A 5 HP Dynamatronic CNC milling center failed its servo amplifier on first power-up from a 10 HP rotary phase converter measuring 237 V on every leg (balanced within 2 V). The machine is labeled 220 V. The ratio 237/220 = 1.077, so the supply sits 7.7% above nameplate, and the amplifier failure is attributed to that overvoltage. The decision is whether that 7.7% is enough to destroy an amplifier, whether something else in the converter wiring did it, and where a step-down transformer belongs if one is needed.
Overvoltage margin at 237 V against a 220 V nameplate
The quantity that decides the case is the DC bus voltage the amplifier sees after its input rectifier, compared with the bus overvoltage limit and capacitor rating in the amplifier datasheet. Ideal no-load rectified peak is VLL x 1.414 (derived; ignores ripple and regulation).
| Supply (line-to-line) | Deviation from 220 V | Ideal rectified peak | Where to read the limit |
|---|---|---|---|
| 220 V | 0% | about 311 V | Amplifier nameplate / datasheet input range |
| 237 V (measured) | +7.7% | about 335 V | Amplifier datasheet: max input voltage and bus overvoltage trip |
| 225 V (237 V through a 5% tap) | +2.3% | about 318 V | Transformer tap table |
| 213 V (237 V through a 10% tap) | -3.2% | about 301 V | Transformer tap table |
Against a 240 V nominal, 237 V is 1.25% low. North American single-phase service is 120/240 V, and rotary converter output should normalize to 240 V. CNC-rated converters typically hold output differential under 10%, and 237 V is inside that. Two cases change the conclusion: if the control or amplifier is rated 208 V only, 237 V is +13.9% high; if it is rated 200 V, it is +18.5% high. Read the machine and amplifier nameplates for the rated voltage and tolerance band before choosing any hardware.
Separating an overvoltage failure from a wiring or control-power failure
The failure occurred on the first power-up. X, Y and Z axes operate; the spindle will not start, so the converter was essentially idle when 237 V was measured. That reading does not prove the voltage under load or the voltage from each leg to ground. Use the table to narrow the cause.
| Candidate cause | Quantity to measure | What decides it |
|---|---|---|
| Sustained overvoltage at the amplifier input | L-L voltage at the amplifier terminals, idle and under load | Reading above the amplifier's stated maximum input voltage |
| Machine rated for 200 V or 208 V, external step-down transformer not supplied or not installed | Machine nameplate voltage; shipping list for a transformer | Nameplate below 220 V, or a transformer listed with the machine |
| Single-phase control power taken from the manufactured (high) leg | Leg-to-neutral or leg-to-ground on each converter output leg; which legs feed control transformer primaries | Control primary connected to the leg reading about 208 V to neutral |
| Converter imbalance or transient at idler start | Voltage on all three legs at start-up and at steady state | Legs unequal by more than a few volts, or a spike on start |
| Amplifier failure unrelated to supply (defect, wiring, motor or encoder fault) | Bench evaluation by the manufacturer; insulation and short checks on motor leads | Failure mode visible in the failed unit; replacement amp trips again at a verified in-spec voltage |
A diagnosis that came from a phone call, without the failed unit inspected, carries less weight than a bench report. Ask for the failed amplifier's fault evidence and the stated overvoltage threshold in writing. The mention of a rotary converter alone often draws a supply-fault response from a vendor, so the numbers decide, not the label.
Manufactured leg placement for single-phase control loads
A rotary converter takes two lines of single-phase service (L1, L2) and the idler generates a third leg. The two line legs sit at about 120 V to the service neutral; the generated leg sits at about 208 V to that neutral (120 x 1.732, derived). Connecting a single-phase load between the generated leg and neutral therefore puts about 208 V on a 120 V circuit. Connecting one between the generated leg and a line leg gives a line-to-line voltage that tracks the converter output, but the generated leg voltage moves with idler loading and balance.
Wire the machine's single-phase control transformer primary and any 120 V auxiliary circuits across the two line legs of the converter (the legs that come straight from the utility service), not to the generated leg. If the controller is single-phase, treat this as a required check: trace the control transformer primary conductors back to the converter output terminals and label which leg each one lands on. A 120 V load from the generated leg to neutral is a direct overvoltage fault.
Step-down options compared
Three approaches lower the voltage the amplifier sees, and one removes the cause at the source.
| Approach | Voltage effect | Transformer duty | Effect on other loads and the converter | Main risk |
|---|---|---|---|---|
| A. Single-phase transformer ahead of the converter | Lowers converter input, so all output legs fall together (output tracks input) | Carries full converter input current, sized to the 10 HP converter, not the 5 HP machine | Every load on the converter sees the lower voltage; idler motor runs at reduced voltage | Oversized, costly transformer; converter starting inrush through the transformer; below-nameplate voltage for other three-phase loads |
| B. Three-phase transformer after the converter, feeding only the machine | Machine-side voltage set by the tap; converter output unchanged | Sized to the machine's rated load only | Other loads on the converter keep 237 V; transformer becomes a load on the idler | Tap choice; grounding and neutral reference on the machine side |
| C. Rebalance the converter and move single-phase loads to the line legs | Reduces the generated-leg error and unwanted stress, does not lower a uniformly high 237 V | None | Improves all loads | Leaves the 7.7% overvoltage in place if the amplifier's limit is below 237 V |
| D. Correct the wiring (control power to line legs) and read the nameplate voltage | None | None | None | None; this is a diagnostic gate, not a substitute for a voltage fix |
Standard autotransformer taps are 5% and 10%. A 5% tap moves 237 V to about 225 V; a 10% tap moves it to about 213 V.
