If your workshop has a 5 HP three-phase lathe but your building only receives single phase power, you have three practical options: a rotary phase converter, a static phase converter, or a variable frequency drive (VFD). The right choice depends on your load size, how many machines you run, and whether you need speed control. One thing is certain up front: a transformer alone cannot create the missing phases — it changes voltage, not the number of phases. This guide explains how each method works, how to size the equipment, where a transformer still fits, and the mistakes that cause most conversions to fail.
Single-phase power delivers AC through two conductors — one live and one neutral — and its waveform crosses zero 100 or 120 times per second. Three-phase power uses three conductors carrying waveforms offset by 120 electrical degrees, so the power delivered to a motor never drops to zero. That overlap is why three-phase motors are smaller, smoother, and more efficient than single-phase units of the same rating.
Utility grids carry three-phase power along streets and industrial zones, but drop to single-phase service for homes and small shops. That is why so many small factories, farms, and woodworking shops end up needing a three-phase conversion and transformer solution: the equipment is three-phase, but the wall socket is not. For a deeper comparison, see our guide on single-phase and three-phase transformers.
No. A conventional transformer is a magnetically coupled device: it transfers whatever waveform and phase count it receives from primary to secondary, changing only the voltage ratio. Feed it single phase and you get single phase out. The rare exceptions — Scott-T and open-delta arrangements — still require a three-phase source on one side or paired units, so they cannot conjure a third phase from a two-wire supply either.
What a transformer can do is handle the voltage side of the problem. The phase gap itself must always be bridged electronically or mechanically first. Our article on whether a transformer can go from three-phase to single-phase covers the mirror-image case in detail.
A rotary phase converter (RPC) uses an idler motor — a three-phase motor spinning on single-phase power — to generate the third leg mechanically, with run capacitors keeping the phase balanced under load. The output is close to true three-phase power and can feed an entire shop, with each machine started and stopped independently.
Sizing rule: choose an RPC rated at 1.5 to 2 times your largest single motor, plus the sum of any other motors running simultaneously. A shop with a 5 HP mill, a 3 HP drill press, and a 2 HP lathe needs roughly a 15 HP converter. The main trade-offs are idle power draw, audible hum, and regular bearing maintenance.
A static phase converter uses capacitors to provide a starting phase shift, then lets the motor run on single phase with one winding acting as an auxiliary. It is the cheapest entry point, but the motor delivers only about two-thirds of its nameplate horsepower, and unbalanced currents shorten winding life. Static converters suit light, infrequent-use loads — not CNC machines or anything running eight hours a day.
A VFD rectifies the single-phase input to DC, then synthesizes a clean three-phase output with PWM switching, giving you full nameplate torque, soft start, motor protection, and speed control. The critical rule: fed from a single-phase supply, the drive must be oversized by roughly 1.5 to 2 times the motor horsepower — a 5 HP motor needs a 7.5–10 HP rated drive. Also remember that one VFD powers one motor only; control circuits and panel electronics must stay on the utility side.
| Yöntem | Typical Cost | Output Quality | Multiple Motors | En Uygun Olan |
|---|---|---|---|---|
| Rotary phase converter | Medium–high | Good balance (±5%) | Evet | Multi-machine shops, lathes, mills |
| Static phase converter | Düşük | Poor after start; ~1/3 HP loss | Hayır | Light, intermittent single motors |
| VFD | Orta | Excellent (synthesized sine wave) | No (one motor per drive) | Pumps, fans, conveyors, speed control |
For wiring specifics once three-phase power is available, our three-phase transformer connection instructions walk through delta and wye configurations step by step.
Suppose you need to run a 5 HP, 230 V, three-phase air compressor on a 230 V single-phase supply. The nameplate full-load current is about 14.5 A, but the single-phase input current must carry the same power through only two wires:
Output kVA ≈ √3 × 230 V × 14.5 A ≈ 5.8 kVA. Input current ≈ 5,770 VA ÷ 230 V ≈ 25 A.
So the VFD needs an input rating of at least 26 A — sold as a “7.5 HP single-phase input” drive — with a feeder breaker sized at 125–150% of that input. A rotary converter for the same motor needs a 7.5–10 HP idler. Either way, budget for inrush current, not just running load.
Phase conversion and voltage conversion are separate problems. A VFD or converter produces three-phase power at your supply voltage — say 230 V — but many imported machines want 400 V, 440 V, or 575 V. That is where a step-up/step-down transformer such as the STU series enters the chain: single-phase supply → phase converter or VFD → step-up transformer → three-phase machine. The transformer must be sized in kVA with headroom; since transformer ratings reflect apparent power rather than mechanical output, review why transformers are rated in kVA before selecting a model.
A 5 HP (≈3.7 kW) motor with 90% efficiency and 0.85 power factor draws roughly 4.8 kVA, so a 5–7.5 kVA step-up unit is the practical minimum with starting margin. For machines rated below 5 kVA — benchtop mills, small pumps, test rigs — a single STU-5000 unit covers both the voltage conversion and the safety margin in one enclosure.
Not by itself. A step-up transformer raises voltage but passes through the phase count it receives. Pair it with a phase converter or VFD for the conversion, then use the transformer for voltage matching.
If you run one motor and want speed control, a VFD is cleaner, quieter, and often cheaper. If you run several machines at different times, a rotary converter serves the whole shop from one unit.
Equipment-side conversion runs roughly USD 150–500 per machine with a VFD, or USD 800–3,000 for a shop-wide rotary converter. A new utility three-phase drop can cost USD 5,000–25,000, which is why most small shops convert locally.
A properly sized VFD or rotary converter delivers full nameplate horsepower. A static converter derates the motor to about two-thirds, and the capacitor-only (Steinmetz) method can derate it by 40–50%.
Size the transformer kVA at least 25% above the machine’s apparent power draw, and more for hard-starting loads like compressors. For a 5 HP motor, that means a 5–7.5 kVA unit.
Converting single phase power to three phase power is a solved problem — the key is matching the method to the load. A VFD is the best fit for a single motor with speed control, a rotary converter powers a whole shop, and a static converter only suits light occasional use. A transformer never creates the third phase, but it is essential for voltage matching and isolation after the conversion. If you are building a conversion chain for workshop equipment, explore the step-up and step-down transformer range or contact Johsun Tec’s engineering team for sizing help with your specific load.
