Can a 3 Phase Motor Be Converted to Single Phase? Swap the Motor or Add a Converter
Rewiring a 3 phase motor won't get it to start on single phase power. Capacitors can get it started, and it then runs with less power: Phoenix Phase Converters puts it at roughly two thirds of the motor's rated horsepower. For full power on single phase, replace it with a single phase motor, or keep it and feed it 3 phase power from a VFD (variable frequency drive) or a rotary phase converter.
A swap to a single phase motor makes sense when:
- The machine has one 3 phase motor.
- An off-the-shelf single phase motor with the same mounting and shaft bolts into its place.
- Your utility's limits on starting current allow a single phase motor that size.
Keep the 3 phase motor and add a VFD or converter when:
- A single phase motor that size would draw more than your circuit or your utility's starting limits allow.
- It uses special brackets or a threaded shaft.
- The machine has other 3 phase motors on it.
- You want variable speed.
- You plan to add more 3 phase machines.
At Built for Production, we tie our figures to the documents they come from, so you can check them.
Can you rewire a 3 phase motor to run on single phase?
A 3 phase motor connected to single phase power is missing one of its three phases, which motor makers call single phasing. Michigan State University's Electrical Tech Note 322 on phase converters says, "A 3-phase motor will not start if energized from a single-phase source". Current flows through only two of the motor's windings, "and a non-rotating magnetic field is produced". Changing the motor's wiring doesn't make that field rotate. ABB's Baldor-Reliance installation and operating manual (MN408) also lists single phasing as a cause of a 3 phase motor overheating.
The capacitor wiring shown in many videos can get the motor running on single phase. It's a static phase converter: capacitors stand in for the missing third phase long enough to get the motor turning. The motor then keeps running, with less power. Phoenix Phase Converters, a converter maker, says in its comparison of static, rotary and digital phase converters that a motor run this way delivers "roughly two-thirds of the motor's rated horsepower."
Can I replace a 3 phase motor with a single phase motor?
You can replace a 3 phase motor with a single phase motor when the new motor matches the old one closely enough to bolt in and turn the machine the same way.
Read these off the old motor's nameplate:
- Horsepower
- Speed in rpm
- Frame number, such as 56, 145T or 184T
- Enclosure: TEFC (totally enclosed, fan cooled) or ODP (open drip proof)
- Voltage and full load amps
- Code letter, which Baldor lists as the "KVA Code" (your utility will ask for it)
Check these on the motor and the machine:
- Shaft diameter and keyway
- Mount: feet, a C-face (the motor bolts to the machine by its flat front face) or a bracket
- Rotation: which way the shaft turns, and whether the machine also runs in reverse
What does the frame number on a motor tell you?
Match the frame number first. A NEMA frame number tells you where the shaft and the bolt holes are. NEMA, the National Electrical Manufacturers Association, sets the standard motor dimensions US makers build to.
Ed Cowern explains the numbers in EC&M's guide to frame sizes. On a three-digit frame, divide the first two digits by 4 to get the shaft height in inches. A 184T has a shaft height of 18 / 4 = 4.5 inches. The third digit stands for body length, and a longer body means the bolt holes in the base sit farther apart. Two motors with the same three digits share the same shaft height and base bolt spacing. The letters after the digits matter too: C or TC means a C-face mount, and S or Z means a short or special shaft. Match the whole frame designation.
Small motors use two-digit frames like 48 and 56. Baldor's Cowern Papers say each of these frame numbers sets the shaft height, shaft diameter and mounting hole pattern.
Does a single phase motor of the same hp fit the same frame?
A single phase motor of the same horsepower can need a bigger frame than the 3 phase motor it replaces. Here are two pairs from ABB's Baldor-Reliance catalog, all TEFC motors that run at about 1,750 rpm:
| Catalog no. | hp | Phase | rpm | Frame | Ship weight |
|---|---|---|---|---|---|
| L3609T | 3 | Single | 1725 | 184T | 80 lb |
| EM3611T | 3 | 3 phase | 1765 | 182T | 73 lb |
| L3612TM | 5 | Single | 1725 | 184T | 100 lb |
| EM3615T | 5 | 3 phase | 1755 | 184T | 92 lb |
At 3 hp, the single phase motor is a 184T and the 3 phase motor is a 182T. Both have a 4.5 inch shaft height and a 1.125 inch shaft. WEG's NEMA dimensions chart puts the front-to-back bolt spacing at 5.5 inches for a 184T and 4.5 inches for a 182T. The single phase motor's feet sit an inch farther apart, so the plate or base the motor bolts to may need new holes. At 5 hp, both motors are 184T frames.
