How to Size a Rotary Phase Converter

The Core Sizing Formula

The formula is simple: Motor HP × Load Multiplier = Minimum Converter HP. Then choose the next standard converter size at or above that number.

The multiplier accounts for starting current (inrush), load type, and operational demands. It ranges from 1.0 for easy loads up to 2.5 for hard-starting equipment.

Load Multipliers by Equipment Type

The multiplier is not arbitrary — it's derived from the ratio of starting current to running current for each equipment category. Air compressors, for example, can draw 7–10× running current on startup. A 2.5× converter sizing provides enough capacity to supply that inrush without stalling the converter.

Step-by-Step: Sizing for a Single Machine

Step 1: Find the Motor HP

Read the motor nameplate on your equipment. Find the "HP" value. This is your starting number. If the nameplate shows FLA (Full Load Amps) but not HP, use the FLA table in our Reference Tables to estimate HP.

Step 2: Identify the Load Type

Use the table above to find your multiplier. A table saw is an easy load — it starts under no load and the load is constant and predictable.

Step 3: Calculate Minimum Converter HP

Step 4: Choose the Next Standard Size

Phoenix converters come in standard HP sizes: 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 100. Round up to the next size if your calculation falls between standard sizes.

Sizing for Multiple Machines

If you plan to run multiple machines simultaneously, you need to account for all of them. The formula is:

Use the full multiplier for the largest machine. Add half the multiplier for each additional machine you might run at the same time.

Multi-Machine Example

Shop with three machines running simultaneously:

Special Cases and Adjustments

Air Compressors

Air compressors are the hardest load a phase converter faces. They use capacitor-start capacitor-run motors with extremely high starting torque requirements. Always use 2.5× for compressors. For compressors with rotary screw heads or 2-stage reciprocating pumps, consider calling us for a custom size recommendation.

CNC Machines

CNC machines have multiple motors running simultaneously — spindle motor, servo drives, coolant pump, hydraulic power unit, axis drives. The spindle motor is usually dominant, but large lathes and machining centers with many servo axes can require more. Use 2.0× as a minimum, and consider the PL Series for tight voltage balance.

Service Factor

Some motors have a service factor (SF) on their nameplate — typically 1.0, 1.15, or 1.25. This means the motor can handle more than its rated HP for short periods. You size the converter for the rated nameplate HP, not the SF HP. The service factor is for the motor's protection, not the converter's sizing.

Dual-Voltage Motors

Many three-phase motors are dual-voltage rated (e.g., 230/460V). Always wire the motor for 230V operation when using a 230V output converter (like the NL or PL Series). Wiring for 460V would cause the motor to run at half speed or not at all.

Common Sizing Mistakes

Mistake 1: Using Nameplate KW Instead of HP

Some motor nameplates, especially on European equipment, show kW instead of HP. Convert: kW × 1.341 = HP. A 7.5 kW motor is approximately 10 HP.

Mistake 2: Forgetting the Multiplier

Sizing a phase converter at 1:1 with motor HP is the most common mistake. It works for very easy loads, but fails on anything with significant starting torque. Always apply the multiplier.

Mistake 3: Sizing Too Small to Save Money

A converter that's 10% undersized for your load will run hot, wear out early, and fail to start your equipment reliably in cold weather when motor resistance is higher. The cost difference between a 20 HP and 25 HP converter is small compared to the cost of converter replacement.

Mistake 4: Not Accounting for Future Equipment

If there's any chance you'll add machinery in the next few years, size up now. A converter is a permanent installation — pulling and replacing it is expensive. Buying one size larger now is almost always the right call.

Quick Reference: Common Equipment Sizing