What Is a Pressure Switch for a Compressor and Why Is It Needed
A pressure switch (pressostat) is the key automation component of any compressor. It starts the motor when pressure in the receiver drops below the set minimum, and stops it when pressure reaches the maximum value. Without a working pressure switch, the compressor will either fail to stop in time (receiver overpressure) or fail to start when pressure in the pneumatic network is insufficient.
The Condor MDR3 10/6.3A handles this for 3-phase compressors with motors up to 5.5 kW — the most common class of industrial piston compressors.
How the Condor MDR3 Pressure Switch Works
Inside the housing is a diaphragm connected to the compressor’s pneumatic system via a 1/4″ port. As pressure rises, the diaphragm pushes against the spring mechanism that controls the power contact.
- When pressure drops below the set minimum — the contact closes and the motor starts.
- When maximum pressure is reached — the contact opens and the motor stops.
- The unloader valve (side port, 6 mm) releases pressure from the line after shutdown, making the next motor start easier.
The difference between cut-in and cut-out pressure (hysteresis) is adjusted manually with a separate regulator.
5 Reasons to Choose the Condor MDR3 10/6.3A
1. German quality and long service life Condor Pressure Control GmbH is a specialist pressure switch manufacturer with a long-established track record. The MDR series is the industry standard for industrial compressors across Europe. Contacts are rated for hundreds of thousands of switching cycles.
2. Integrated thermal motor protection The thermal relay trip range of 4–6.3 A automatically disconnects the motor under current overload — without a separate thermal relay in the circuit. This reduces installation cost and simplifies the electrical schematic.
3. Precise adjustment for your compressor Independent adjustment of cut-out pressure (p₁) and hysteresis (Δp) allows the 4–11 bar range to be set precisely to the requirements of a specific compressor and pneumatic tooling.
4. IP54 protection for real working conditions The die-cast aluminium housing with gasket withstands water splashes, oil dust, and vibration in workshop, service centre, or production floor environments.
5. Easy replacement and wiring The MDR3 uses a standard form factor compatible with most piston compressors. Terminal block for 3 phases + PE, connector for the 6 mm unloader valve tube. Replacement takes 20–30 minutes.
Which Compressors Is the MDR3 10/6.3A Compatible With
The switch suits 3-phase piston and screw compressors with motors of ~4–5.5 kW at 400 V. Typical applications:
- Piston compressors Remeza AirCast LB50, LB75, LB40 and equivalents with 3-phase motors
- Compressor stations in vehicle service centres — air pressure control for pneumatic tools
- Industrial pneumatic lines — working pressure stabilisation in production workshops
- Agricultural equipment — compressors for blowing, inflation, and spraying
- Pump systems — pump control by line pressure
⚠️ Important: The MDR3 10/6.3A is designed for 3-phase systems (3Ph + PE, 400 V). For single-phase compressors (220 V) use the MDR1 series. Confirm your motor specifications before ordering.
Technical Specifications
| Parameter | Value |
|---|---|
| Type | Electromechanical pressure switch (pressostat) |
| Series | MDR 3 |
| Thermal relay current range | 4–6.3 A |
| Supply voltage | ≤ 400 V AC, 50/60 Hz |
| Protection class | IP 54 |
| Contact | 3-pole normally closed (NC) |
| Maximum motor power | ~5.5 kW (at 400 V) |
| Pressure adjustment range | ≈ 4–11 bar |
| Maximum allowable pressure | ≤ 35 bar |
| Housing material | Die-cast aluminium |
| Maximum medium temperature | up to 80 °C |
| Connection port | G 1/4″ |
| Unloader valve | 6 mm port (for Rilsan tube) |
| Standard | EN 60947, UL/CSA variants |
| Manufacturer | Condor Pressure Control GmbH (Germany) |
Wiring the MDR3 Pressure Switch: Key Points
Unloader valve (bleed port) A flexible tube (Rilsan type or equivalent) runs from the side 6 mm port to the compressor’s check valve. This is a mandatory connection: after the motor stops, the valve releases pressure from the line between the check valve and the compressor block. Without it, the next start will be under load — the motor will fail to start or burn out.
Phase sequence When connecting a 3-phase switch, always observe the correct phase sequence (L1–L2–L3). Incorrect phase sequence reverses the direction of compressor shaft rotation — the valves work in reverse, the compressor fails to build pressure, and rapid failure follows.
Pressure adjustment
- Large spring (p₁ regulator) — cut-out pressure (upper)
- Small spring (Δp regulator) — difference between cut-in and cut-out pressure (hysteresis)
- Typical setting for a 10 bar compressor: p₁ = 10 bar, Δp = 2 bar → cut-in at 8 bar
Expert Opinion
“The Condor MDR3 is the pressure switch I recommend when replacing a failed relay on a 3-phase piston compressor. Two things are critical during replacement: first — don’t forget to connect the unloader valve tube to the check valve; otherwise the motor will start under pressure every time and burn out quickly; second — check the phase sequence after wiring, run the compressor, and confirm it is building pressure rather than releasing it. On the MDR3 this takes two minutes — just swap two phases at the terminal block.”
Vitaly Boluchevsky, Chief Engineer at Acvatron SRL, 15 years in the industry
FAQ
❓ What is the difference between the MDR3 and MDR1 pressure switches? MDR1 is for single-phase compressors (220 V, 1 phase). MDR3 is for 3-phase compressors (400 V, 3 phases). The housings and dimensions look similar, but the internal circuit and terminal block are different. Fitting an MDR3 to a single-phase compressor or vice versa is not permitted.
❓ How do I tell if the pressure switch has failed? Characteristic signs: the compressor does not stop when maximum pressure is reached; the compressor does not start at sufficient network pressure; the compressor starts immediately without pressure (unloader valve not working); sparking or a burning smell from the switch housing.
❓ Can the switch be set above 11 bar? The MDR3 10/6.3A is rated for a range up to 11 bar. For compressors with working pressure of 12–16 bar, use the MDR3 16 model (adjustment range up to 16 bar). Exceeding the rated pressure results in inaccurate switching and accelerated wear.
❓ Should anything else be replaced at the same time as the pressure switch? When replacing the switch, we recommend simultaneously checking the condition of the check valve (if it leaks under pressure, the unloader will not work) and the unloader tube (must not be blocked or kinked). On older compressors, also check the start capacitors.
❓ Is the MDR3 10/6.3A in stock in Chișinău? Yes, the Condor MDR3 pressure switch is permanently stocked at Acvatron SRL in Chișinău. Same-day collection after payment, delivery across Moldova. Call: +373 22 503-580.
