What the A.40 alarm means

The DC bus voltage in the drive exceeded the upper limit; typically caused by high incoming supply or un-dissipated regenerative energy. Our engineers run into this alarm every week on ESTUN ERC / ED3L controller + ESTUN servo controllers, and a single on-site inspection usually identifies the problem.

Most common causes

  • Incoming AC supply above the rated range
  • Braking resistor open or disconnected
  • Voltage spikes from heavy equipment on the same feeder
  • Drive voltage detection circuit fault
Field note: In our repair stats the first cause has the highest hit rate, so confirm it first before digging further.

Step-by-step field fix

  1. Measure incoming three-phase voltage at the cabinet
  2. Verify the braking resistor and its wiring first — this is the usual culprit
  3. Check for voltage spikes when nearby welders or presses switch
  4. Reduce deceleration and retry
  5. Replace the drive only after supply and regeneration are proven healthy
Safety first: For ESTUN a-40, this alarm family involves the DC bus, which sits at several hundred volts and stays charged for minutes after power-off - touching terminals is a shock hazard. On site you may not carry out the first step yourself: Measure incoming three-phase voltage at the cabinet. Beyond that it is the DC bus and power stage work - leave it to a professional. On ESTUN a-40: checking incoming voltage at the disconnect and looking at breakers and fuses is fine to do yourself; opening the cabinet to touch bus terminals or pulling power modules needs a qualified electrician.

How we diagnose drive alarms on site

ESTUN a-40 belongs to the drive & power stage family. On ESTUN a-40 the failing stage is matched servo drives (in-house / Inovance-class servos) on the ESTUN robot controllers (motion-control-platform based) - the fault sits on the energy chain: mains input, rectification, DC bus, braking resistor, inverter power devices and the drive board. We start reading ESTUN a-40 at the pendant alarm list (A.xxx / E.xxx segments) and the servo panel display code.

The chain involved in ESTUN a-40 runs: three-phase mains -> rectifier -> DC bus (bus capacitors) -> brake chopper / braking resistor -> inverter (IGBT) -> motor.

When ESTUN a-40 fires, the site usually reports Incoming AC supply above the rated range first, but the real cause is often cooling: More than half of power-stage alarms are not a dead device but a device cooked by failed cooling. A stopped cabinet fan, a packed filter mat or a dusted air duct keeps IGBT junction temperature high: intermittent over-temperature first, hard fault days later. On these alarms we open the power module and look for cracked thermal paste and felted dust in the duct, then measure phase-to-phase output and insulation to frame. Swapping the module without cleaning the duct just buys the same failure again.

Field notes on this page cover ESTUN a-40 (Main Circuit Overvoltage); written by the Henghuan repair team, updated 2026-10-01.

If you can't fix it, reach our engineers directly

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Related Drive & Power Stage alarms

A.02 — Parameter Setting Error A.10 — Overcurrent A.30 — Regeneration Error / Regenerative Overload A.41 — Main Circuit Undervoltage A.71 — Overload — Instantaneous High Load A.72 — Overload — Continuous Over-torque
Same family: this code belongs to the Drive & power stage alarms family - the page collects all Drive & power stage-family codes we have handled, with the common inspection order and spares.
Other brands: we also keep field-fix libraries for INOVANCE · EFORT · JAKA and more — see the complete alarm code hub.

Frequently asked questions

QWhat does the ESTUN a-40 alarm mean?

The DC bus voltage in the drive exceeded the upper limit; typically caused by high incoming supply or un-dissipated regenerative energy. Our engineers run into this alarm every week on ESTUN ERC / ED3L controller + ESTUN servo controllers, and a single on-site inspection usually identifies the problem. ESTUN a-40 sits in the drive & power stage family; on our work orders the highest-frequency trigger for this code is "Incoming AC supply above the rated range".

QCan I clear a ESTUN a-40 alarm myself?

Whether ESTUN a-40 clears on its own depends on the outcome of "Measure incoming three-phase voltage at the cabinet". Finish the flow with "Replace the drive only after supply and regeneration are proven healthy" to confirm ESTUN a-40 - do not skip it. If that step is clean, the second most frequent cause on ESTUN a-40 is "Braking resistor open or disconnected". Do not chase takt through a repeating ESTUN a-40. Left alone, drive & power stage faults tend to graduate from repairable to must-replace.

QFor drive/power alarms like ESTUN a-40, what spares do you bring on site?

The commonest causes we see on ESTUN a-40 are "Incoming AC supply above the rated range, Braking resistor open or disconnected, Voltage spikes from heavy equipment on the same feeder", so we stock the van against those. For ESTUN a-40 specifically, we stock the van along the energy chain: braking resistors, bus capacitors, input fuses, drive-board supply parts and the common power modules for the brand. For ESTUN a-40, drive alarms usually stop the whole line, so they sit at the front of our dispatch queue. Before we roll, tell us what ESTUN a-40 shows on the pendant alarm list (A.xxx / E.xxx segments) and the servo panel display code and what the machine was doing - phone or video triage is free.

The notes on ESTUN a-40 were summarized by the Henghuan Robotics repair team from on-site service. The alarm interpretation is our empirical summary for troubleshooting reference and may vary by controller and software version — please check ESTUN documentation or contact us with questions. Henghuan is an independent service provider, not affiliated with or endorsed by ESTUN.