How to Choose a Digital Phase Converter

To choose a digital phase converter, first check which design the maker means by "digital." The word is used for a solid state converter, a rotary converter with a digital control, and units built on a VFD (variable frequency drive). This guide sizes the solid state kind, using the rule Phase Technologies publishes for its Phase Perfect converters. The rule has two parts. First, the converter's hp rating must be at least your largest motor's hp. Second, the amps of every machine running at once must stay under the converter's output amps.

At Built for Production, we read each maker's manual and show you its own figures, with the document named, so you can check the math. Most numbers below come from Phase Technologies' manual for its Gen 4 Phase Perfect units (the Gen4 manual), which lists output amps, input amps and standby watts for each model.

What is a digital phase converter, and how does it work?

A single phase line brings two hot legs, L1 and L2, into your shop. A 3 phase motor needs three. In the strict sense, a digital phase converter is a solid state unit that makes the third leg with power electronics. It has no idler motor, the spinning motor a rotary converter uses to make that leg.

Phase Technologies' manual for its older DPC series puts it this way: "L1 and L2 of the single-phase input pass directly through the phase converter to provide two legs of the three-phase output." The electronics make the third one, the manufactured leg (the generated or wild leg).

What "digital" means on a product listing

Product listings also use "digital" for a fourth kind of product, the digital phase shifter. Phoenix Phase Converters makes rotary converters and its own digital phase shifter. It is the source for two rows below. Here's how makers describe each kind.

Before anything else, ask whether the unit has an idler motor. If it does, it's a rotary. From here on, "digital converter" in this guide means the solid state kind. For the full comparison of rotary, digital and VFD, see our rotary vs digital vs VFD guide.

Does a digital phase converter make real 3 phase power?

A solid state converter makes real 3 phase power, according to Phase Technologies. Its FAQ says: "Phase converters provide three-phase, 60 Hz power like what a utility would provide".

Phoenix says the output of its digital phase shifter is "still not equivalent to a rotary or utility three-phase source."

How close any unit comes is measured by voltage balance: how evenly the three legs match. Makers publish it as a percentage, covered in the CNC section below.

Is a digital converter the right type for your shop?

A digital converter fits when your machines fit within its amp rating and you want three things:

• A low published imbalance figure, meaning the three legs match closely
• Low standby power, the power you pay for while it idles
• A quiet unit

Another type may fit better in three cases:

• One machine with one motor. A VFD with single phase input may be the simpler answer.
• Loads that send power back, such as a hoist lowering a load. Phoenix says units in its "Digital (VFD)" category "Can malfunction with regenerative loads such as machines that return energy to the system when braking". That warning is about VFD-based units. If you run a load like that on any converter, ask its maker in writing before you buy.
• You want a long warranty. Phase Technologies lists a one year term for Phase Perfect, with extensions covered below. Several rotary makers publish lifetime coverage. American Rotary's ADX page says "Every converter is backed for life." Our phase converter warranties by maker post compares the terms.

How do you size a digital phase converter?

Size a digital converter from your machine's nameplate, using the converter maker's own rating. For Phase Perfect, that rating has two parts: horsepower for starting and amps for running.

Horsepower: what it can start

Phase Technologies publishes separate manuals for its Gen 3 and Gen 4 Phase Perfect units, so check which generation a unit or quote is before you apply a manual's rules. On a Gen 4 unit, the label inside the front cover reads "GEN 4", per the Gen4 manual. The Gen4 manual covers the PT models in the spec table below. It says: "All Phase Perfect digital phase convertors are rated to across the line start motors up to the nameplate horsepower rating of the convertor except for 50 HP models and larger." Across the line means the motor starts on full voltage, with no soft starter. So the PT010, a 10 hp model, is rated to start a 10 hp motor.

The Gen4 manual sets no starting rating for its 50 hp models and larger. The manual for the earlier Gen 3 units says: "Phase converters 50 HP and larger should not be used to cross-the-line start motor loads." That is a Gen 3 rule. For a Gen 4 model of 50 hp or larger, ask Phase Technologies what it will start.

Rotary makers publish different rules for their own units. Here is the same made-up 7.5 hp CNC mill under each maker's rule, from each maker's own document:

Phoenix publishes its two CNC figures in two documents. Its sizing guide's 2.5x is for CNC machines with servo rapid traverse. Its CNC guide's 2x is based on the largest spindle motor and doesn't mention servo rapids. Each rule sizes that maker's own units. Our phase converter sizing guide shows four makers' rules side by side: American Rotary, Phoenix, NAPCco and Phase Technologies.

