[Al_The_Man]

Over heating of a servo can be caused by allowing too high a current.
Continuous too high a voltage will cause excessive rpm, in itself will not cause overheating. It just exceeds the safe recommended manuf. rpm.
Most drive manufacturers will suggest voltage supply to be anywhere from 110% to 150% of the max motor rated voltage, provided the drive can handle the voltage.
With modern PWM drives, the max current can be controlled by the drive, and the higher voltage, in and off itself, will not damage a motor, providing the rpm is kept within specs.
Although this high voltage is applied to the motor, it is not continuous, as would be a direct connection to the PS, the same applies to the control of current.
Using a PS/drive with a high current rating, but lower voltage rating than the motor, will allow a high torque to be obtained, but the max motor voltage (max rpm) will not be achieved.
Al.