Rotary vs Digital Phase Converter vs VFD: Which Fits Your Shop?
Count the motors and the machines. One motor on one machine, inside a drive's single phase input rating: a VFD with single phase input. Lenze's application note says most drive makers stop at 3 hp on single phase input; Invertek's E3 specification lists up to 5 hp. Several machines, or a machine with more than one motor: a rotary phase converter, which Phoenix Phase Converters says "can power your entire shop", or a digital converter its maker rates as a power supply. A CNC control that needs a tight balance figure: compare the figure each maker publishes for the exact model, because they run from 1% to 5%. This guide puts the makers' own numbers side by side: machines at once, balance, noise, what each costs to leave on, upkeep and warranty. Built for Production picks none of them for you. The numbers do that.
Which one do you need?
Start with the row that matches your shop.
| Your shop | Look first at | The maker's own words |
|---|---|---|
| One machine, one motor, inside a drive's single phase rating | A VFD with single phase input | Phoenix's CNC guide: "VFDs work well for single-motor applications like spindle-only conversions" |
| Several machines, or one machine with several motors | A rotary phase converter | Phoenix's comparison: "one rotary converter can power your entire shop" |
| A CNC control with a balance requirement | Whichever unit publishes a figure your control accepts | Figures below run from 1% to 5%, by maker and model |
| A room that has to stay quiet | Each maker's published dB figure, with its distance (see "How loud is each one?") | Phoenix's table: its GPX digital line "Very Quiet (fan only)"; rotary "Moderate (idler motor hum)". Phase Technologies' 2021 and 2022 documents print 70 to 80 dBA for Phase Perfect units, with no distance given |
| A compressor or another hard-starting load | A rotary converter sized with the maker's adder | Phoenix: 2.5× for hard-starting loads; American Rotary: "add 25-50% capacity" |
| A load that drives the motor backward (a lowering hoist) | The maker's engineers | Phoenix marks drive-based digital units "Generally troublesome to use" with regenerative loads. That is one rotary maker's claim about the other design |
What each one is, in the makers' words
A static converter, from Phoenix's comparison of the three converter types: "A static phase converter uses capacitors and a start relay to momentarily generate a third phase of power." It starts the motor and drops out. It is here for comparison only; the static converter guide covers it.
A rotary converter, same page: it "uses an idler motor", a 3 phase motor with no load on its shaft, "combined with a control panel to generate continuous, balanced three-phase power."
A digital converter, same page: "also called an electronic phase converter", it uses power electronics to "synthesize the third phase digitally". Phase Technologies' DPC series manual describes how its Phase Perfect does it: "L1 and L2 of the single-phase input pass directly through the phase converter to provide two legs of the three-phase output." The unit builds the third.
A VFD, from Yaskawa's application note on single phase input: "Standard Pulse-Width-Modulated (PWM) VFDs use a 6-pulse diode rectifier because of its simplicity and low cost structure." The drive turns the input into DC, then builds 3 phase output for one motor, at whatever speed you set.
How many machines can each one run?
Phoenix's comparison table answers in one row, "Multiple Machines?": static "No", rotary "Yes", digital "Limited". The table covers Phoenix's own lines.
A rotary converter feeds a 3 phase panel, and every machine on that panel. Phoenix: "one rotary converter can power your entire shop". The sizing rule for several machines is each maker's own; the sizing guide has them side by side.
A digital converter depends on the design. Phase Technologies' DPC manual: "Phase Perfect can be configured as a three-phase power supply or as a motor starter depending on the application." In power supply mode it feeds "three-phase power to multiple loads of any type". A digital unit built on drive electronics is another matter. Phoenix's digital (VFD) vs rotary article says those are "Typically rated for one machine at a time". That is a rotary maker's description of the other design; ask the digital maker how it rates its unit.
A VFD runs one motor. Phoenix's CNC guide, on VFDs: "Cannot power multiple motors simultaneously". A machine with a spindle motor, a coolant pump and a power feed has three motors.
Which one holds voltage balance for a CNC?
