A Bandit controller can operate in a shop supplied with single-phase power only after separating the machine's actual loads. The spindle may require three-phase power while the controller, motion electronics, contactors, and auxiliaries may receive single-phase power through a control transformer. That arrangement is common, but it must be verified on this machine before selecting a rotary phase converter or variable-frequency drive (VFD).
Machine power architecture
The term power architecture here means the path from the machine disconnect to the spindle motor, control transformer, motion system, and auxiliary loads. Do not infer that path from the Bridgeport or Bandit names. Trace it from the installed wiring and component nameplates.
- De-energize the machine and apply the required isolation procedure.
- Locate the machine input terminals and record every terminal designation exactly as marked.
- Locate the spindle motor conductors and trace them back to their starter, contactor, or drive branch.
- Locate the control transformer, if fitted. Trace its primary conductors to the incoming supply and its secondary conductors toward the controller.
- Identify pumps, fans, lubrication equipment, coolant equipment, and motion-power supplies that may form additional loads.
Some mills use three-phase power only for the spindle. Others distribute it to auxiliary motors or transformer primaries. A separate single-phase controller supply is practical only when the circuit can be isolated without defeating disconnecting, protective, or emergency-stop functions.
Check 1: Expect a documented one-line sketch showing which incoming conductors feed the spindle branch and which feed the control branch. Stop if any conductor cannot be traced.
Nameplate and load inventory
Record ratings rather than sizing a converter from appearance or machine reputation. Capture the machine input plate, spindle motor plate, control-transformer primary and secondary plates, and every auxiliary motor plate. Record voltage, phase, frequency, full-load current, and transformer tap information wherever shown.
A comparison discussed for similar equipment used a spindle-motor range of 6-10 A at 220 V, three phase, and a cabinet-input range of 20-24 A. Those are example values, not ratings for this machine. Cabinet current can include control and auxiliary loads, transformer losses, and branch-loading assumptions; it is not automatically the required three-phase output current.
| Item | Required observation | Decision affected |
|---|---|---|
| Machine input plate | Voltage, phase, frequency, current | Feeder and whole-machine converter requirements |
| Spindle motor | Voltage, phase, full-load current, connection | VFD or converter output selection |
| Control transformer | Primary taps, secondary voltage, load rating | Permitted control-supply arrangement |
| Auxiliary motors | Number of phases and current | Whether spindle-only conversion is sufficient |
| Existing protection | Fuse, breaker, overload, and contactor markings | Coordination and fault diagnosis |
Check 2: Expect every load on the sketch to have a verified nameplate rating or a marked instruction to obtain that rating before equipment is ordered.
Control-supply tracing
A rotary phase converter creates a third output conductor, commonly called the manufactured leg. Its voltage to either utility-supplied conductor can respond differently as load changes. A control transformer connected through that leg may therefore expose the controller to a less stable supply during spindle starting or load transitions.
Trace the control-transformer primary back to the disconnect. Identify which pair of incoming conductors supplies it and whether that pair also supplies contactor coils, motion power, or auxiliaries. If the transformer has selectable primary taps, compare the installed tap with the measured source voltage and the transformer diagram. Never move a tap merely to compensate for a poorly balanced converter.
A separate single-phase control circuit is an option when the machine architecture permits it. The design must retain a coordinated means of isolation, protective grounding, overcurrent protection, and emergency removal of hazardous motion. Two independent sources must not leave part of the cabinet energized after the apparent main disconnect is opened.
Check 3: With power still isolated, expect continuity tracing and conductor labels to identify the exact incoming pair feeding the control transformer. The manufactured-leg decision cannot proceed from an assumed terminal position.
Conversion topology selection
Select the topology after the load inventory is complete. A rotary phase converter can serve the spindle and other genuine three-phase loads and may be attractive when the shop also has a three-phase lathe or surface grinder. Each connected machine still requires its own terminal mapping and load assessment.
A VFD is the focused choice when the spindle motor is the only three-phase load and its nameplate is compatible with the proposed drive. Feed the controller and verified single-phase auxiliaries from the appropriate single-phase circuit; feed only the spindle motor from the VFD output. Existing reversing contactors, speed controls, braking arrangements, and interlocks must be reviewed because a VFD changes how the motor is started, stopped, and reversed.
A label such as “CNC-rated” does not define the required electrical performance by itself. Obtain the converter manufacturer's permitted load range, starting capability, line-to-line voltage limits under load, and connection instructions. Tell the supplier that the load is a CNC mill with electronic control and provide the recorded motor and cabinet ratings.
| Condition | Preferred path to evaluate | Blocking question |
|---|---|---|
| Several machines need three-phase power | Rotary phase converter | Can it start and carry the intended load combination? |
| Only the spindle needs three-phase power | Spindle-dedicated VFD | Can the control and auxiliaries be supplied separately? |
| Three-phase auxiliary loads are present | Whole-machine converter or separate engineered supplies | Which loads must operate concurrently? |
| Power architecture remains unclear | Continue tracing | Which incoming pair feeds the control transformer? |
Check 4: Expect the selected topology to account for every load and preserve every stop and protective function before wiring begins.
