overvoltage protection for psu ?

[andy55]

Hi group,

anyone have any circuit ideas for overvoltage protection for a servodrive psu ?

i.e. to dump exess current generated from back-EMF during abrupt braking.

would a comparator driving a relay work ? or is a normal relay too slow ?

what size and wattage resistor would be recommended ?

thanks,

[Al_The_Man]

Most of the circuits of this nature for dissipating the over-voltage produced by braking with VFD/servo's etc use a power hexfet with a voltage comparator input to the Gate, and switch in a high wattage resistor during the on-time.
It is a fairly simple circuit, I designed a similar one for high voltage field regulation on DC generators (240vdc).
The resistor and HexFet size would depend on the energy to dissipate, and the duty cycle.
Al.

[ViperTX]

Are you concerned with the Power Supply Not being able to absorb the back EMF? Do you have any idea what the back EMF is? Usually the back EMF is only hazardous to the FET or Transistor Drivers and most of those have bypass Shottky Diodes to feed the back EMF to the supply.

[NC Cams]

My Fanuc powered lathe has SCR drives of a brushed DC motor.

They put an inductor in SERIES with one of the legs and a 100 ohm high wattage resistor across the output legs - it is NOT switched off line at any time according to the schematics. The resistor would shunt off back fed EMF.

The system does have a relay contact that "breaks" the SCR free of both motor terminals. The relay is hooked to the MOL (motor overload) circuitry which sensed output current via a current sense resistor. Can't tell if they have a comparator to look for over induced over voltage as the MOL voltage being read is being magically processed somewhere that is not readily apparent - perhaps in the relay itself.

Shunting diodes (usually Schottky's) are commonly used to shunt back EMF around fets and/or power transistors.