What Is a Phase Converter, and How Does a Rotary Converter Work?
A phase converter lets you run a 3 phase machine, like a lathe, mill, planer or air compressor, on single phase power, the kind most homes and small shops have. A 3 phase motor needs power on three wires. Single phase brings in two. The converter makes the power for the third wire.
There are three types: static, rotary and digital. A rotary phase converter uses an idler. Phoenix Phase Converters, one of the makers, and a Michigan State University tech note both describe it as a 3 phase motor that runs with no load on it. Your two incoming wires run straight through the converter to your machine. The same two wires also spin the idler. The spinning idler puts out the third wire's power, so your machine gets all three.
At Built for Production, we read the rotary converter manuals, FAQs and guides from four makers. Their numbers don't always agree, so every figure below names who published it.
- It draws the two incoming wires, the idler and the third leg, one step at a time.
- It shows what each part does, and what happens from switch-on to a running machine.
- It explains why one leg reads high on a meter, and why a rotary hums with nothing running.
Single phase vs 3 phase: why a 3 phase motor won't start on single phase
3 phase power comes in on three wires, and the power on each wire is called a leg. Each leg's voltage rises and falls in a wave. The three waves are 120 degrees out of step, per Michigan State University's tech note on phase converters. In practice, each leg peaks at a different moment, a third of a cycle apart.
Inside a 3 phase motor are three windings, coils of wire that each take one leg. Because the three legs peak one after another, the windings make a magnetic field that rotates. The rotor follows that field.
Feed the same motor single phase and current flows through only two of the three windings, in step with each other. The field doesn't rotate, so the rotor has nothing to follow. Michigan State's note says it plainly: "A 3-phase motor will not start if energized from a single-phase source." A phase converter supplies the missing third leg. Our post on single phase vs 3 phase power explains the two kinds of service in more detail.
Types of phase converters: static, rotary, digital and VFD
Static, rotary and digital phase converters each make the third leg a different way. Buyers also weigh a fourth option, a VFD (variable frequency drive). A VFD controls a motor's speed. It's on this list because some VFDs take single phase in and run one 3 phase motor.
| Type | How it makes the third leg | Machines at once | Power at the motor |
|---|---|---|---|
| Static converter | Capacitors, parts that store electrical energy, make a third leg for a moment, only to start the motor (Phoenix) | One (Phoenix) | Roughly two thirds of the motor's rated hp (Phoenix) |
| Rotary phase converter (RPC) | A spinning idler makes the third leg the whole time the converter runs | Several, generally started one at a time (Michigan State) | Full rated power (American Rotary). Phoenix's sizing rule makes the converter larger than the motor so it can supply the surge of current the motor draws as it starts |
| Digital phase converter | The two incoming legs pass straight through. Electronics turn some of the incoming power into DC (direct current) and use it to build the third leg (Phase Technologies, maker of Phase Perfect) | Phase Perfect DPC series units can be set up to feed several machines (Phase Technologies' DPC manual) | A Phase Perfect Gen4 model under 50 hp starts a motor up to its own hp rating, so a 10 hp unit starts a 10 hp motor (Phase Technologies) |
| VFD with single phase input | Turns the input into DC, then builds 3 phase output for the motor (drive maker Yaskawa) | One motor (Phoenix) | A drive built for 3 phase input can also be fed single phase. Yaskawa says its drives used that way must be derated, meaning rated for less power, so the same motor needs a bigger drive |
Our digital phase converter guide covers how the electronics work. To choose between a rotary converter, a digital converter and a VFD for your shop, see Rotary vs Digital Phase Converter vs VFD.
How does a rotary phase converter work?
A rotary phase converter works as a motor and a generator at once. Single phase power goes to two of the idler's three leads, the wires that come out of the motor, and spins it like a motor. Once it's spinning, the idler produces the third leg, as Michigan State's tech note describes it. A control panel holds the parts that start the idler and keep the three legs' voltages close to each other.
The parts of a rotary phase converter
Phoenix's and American Rotary's rotary converters share four main parts: the idler, the control panel, run capacitors and start capacitors. Phoenix's design adds a start relay. American Rotary's manual says its capacitors help with starting and with keeping the three voltages even.
- Idler. The 3 phase motor that spins and makes the third leg.
- Control panel. The box that holds the capacitors. On Phoenix's units it also holds the start relay.
- Run capacitors. These stay in the circuit the whole time the converter runs. Phoenix says they "continuously balance voltage across all three legs."
- Start capacitors. A second set, used for starting. American Rotary's manual says they engage only while the converter starts and while a machine starts.
- Start relay (Phoenix). A switch that senses when the idler is up to speed and takes the start capacitors out.
What happens from switch-on to a running machine
- Two legs come in and pass through. Single phase power arrives on two wires, or legs, labeled L1 and L2. Inside the converter they connect to two of its output terminals, T1 and T2, and also feed the idler. American Rotary's manual warns that these two legs stay live even when the converter is off.
- You switch on the converter with no machine running. Michigan State notes that a motor generally needs more capacitors in the circuit to start than to run. So in Phoenix's and American Rotary's designs, the start capacitors kick in now.
- The idler reaches full speed in seconds. American Rotary's manual says its converter "should start within a second or two." In Phoenix's design, the relay then disconnects the start capacitors.
- The idler makes the third leg. With the idler at speed, the third output terminal, T3, carries the leg the idler makes. Makers call it the manufactured leg (the generated or wild leg). The run capacitors stay in to keep the three voltages close.
- You start the machine once the converter is at full speed. Michigan State's tech note says the rotary unit must be "up to full rpm before any loads can be applied."
