Moog Animatics offers several shunt options for use with DC input servo motors in 100, 200 and 300 Watt size ranges. Shunts are needed to protect the servo controller and drive stages from overvoltage, which can originate from the following sources:
Back EMF due to back driving the motors
Sudden or hard decelerations
Hard stop crashes (immediate deceleration to zero speed)
Vertical load drops 
When voltage exceeds the trigger level, the shunts automatically add an additional load to the DC bus by connecting large load resistors across the bus. Trigger voltage is typically 49.5 VDC.
 
NOTE: Shunts start loading the bus at 49.5 VDC and rising, and will be fully loaded at 53 VDC.
 
WARNING: If the shunt is connected to an adjustable power supply, the output voltage must be set at or below 48 VDC. If the output voltage is sustained above the trip point of the shunt, overheating and damage may result.