How to Use the CNC Machine Tool Automatic Power Failure Function
This guide is for machine operators, maintenance technicians and process engineers who need the automatic power failure function to stop an axis before it damages a part or a spindle. It covers what the function monitors, how to set the thresholds, how to test the circuit, and what to do after power returns.
In this article
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Key takeaways
What the CNC machine tool automatic power failure function actually monitors
The function is a monitor plus a trip. It reads the incoming supply and the drive load many times per second, compares both against limits you set, and opens the main contactor or drops the axis enable when a limit is crossed. On most controls the monitored values are line voltage, phase balance, DC bus voltage, spindle and axis current, and the state of the emergency stop loop.
Two different faults get grouped under the same alarm. A supply fault comes from outside the machine: a brownout, a lost phase, a voltage sag when a nearby welder starts. A load fault comes from the cut: a dull tool, a chip jam, a fixture that moved. The voltage window catches the first. The current window catches the second. If you only set one, the other will keep tripping you.
Decide the response before you decide the number. Three responses are common. A full trip opens the contactor and everything stops. A controlled stop holds the axes, retracts Z and stops the spindle with the drives still powered. A ride-through keeps the machine running through a short sag of 20–100 ms. The response you choose sets the delay, because a delay that is fine for a controlled stop is fatal for a full trip.
Every parameter in this section lives in the PLC or the drive, not in the NC program. That matters at handover. If the settings are not written down in the maintenance file, the next technician will tune them by feel, and the machine will trip on a different fault every month.
Setting undervoltage, overvoltage, phase loss and overload thresholds
Start with the nameplate. A machine rated for 380–415 V three-phase should trip on undervoltage near 90% of nominal, so about 340–360 V, and on overvoltage near 110%, about 440–460 V. For a 480 V supply the same percentages give roughly 430 V and 530 V. Do not set the undervoltage trip above 95% of nominal. Normal load switching will cross it and you will chase phantom alarms.
Phase loss needs a shorter window than undervoltage. A dropped phase drives the remaining two phases hard and heats a motor in seconds. Set phase imbalance to trip at 3–5% and keep the delay at 0.5–2 s. A voltage sag from a nearby welder is different: it recovers in tens of milliseconds, so a 0.2–0.5 s delay on the undervoltage trip rides it out without letting a real brownout through.
Overload is a thermal setting, not a voltage one. Set the spindle overload at 110–120% of the continuous rating and give it 3–10 s, because a heavy roughing pass is a normal short overload. Set the axis overload tighter, 105–115%, with a 0.5–2 s delay, since a jammed axis climbs faster than a spindle. A setting at 150% with a 30 s delay will not protect the motor. It just decides which part fails first.
The emergency stop loop is separate and must not share a delay. It should open the contactor directly, without PLC logic in the path, and it should drop the spindle brake and the axis enable in the same scan. If your E-stop goes through the same timer as the undervoltage trip, the safety function has been weakened to make the machine feel smoother.
How the trip behaves under spindle and axis load
A trip under load is harder on the machine than a trip at idle. The spindle carries stored energy in the tool and the rotor. A full contactor trip at 8,000 rpm lets the spindle coast, and on a heavy face mill that coast can drag the tool across the part. A controlled stop handles the same event by retracting Z first and braking the spindle with the drive, which takes 1–3 s but leaves the surface intact.
Axis behavior follows the same logic. When the drives lose enable, a vertical axis can fall if the brake does not engage in time. Check the brake timing against the enable drop. A brake that engages in 80 ms on an axis that loses torque in 20 ms will hold. A brake that needs 300 ms will not, and the axis will drop a few hundredths of a millimeter before it stops. That is enough to scrap a finishing pass.
Cutting parameters change how often the trip fires. Deep radial engagement in 7075 aluminium pulls a steady spindle load and rarely trips on current. Interrupted cuts in 304 stainless produce load spikes; if the overload delay is too short, the machine stops on a normal entry. Set the delay from a recorded load trace, not from a guess, and watch the trace over a full roughing pass.
Thermal drift is the slow fault. A spindle that runs at 80% load for four hours will heat the cabinet and the drive, and the trip point can drift with temperature. Log cabinet temperature next to trip events for a month. If trips cluster in the afternoon, the problem is cooling, not the supply.