Two makers publish what a CNC control needs. American Rotary's CNC page: "Generally speaking, three-phase CNC machines require a voltage balance of +/- 5% or better." Phoenix's CNC guide: "Phase balance should be within 2-3% for optimal CNC performance." Your CNC maker's manual decides. Here is what each converter maker publishes for its own units.
| Maker and unit | Published balance figure | Where it says so |
|---|---|---|
| American Rotary, rotary lines | "typically within 2–5% voltage variation between phases under load" | Phase converter FAQ |
| American Rotary, MicroSmart control | "precise voltage balance at 1-5%" | Phase converter for CNC page |
| Phase-A-Matic, rotary converter paired with a voltage stabilizer (CNC PAC) | "close to + 5% or less between legs" | CNC PAC and voltage stabilizer brochures |
| Phoenix NL series, rotary | "3-5% voltage balance" | Pricing guide |
| Phoenix DualZone, digital | "1-2% voltage balance" | Pricing guide |
| Phoenix GPX, digital | "Superior (±1%)" | Comparison table |
| Phase Technologies, Phase Perfect Gen4 | "limit voltage imbalance between the three legs to ≤ 2%" | Gen4 manual |
| Phase Technologies, DPC series | "better than 1%" | DPC series manual |
| VFD with single phase input | No converter-style balance figure published by any drive maker in our reading | None |
A VFD feeds one motor from its own output, so drive makers do not publish a phase balance figure the way converter makers do. That is not a mark for or against; it is a different product.
Which costs the most?
Phoenix's comparison table puts static lowest, rotary next and digital highest, read 2026-09-22. Within each type the spread is wide, and one maker's lowest overlaps another's highest. Each maker's list changes; we don't print prices in guides.
How loud is each one?
A rotary converter has a motor spinning in it. Two makers publish a number, under different conditions, so they are not compared with each other. Phoenix's article on noise: "All of our phase converter models produce 43 Db from ten feet away." For scale, the same article puts a whisper at 30 dB and a running box fan at 60 dB. American Rotary's FAQ: "American Rotary phase converters produce moderate noise levels similar to a bathroom exhaust fan running." It lists 60 to 70 dB for small, medium and large units, with no distance stated, and adds: "All of our phase converters produce the same level of noise, no matter the size."
A solid state digital converter has fans and no idler. Phoenix's comparison table rates its GPX digital line "Very Quiet (fan only)", and Phoenix's how-it-works article rates a digital converter at "< 40 dB (fans only)", with no distance given. Phase Technologies' 2021 and 2022 documents print 70 to 80 dBA for its Phase Perfect units, by model and series, with no distance given; its current documents print no figure.
A VFD has no idler. ABB's ACS355 manual lists 50 to 62 dBA for its fan-cooled drives, and WEG's CFW300 catalog lists "<60 dB". Neither gives a distance. The dB figures above were taken under different conditions, so they don't line up against each other. Every figure, with its distance and document, is in our rotary converter noise guide.
What does it cost to leave on?
A rotary converter that is switched on but not cutting draws power to spin the idler. American Rotary's FAQ: "The converter uses approximately 3–10% of rated HP when running but unloaded (idler running only)". On a 10 hp unit that is the range of 0.3 to 1 hp, by American Rotary's figure.
Phase Technologies publishes standby watts per model in its Gen4 manual: PT005 68 W, PT010 74 W, PT020 80 W, PT030 175 W. It lists efficiency at 98.7% for its standard models. The older DPC series manual lists 100 W for the DPC-A10, 240 W for the DPC-20 and 400 W for the DPC-30, at "95-97% typical" efficiency.
No drive maker in our reading publishes a standby figure for a VFD. Our idle power guide has the figures by maker and model, and our cost to run calculator runs them at your electric rate.
What upkeep and what warranty?
Upkeep, where a maker publishes any: Phase Technologies' DPC manual says "Line filter capacitors should be inspected annually at a minimum." and "Replacement of the capacitors every three years is recommended." Its Gen4 manual calls for the converter to be inspected and cleaned once a year, more often in a hot or dirty spot. Rotary makers in our reading publish no service interval.
