You close the disconnect on a freshly moved MV-35 fed from a rotary phase converter and one of four things happens: nothing at all, the control comes up clean, the main contactor chatters and drops out, or the spindle drive faults the moment it draws real current. Each of those points at a different check. Work them in order — voltage window first, balance second, leg assignment third, rotation fourth. Skipping to rotation because it is the easiest to measure wastes an afternoon.
Start Here: Voltage Window and Transformer Tap
The manual gives 200 V/220 V with a 10% variance. That is two windows, not one, and the primary tap strapped on the machine's main transformer decides which one applies to you.
- 220 V tap: 198 V to 242 V
- 200 V tap: 180 V to 220 V
Measure all three line-to-line voltages at the line side of the main disconnect with the converter running and the machine breaker still open. A US 240 V single-phase service commonly sits at 240–248 V at light load, and the manufactured leg on an idling converter frequently reads higher than the two utility legs. If the cabinet is strapped for 200 V and you are feeding it 244 V, you are 22% over before the machine has done any work.
That matters because everything downstream scales linearly. Control transformer secondaries, the 100 V and 24 V supplies, and contactor coils all ride the input. The drive DC bus sits at roughly 1.35 × VLL loaded and near 1.41 × VLL unloaded, so a 200 V-designed bus lands about 60 V high. Open the cabinet, find the primary tap links on the main transformer, and move the strap to match what you measured. If no tap covers your feed, put a buck transformer on the converter output. Do not energize the control to see what happens.
Branch: all three legs inside the window for the strapped tap → go to the balance check. Any leg high → retap or buck. Any leg low with the converter loaded → converter sizing or service voltage, not a machine problem.
Measure Unbalance Under Load, Not at Idle
A no-load spread of 1% between legs tells you the run capacitors are in the ballpark. It does not predict anything. A rotary converter passes two utility legs straight through and generates the third from the idler; that manufactured leg's voltage is set by the idler back-EMF and a fixed bank of run capacitance, which is only correct at one load point. Load the machine and the manufactured leg moves while the other two barely do. Spreads that read 1% at idle routinely open to 5–10% with the spindle at speed.
Use the standard definition and compute it yourself:
% unbalance = 100 x (max deviation of any L-L reading from the average) / (average of the three L-L readings)
Take the set of three readings at four points: converter idling with the machine breaker open; control energized with hydraulics and lube running; spindle at maximum rpm unloaded; spindle accelerating. The worst case is almost always acceleration.
Then clamp each incoming conductor at the same load points. Current unbalance is what heats iron, and in an induction motor it runs several times the voltage unbalance. The MV-35's direct-on-line motors — hydraulic power unit, coolant pump, chip auger, magazine drive — are what cook, not the drive-fed spindle. Check the motor nameplates against the NEMA MG 1 derating curve for unbalanced voltage rather than assuming a threshold.
Read the Symptom, Then Take the Named Reading
| Symptom at hookup | Likely cause | Reading that decides it |
|---|---|---|
| Main contactor will not seal in; no control power | Phase sequence / phase loss relay sees reversed rotation or a missing leg | Rotation meter on the line terminals; L-L on all three pairs |
| Control comes up, spindle drive faults on input phase loss under load | Manufactured leg collapsing at load; failed or undersized run capacitor bank | Three L-L voltages during spindle accel |
| Control transformer hot, lamps bright, coils buzzing | Machine strapped to the 200 V tap on a 240 V feed | Primary tap strap position vs measured input |
| Hydraulic pump noisy, no pressure build; coolant pump runs, no flow | Reversed phase rotation into direct-on-line motors | Rotation meter; pump discharge gauge |
| Legs within 1% at idle, 8–10% apart at spindle speed | Idler undersized or capacitance wrong for this load point | Clamp amps per leg at load, plus idler frame temperature |
| Converter trips on machine power-up only | Main transformer magnetizing inrush plus drive bus precharge | Inrush at the converter output; idler HP vs machine kVA |
Decide Which Terminal Gets the Manufactured Leg
Two different things get called the wild leg and only one of them applies here. On a utility high-leg delta, the stinger sits about 208 V to neutral and its position is fixed by code. On a rotary converter, the manufactured leg is not referenced to a neutral at all — it is simply the leg whose voltage moves with load. Treat it accordingly.
- Open the machine schematic and trace the control transformer primary. Note which two of the three incoming terminals it lands on.
- Trace any other single-phase load — cabinet fan, cooler, single-phase pump — and note its pair.
- Land the manufactured leg on the terminal those single-phase loads do not use. Then their supply comes from two stiff utility legs and stops swinging with spindle load.
