The first time a servo drive throws an alarm, it looks serious. Flashing red, a code you have never seen, a machine that just stopped mid-job. But here is what we tell every customer who calls us about it: most drive alarms are the same handful of problems wearing different code numbers.
We have been making and supporting servo drives since 2010 - our T3D series is one of our best-selling products, with the full manual available in English, Russian and Portuguese - and a large share of the support calls we get are solved by the same three checks, in the same order. Walk through them before you touch any settings.
First, the safe stuff
Before anything else: disconnect power and wait at least five minutes before touching any terminals. The DC bus inside a servo drive holds dangerous voltage well after the power is off. This is in the safety section of the manual for a reason - we have seen people skip it and regret it.
Check 1: What was the machine doing when it faulted?
This sounds obvious, but it is the fastest diagnostic filter there is:
- Alarm during rapid acceleration or a heavy cut? That points at overload or overcurrent - the drive is being asked for more current than the motor or the wiring can deliver.
- Alarm right after power-on, before any motion? That is usually wiring, encoder connection, or a parameter problem, not a load problem.
- Alarm after the machine sat unused for a while? Check the absolute encoder battery. Multi-turn absolute-value servo systems keep position using a battery when power is off, and a dead battery shows up as an encoder alarm on startup. Replacement is a straightforward procedure - it is covered step by step in the manual's encoder battery section, and we can walk you through it on a call.
Check 2: Power supply and wiring, in that order
The most common root cause we see in returned units is not the drive at all - it is what was feeding it.
- Voltage first. A drive rated for 220VAC fed from a higher supply will fault and, eventually, fail. Verify input voltage at the terminals with a meter instead of trusting the label on the switch.
- Phases. On 3-phase input models, a missing phase produces symptoms that look exactly like a drive fault but are actually a supply problem.
- Motor wiring U, V, W. These must connect one-to-one between drive and motor. Swapped phases are a classic cause of "motor runs wrong direction" or overspeed faults.
- Encoder cable. On absolute encoder systems, this is the second most common fault source. Damaged or poorly shielded encoder cable causes intermittent position alarms that are maddening to chase - replace the cable before you suspect the drive.
Check 3: Parameters - and do not skip the reset test
If power and wiring check out, the next question is whether the drive's parameters match the motor and the application. A drive loaded with the wrong motor parameters will alarm on overcurrent even with everything wired correctly.
Our standard advice: write down your current parameter values, then restore the drive to factory defaults using the panel operation (the manual covers default value recovery in its parameter management chapter). Run the motor unloaded with default settings using the keypad speed trial run function - on the T3D that is the Fn-20 procedure. If it runs clean unloaded and only faults under load, you have a tuning or sizing problem, not a hardware one. If it faults unloaded with defaults, the problem is physical.
What the alarm code list is telling you
Every drive's manual has an alarm code table - the T3D manual lists all alarms with causes and handling in its diagnostics chapter. The codes cluster into a few families:
- Overcurrent / overload - mechanical binding, wrong motor parameters, or the motor is simply undersized for the job. If the current rating is borderline, check the servo motor ratings before assuming the drive itself is at fault.
- Overvoltage / undervoltage - supply issues, or on overvoltage, a load that regenerates too much energy back into the bus without a proper braking resistor. The T3D has a built-in braking resistor, and the manual covers external resistor connection for heavier cases.
- Encoder / position errors - encoder cable, battery, or a loose connector. Check the mechanical connection before replacing parts.
- Communication faults - on RS485 setups, check bus termination and wiring first.
Those same families turn up on stepper axes too. If the machine runs something like our HBS57 stepper driver instead of a servo, overcurrent and undervoltage are still behind most of the alarms we hear about, and the order of checks below does not change.
Servo drive alarm code quick reference
The code strings differ between brands, so do not hunt for your exact number first - match the family, then work the row. The numbers below are the ones we see most often on our own drives.
