| Supply voltage | Measure the nominal voltage and confirm the allowable variation at the controller terminals. | Common AC classes include 120 V, 230 V, 400 V, and 480 V. Common DC classes include 12 V, 24 V, 48 V, and higher DC-link voltages. | Select a controller whose rated input range includes the system's minimum and maximum operating voltage. | Do not match by nominal voltage alone; overvoltage and undervoltage limits must also be compatible. |
| Phase and frequency | Identify whether the supply is single-phase or three-phase and record the AC frequency, commonly 50 Hz or 60 Hz. | Controllers may be designed for single-phase input, three-phase input, or both; frequency ranges often cover 50/60 Hz systems. | Use a three-phase controller for a three-phase motor or balanced three-phase load unless the design specifically permits another arrangement. | Phase loss, phase sequence, and neutral requirements can affect operation and protection. |
| Load power and current | Determine the continuous load current, rated power, duty cycle, and the highest expected operating temperature. | Current ratings commonly range from less than 1 A for signal-level equipment to hundreds of amperes for industrial power controllers. | Choose a continuous current rating at or above the calculated load current, with allowance for ambient temperature and enclosure derating. | A controller's short-term peak rating does not replace its continuous current rating. |
| Current calculation | Use the load power, voltage, power factor, and efficiency where applicable. | For single-phase AC: I = P ÷ (V × PF × η). For three-phase AC: I = P ÷ (√3 × V × PF × η). For DC: I = P ÷ V. | Calculate using the worst-case operating point, not only the average load. | Motor nameplate current and manufacturer application data should take priority over a simplified estimate. |
| Load type | Classify the load as resistive, inductive, capacitive, motor-driven, transformer-based, LED, or electronic power supply. | Controllers may be rated specifically for resistive loads, motors, heaters, lamps, transformers, or general-purpose electronic loads. | Select a controller with an explicit rating for the actual load type and switching method. | A resistive-load rating should not automatically be used for motors, transformers, or capacitive input equipment. |
| Starting and inrush current | Check motor starting current, transformer energization current, lamp inrush, and capacitor-charging current. | Some controllers support controlled ramp-up, current limiting, soft start, or specified short-duration overload capacity. | Confirm both the peak-current rating and the permitted duration of the inrush event. | A controller that survives continuous current may still fail during repeated starts. |
| Control signal and interface | Identify required command signals, isolation needs, feedback devices, and communication protocols. | Common interfaces include dry contact, 0–10 V, 1–5 V, 4–20 mA, pulse or frequency input, encoder feedback, and industrial Ethernet or serial communication. | Match signal range, input impedance, polarity, isolation, connector type, and software configuration. | A signal may be electrically compatible but functionally unsuitable if scaling or fail-safe behavior differs. |
| Output method | Determine whether the application needs on/off switching, phase-angle control, pulse-width modulation, variable frequency, or regulated DC output. | Typical technologies include electromechanical relays, solid-state switching, thyristor control, transistor switching, and variable-frequency power conversion. | Choose the method that produces the required voltage, frequency, waveform, speed range, and thermal performance. | Switching method affects electromagnetic interference, acoustic noise, heat generation, and load compatibility. |
| Protection functions | Review risks from short circuits, overloads, overtemperature, ground faults, phase loss, and incorrect wiring. | Useful functions include overcurrent, overvoltage, undervoltage, thermal, short-circuit, ground-fault, stall, and phase-loss protection. | Prioritize protection functions that address the actual failure modes of the installation. | Integrated protection does not eliminate the need for correctly sized upstream overcurrent protection and disconnects. |
| Thermal conditions | Record ambient temperature, altitude, ventilation, enclosure size, mounting orientation, and nearby heat sources. | Many power controllers require derating above a specified ambient temperature or altitude and may need forced ventilation. | Select a unit whose derated current remains above the real load current under the worst installation conditions. | Heat-sink temperature and enclosure airflow can be more important than the laboratory-rated current. |
| Enclosure and environment | Assess dust, water, oil, chemicals, vibration, condensation, UV exposure, and indoor or outdoor installation. | Enclosures may be specified by IP or NEMA protection levels, depending on the applicable standard and market. | Choose an enclosure rating that is equal to or higher than the environmental requirement, including cable-entry conditions. | An enclosure rating applies only when the complete installation, including glands and covers, maintains the required protection. |
| EMC and harmonics | Check nearby sensitive equipment, cable length, grounding arrangement, radio-frequency limits, and utility power-quality requirements. | Options may include built-in filtering, shielding provisions, input reactors, output filters, or adjustable switching parameters. | Use the controller's EMC installation instructions and verify compliance with the applicable emissions and immunity requirements. | Correct cable routing, bonding, shielding, and grounding are part of EMC performance. |
| Safety and compliance | Identify required electrical, machinery, building, and workplace-safety rules for the installation location. | Look for documented conformity with applicable IEC, UL, CSA, CE, or other locally recognized requirements. | Confirm that the certification covers the controller's exact configuration, voltage class, installation category, and intended use. | Certification marks alone do not confirm that a controller is suitable for every application. |
| Wiring and installation | Verify conductor size, terminal capacity, cable temperature rating, grounding, disconnect location, and required clearances. | Controllers specify allowable wire sizes, tightening torque, stripping length, creepage, clearance, and mounting requirements. | Use the installation manual to size conductors and maintain the required separation between power and control wiring. | Poor termination, inadequate cooling space, or incorrect grounding can cause overheating and nuisance trips. |
| Duty cycle and service life | Define operating hours, switching frequency, acceleration cycles, standby periods, and expected service life. | Controllers may specify continuous duty, intermittent duty, maximum switching frequency, relay electrical life, or capacitor service-life limits. | Select a design whose duty rating exceeds the real operating cycle, including repeated starts and stops. | Frequent switching and high temperature generally reduce the life of mechanical contacts and power semiconductors. |
