# Question about AC servo motor driver

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.

• Can you link to the user manual and give a page reference? Hit the edit link below your question ... – 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.

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.

• 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? – Davor Krpan Feb 1 at 10:30
• 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. – Transistor Feb 1 at 10:34
• 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. – 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.