Rotary Phase Converter Idler Sizing
The minimum rotary-phase-converter idler to make three-phase from single-phase: idler HP = max(start factor x largest single motor, total HP running at once). The idler must both start the largest motor across the line (a ~2x factor for normal loads, ~3x for a high-inertia or hard-starting load) and run the whole aggregate load. A 10 HP lathe with a 5 HP mill also running is max(2x10, 15) = 20 HP; a shop with a small 5 HP largest but 30 HP total running is governed by the 30 HP load. Undersized, the converter stalls or will not start the big motor; oversized, it wastes idle power. A rule-of-thumb screen; the converter manufacturer's data and the motors' locked-rotor current govern.
Formula and source
start_demand_hp = start_factor x largest_motor_hp; idler_hp_min = max(start_demand_hp, total_running_hp); start_factor ~2 (normal) or ~3 (high-inertia).
Rotary phase converter idler-sizing rule of thumb (industry / converter-manufacturer guidance), by name; NEMA MG-1 across-the-line inrush behavior. The converter manufacturer governs.
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