Amps: what it can run

Phase Technologies' FAQ says to compare the equipment's full load amps (FLA) with the converter's output rating. The worked example below uses FLA from the nameplate. A CNC mill often has more than one motor, such as a spindle motor and a coolant pump, so add the ones that run together.

Several machines on one converter

One Phase Perfect can run several machines if their amps fit under its output rating. Phase Technologies words that limit two ways:

• Its FAQ counts the machines running at the same time. One converter can run several loads "as long as the total amp draw of the equipment, that will be running at the same time, is less than the output rating of the phase converter."
• Its cut sheet, the Phase Perfect spec sheet, counts every machine connected. It says to "select a converter rated for the total current of all connected loads."

If your machines never all run at once, the FAQ's wording can allow a smaller model than the cut sheet's wording. Ask Phase Technologies which wording applies to your shop.

How do you read a digital converter's spec sheet?

The Gen4 spec table gives three numbers for each model:

• Max steady state output amps: what your machines can draw together.
• Max AC input amps: what the converter pulls from your single phase panel.
• Standby watts: what it uses while idling with nothing running.

Here are Phase Technologies' figures for its Gen4 standard models in the 240V class, from the Gen4 manual, Table 2. Every model in this table takes 187 to 260 VAC single phase input.

Phase Technologies also makes a Residential Pro line, the PTR models from 3 to 20 hp, which it pitches "for your home shop or small business." Its manual lists lower output amps at the same hp. The PTR010 puts out 32 A, against the PT010's 36 A. The PTR020 puts out 60 A, against the PT020's 64 A. Ask which line a quote is for. Then compare output amps, not only hp.

A worked example

Here's a made-up nameplate to show the math.

• CNC mill spindle motor: 7.5 hp, 22 A FLA at 230V
• Coolant pump on the same mill: 2 A FLA
• Both run together: 22 + 2 = 24 A

The largest motor is 7.5 hp, so Phase Perfect's starting rule calls for a 7.5 hp model or larger. The PT007 is rated 7.5 hp and 26 A of output. 24 A is under 26 A, so the PT007 fits the FAQ's rule with 2 A to spare.

On the Residential Pro line, the PTR007 puts out 24 A. The FAQ says the total must be "less than" the output rating. 24 A equals 24 A, so the PTR007 doesn't meet that wording. The next size up, the PTR010 at 32 A, does.

Now add a 5 hp air compressor with a 15 A FLA nameplate. A compressor cycles on by itself, so count it as running with the mill unless you switch it off: 24 + 15 = 39 A. That's over the PT010's 36 A and under the PT015's 52 A, so the PT015 is the smallest Gen4 model that fits. On the Residential Pro line, it's the PTR015 at 48 A. The cut sheet's rule adds every connected machine, running or not, so it also gives 39 A and the same models.

Will a digital converter start an air compressor?

Phase Technologies rates Phase Perfect to start a compressor motor the same way as any other motor: up to the converter's own hp, across the line. It publishes no separate sizing rule for hard starting loads such as compressors. The FAQ on its Phase Perfect product page lists welders, air compressors and machine tools as home shop uses for its Residential Pro line.

The Gen4 manual also has a setting for heavy starts. By default, the converter lowers (folds back) the voltage on the generated leg when a starting motor draws more than 400% of the converter's full load amps. Turning off a setting called Fold Back Voltage makes the generated leg "maintain nominal voltage during heavy startups," so it holds its normal voltage while the compressor comes up to speed.

If the compressor is your only 3 phase machine, read our post on a VFD for an air compressor vs a phase converter.

Is a digital phase converter good enough for a CNC?

Compare the converter's published balance figure with what your CNC maker's manual accepts. Here's what Phase Technologies publishes:

• Phase Perfect Gen4 models: "Output Voltage Imbalance ≤2%" (Gen4 manual, Table 1)
• Phase Perfect DPC series, an older line: "Phase to phase voltage balance: better than 1%" (DPC Series manual)

The two Phase Perfect figures cover different models.

Balance matters because a small voltage gap makes a much bigger current gap. The U.S. Department of Energy's tip sheet on voltage unbalance says: "The magnitude of current unbalance may be 6 to 10 times as large as the voltage unbalance." Its example is a 100 hp motor at full load. When the voltage was 2.5% out of balance, the current was 27.7% out of balance. Uneven current heats the motor windings.

The tip sheet recommends keeping voltage unbalance at the motor terminals at 1% or less. Above 1%, it says the motor must be derated, which means run below its nameplate rating. The Gen4 manual's published maximum of 2% is above that 1% line. The DPC series' "better than 1%" is below it. Your CNC maker's manual decides what its control accepts, so check that number before you pick a model.

