An axis overcurrent alarm almost never means "the motor is dead", even though that is the first thing people assume. It means the drive is delivering more current than expected to move that axis, and the causes range from seized mechanics to a power cable with a phase to ground. Swapping the servomotor without diagnosing is the most expensive mistake made on the shop floor with this fault: the motor turns out fine in 60-70% of cases and the problem returns on the first restart.
What the alarm is actually telling you
Every manufacturer labels it differently, but the mechanism is the same: the amplifier measures the current flowing through the motor phases and trips when it exceeds a threshold for a set time. On FANUC you will see SRVO families related to overcurrent or "HCAL" (hardware current alarm). On KUKA it appears as a servopack/KSP fault referencing axis current. On ABB it trips as a drive unit or axis error. What matters is not the code, it is telling whether current rises because the axis is hard to move (mechanical cause) or because the electrical system has failed.
Causes ordered by real frequency
- Abnormal mechanical resistance. Gearbox seized from lack of oil, a brake not fully releasing, a previous collision that left the axis binding, or a mis-declared load/tool. The motor demands more current to overcome the extra torque. This is cause number one and the most ignored.
- Damaged power cable. Broken insulation in a flex zone, intermittent phase to ground, corroded connector. Gives overcurrent that comes and goes depending on arm position.
- Stuck or poorly powered axis brake. If the brake does not release and the drive tries to move, current spikes at the instant of start. Typical after moisture or a long shutdown.
- Degraded drive/amplifier module. Power stage with IGBTs on their last legs or a mis-calibrated current sensor. Gives false readings or genuinely excessive current.
- Servomotor with internal fault. Inter-turn short, seized bearing raising resistive torque, faulty thermal sensor. It is real, but it is the last thing to confirm, not the first to assume.
How to isolate the source step by step
The goal is to separate mechanics, cable, drive and motor before ordering a spare. Do it cold and with the robot in safe mode:
- Read the alarm history. Note at which axis position it trips, whether it is always the same and whether it coincides with start-up, high-speed motion or a load change. A position pattern points to cable; at start-up, to brake; at high speed and torque, to mechanics or motor.
- Move the axis by hand with the brake released (manufacturer procedure). If you feel hard spots, friction or noise, the cause is mechanical. Check the gearbox oil level and condition.
- Verify the brake releases. Listen for the characteristic clack and confirm the axis is free. A brake that does not release properly generates overcurrent and heat.
- Inspect that axis power cable. Flex the dressing section and try to reproduce the alarm. Measure continuity and insulation phase-to-phase and phase-to-ground with the motor disconnected. Check connectors for corrosion.
- Measure resistance between motor phases. With the connector open, the three phases should read practically equal. A clear imbalance points to damaged windings. Also measure insulation to ground.
- Swap the suspect axis with another drive of the same type (if the controller allows it and with a prior backup). If the alarm moves with the drive, the problem is the amplifier; if it stays with the axis, it is motor, cable or mechanics.
This order avoids blind spending. Most overcurrents are resolved in points 1 to 4 without touching motor or drive.
Common shop-floor mistakes
- Replacing the motor without measuring cable insulation: the new motor "works" for a while and trips again due to the cable to ground.
- Resetting the alarm repeatedly and carrying on producing. Each restart with overcurrent stresses the power stage and can take out the drive.
- Ignoring that the tool or part was changed without updating the load and TCP. A mis-declared load spikes current during accelerations.
- Confusing overcurrent with motor overtemperature: different alarms with partly shared causes. If in doubt, also review the thermal management of the robot.
Signs that force you to stop and call a specialist
- Current spikes the instant of start-up even with no motion command: suspect a short in motor or cable, do not reset.
- Burning smell, discoloration on the connector or a cable hot to the touch.
- The alarm persists after checking mechanics, brake and cable: it is time to measure the drive and motor with instruments, service-technician territory.
- Resistance imbalance between motor phases or insulation to ground below normal.
In these cases, forcing a restart only adds damage. A corrective maintenance diagnosis with real current and insulation measurement locates the source without swapping parts blindly, and if a motor or amplifier is needed, the spare parts and exchange route cuts downtime versus waiting for a new part.
How to stop it coming back
Overcurrent almost always has a mechanical or wiring root, so prevention comes down to up-to-date lubrication, dressing inspection and periodic brake verification. A preventive maintenance plan that includes measuring motor currents on a reference cycle detects drift before it reaches the alarm threshold: if an axis starts demanding more current than six months ago for the same move, there is your early warning.