Does anyone know what means when AC servo motor driver displays error code for "regenerative discharge", and stops working. What this mean in practical situation. This happens from 1 to 10 minuts after turning on. If it helps, servo motor is for one axis of cnc cutting machine. AC servo motor driver is "panasonic msda023a1a", and error code is "Err_18". Thanks.

  • \$\begingroup\$ Can you link to the user manual and give a page reference? Hit the edit link below your question ... \$\endgroup\$ – Transistor Feb 1 at 8:58


simulate this circuit – Schematic created using CircuitLab

Figure 1. A simplified diagram of an AC motor driver. The switches represent the transistor switches in the drive.

With reference to Figure 1:

  • A rectifier rectifies the mains supply giving out \$ \sqrt 2 V_{mains} \$ peak. Larger drives will use a three-phase input.
  • C1 represents the smoothing or DC-link capacitance.
  • R1 is the braking resistor.
  • The output stage switches the motor phases and generates the pseudo three-phase output of the required frequency.

When a motor is decelerated it will act as a generator and this power must be absorbed by the drive. This will result in the DC-link voltage starting to rise. There will be a maximum allowed DC voltage and if exceeded any of the components attached to the DC-link may be damaged. To prevent this a braking resistor, R1, is switched in to dissipate the excess power. That, I suspect, is the "regenerative discharge".

I have seen Yaskawa servo drives using something like a toaster element for R1 and these have burnt out. I have replaced these with a high-wattage cartridge heater of similar resistance and had no more trouble with the drives.

If you feel competent you can have a look for the braking resistor - usually on the rear of the drive - disconnect one lead and measure the resistance. If it is open-circuit then check the good drive to see what the resistance should be.

If the braking resistor is OK then SW1, a transistor, may be dead. You are unlikely to be able to repair that yourself as it will be part of the power block.

enter image description here

Figure 2. A typical IGBT VFD module. Source: Fuji Electric.

The module shown in Figure 2 includes a three-phase rectifier, the P-N output to the smoothing capacitor, P1-N1 input to the IGBT section and the UVW outputs to the motor. The braking resistor will be connected between P1 and B.

  • \$\begingroup\$ Thanks for that, but why do I get that error code when motor is not running, after 2-3 minuts when it stops working? I have UPS, that started about same time to beep for overloading. I didn't change anything in configuration. Can UPS do that or is it beeping because some fault in motor or driver? \$\endgroup\$ – Davor Krpan Feb 1 at 10:30
  • \$\begingroup\$ I don't know. I note that you didn't provide links as requested. There is no mention of UPS in your question and it is not clear what this has to do with the question. \$\endgroup\$ – Transistor Feb 1 at 10:34
  • \$\begingroup\$ Sorry, here it is.... industrial.panasonic.com/content/data/MT/PDF/minas_a_e.pdf user manual, page 67 for error code Err_18. And I have UPS that started to beep for overloading same time this alarm started, but not always at same time. \$\endgroup\$ – Davor Krpan Feb 1 at 10:44

Transistor did post a comprehensive answer. I will just add some suggestions:

  • Remove UPS. In the industry you won't find any machine fed by UPS. If there is no mains power you can't run a CNC machine, that's obvious.

  • Install an external regenerative discharge resistor as described in the manual.


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