If your CNC already faults on an existing converter, take the converter's published balance figure to both the CNC maker and the converter maker.

What voltage does a digital phase converter take and put out?

A standard 240V class Phase Perfect puts out 3 phase at about the same voltage it takes in, per the Gen4 manual.

Phase Technologies also lists voltage doubling models, the PT207 to PT230 (7.5 to 30 hp), with output about twice the input voltage. If your machine runs only on 480V and your shop has 240V, take that to the maker and a licensed electrician before you buy anything. Our post on 3 phase voltage for shop machines explains 208V, 230V, 240V and 480V.

What electrical service does a digital converter need?

A digital converter draws more amps from your panel than it puts out. In the worked example above, the PT015 supplies up to 52 A of 3 phase but can pull up to 90 A from your panel. Pick a model by its output amps. Your panel, breaker and wire have to supply its input amps.

Phase Technologies' Gen4 manual says: "This device should be installed and serviced only by trained, licensed, and qualified personnel." A licensed electrician sizes the breaker, the wire and any service upgrade to the NEC (National Electrical Code) and local code. Bring the model's max input amps to that conversation.

Check where it will sit, too. The Gen4 manual says its converters can be ordered in a NEMA 1 enclosure for indoors or a NEMA 3R enclosure for outdoors. A NEMA 3R unit needs a rain hood. The manual says an outdoor install without one voids the warranty. It gives an operating range of "-10°C (14°F) to 50°C (122°F)." An unheated garage in a northern winter can drop below 14°F.

What to check on safety listing, warranty and upkeep

Safety listing. A safety listing means a recognized test lab, such as UL, has tested the product and lets the maker put its mark on it. Your local inspector may look for one. Ask the maker, in writing, for the exact listing wording for your model. Before you order, ask your local inspector what the inspection requires. "UL-listed components" means some parts inside carry a listing. It doesn't say the converter itself is listed.

Warranty. Phase Technologies lists Phase Perfect at "One Year," covering "both parts and labor." Its warranty page says "Coverage can be extended for up to five years on eligible systems." Warranty service runs through an authorized Phase Technologies service center or its headquarters in Rapid City, South Dakota.

Upkeep. Phase Perfect units have line filter capacitors that wear out. Phase Technologies' FAQ gives a capacitor schedule that depends on when the unit was built:

• Built before 2019: check the capacitors once a year. The FAQ says they "should typically be replaced every three years."
• Newer units: these use "film type line filter capacitors that should see a longer service life," per the FAQ. They should still be checked once a year.

The older DPC Series manual gives one schedule, with no split by year: "Line filter capacitors should be inspected annually at a minimum. Replacement of the capacitors every three years is recommended."

The Gen4 manual adds: "The converter should be inspected and cleaned annually or more frequently if located in a hot or dirty environment." Shop dust counts.

Looking at a used unit? Ask its build year and when the capacitors were last replaced. If it's from the older DPC series, ask how it was set up. That manual says it can be set up "as a three-phase power supply or as a motor starter." A unit set up as a motor starter is meant for "single-motor loads": one motor, not a shop full of machines.

What does a digital converter cost to run, and how loud is it?

Standby power is listed per model. The PT010, for example, draws 74 W while idling with nothing running, per the Gen4 manual. Our post on phase converter cost to run compares idle power by maker.

Phase Technologies' Phase Perfect product page calls it "Near silent" and points to its lack of moving parts. We found no dB figure in Phase Technologies' documents. For the figures rotary makers publish, see rotary converter noise.

What should you ask the maker before you buy?

Ask these in writing, and keep the answers:

• Which design is this: solid state, a rotary with a digital control, or VFD-based?
• Which product line is this quote for, and what are the model's rated hp and max steady state output amps?
• Will it start my largest motor across the line?
• Does my compressor or another hard starting load need a starting setting changed, such as Fold Back Voltage?
• What balance figure do you publish for this exact model?
• Which input voltage does it take, and what does it put out?
• Can it run several machines at once? Do I count only the machines running at the same time, or every machine connected?
• What is the exact listing wording for this model?
• How long is the warranty, can I extend it, and who does repairs?
• What is the capacitor service interval for this unit?
• I run a load that brakes or lowers. Is that a problem for this model?

Then ask your electrician whether your service can carry the model's max input amps, and what breaker, wire and permit the install needs.

Got your machine's nameplate in front of you? Put the numbers into our phase converter sizing calculator, which shows makers' published sizing rules applied to your numbers, with the arithmetic and each maker's document named. Or call (855) 923-2610 with the nameplate in hand, and we'll check it against the maker's rule and show you the math.