- Never derive a neutral or a line-to-neutral load from the manufactured leg. Most 200 V Japanese machines are line-to-line only and need no neutral at all.
- Bond the converter output ground to the machine PE at one point. Do not create a second bond at the machine if the converter panel is already bonded.
Size the idler on the machine's largest across-the-line motor start plus the main transformer kVA. The drive-fed spindle limits its own inrush, so the biggest single event at power-up is transformer magnetizing inrush combined with DC bus precharge.
Set Rotation — and Know What It Does Not Affect
Put a rotation meter on the line side of the main disconnect and record the sequence before you touch anything else.
Incoming rotation does not set spindle or axis direction. Those drives rectify the three-phase input to a DC bus and synthesize their own output phases; the spindle turns the way the M-code and the drive parameters tell it to regardless of what comes in the door. Chasing spindle direction to diagnose rotation is a dead end.
Incoming rotation does set direction on every motor started direct-on-line: hydraulic power unit, coolant pump, chip conveyor, three-phase lube pump, magazine drive. A unidirectional vane or gear pump run backwards makes noise, builds no pressure, and can damage itself in seconds. Kill it if the gauge does not move.
About the MGFOR and MGREV buttons above the disconnect with the CW/CCW arrows: on machines of this generation, MG most often means magazine, and those are carousel jog buttons for ATC setup, not a rotation setting. Confirm it before you press either one. Pull the button wire numbers, find them on the ladder, and see where they land — magazine motor contactor coils or a PMC index input means magazine jog. Either way, do not use them as a rotation test.
If rotation is wrong, swap two incoming conductors at the disconnect — but swap the two utility legs and leave the manufactured leg on the terminal you selected above.
Power Up, Air Up, Verify
- Converter running, machine breaker open. Record three L-L voltages and the rotation sequence.
- Confirm the transformer primary strap matches the measured input.
- Close the main breaker with the control off. Listen for transformer hum, then check secondaries against the schematic values.
- Control on. Confirm no phase-loss or sequence alarm and record L-L again.
- Start hydraulics and lube. The pressure gauge should build immediately. No build in a few seconds means rotation — shut it down.
- Run the coolant pump and verify flow, not just motor rotation.
- Connect air. Set the machine's regulator to the value marked at the air unit; this class of machine typically wants 0.5–0.7 MPa (about 70–100 psi), with 0.6 MPa the common setting. Read the low-pressure switch setpoint in the schematic and confirm the switch is made.
- Filter and dry the supply before the machine inlet. Water reaching the drawbar and taper blow-off destroys spindles.
- Jog each axis at low feed, then zero return.
- Ramp the spindle to maximum in steps. At each step record the three L-L voltages and clamp each leg.
If the ATC hangs mid-cycle with a normal static gauge reading, watch the gaugeduring unclamp — that is a compressor flow problem, not a pressure setting.
When to Stop and Escalate
Stop if you cannot hold all three legs inside the tap window with the spindle loaded, if the spindle drive keeps faulting on input phase loss after balance is verified, or if the transformer taps in the cabinet do not match anything on your prints. Those are converter sizing and machine transformer questions, not wiring errors, and guessing at them costs drives. Take your recorded voltage and current tables to DMG MORI service with the machine serial number, and the same tables to the phase converter manufacturer.
FAQ
Why does my phase converter read 1% unbalance at idle and 8% with the spindle running?
The run capacitor bank that balances the manufactured leg is fixed, so it is only correct at one load point. As the machine draws current the idler's magnetizing contribution changes and the generated leg moves while the two utility legs stay put. Always judge a converter on loaded readings.
Why does the MV-35 spindle turn the right way even with reversed incoming phase rotation?
The spindle and axis drives rectify the incoming three phases to a DC bus and generate their own output waveform, so line sequence never reaches the motor. Rotation only affects direct-on-line motors such as the hydraulic pump, coolant pump, chip conveyor, and magazine drive.
Why does the hydraulic pump build no pressure right after hookup?
Reversed rotation on a unidirectional vane or gear pump. Shut it down before it seizes, swap the two utility legs at the disconnect — leaving the manufactured leg on its assigned terminal — and re-verify with a rotation meter.
Why does the manual list both 200 V and 220 V, and which tolerance applies?
The machine transformer has selectable primary taps. On the 220 V strap the window is 198–242 V; on the 200 V strap it is 180–220 V. Check which strap is fitted before you decide whether your measured supply is legal.
Why does the ATC stall mid-cycle when the air gauge reads normal?
Static pressure is not flow. Watch the gauge during the unclamp blast — if it dips below the pressure switch setpoint the cycle stops. Size the compressor and the supply line for peak CFM at the drawbar, and set the regulator to the value marked on the machine's air unit, typically around 0.6 MPa.