| Code (display) | What it means | First thing to do on site |
|---|---|---|
| OC / AL.01 - Overcurrent | Phase-to-phase short, swapped U/V/W, or a jam that stops the motor while current is still rising. The drive trips somewhere around 200-300% of rated current, within microseconds. | Power off, wait five minutes, then disconnect the motor cable and check for a short across U-V, V-W and W-U with a meter. After that, push the axis by hand: if the table is stiff, the fault is mechanical and the drive was only reporting it. |
| OL / AL.02 - Overload | Sustained current above the motor rating - typically 150% for 3-5 seconds, or 110% held for about a minute. This is the drive protecting the motor, not failing. | Watch the load percentage on the panel through one full cut. Drop feed rate or depth of cut one step and run it again. If it trips while showing under roughly 60% of rated torque, the axis is undersized or binding - no parameter will fix that. |
| OU / AL.03 - Overvoltage | DC bus above roughly 400 VDC on a 220VAC drive, or 800 VDC on a 400VAC drive. Nine times out of ten it is energy coming back from a decelerating load. | Double the deceleration ramp time first - it costs nothing and clears most cases. If the axis is vertical or carries high inertia, fit an external braking resistor. The T3D has one built in; heavier loads need the external connection shown in the manual. |
| LU / AL.04 - Undervoltage | Bus below about 200 VDC on 220VAC input, or 380 VDC on 400VAC input. On 3-phase models, a missing or unbalanced phase reads the same way. | Measure at the drive terminals while the fault is showing, not after it clears. The three causes we find most: a loose terminal screw, undersized supply cable, and a contactor that chatters under load. |
| PE / AL.20 - Encoder error | The drive lost feedback: broken or pinched cable, moisture in the connector, or a flat absolute-encoder battery. Our battery packs are 3.6V lithium - below about 2.9V the multi-turn position is already gone. | Swap in a known-good shielded encoder cable before suspecting anything else. On battery systems, replace the battery with control power still on, then re-home the axis and re-check the reference point. |
| EC / AL.21 - Excess position error | Commanded and actual position drift apart by more than the allowable error setting. Usually a coupling that slipped, a loose pulley, or a velocity gain set too low for the load. | Grab the coupling and try to rotate the motor shaft by hand with power off. Any movement means mechanical slip, and that shows up as a position alarm every time. Only look at gain after the coupling is proven tight. |
| OH / AL.30 - Overheat | Heatsink or module temperature over roughly 80-85 °C, or the internal thermal model tripping after a long overload. | Check cabinet ambient first - above 40 °C inside the enclosure is the real problem, not the drive. Clean the fan filter, confirm the fan actually turns, and open up ventilation before touching any parameter. |
| CE / AL.90 - Communication error | RS485 or CAN frames not arriving: wrong baud rate, missing termination, or two stations sharing an address. | Fit 120 Ω termination at the two physical ends of the bus only - not at every node - and confirm each station number is unique. On long cable runs, drop from 115200 to 38400 baud and see whether the alarm interval changes. |
| OS / overspeed - Or: motor runs the wrong way | U/V/W swapped on the motor cable, or the encoder counting direction set opposite to the motor direction. | Swap any two motor phases, never the encoder wires, or flip the direction parameter. On an absolute-value system, fix the wiring rather than forcing the parameter, or the reference point will not hold. |
| No code, axis just stops | Not every stop is an alarm. A drive sitting with no display code is usually waiting for an enable signal or reading a limit switch. | Look at the enable/DI input LED on the drive and the limit switch state in the I/O monitor before power-cycling anything. Power-cycling clears the display, not the cause. |
Two things worth knowing about the table. The thresholds above are the ones we work with on 220VAC single-phase and 380/400VAC three-phase models - your manual lists the exact trip point for your unit, and if you are between two values, trust your meter over the table. And do not chase two alarms at the same time. Fix the first one, clear it, and see what is left. On overload trips especially, the second alarm is often just a consequence of the first.
Nine out of ten alarms we get asked about are power, wiring or parameter problems - not a dead drive. Work the three checks in order before you box anything up for return.
If the code still will not clear, send us the code number, the motor model and a photo of the terminal wiring through the inquiry page. We keep the T3D manuals on hand and will tell you plainly whether it is a setting or a hardware failure.
When to send it back
If you have verified supply voltage, wiring, encoder cable, and defaults - and the drive still faults - then it is genuinely worth a repair or replacement conversation. That is what we are here for: tell us the model (it is printed on the label), the alarm code, and what you have already checked, and we will tell you straight whether it sounds like a drive problem or something upstream of it.
For the full alarm list, parameter tables and wiring diagrams, the complete T3D full manual is free to download - all our drive and controller manuals are collected on the downloads page. And if you are still stuck after going through these checks, put the code and the model in an inquiry with your alarm code, or email admin@hengli-hltnc.com, and we usually come back with a likely cause the same day.
