Question about AC servo motor driver
simulate this circuit &ndash; 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.