Warranty, in each maker's words, read 2026-09-22 (Phase-A-Matic's brochures, October 2026). The maker's warranty applies; the terms are theirs. What each term covers and what voids it is in our phase converter warranties guide.
| Maker | Published term |
|---|---|
| American Rotary | "Lifetime Warranty on Major Components" and "2-Year Warranty on Idlers" (phase converter FAQ) |
| Phoenix Phase Converters | Warranty page, read 2026-09-22: rotary control panels are "backed by a lifetime warranty, safeguarding against defects in materials and craftsmanship"; "The Electric Motor/Idler is protected under a three-year warranty provided by North American Electric Motors or Techtop."; the rotor "comes with a lifetime warranty for the original owner". Exclusions include "installations not compliant with local electrical codes" |
| NAPCco | "Most NAPC converters include a lifetime warranty." The TritonPro Maxx digital line carries its own terms (phase converters category page) |
| Phase-A-Matic | Rotary brochure: "4 YEAR WARRANTY". Static UL series brochure: "GUARANTEED 12 MONTHS AGAINST MANUFACTURING DEFECTS" |
| Phase Technologies, Phase Perfect | "One Year", which "May be extended at time of purchase or within warranty period" (warranty table); the Gen4 manual: "One Year Warranty" |
| Phase Technologies, drives | ES and SD series "Three Years"; SDE, 2XD, 1LH, 3LH and DX "Two Years" (warranty table) |
| Invertek, Optidrive E3 | "Up to 5 Year Warranty" (E3 specification) |
"Digital" means three different things
Search results mix three designs under one word.
A solid state converter builds the third leg with electronics and no idler. Phase Perfect is one; the DPC manual's pass-through sentence above describes it.
A rotary converter with a digital control is still a rotary converter. American Rotary's ADX page describes a "Custom Baldor generator, digital MicroSmart™ control". The idler is there; the control is digital.
A converter built on drive electronics is what Phoenix's comparison article heads "Digital (VFD)". Its limits follow the drive: one machine at a time, by Phoenix's account.
Before you compare "digital" units, read which design each maker means. Every product page here names the design in the maker's words, and our guide to choosing a digital phase converter goes deeper on the solid state kind.
When a VFD is the answer
One motor, and speed control comes with it. Phoenix's CNC guide: "VFDs work well for single-motor applications like spindle-only conversions, but rotary converters are better for complete CNC systems." The same guide notes that "Frequency variation can affect spindle operation". That is Phoenix's preference, not a ban.
The ceiling is the limit to watch. Lenze's application note on derating VFDs for single phase input: "Most manufacturers of VFDs only offer products up to 3 Horsepower (HP) for single phase input." Invertek's Optidrive E3 specification lists "230V 1 Ph Input: 0.37–4kW (0.5–5HP)", and a smaller line, "110V 1 Ph Input: 0.5–1.5HP (230V 3 Ph Output)". Its E3 lists "Sensorless Vector Speed Control" and an overload of "150% for 60 Seconds".
Above the ceiling, drive makers publish a way to feed a 3 phase drive from single phase, with the drive run below its nameplate. Yaskawa's application note: "Therefore, single-phase use requires the three-phase VFD power rating be reduced (derated) to avoid over stressing the rectifier and DC link components." Under single phase, "the DC bus ripple becomes 120 Hz and the VFDs DC bus circuit is subject to higher stress in order to deliver equivalent power." Lenze's note sizes the drive by current: the drive's input rating has to be at least the motor's current times 1.73, so its 10 hp example, a motor at about 27 A, needs a drive rated for 46.71 A. Yaskawa adds a warning about its own drives: "Disabling input single-phase protection will result in permanent drive damage and void the warranty." At 480V, ABB's Technical Note 091: "Simply doubling the VFD size compared to the motor will cover the added input current but not always be enough to handle the extra DC bus ripple on 480 VAC drives." Each rule belongs to its own maker's drives. Ask the drive maker before you derate, and let a licensed electrician do the install.
When a rotary converter is the answer
Several machines on one panel, or one machine with several motors. Hard-starting loads, sized with the maker's adder: Phoenix's sizing guide uses 2.5× for "compressors and other hard-starting loads" and 3× for "dust collectors and large fans with high-inertia impellers"; American Rotary's FAQ says "For hard-starting loads (compressors, punch presses), add 25-50% capacity." The calculator runs both. For one compressor, a drive is the other route; our VFD for an air compressor article lays out what each maker says. Regenerative loads, by Phoenix's account: its table says a rotary converter "Can be used easily, even with regenerative load". That is one rotary maker's claim about its own design; confirm it with the maker of the unit you pick.
