The Use Case

A shop or field site has only 230 V (or 240 V) single‑phase utility available but needs to run a large industrial air compressor rated at roughly 20 HP. The compressor will be the sole load on the converter, so it must start reliably and deliver full torque for continuous operation. Customers often call asking “What size phase converter do I need?” without knowing what information is required.

What Usually Matters for Sizing

  1. Motor name‑plate data – HP (or kW), voltage, full‑load amps (FLA) or locked‑rotor amps (LRA). For a 20 HP compressor the LRA can be 2–3× the FLA, and starting torque is critical.
  2. Load type – Air compressors are classified as *compressor‑duty* (sometimes called HVAC duty). They have high inrush currents and may run intermittently with frequent starts. Phoenix recommends using AutoLink or compressor‑duty sizing rather than a generic NL/PL model.
  3. Service voltage – Single‑phase input is typically 230 V/240 V. The output must be three‑phase at the compressor’s rated voltage (often 460 V). If the compressor is 460 V, a transformer package may be needed to step up the single‑phase input before conversion.
  4. Starting behavior – Does the compressor have a soft‑starter or direct‑on‑line start? Soft starters reduce LRA but still require a converter that can handle the peak draw.

All of these factors combine into a *converter rating* expressed in kilowatts (kW) or horsepower equivalent. Phoenix’s internal calculators use the name‑plate HP multiplied by a duty factor (≈2 for compressor‑duty) to arrive at a minimum converter size.

Common Mistakes to Avoid

  • Choosing a bare NL/PL model – Those are sized for light, balanced loads and will stall or overheat on a 20 HP compressor.
  • Ignoring LRA/FLA – Selecting based only on continuous horsepower can leave the converter undersized for start‑up, causing voltage sag and possible motor damage.
  • Skipping transformer considerations – Trying to run a 460 V compressor directly from 230 V single‑phase without a step‑up transformer will force the converter to work beyond its design limits.
  • Assuming one size fits all – Even within “20 HP” compressors, variations in efficiency and motor construction affect LRA dramatically.

Practical Phoenix Recommendation

Based on anonymous call patterns where a 20 HP air compressor was the sole load, Phoenix typically recommends an *AutoLink/compressor‑duty* converter such as the GP50PLA series. This model provides roughly 2× the horsepower rating (≈40 HP equivalent) and includes built‑in overload protection suitable for high LRA starts.

If the compressor is rated for 460 V three‑phase, pair the GP50PLTA package with a 230 V → 460 V transformer package (often sold as a matched kit). The transformer steps up the input voltage, allowing the converter to generate a clean three‑phase output without excessive current draw on the single‑phase side.

Final sizing must be verified against the compressor’s nameplate: confirm HP, rated voltage, LRA, and FLA, then apply the compressor‑duty factor. Phoenix engineers will run those numbers and may suggest a slightly larger unit if the start is particularly hard or if future expansion is anticipated.

What to Have Ready Before Calling

  • Name‑plate photo or data – HP, voltage, LRA, FLA, service frequency (60 Hz/50 Hz).
  • Load profile – Continuous run time vs. duty cycle, any soft‑starter or VFD in use.
  • Available single‑phase service – Voltage (230 V or 240 V), amperage rating of the feeder, and distance from panel to converter location (to assess voltage drop).
  • Space & mounting constraints – Dimensions, ventilation requirements, and whether a floor‑standing or wall‑mounted unit is needed.

Having these details on hand speeds up the quote process, eliminates back‑and‑forth clarification, and ensures you receive a converter that will start your 20 HP air compressor reliably every time.