UPS Output for CNC and Servo Drives: What to Check Before Sizing

“The CNC machine has a 15 kW spindle, so will a 20 kVA UPS run it?” The numbers alone cannot answer that. A machining centre is not one load: it has a spindle, servo axes, controls, drives and auxiliaries that behave differently when power disappears. The spindle's motor rating is not the power drawn at the proposed UPS connection point. First decide what the machine must do when utility power fails.

A UPS for a CNC machine might only keep the controller, HMI, communications and lubrication controls alive long enough for a controlled stop. Another project may require the axes and spindle to keep moving and finish a cut. The second duty puts a very different demand on the UPS and its battery. The machine builder has to confirm the permitted power-loss sequence before either arrangement is selected.

For a controlled stop, list the circuits that stay powered and the deceleration time the axes need. Confirm whether brakes, coolant or lubrication must also run. Separate the machine's main drive supply from auxiliary and control supplies on the wiring diagram. A small UPS on a control cabinet is not automatically permission to keep the spindle running.

For continued machining, write down the operating cycle that must survive: tool position, spindle speed, simultaneous axis moves, acceleration, cutting load and the time until power returns or the job stops. Some machines deliberately stop on a mains-loss signal even if AC power remains available elsewhere. Drive undervoltage, safety interlocks and restart settings can also stop motion. The OEM must approve the intended behaviour; feeding AC power to the machine does not override its controls.

Get the machine's electrical schedule. If it is installed, measure at the proposed UPS output connection. Record three-phase voltage, running kW and kVA, RMS current, power factor and the highest input current during power-up, spindle acceleration, simultaneous axis moves and braking. Capture the duration of each peak. A spindle motor nameplate and the sum of individual servo ratings can be useful context, but they do not show the combined AC input seen by the UPS.

Input converters and drives can draw nonlinear current. That matters because the UPS has both a continuous kW/kVA rating and a model-specific overload or current limit on battery. Check the drive maker's input requirements, any line reactor or filter and the UPS output-voltage response against the measured waveform. A universal “motor starting multiplier” does not fit every servo system; use the drive's actual input data. Our guide to rated, actual and peak UPS load power explains why those values must stay separate.

When a spindle or axis decelerates, the motor may return energy to the drive's DC link. Some systems dissipate that energy through a braking chopper and resistor. Others use a regenerative supply that can send power back toward the AC line. SEW-EURODRIVE's documentation makes the practical limit clear: feeding energy back requires a suitable line supply. For one of its regenerative supply designs, SEW specifies a braking resistor as a controlled-stop option when energy cannot be fed back. The drive model and wiring, not the word “servo,” identify which arrangement is installed.

If the line module can export power toward the UPS output while the UPS is on battery, the machine builder and UPS supplier must agree how that energy is handled. Do not assume the UPS inverter can absorb it. A resistor-braking design has a different question: whether the resistor and drive settings can handle the agreed stopping or operating cycle. Neither option can be selected from the motor kW figure alone. The same interface issue appears in our VFD elevator UPS guide, although a CNC machine has its own motion and safety sequence.

Trace the supplies for the CNC controller, PLC, HMI, servo controls, contactors, brakes, encoders and any external 24 V power supply. Decide which ones stay alive for the agreed stop or continued operation. A drive with an energized power stage may still stop if its control circuit or enable signal drops. Conversely, keeping the control supply alive does not mean the main DC link can drive the motors.

Review the OEM's mains-loss, undervoltage and restart settings. SEW-EURODRIVE documents different responses to mains loss: a drive may try to keep operating from recovered motor energy, disable its output or execute a rapid stop, depending on its setting. That is drive-specific behaviour, not a general substitute for a UPS. The machine's emergency-stop and safe-torque-off functions must behave as its safety design requires; a UPS proposal must not bypass them.

Give the UPS supplier the machine and drive models, single-line diagram, proposed UPS supply point, measured input trace, required outage sequence and target minutes. Include the braking arrangement and any line filter, reactor or transformer. The supplier should check continuous output, short-duration current and battery-mode overload behaviour against that data. If the UPS transfers to bypass during an overload, confirm whether a usable bypass source exists during the outage.

Then size the battery for the agreed load schedule, not for the spindle nameplate. Our UPS battery runtime guide shows the load-to-discharge-data method. The site acceptance test should cut the normal supply under a controlled, OEM-supervised procedure and run the agreed stop or machining sequence with the intended load. Record UPS output voltage and current, alarms, battery discharge, drive faults and the machine's final state.

The useful quotation is not “20 kVA for a 15 kW spindle.” It states which parts of the machine remain powered, what motion is permitted, how braking energy is handled and what the tested UPS must carry while utility power is absent.

SEW-EURODRIVE braking-power guidance — regenerative feedback needs a suitable line supply.

SEW-EURODRIVE braking-resistor guidance — a model-specific controlled-stop option when feedback is unavailable.

SEW-EURODRIVE mains-loss stop control — drive response options after supply loss.

Use this as a checklist, not as an approval. The machine builder's supply and safety design, the drive manuals and the UPS model's data still govern the installed system and its test.