The machine was cheap because nobody else could plug it in. Used three-phase equipment sells at a discount in most markets for one reason: most buyers have a single-phase supply, and powering the machine costs more than the machine did.
So you are weighing up phase converter vs generator. Almost every page on that question was written by someone who sells converters, and all of them reach the same conclusion.
This article takes the other side honestly. We build generator sets, and for a workshop on a sound single-phase service, a converter is often the right answer. What we can add is the arithmetic that decides it: what each route costs to buy and run, and the run hours at which the answer flips. We cover the power-quality difference that settles whether a route suits your machine. We manufacture three-phase sets up to 3,000 kW, sized against real load lists.
Want this worked for your machine? Send us the nameplate photo and your annual run hours, and we will tell you which route pays. Talk to our engineering team →
Key Takeaways
- There are six routes, not two: static, rotary and digital converters, a single-phase-input VFD, a generator, and a utility service upgrade.
- A converter is cheaper to buy and run but needs the grid. Utility power runs 0.10−0.13/kWh∗∗against∗∗0.10−0.13/kWh∗∗against∗∗0.30-0.46/kWh for diesel.
- A generator never wins on running cost when the grid is available. It wins on capability.
- Run hours decide it: 2,000 a year should pay for a proper supply; 200 should not.
- Power quality runs both ways: a generator wins voltage balance at ±1% against 3-8% for a rotary converter, and loses on frequency to 3-5% governor droop.
Phase Converter vs Generator: The Comparison Is Missing Options
Most of these pages present a two-way choice. That framing is wrong, and it costs money. The real decision covers six routes:
- Static converter, capacitors and often a transformer generating a third leg
- Rotary converter, a static converter plus an idling motor that balances the output
- Digital converter, an inverter that synthesises a genuine third phase
- Single-phase-input VFD, one drive feeding one motor
- Generator, a three-phase set making its own power
- Utility three-phase upgrade, a new or extended service from the supply network
Two-option framing dominates because the pages are written by converter sellers. Our own three phase diesel generator guide makes the same simplification, because it covers the machine’s output rather than how you get the supply.
Two of the six, digital converters and single-phase-input drives, are absent from almost every comparison we read, and they are the two that change the answer for CNC work. For the electrical difference between the machine types, see our single phase vs three phase generator comparison.
What Each Route Actually Does
Briefly, what matters here is cost and power quality. A static converter derives a third leg from capacitors: poorly balanced, below nameplate output, and suited to one motor. A rotary converter adds an idling motor that smooths that leg so several motors can run, at the price of a rotating machine with bearings and losses. A digital phase converter synthesises all three legs electronically, balanced to within a couple of percent at any load. A single-phase-input VFD runs one motor with speed control, one drive per motor. A generator makes its own output from an engine, needs no grid, and is limited only by what you buy. Set ratings follow ISO 8528-1.
The Two Numbers That Decide It: Purchase Cost and Cost per kWh
What Each Route Costs to Buy
The ranges below are market-reported figures for a 15-25 HP workshop load. They vary by market and by how much electrical work the site needs, so treat them as orders of magnitude rather than quotes.
| Route | Typical purchase cost | Notes |
|---|---|---|
| Static converter | ~$80-500 | Cheapest route; effectively one motor |
| Rotary converter | $1,800-3,500 | Mid-range; several motors |
| Digital converter | $6,000-10,000 | Several times a rotary unit |
| Single-phase VFD | low, per motor | Multiplies with motor count |
| Generator | $10,000-25,000 | Plus fuel storage and a base |
| Utility upgrade | $8,000-25,000 | Line extension reported at roughly $30,000-90,000 per mile |
The utility line surprises people. It is priced by distance and the customer usually pays, so a site a few hundred metres from the network can be quoted five figures before any work inside the building.
What Each Route Costs to Run
Running cost separates the routes far more sharply:
- Utility power, through a converter or a direct three-phase service: roughly $0.10-0.13/kWh
- Rotary converter: about $0.12/kWh at roughly 85% efficiency, plus a continuous idling draw
- Digital converter: about $0.12/kWh at roughly 97% efficiency, with negligible idle draw
- Diesel generator: roughly $0.30-0.46/kWh at about 30% fuel-to-electric efficiency, plus servicing
A converter is cheap on both counts and tied to the grid. A generator is expensive on both and works anywhere; see the generator fuel consumption chart for the per-hour figures and kVA vs kW generator capacity.
