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IFPS CFPS: Electrohydraulics and Electrical Controls

IFPS CFPS · Work through your exam, one topic at a time.

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Electrohydraulics and Electrical Controls

Electrohydraulics and Electrical Controls tests the interface between electrical logic and fluid power: solenoids, relays, PLC I/O, sensors, proportional and servo valves, feedback, and fail-safe wiring. The exam expects you to read ladder logic and verify that an electrical output can safely operate a valve.

Core rules

  • A solenoid must overcome spring force, flow forces, and friction. AC solenoids have higher inrush than holding current and need proper shading or DC operation to avoid chatter. Check duty cycle and voltage tolerance.
  • Ladder logic: series contacts = AND, parallel contacts = OR. A seal-in branch maintains a run condition; a normally closed stop contact opens the rung. Fail-safe design de-energizes to stop.
  • PLC outputs may be relay, transistor, or triac. Verify sinking/sourcing compatibility, output current rating, and surge suppression. Analog signals such as 4–20 mA must be scaled and checked for resolution.
  • Proportional valves are current-controlled and use dither to reduce hysteresis; null adjustment centers the spool. Servo valves are closed-loop and use feedback such as an LVDT. Amplifier gain affects stability.
  • Pressure switches, proximity sensors, limit switches, and flow sensors provide feedback. Grounding, wire size, voltage drop, and emergency-stop wiring are exam targets.

Worked example

A 24 VDC directional-valve solenoid is rated 18 W, so holding current = 18/24 = 0.75 A; inrush may be about twice that, or 1.5 A. A PLC output rated 0.5 A cannot drive it directly. Use an interposing relay with a 24 VDC, 1.2 W coil: coil current = 1.2/24 = 0.05 A, well within the PLC output. Specify a relay contact rated at least 2 A to handle the 1.5 A inrush. The reasoning: the PLC drives the relay coil, and the relay contact carries the solenoid current and inrush.

Traps

Wiring a solenoid directly to an undersized PLC output, ignoring inrush current, confusing a normally open valve with a normally open ladder contact, operating an AC solenoid on DC or with a half-wave supply, mis-scaling an analog command, and treating a proportional valve as a closed-loop servo valve.