When won't a single phase motor bolt in?
An off-the-shelf motor won't drop in when the machine was built around a special one. John White wrote in Fine Woodworking that a swap may not be possible on some machines because "the original motor has special mounting brackets or the drive shaft has custom threads or splines." He added: "Unfortunately, specialized motors are fairly common on tablesaws." He also wrote that if the machine's maker is still in business, you may be able to get a single phase motor from it.
If the old motor's shaft or mount doesn't match a NEMA frame, a catalog NEMA motor won't fit. Check with the machine's maker, or keep the motor and give it 3 phase instead.
How big a single phase motor can you run on a home shop circuit?
Your circuit and your utility set the practical limit on a single phase motor's size. Single phase motors go up to 15 hp in ABB's Baldor-Reliance line.
The utility has a say because a single phase motor pulls a surge of current from the moment it starts until it gets up to speed. Baldor lists a starting current of 105 A at 230V for its 3 hp single phase motor, about 7.5 times its 14.0 A running current. Utilities judge that surge by the motor's starting current and its code letter, not by horsepower alone.
Florida Power & Light (FPL), a Florida utility, publishes service standards with a table of single phase motors it accepts for starting straight off the line, with no special starter, at most locations. The table runs to 5 hp and sets a code letter range and a maximum step in starting current for each size. At 230V, a 3 hp motor can be code A through G and draw up to 80 A. A 5 hp motor can be code A through F and draw up to 120 A. Baldor's example motors start above those lines: the 3 hp L3609T lists 105 A and code K, and the 5 hp L3612TM lists 163 A and code J.
The table also assumes one motor that starts no more than four times an hour, and a cycling compressor can start more often. For motors beyond the table, FPL says the customer "may be required to provide reduced voltage or incremental starting equipment", which means a starter that limits the surge, and that "FPL shall be consulted". PSEG Long Island's Redbook says the utility "must be notified of any single phase motor installation totaling 3 HP or more".
Take the motor's starting current and code letter to your utility, and get its answer before you buy the motor. A licensed electrician sizes the circuit.
What else changes when you swap to a single phase motor?
Swapping to a single phase motor can also change the machine's controls, how it reverses, whether you can change its speed, and what happens to any other 3 phase motors on it.
Controls built for 3 phase
A 3 phase machine's starter, overload relay and reversing switch were picked for the 3 phase motor. The overload relay, for example, shuts the motor off when it draws too much current, and it's set for the old motor's full load amps. A single phase motor of the same horsepower draws more current, so the old setting doesn't fit the new motor. A licensed electrician makes the connections and decides which parts stay.
Reversing a single phase motor
ABB's 2003 general purpose motors catalogue, in its single phase section, says a single phase motor, as a rule, has to stop completely before it can reverse. Not every single phase motor can reverse, Baldor's MN408 manual says. If your machine runs in reverse, check that the new motor can. Before you buy, check the shaft rotation listed on the motor's catalog page. Baldor's page for its 3 hp L3609T, for example, lists it as "Reversible." Once you have the motor, the MN408 manual says, "For single phase motors, check the connection diagram to determine if the motor is reversible".
One speed
A standard single phase replacement motor runs at one speed. A VFD on the original 3 phase motor lets you set the speed.
Other 3 phase motors on the machine
Check the nameplate of every motor on the machine. A mill or lathe can carry a coolant pump or a power feed with its own motor. If one of those is a 3 phase motor, it still needs 3 phase power after you swap the main motor. Replace each of those with a single phase motor too, or keep 3 phase power on the machine with a rotary phase converter, covered below, which can run all of them.
Keep the old motor
If you swap, keep the 3 phase motor. It lets you put the machine back to original if you sell it.
Can a 3 phase motor run on single phase with a VFD or phase converter?
A 3 phase motor can run on single phase power when a VFD or a phase converter supplies the 3 phase it needs. Which unit fits depends on how many motors you're running and how big they are.
VFD with single phase input
A VFD with single phase input takes 240V single phase in and runs one 3 phase motor, with speed control. Two Yaskawa America staff writing in Automation.com say one drive can run several motors only when each motor needs the same speed. Motors that don't all start together need a bigger drive. So one drive can't give a spindle, a coolant pump and a power feed different speeds.
Most drives built for single phase input stop at 3 hp:
- Lenze Americas' post on derating VFDs for single phase power says, "Most manufacturers of VFDs only offer products up to 3 Horsepower (HP) for single phase input."
- TECO-Westinghouse's E510 compact brochure rates the E510 at 0.5 to 3 hp on single phase input.