Recommended: a three-phase step-down transformer on the converter output
Use approach B, and do C and D as preconditions. A transformer on the output side of the converter is sized for the 5 HP machine instead of the converter's full input, only the machine sees the reduced voltage, and the converter's idler and any other loads stay at their normal voltage. Put the transformer after the converter as a three-phase unit, not in front as a single-phase unit, because the front-mounted unit must carry converter input current and lowers voltage for everything on the converter.
Size the transformer from the machine nameplate, not from the 5 HP figure alone:
kVA = sqrt(3) x V_LL x I_line / 1000
V_LL = machine rated voltage (secondary), from nameplate
I_line = machine full-load line current, from nameplate (three-phase)
Add margin for servo and spindle acceleration peaks and select a unit whose kVA rating covers the nameplate value with margin; read the exact Iline from the machine nameplate or electrical drawings. If the machine expects a grounded neutral reference or isolation, use an isolation transformer with a properly bonded secondary instead of an autotransformer; check the machine's electrical documentation. An autotransformer gives no isolation and passes the converter's ground reference straight through.
Tap selection: with 237 V primary, the 5% tap gives about 225 V, which sits 2.3% above 220 V and inside typical tolerance; the 10% tap gives about 213 V, which is 3.2% below 220 V. If the amplifier datasheet states a band, choose the tap that puts the secondary near the middle of it, and confirm both idle and loaded values.
Procedure for installing and commissioning the step-down
- Lock out the converter and the machine. Read the machine nameplate for rated voltage, phase, full-load current and control-power source; read the amplifier datasheet for the input voltage band and bus overvoltage limit.
- Trace every single-phase load (control transformer primary, 120 V auxiliaries) back to the converter terminals. Move any load fed from the generated leg to the two line legs.
- Measure line-to-line voltage on all three legs, and leg-to-neutral on each, with the converter running idle, then with a load applied (spindle motor or a representative three-phase load). Record all six values.
- Calculate transformer kVA from the formula above using nameplate current; select a three-phase unit with a 5% tap (and a 10% tap if the amplifier band requires it).
- Install the transformer between the converter and the machine disconnect, with overcurrent protection sized for the transformer and machine per local code and the transformer manufacturer's instructions.
- Energize the transformer with the machine disconnect open. Measure secondary line-to-line voltage on all legs, no-load, and compare with the amplifier band.
- Only then install the replacement amplifier and close the disconnect.
Checks before the replacement amplifier is energized
A replacement amplifier on order should not see power until every value below sits inside its limit. Record readings in a table for the service file.
| Check | Pass condition | Where to read the limit |
|---|---|---|
| Machine-side L-L voltage, idle | Within amplifier input band on all three legs, legs within a few volts of each other | Amplifier datasheet input voltage range |
| Machine-side L-L voltage, loaded | Still inside the band; sag under load does not fall below the minimum | Amplifier datasheet; machine nameplate tolerance |
| Control transformer primary leg assignment | Both conductors on line legs, none on the generated leg | Converter output terminal labeling |
| Leg-to-neutral on every 120 V circuit | About 120 V, never about 208 V | Circuit schematic |
| Motor lead insulation and phase-to-phase shorts on servo and spindle motors | No shorts, insulation resistance per motor maker | Motor datasheet |
| Start-up transient on idler start | No voltage excursion above the band on a scope or logging meter | Amplifier datasheet bus overvoltage limit |
The spindle not turning after the failure is a symptom of the failed amplifier, not a load condition; the idle measurement therefore represents a lightly loaded converter. Repeat the voltage readings with the spindle running once the new amplifier is installed, because the generated leg shifts with load.
How do I know if 237 V is too high for my CNC amplifier?
Compare the measured L-L voltage with the input range on the amplifier datasheet or nameplate. 237 V is +7.7% over 220 V but only 1.25% below 240 V; it is out of band only if the amplifier or control is rated 208 V or 200 V.
How do I size a step-down transformer for a 5 HP CNC on a rotary converter?
Use kVA = sqrt(3) x V_LL x I_line / 1000 with the machine's nameplate secondary voltage and full-load line current, then add margin for acceleration peaks. Size to the machine, not the 10 HP converter, when the transformer sits on the converter output.
How do I choose between a single-phase transformer before the converter and a three-phase one after it?
Choose the three-phase transformer after the converter when only the machine needs lower voltage: it is sized to the machine load and leaves the idler and other loads at full voltage. A single-phase unit ahead of the converter must carry the whole converter input current and lowers voltage for everything.
How do I wire single-phase control power from a rotary phase converter?
Connect single-phase control transformer primaries and 120 V auxiliaries across the two line legs of the converter, the legs that come straight from the utility service. Never load the manufactured leg to neutral; it reads about 208 V and overvoltages a 120 V circuit.
When should I stop and escalate to the machine or converter manufacturer?
Stop if the replacement amplifier trips or fails again with a verified in-spec voltage, or if the machine nameplate voltage, tap choice, or grounding requirement is unclear. Contact Dynamatronic technical support for the amplifier fault analysis and the converter manufacturer for output-voltage and balance specifications before re-energizing.