AirCompDX

Pilot-duty pressure controls

Contactor-piloting switches for large recips, screws and plant air

All pressure switches
Industrial pilot duty · Allen-Bradley 836T / Danfoss KP-RT / Telemecanique XML

Pilot-duty pressure controls

Once the motor is above roughly 7.5 HP, or on any three-phase machine without an integrated starter, the pressure control stops carrying motor current and becomes a pilot device: it makes and breaks the contactor coil circuit or feeds a controller input. Ratings are stated in VA and control-circuit amps, not horsepower.

Specifications

Action
Standard — contacts open on rising pressure
Adjustable range
Wide — vacuum through several hundred psi depending on the sensing element
Differential
Adjustable on essentially all industrial pilot controls (adjustable)
Factory settings
Field set — these ship unset
Ports
1/4 in. NPT typical; bellows, piston or diaphragm sensing depending on model and media
Unloader
None — unloading is handled by a separate solenoid or the compressor controller
Lever / disconnect
None; the starter provides hand/off/auto
Enclosure
NEMA 1 through NEMA 4X depending on model
Temperature
Model dependent

Electrical

  • Pilot duty rated (VA at 120/240 VAC control voltage)
  • SPDT or DPDT contacts so you can drive a coil and an alarm/telemetry input
  • Never wired in series with motor leads

Size against motor full-load amps with the motor amperage calculator, and check feeder sizing with the voltage drop calculator.

Adjustment

  1. Set the cut-out (range) first with the machine running, then dial the differential for the starts-per-hour target.
  2. Check the contactor coil inrush against the switch's pilot-duty VA rating before wiring.
  3. On rotary screws the pressure switch is usually only a backup to the electronic controller — confirm the control philosophy before changing setpoints.
Before you open the cover

Kill power at the disconnect and lock out. Bleed the tank to 0 psi. Never set cut-out above the tank data-plate MAWP. Verify your numbers first in the cut-in / cut-out calculator.

Best fit

  • Compressors above 7.5 HP
  • Three-phase installs with a starter
  • Plant air headers and remote-mounted controls

Watch out

  • The most common failure is a technician wiring a pilot-duty switch as a power switch. It welds shut on the first start.
  • On a rotary screw, adjusting a backup pressure switch will not change load/unload behaviour — that lives in the controller.

Cross-references

  • Square D 9013GHG with a contactor
  • Condor MDR 3 for small three-phase machines