil differential pressure switch

1. connection to the pressure side of the lubrication system, OIL
2. connection to the suction side of the refrigeration system, LP
3. adjusting washer (MP 55 and MP 55A)
4. reset button
5. tester cable

The function of the pressure switch is only due to the differential pressure (pressure difference) between the two oppositely acting corrugated tube elements, while it is independent of the absolute pressures acting on the corrugated tubes. MP 55 and 55A can be adjusted to different differential pressures by means of the pressure adjusting disc (3). The set differential pressure is read on the inside scale. MP 54 is fixed and has no pressure adjustment disc. The factory-set differential pressure is hammered into the front panel of the device.

Technical specifications

Function

If the oil pressure fails during start-up or falls below the set value during operation, the compressor comes to a standstill after the tripping time has elapsed. The electrical circuit was divided into two separate circuits, a safety circuit and a differential circuit.

The timing relay (e) in the safety circuit is activated when the effective lubrication oil pressure, the oil differential pressure (the differential pressure between the pump pressure and the suction pressure), is less than the set value. The timer is deactivated when the oil differential pressure is greater than the setpoint plus the contact difference.

Electrical diagram

The two terms oil differential pressure and contact difference are shown in the two diagrams below. Both must be taken into account when using oil differential pressure switches.

The first shows the function of the differential pressure switch at start-up, the second shows the function during operation.

At the decrease

Pos. A: Proper start During commissioning, the build-up of the lubricating oil pressure to the set / fixed difference plus the contact difference occurs before the time relay switches off (here after 45 seconds). In point A the contact T1-T2 opens and the time relay is switched off. Normal oil pressure for the compressor is sufficient.

Pos. B: The lubricating oil pressure does not rise to the set / fixed difference plus the contact difference before the timer expires. At point B, the time relay interrupts the operating circuit LM and the compressor switches off. Any signaling device connected to terminal S is activated. A restart can be made after about 2 minutes by pressing the reset button and correcting the causes of the error.

Operational

Pos. C: The lubricating oil pressure falls below the set / fixed difference during operation. At point C, the safety circuit T1-T2 is closed, and the time relay is activated.

Pos. D: The lubricating oil pressure rises to the set / fixed difference plus the contact difference before the timer expires. In section D the safety circuit T1-T2 opens and the time relay is switched off. Normal oil pressure for the compressor is sufficient.

Pos. E: The lubricating oil pressure falls below the set / fixed difference during operation. At point E, the safety circuit T1-T2 is closed and the time relay is activated.

Pos. F: The lubricating oil pressure remains below the set / fixed difference. In point F, the time relay interrupts the operating circuit LM and the compressor switches off. Any signaling device connected to terminal S is activated. A restart can be made after about 2 minutes by pressing the reset button and correcting the causes of the error.

After the start

After commissioning, check whether the differential pressure switch works as desired.

Press the test device (left in the housing) for this check.

Holding the device down turns off the compressor motor after the trip time indicated on the timer has expired.

measurements and weight