- More than one machine? Start them one at a time. Michigan State says "it is generally necessary to start the loads individually."
A licensed electrician makes the connections. American Rotary's manual says installation "should be done by a licensed electrician who is familiar with phase converter installations."
Does a rotary phase converter make real 3 phase power?
A rotary phase converter makes 3 phase power that a 3 phase motor runs on. The voltages on its three legs are close, but not equal. Two of the legs are your single phase lines, and the idler makes the third.
Makers publish how close the legs are as a balance figure: how far apart the three leg voltages are, as a percent. A lower number means the legs are closer together.
| Maker | Published balance | Conditions | Where it says so |
|---|---|---|---|
| American Rotary | 2 to 5% between phases | "Typically," under load | Phase converter FAQ |
| Phoenix Phase Converters | 2 to 3% | "Typically" | The Complete Guide to Phase Converters for CNC Machines (2026) |
| Phoenix Phase Converters | 3 to 5% | Under load, with run capacitors | How a Phase Converter Works |
| Phase-A-Matic | Close to 5% or less between legs | Rotary plus Phase-A-Matic's Voltage Stabilizer, properly sized, not the rotary alone. Phase-A-Matic says the stabilizer is designed to bring the manufactured leg's higher voltage down near the input voltage | Voltage Stabilizer brochure |
| Phase Technologies (digital, for contrast) | 2% or less | Phase Perfect Gen4 models | Phase Perfect Gen4 manual |
Phoenix gives a different figure on each of its two pages, so both are shown.
Running a CNC machine? Check the CNC maker's manual for how much imbalance the machine's control accepts, or ask the CNC maker. Our post on phase converters for CNC machines covers that.
Why does one leg read different on a meter?
One leg on a rotary converter reads different from the other two. It's the manufactured leg, the one the idler makes, and makers expect it to read higher. Their manuals say by how much:
- American Rotary's manual. The reading between T1 and T3 "should be up to 5% higher than the T1 to T2 single-phase voltage." On 240V, 5% is 12 volts. The manual says to measure between two legs, not from a leg to ground. It says readings are most accurate with a 3 phase machine running.
- North America Phase Converters' manual. T1 to T3 and T2 to T3 can read 5 to 15 volts higher than T1 to T2 with no machine running. The readings come closer once a machine runs.
Some parts of a 3 phase machine run on single phase, such as the control circuit that starts and stops the motor. American Rotary's manual says not to use the manufactured leg for single phase loads like these. Michigan State's tech note says a single phase load there can make control parts fail or malfunction. The National Electrical Code (NEC) forbids it, per EC&M's summary of NEC Section 455.9. Your electrician decides where each load connects. Our post on running a machine with its own VFD on a rotary converter goes further into the manufactured leg.
Why does a rotary converter hum and use power with nothing running?
A rotary converter hums and uses power with nothing running because its idler spins the whole time the converter is switched on. American Rotary's FAQ puts that idle use at about 3 to 10% of the converter's rated hp. Our posts on rotary converter noise and phase converter cost to run give each maker's figures.
Will my machine lose power on a phase converter?
Your machine shouldn't lose power on a rotary phase converter: American Rotary's FAQ says equipment runs at full rated power. The converter is bigger than the motor because, as Phoenix's sizing guide explains, a motor draws a surge of current as it starts, and the converter needs enough capacity to supply it.
A static converter does cost you power. Phoenix's comparison article says the motor gets roughly two thirds of its rated hp. Our static phase converter guide covers that trade-off.
Does a phase converter change the voltage?
A rotary phase converter doesn't change the voltage. Phoenix says a rotary on its own "produces a three-phase voltage that matches the input voltage." So 240V single phase in gives you about 240V 3 phase out.
If your machine is 480V and your shop is 240V, a rotary converter alone won't give the machine the voltage it needs. A transformer is the part that changes voltage, but Hammond Power Solutions says "Transformers cannot convert a single-phase source to a three-phase source." For a 480V machine on 240V, ask the converter maker's engineers before you buy. Our posts on 3 phase voltage and whether a transformer can make 3 phase cover the options.
What size rotary phase converter do I need?
A rotary phase converter needs to be bigger than your motor. How much bigger depends on the maker's rule. Here is each maker's rule for one machine, with the math for a 5 hp lathe and a 5 hp air compressor:
| Maker and document | Rule | 5 hp lathe | 5 hp air compressor |
|---|---|---|---|
| American Rotary, FAQ | At least 2x the motor's hp. Add 25 to 50% capacity for hard-starting loads like compressors | 5 x 2 = 10 hp | 10 hp, plus 25 to 50% more capacity. The FAQ doesn't say what the 25 to 50% is added to |
| Phoenix, sizing guide | 2x for most motors. 2.5x for compressors and other hard-starting loads. 3x for dust collectors and large fans | 5 x 2 = 10 hp | 5 x 2.5 = 12.5 hp, then round up to the next size |
| Phase-A-Matic, rotary instruction sheet | "50% larger or more" than your largest motor, for most applications | 7.5 hp or more | The rule doesn't name compressors |
Our phase converter sizing calculator asks for your machine type and its largest motor's nameplate hp, amps or kW. It applies the published rules from American Rotary, Phoenix, North America Phase Converters and Phase Technologies, which makes digital converters. It doesn't include Phase-A-Matic's rotary rule, so use the table above for that one. The sizing guide sets those four makers' rules side by side.
Got the machine in front of you? Read the nameplate hp and amps, then call us at (855) 923-2610 and we'll run each maker's rule on them with you. You can also reach us through our contact page.