Tritex II™ Brake / Shunt Resistor Considerations

Many applications require a brake resistor (or “braking resistor” or “shunt resistor”) to dissipate energy regenerated from decelerating an inertial load, lowering a vertical load, or relaxing a return spring. There is little energy storage in internal capacitors and Tritex II™ AC does not return energy to the AC line. With gear motors and linear actuators, inertial energy is often not a significant factor. Vertical loads and return springs must always be considered a source of regenerated energy. A high bus fault will occur if an application requires a braking resistor but does not have one installed.

Terminals R1 and R2 on the power terminal block are provided for connection of an external brake / shunt resistor only. Each drive must have an independent resistor connected to only one drive. No other connections are allowed at the R1 and R2 terminals. Refer to the warnings in the power terminal wiring and power connector wiring sub-sections in this document pertaining to these connections.

Protection against Overloaded Brake Resistor Required

It is essential that the external braking resistor be protected against overload caused by a failure of the brake control. The protection can simply open the circuit, such as a fuse or over temperature switch, or be wired to a contactor that isolates input power to the actuator on an over temperature condition at the resistor. A thermal switch built into the resistor assembly is best. Fuses are difficult to size properly.

The resistor used must be rated for dynamic braking applications for 240 Vac drives, meaning it is capable of peak working voltage of at least 400V and can absorb high energy within a short period. The minimum resistance is 40 ohms. Recommended range is 40 to 50 ohms. A higher ohms value has no advantage and may not be able to keep up with peak regenerative power, resulting in a high bus fault.

It is best to consider the energy in a regenerative event and the time between these events when sizing a braking resistor for energy and power. An example may provide the clearest presentation. A 100kg vertical load is lowered 0.2m in a move repeated every 10 seconds. Since 100kg exerts 980 Newtons, the move results in 196 Nt-m (or Joules) of energy. Assuming roughly 90% conversion efficiency, the regen event results in 180 Joules delivered to the braking resistor. Since this happens every 10 seconds, the average power to the resistor is only 18 W. Though most resistors are not directly rated for energy, short term overload ratings can be used, though these ratings usually are for isolated events where the resistor can fully cool before another event. Using half the energy rating with repetitive events is generally appropriate. A resistor rated 100 W with an overload rating of 2 times continuous for 5 seconds which is 1000 Joules would be able to handle repetitive 500 Joule events, so would be adequate for the application.

See the Tritex™ manual for more information.



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