Rotary phase converter connection
Connect a rotary converter according to both the converter instructions and the machine's traced power architecture. The recurring rule is to keep the control-transformer primary off the manufactured leg unless the machine documentation or a verified design explicitly requires another arrangement.
A different CNC mill was reported to require its manufactured leg on L2. That terminal assignment is installation-specific and must not be transferred to a Bandit-equipped Bridgeport. Terminal letters do not establish internal loading; conductor tracing does.
- Mark the converter's two utility-derived conductors and manufactured conductor.
- Assign the utility-derived pair to the verified control-transformer input pair.
- Connect all three conductors to verified three-phase loads through the machine's protective devices.
- Before enabling the controller, measure each line-to-line voltage at the machine input.
- Repeat the measurements while the spindle starts and while it carries a representative cutting load.
Compare measurements with the machine nameplates and the documented tolerances supplied for the controller and converter. A no-load measurement alone cannot expose starting sag or load-dependent imbalance.
Check 5: Expect the control-transformer primary to remain on the verified utility-derived pair and all three line-to-line readings to stay within the applicable equipment limits during spindle operation.
Spindle-only VFD connection
Do not connect a general spindle VFD output to the complete machine cabinet. Its synthesized output is intended for a compatible motor load, not as a substitute utility source for the Bandit controller, control transformer, contactor coils, or unrelated electronics.
- Verify the spindle motor voltage, current, frequency, and connection from its plate.
- Select a drive whose documentation explicitly permits the available single-phase input and the recorded motor load. Account for any manufacturer-required input derating rather than using only the drive's headline output rating.
- Route the VFD output directly to the spindle motor under the drive manufacturer's permitted switching arrangement.
- Configure motor data from the nameplate. Set acceleration, deceleration, minimum speed, maximum speed, and control inputs from the machine's mechanical requirements and the drive documentation.
- Integrate run permission, stop, fault, and emergency functions so a controller command cannot bypass the machine's protective chain.
- Test spindle direction uncoupled or under the least hazardous mechanical condition available. Correct direction using the documented motor-output procedure, not by changing the input supply.
An uninterruptible power supply may protect a compatible control circuit from short disturbances, but it does not correct wrong transformer taps, an unstable converter, poor grounding, or incorrect manufactured-leg placement. Match any such device to the controller load and grounding arrangement.
Check 6: Expect the Bandit control to receive its traced single-phase supply while the VFD output terminates only at the verified spindle-motor circuit.
End-to-end commissioning verification
| Symptom | Likely electrical cause | Next measurement |
|---|---|---|
| Controller resets when the spindle starts | Control transformer on the manufactured leg, source sag, or undersized supply | Control secondary voltage during spindle acceleration |
| Controller operates but spindle does not | Open spindle branch, converter/VFD fault, or missing run interlock | Input status and motor-branch voltage using the equipment procedure |
| Transformer noise or heating | Wrong primary tap, excessive voltage, imbalance, or overload | Primary voltage, secondary voltage, and load current |
| Motion becomes erratic under spindle load | Control-supply disturbance, grounding problem, or converter imbalance | Control supply and all line pairs under representative load |
| Protection opens during starting | Fault, incorrect protection, excessive inrush, or inadequate source | Starting current and protective-device identification |
- Check 7: Energize the control without commanding motion. Expect a stable display, normal startup, and control-secondary voltage within the transformer's documented requirement.
- Check 8: Exercise each axis through a limited, obstruction-free move. Expect correct direction, stable motion, and no controller reset.
- Check 9: Start and stop the spindle while monitoring the control supply. Expect correct rotation and no control dropout during acceleration or deceleration.
- Check 10: Run the spindle with a representative mechanical load and repeat all input line-pair and control-secondary measurements. Expect readings within the documented equipment limits, with no overheating, protection operation, or erratic motion.
FAQ
What happens if the manufactured leg feeds the Bandit control transformer?
The control voltage may shift or sag as spindle load changes, causing resets or erratic operation. Trace the transformer primary and place it on the verified utility-derived conductor pair unless the machine's documented design specifies otherwise.
What happens if a spindle VFD feeds the whole Bridgeport cabinet?
The VFD output can expose the transformer, controller, contactors, and auxiliaries to a waveform and switching arrangement for which they were not selected. Connect the VFD output only to the compatible spindle motor circuit.
What happens if the spindle runs but the Bandit resets?
Measure the control-transformer secondary during spindle acceleration and under representative load, then inspect manufactured-leg placement and source sizing. Final verification: repeat spindle starts, axis moves, and a loaded run; expect stable control voltage, no reset, correct motion, and all measured voltages within the documented equipment limits.