The Break-Even Is Your Run Hours
Here is the calculation nobody publishes, and it decides the answer. The dearer-to-run option saves capital, and you burn through that saving at a fixed rate per hour. Divide the capital difference by the hourly running-cost difference and you get the hours before the cheaper capital is wiped out.
Consider a specific example: if a power supply system upgrade is quoted at $30,000 while installing a generator costs $15,000, there is an initial saving of $15,000. The cost of diesel-generated power is $0.35 per kWh, compared to $0.12 per kWh for grid power—a difference of approximately $0.23 per kWh; this means that for a 15 kW load, operating for one hour incurs an additional cost of about $3.45. Based on this calculation, the initial savings would be offset after approximately 4,300 hours of operation.
At 2,000 hours a year, eight hours a day for 250 days, the utility line repays itself in a little over two years and then runs cheaper forever. At 200 hours a year the same calculation takes more than two decades, past the point where the equipment is replaced anyway.
Banded estimates, so your numbers will differ; the shape of the answer will not. High run hours favour paying for a proper supply. Low, intermittent hours favour the cheaper capital option.
Chidi worked through this on a joinery shop in Enugu. The three-phase machines had been bought at auction for well under half the single-phase equivalent, and the utility wanted a five-figure sum to bring three-phase power to the unit. The arithmetic against 1,600 hours a year pointed to a generator, and two machines would later move to a second site a converter could not follow them to.
Why Inrush Sets the Generator’s Price and Not the Converter’s
There is a second reason the generator costs more, and it has nothing to do with the engine.
A converter draws its starting current through the utility. The network absorbs that surge from capacity the site never paid for, and a breaker tolerates it for the second or two it lasts. The converter’s rating covers steady load, not the start.
A generator must manufacture every one of those amps itself. No grid sits behind it and no short-circuit capacity is there to borrow, so the set must supply the largest starting demand on site while carrying everything else already running.
Part of what you pay for in a generator is capacity that exists for a few seconds and then sits idle. A digital converter can ride a large short-term overload because the utility behind it can. A generator can’t, so it’s sized by the worst moment in the day rather than by the load it carries all afternoon.
That sizing method is a subject in its own right, and our three-phase diesel generator guide works through it, as does Caterpillar’s Electric Power Applications guide. The price effect is what matters here: the starting requirement, not the running load, widens the gap. Our three phase generator amps guide covers the current figures your switchgear must carry.
Power Quality: Where the Generator Wins and Where It Loses
Running cost is only half the decision. The other half is whether the route suits the machine, and here the honest answer runs both ways.
Voltage Balance: the Generator Is Ahead
Voltage balance is how closely the three legs match. Motors are sensitive to it, because imbalance produces heating without producing work.
- Rotary converter: 3-8% imbalance, derating a motor to roughly 85-91% of nameplate output and shortening insulation life
- Digital converter: about ±2%
- Generator with a well-regulated AVR: about ±1%
That ordering is not the one converter marketing implies. A properly governed generator holds a closer balance than any converter, which matters for a motor loaded through long shifts. The derating basis is set out in NEMA MG-1 and in our generator alternator specifications guide.
Frequency Stability: The Converter Is Ahead
Against that, a converter inherits the utility’s frequency, held to a fraction of a hertz and never drifting. A generator’s frequency is set by the engine governor, and a mechanical governor shows 3-5% droop between no load and full load, a swing of about 1.5-2.5 Hz on a 50 Hz system.
Harmless for a pump or a conveyor. Not harmless for a CNC control, which cares about frequency, voltage and recovery time together. A generator can be regulated closely enough for sensitive equipment, but only if it’s specified for it.
The Regeneration Problem on CNC Deceleration
There is a failure mode here that is rarely written down, and it affects the route most people recommend.
When the spindle or servo system decelerates a heavy rotating load, the motor switches to generator mode and feeds energy back into the power supply network. Since the rotary converter set lacks an effective circuit to absorb this energy, the current circulates between the idling motor rotor and the running capacitors, converting into heat.