Step by step: configuring and testing the function
- 11. Record the baseline supplyWith the machine idle, log line voltage, phase imbalance and frequency for 24 hours. Note the lowest and highest values. Every threshold you set later must sit outside this band.
- 22. Enter the voltage windowSet undervoltage near 90% of nominal (about 340–360 V on a 400 V supply) and overvoltage near 110%. Use a 0.2–0.5 s delay on undervoltage and 0.5–1 s on overvoltage.
- 33. Set phase loss and imbalanceTrip at 3–5% imbalance with a 0.5–2 s delay. Confirm the phase sequence relay is wired ahead of the contactor, not behind it.
- 44. Set overload limitsSpindle at 110–120% of continuous rating with a 3–10 s delay. Axis drives at 105–115% with a 0.5–2 s delay. Write both values in the maintenance file.
- 55. Choose the stop responseFull trip for unattended roughing. Controlled stop with Z retract and spindle brake for finishing and for any cut where the tool is in the part.
- 66. Dry-test the tripRun the spindle at 500 rpm with no tool in the cut. Pull one phase or open the test switch and confirm the contactor opens, the brake engages and the alarm appears within the set delay.
- 77. Test under a real loadTake a light roughing pass in aluminium and trigger a test trip. Check for tool marks on the part and for axis drop. Repeat once in the hardest material you run.
- 88. Log and hand overRecord the trip time, fault code, load at trip and cabinet temperature. Store the parameter list with the machine file so the next technician does not retune by feel.
Trip settings by fault type
Typical values for a 400 V three-phase machine. Adjust to your nameplate.
| Fault | Typical threshold | Delay | Best stop response |
|---|---|---|---|
| Undervoltage | 90% of nominal, 340–360 V | 0.2–0.5 s | Ride-through, then controlled stop |
| Overvoltage | 110% of nominal, 440–460 V | 0.5–1 s | Full trip |
| Phase loss | 3–5% imbalance | 0.5–2 s | Full trip |
| Spindle overload | 110–120% of rating | 3–10 s | Controlled stop, retract Z |
| Axis overload | 105–115% of rating | 0.5–2 s | Full trip |
| Emergency stop | Direct contactor open | No delay | Full trip plus brake |
Common questions
Should the automatic power failure function stop the machine or ride through a sag?
It depends on the cut. For roughing with no tool in a finished surface, a full trip is simpler and safer. For finishing, a controlled stop that retracts Z and brakes the spindle protects the part.
A ride-through of 20–100 ms is fine for short sags from nearby equipment. Anything longer than that should trip, because the drives cannot hold the axes without bus voltage.
Why does the machine trip in the afternoon but not in the morning?
Heat is the usual cause. Cabinet temperature rises through the day, drive and contactor behavior drifts, and the same load crosses a threshold that it cleared at 8 a.m.
Log cabinet temperature next to each trip for two weeks. If the pattern holds, fix the cooling and airflow before you widen the trip window.
Can a voltage sag from a nearby welder cause a real trip?
Yes, if the undervoltage delay is too short. A welder start can pull the line down for 50–200 ms.
Set the undervoltage delay to 0.2–0.5 s. That rides out the sag and still trips on a brownout that lasts longer.
What has to be checked before restarting after a trip?
Clear the alarm and confirm the cause. Check the tool for chipping, check the fixture for movement, and check the part for a witness mark where the tool stopped.
Then reference the axes, verify the work offset, and restart from a safe block rather than from the middle of a cut.
Does the E-stop share the same delay as the undervoltage trip?
No. The E-stop loop should open the contactor directly, without a timer in the path, and drop the spindle brake and axis enable in the same scan.
If the E-stop waits on the same delay as the supply trip, the safety function is slower than the label says.
How often should the trip circuit be tested?
Once at commissioning, then at every scheduled maintenance interval. A dry test at low spindle speed takes a few minutes and confirms the contactor, the brake and the alarm path.
Test under a real load at least once a year, in the hardest material the machine runs.
Send us the drawing and the machine parameters
We build machined parts to ±0.005 mm and inspect 100% before shipment. If you need a part that survives a power trip mid-cut, or a fixture that holds it, send the file.
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