- WEG's CFW300 catalog lists single phase input models from 0.25 to 3 hp.
- Invertek's Optidrive E3 goes further. Its specification page lists single phase input models up to 5 hp.
For a bigger motor, you can feed single phase to a drive built for 3 phase input, if you derate it. Derating means the drive carries less than its nameplate rating on single phase, so you buy a bigger drive than the motor. Yaskawa's application note AN.AFD.15, for example, lists single phase ratings for its E7, F7 and P7 drives in tables. Ask the maker of the drive you're pricing for its single phase rating.
Static phase converter
A static phase converter, the capacitor method from the first section, runs one 3 phase motor at a time. Phoenix's two-thirds figure means a static converter leaves a 5 hp motor with about 3.3 hp. White's Fine Woodworking article says static units struggle to start heavy loads such as compressors and dust collectors. Our guide to static phase converters has the rest.
Rotary phase converter
A rotary phase converter (RPC) makes the third phase with an idler, a 3 phase motor that spins with no load on it, plus a control panel. One rotary converter can run several machines.
Makers size a rotary converter by comparing its horsepower with the motor's:
- American Rotary's FAQ says to choose a converter rated at least twice the motor's horsepower. For hard-starting loads, which it lists as "compressors, punch presses", it adds 25 to 50% more capacity. The FAQ doesn't say what that percentage is added to.
- Phoenix's sizing guide says 2x the motor for lathes, mills and saws, and 2.5x for piston compressors.
For a 3 hp mill, that's a converter of at least 3 x 2 = 6 hp under either rule. Phoenix's guide says to round up to the next size sold. Our phase converter sizing calculator runs each maker's rule on your nameplate numbers. To choose between a rotary converter and a VFD, see our guide on how rotary converters and VFDs compare.
Swap or converter: which fits your machine?
| Your situation | Swap to a single phase motor | Keep the 3 phase motor |
|---|---|---|
| One motor, 3 hp or less, standard NEMA frame | Fits if one speed is enough, the machine has no other 3 phase motors, your utility accepts the motor's code letter and starting current, and you're willing to change the controls | A VFD with single phase input also fits. It adds speed control and keeps the original motor |
| One motor, over 3 hp and up to 5 hp | Fits if the frame matches and your utility accepts the motor's code letter and starting current | A rotary converter, a derated drive built for 3 phase input, or Invertek's Optidrive E3, which takes single phase input up to 5 hp |
| One motor over 5 hp | FPL's table covers motors only up to 5 hp. Ask your own utility what it allows before you buy a bigger single phase motor | A rotary converter, or a derated drive built for 3 phase input |
| Special brackets, threads or splines | An off-the-shelf motor may not bolt in | Keep the motor and add a VFD or converter |
| More than one 3 phase motor on the machine | Every 3 phase motor needs its own replacement | A rotary converter runs them together. One VFD runs them all at one speed |
| You want variable speed | One speed, from the nameplate | A VFD gives you speed control |
| More 3 phase machines may follow | Each machine needs its own swap | One rotary converter can run several machines |
| Piston air compressor | Fits when the frame and shaft match and your utility accepts the motor's starting current and how often it starts | A VFD with single phase input, within its rating, or a rotary converter sized for a hard start. Phoenix sizes it at 2.5x the motor. American Rotary adds 25 to 50% more capacity. Static units struggle with compressors |
For a compressor, our post on a VFD for an air compressor vs a phase converter goes through each option.
What do you pay for with each option?
A swap means buying a new single phase motor and often new control parts. A VFD or converter means buying the unit and keeping the 3 phase motor you already own. On motor prices, Chuck Yung, a senior technical support specialist at the Electrical Apparatus Service Association (EASA), writes in Plant Engineering that "a three-phase motor is usually less expensive to buy" than a single phase one. So the single phase motor you'd buy for a swap usually costs more than a 3 phase motor of the same size.
- Motor swap: the single phase motor in the frame you need, any new starter or switch, fitting work such as new mounting holes, any circuit change your electrician calls for, and the electrician's time.
- VFD: one drive rated for your motor on single phase input, plus the install.
- Phase converter: a converter sized to your load by the maker's rule, plus the install.
Not sure which way to go?
If you've got the nameplate in front of you and still aren't sure, call us at (855) 923-2610 or email contact@builtforproduction.com. Send the machine type, whether it starts under load, and the motor's hp, full load amps, voltage, rpm and frame. List every motor on the machine.
We'll run each maker's sizing rule for your motor. If a single phase motor swap looks like the better fit, we'll tell you. You can also reach us through our contact page.