The symptoms follow a pattern. The manufactured leg climbs several volts above the others during deceleration. The idling motor runs hotter than it should. Spindle drives fault on overvoltage. And run capacitors bulge or leak within eighteen months rather than the five to ten years the maintenance schedule assumes.
A digital converter with an active front end regenerates cleanly, and a generator avoids the problem too, though it faces the opposite risk of over-frequency and over-voltage. Braking resistors on the drive’s DC bus are the standard fix for both.
Lars hit this on a machining cell in Jutland. The machine ran acceptably on a rotary converter for a little over a year, then a long spindle ramp-down started tripping the drive and an inspection found the run capacitors swelling. Braking resistors and a replacement capacitor bank solved the fault, but consumed the price advantage the rotary unit had been chosen for.
| Property | Converter | Generator | Winner |
|---|---|---|---|
| Voltage balance | 3-8% (rotary), ±2% (digital) | about ±1% | Generator |
| Frequency stability | Locked to the utility | 3-5% governor droop | Converter |
| Works without a grid | no | yes | Generator |
| Purchase cost | $80-10,000 | $10,000-25,000 | Converter |
| Cost per kWh | $0.10-0.13 | $0.30-0.46 | Converter |
Phase Converter vs Generator: Matching the Route to the Load
With the cost structure and the power-quality verdict in hand, the choice comes down to what is on the site.
One Machine, One Motor
A single motor needing variable speed is usually best served by a single-phase-input drive. One that simply needs a supply, with modest starting demand, is the rotary converter’s home ground, sized at 1.5 times the motor’s rating and about 2 times for hard-starting loads such as compressors. For a CNC or anything carrying sensitive electronics, rule out the rotary option first.
Several Machines, Mixed Loads
Several machines starting and stopping independently is where a shop-wide supply beats individual drives, because one drive per motor adds up quickly. A rotary converter can serve a subpanel provided total load and the largest single start stay within its capacity; where machines start together, the required capacity climbs toward the price of a generator.
No Grid, Mobile Work, or a Service Upgrade Quoted at Five Figures
The generator’s position is narrow but firm. It’s the only route that works with no grid connection, and the only one that moves between sites. It’s often the cheapest way to get substantial three-phase capacity when a line extension is quoted in the tens of thousands.
Our diesel generator range covers the sizes these cases usually land in.
Frequently Asked Questions
Is a phase converter cheaper than a generator?
Yes, on both counts. It costs less to buy than a generator of comparable capacity and runs on utility power at roughly 0.10−0.13/kWhagainstroughly0.10−0.13/kWhagainstroughly0.30-0.46/kWh for diesel. The catch: it only works where a single-phase supply already exists and can carry the load.
Can a phase converter or generator run a CNC machine?
Usually, but not a rotary converter on a spindle machine. Its 3-8% imbalance stresses CNC electronics, and the manufactured leg has no path to absorb the energy a spindle pushes back during deceleration, which shows up as drive faults and failed run capacitors. A digital converter suits CNC work, and a generator can too if frequency and voltage regulation are specified for the job.
Can a phase converter run off a single-phase generator?
Only where the generator can carry the extra load. A converter needs a single-phase supply at the right voltage with enough capacity, and draws its starting current through that supply as it would from the grid. Where the set is large enough, a converter or a suitably rated drive downstream will run the three-phase motor, with the same capacity limits that apply to the converter. Our three phase diesel generator guide covers whether the motor can be reconnected instead.
Conclusion
Phase converter vs generator comes down to this: the converter is cheaper to buy and run, and depends entirely on a supply that already exists. A generator costs more on both counts and works anywhere. Neither fact decides it alone.
The arithmetic does. Divide the capital difference by the running-cost difference per hour and compare against the hours you will actually run. High hours favour a proper supply; low, intermittent hours favour cheaper capital, and a machine that has to move favours the generator regardless.
Then check the power-quality column: a generator holds a closer voltage balance and a looser frequency. For a pump, that is settled. For a CNC spindle, it is the whole decision.
Send us the load list and the run hours, and we will size the routes against each other rather than argue for one of them.