When a digital converter is the answer
A published balance figure at the tight end: 2% or less on Phase Perfect Gen4 models, "better than 1%" on the DPC series, "1-2%" on Phoenix's DualZone. Published standby watts, so you know what it costs to leave on. A fan instead of a motor. Weigh against those a one year published warranty at Phase Technologies, shorter than the rotary terms in the table, and a capacitor interval to plan for.
What about a static converter?
It is the least of the four: about two thirds of the motor's horsepower under Phase-A-Matic's Method No. 1, one machine at a time, and, in Phoenix's words, "Static converters cannot run CNC machines, sensitive electronics, or any equipment requiring balanced three-phase power." The static converter guide has the numbers.
Which questions no table settles
- A 480V machine on 240V service. Phoenix's 480V page: "On its own, a rotary phase converter produces a three-phase voltage that matches the input voltage." The converter maker sizes the converter and transformer package. Our 3 phase voltage guide explains 208V, 240V and 480V.
- Regenerative loads. Ask the maker of the unit you are considering.
- Elevators. NAPCco's elevator page: "Some elevators have strict voltage balancing modulation systems, requiring as tight as +/- 3% voltage balance between phases." A maker's engineering job.
- Your single phase service. American Rotary's FAQ publishes a minimum service for each of its converter sizes, from "Min of 20 AMP" at 5 hp to "Min of 200 amp" at 75 hp. If the unit you need outgrows your panel, stop there. A licensed electrician sizes the feed to the National Electrical Code (NEC) and local code; we publish none of those figures as our own.
Call (855) 923-2610 with the nameplate numbers and we point you to the right maker's line.
Questions
Can a rotary converter feed a VFD?
American Rotary says its units can: "Yes, American Rotary phase converters are fully compatible with Variable Frequency Drives (VFDs) and other electronic motor controls." That is one maker's statement about its own units. Ask the converter maker and the drive maker about the pair you have in mind. Our article on running a machine with its own VFD on a rotary converter has each maker's statement.
Which one is cheapest to run?
The only published figures are American Rotary's idling range, 3 to 10% of rated hp unloaded, and Phase Technologies' standby watts per model. No drive maker in our reading publishes a standby figure. Compare the two published figures for the sizes you need, and switch a rotary converter off when the shop is idle.
Do I need a licensed electrician for any of them?
Yes, for all of them. American Rotary's AD(X) owner's manual: "Installation should be done by a licensed electrician who is familiar with phase converter installations." Phoenix's installation guide: "Phase converter installation should always be performed by a licensed electrician." Wiring, breaker and service questions go to your electrician.
Sources named in this guide
Phoenix Phase Converters' comparison of static, rotary and digital converters, pricing guide, complete guide to phase converters for CNC machines, digital (VFD) vs rotary article, sizing guide, article on how loud phase converters are, 480V page, warranty page and installation guide. American Rotary's phase converter FAQ, phase converter for CNC page, ADX page and AD(X) owner's manual. North America Phase Converters' (NAPCco) phase converters category page and elevators page. Phase Technologies' Phase Perfect Gen4 manual, DPC series manual and warranty table. Phoenix's article on how phase converters work. Phase Technologies' 2021 and 2022 product specifications and 2021 brochure. Phase-A-Matic's static converter instruction sheet and its rotary, static UL series, voltage stabilizer and CNC PAC brochures. Lenze's application note on derating VFDs for single phase input. Invertek's Optidrive E3 specification. ABB's ACS355 user's manual. WEG's CFW300 catalog. Yaskawa's application note AN.AFD.15. ABB's Technical Note 091. We name maker documents and don't link maker sites.
Size the unit you land on in the sizing calculator, or read the sizing guide for each maker's rule. Start from the top with the guide to running 3 phase machines on single phase power. Browse rotary phase converters, digital phase converters and VFDs with single phase input, or read the FAQ. Sizing help is free. Call (855) 923-2610.