USA57 Revision
1.2
87
Appendix C
Heat Dissipation
Most stepper applications require intermittent moving of the motor. In the Accuriss, the current is
increased to the “move” current, the move is performed, and the current is then reduced to the
“hold” current automatically. The heat dissipation in the drive is proportional to the current
flowing in the drive and, therefore, the dissipation occurs primarily during the “move”.
When the Drive generates heat, the heat first warms the circuit board and housing.
Only then does the heat transfer to the surroundings. For intermittent moves that are less than
one minute in duration, the Drive primarily cools using this thermal inertia of the board and
housing, and not by steady state dissipation to the surrounding ambient.
The electronics for the Accuriss are fully capable of running at the rated voltage and current.
However, due to the small size of the boards, which limits the steady state heat transfer to the
ambient, care must be taken when the drive is used in high duty cycle and/or high current
applications. For conservative operation, it is recommended that the duty cycle be reduced
linearly, from 100% duty at 50% of rated current, to 25% duty at 100% of rated current. (Duty
cycle means the percentage of the time that the drive is moving the load – averaged over 5
minutes). Conservatively, the maximum continuous run at 100% current is about 1 minute. An
on board thermal cutout typically trips after about 2 minutes at 100% current. (This cutout is self-
resetting when the drive cools). Of course, at 50% of current, the drive will run continuously with
no time limit.
Most “intermittent move” applications will NOT require derating of the drive.
In case the Accuriss is required to run 100% of the time at 100% current, forced air cooling or
mounting to an aluminum structure is required.
Accuriss drives are designed with parts rated at 85°C or better. This means the PCB copper
temperature must remain below 85°C. The ambient air temperature allowed depends on the
airflow conditions.
MTBF is 20000Hrs at 85°C PCB copper temperature and doubles for every 10°C lower
than 85°C.