Exlar Curtiss-Wright Tritex II Series, Installation And Service Manual

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Curtiss-Wright | TRITEX II AC 75 Rev. K PN61928 7/12/2022 43
resistor assembly is best. Fuses are difficult to size properly; the
intension on the fuse is not to protect the Tritex, but to protect the wiring
and resistor from overheating.
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.
Braking Resistor Connections
R1
R2
Bus +
Bus -
Braking resistor
40 ohms minimum
External overload protection to
protect against resistor overheating