Why CNC Is a Hard Load (The 2.0× Multiplier)
Standard equipment like table saws and drill presses have simple, predictable loads. A CNC machining center is fundamentally different:
- Multiple simultaneous motors — spindle drive, servo axes, coolant pump, hydraulic unit, lube pump, tool changer
- Highly variable loading — the spindle goes from 0% to 100% load in milliseconds as a cut begins
- Sensitive electronics — CNC controllers, servo amplifiers, and feedback systems are voltage-sensitive
- High starting torque — spindle motors on large machining centers can start under partial load
The 2.0× sizing multiplier accounts for the combination of high starting inrush and complex multi-motor loading. Never size a phase converter for CNC at 1:1 with the spindle motor HP.
Voltage Balance: The Critical Spec
CNC manufacturers specify maximum allowable voltage imbalance at their equipment's three-phase input. Imbalance causes uneven heating in motor windings and drives, and can cause servo faults, spindle trips, and erratic axis behavior.
| CNC Brand | Max Voltage Imbalance | Notes |
|---|---|---|
| Haas Automation | 2% | Haas spec from their installation manual |
| Mazak | 2–3% | Varies by model; check manual |
| Okuma | 2–3% | Older models may be more tolerant |
| Fanuc controls | 3% | Applies to any brand using Fanuc CNC |
| Doosan/Daewoo | 3% | — |
| DMG Mori | 2% | European manufacturing, tight tolerances |
| Hurco | 3% | — |
| Bridgeport (Hardinge) | 3–5% | Older Bridgeports more tolerant |
The PL Series phase converter typically achieves <2% voltage imbalance under load — meeting the tightest CNC manufacturer requirements. The standard NL Series may exceed 2% imbalance on the generated leg, especially under variable loading typical of CNC.
Calculating Voltage Imbalance
If you want to verify your converter's balance with a voltmeter after installation, use this formula:
Average = (233 + 228 + 231) / 3 = 230.7V
Max deviation = |228 - 230.7| = 2.7V
Imbalance = (2.7 / 230.7) × 100 = 1.17% — well within Haas spec
Sizing for Specific CNC Machines
Always size by spindle motor HP × 2.0 as a minimum. For machines with large servo axes or multiple coolant/hydraulic pumps, consider going one size up.
| Machine | Spindle HP | Calc | Min. Converter | Recommended |
|---|---|---|---|---|
| Haas VF-2SS | 30 HP | 30 × 2.0 | 60 HP | GP60PL |
| Haas VF-4 | 20 HP | 20 × 2.0 | 40 HP | GP40PL |
| Haas ST-20 | 20 HP | 20 × 2.0 | 40 HP | GP40PL |
| Haas Mini Mill | 15 HP | 15 × 2.0 | 30 HP | GP30PL |
| Mazak QT-10N | 10 HP | 10 × 2.0 | 20 HP | GP20PL |
| Mazak Nexus 410A | 25 HP | 25 × 2.0 | 50 HP | GP50PL |
| Bridgeport Series II | 2 HP | 2 × 2.0 | 4 HP | GP5NL |
| Fadal VMC 4020 | 15 HP | 15 × 2.0 | 30 HP | GP30PL |
Recommended Converter: PL Series
For any CNC machine with a control system (Fanuc, Siemens, Haas, Mazak control), we recommend the PL Series over the standard NL Series. The reasons:
- Enhanced output filtering maintains <2% voltage imbalance under variable CNC loads
- Better performance as the spindle motor cycles between rapids and cutting
- Reduced harmonic content on the generated leg — less interference with servo amplifiers
- Price premium over NL is small compared to the value of the CNC machine it's protecting
Running Two CNC Machines from One Converter
Yes, you can run two CNC machines from a single phase converter. Size using:
(30 × 2.0) + (20 × 1.0) = 60 + 20 = 80 HP → Choose GP100PL
Common CNC Issues and Solutions
Spindle Alarm / Drive Fault on Startup
Usually caused by voltage imbalance on the generated leg during the high-inrush startup period. Solution: Upgrade to PL Series, or go one size larger on your current converter. Also verify your input breaker is properly sized — an undersized breaker can cause voltage sag during startup.
E-Stop Trips During Heavy Cutting
The converter may be undersized. During heavy cutting, the spindle motor draws close to full load, and the generated leg voltage may sag or imbalance spike. Verify converter sizing — you may need to go one size up.
Servo Axis Following Errors
Following errors (servo drive faults) can be caused by voltage noise or imbalance on the three-phase supply. The PL Series with enhanced filtering is the fix here. Also verify the grounding is complete — poor grounding can cause servo noise issues.
Machine Won't Power Up at All
If the CNC control panel doesn't power up, check: (1) Are all three phase voltages present and roughly equal? (2) Is the input voltage correct for the machine? (3) Has the converter fully spun up before you powered on the machine? Most CNC machines should not be powered until the converter has been running for 5–10 seconds.
Running a CNC machine? Call us first.
We've sized hundreds of CNC installations. Tell us your machine brand, model, and spindle HP — we'll tell you the exact converter and any installation notes for that specific machine.