Operation
63
4
Figure 4-10:
Current
Foldback
Processing
Example
Each Atlas amplifier has particular default and maximum allowed values for both the continuous current limit and
energy limit. These values are designed to protect the Atlas from excessive heat generation. Refer to
“Atlas Settings Defaults and Limits”
for a complete listing of these defaults and limits.
Setting continuous current limit and energy limit to less than the maximum supported by the Atlas is useful if the
current limit is due to the motor, or to some other factor external to Atlas.
To set the continuous current limit and the energy limit the command
SetCurrentFoldback
is used. The values set
using this command can be read back using
GetCurrentFoldback
. The provided current value is an unsigned
16-bit integer. Refer to
Section 3.11, “Atlas Conversion Factors”
for applicable units and scale factors for the specific
Atlas unit you are using.
The instantaneous state of the current foldback mechanism (whether the foldback limit is active or not) is available in
the Drive Status register and can be read using the command
GetDriveStatus
. If a foldback event has occurred, this
event is recorded in the Event Status register as well as the Drive Fault Status register, and can be read back using
GetEventStatus
and
GetDriveFaultStatus
respectively.
4.8.9.1
Current Foldback in Voltage Mode
Atlas unit’s current foldback mechanism still operates when Atlas is in voltage mode (current loop disabled). When in
this mode, the I
2
t energy calculations and condition testing are identical as when Atlas is operating in current control
mode.
Nevertheless, when in voltage mode, there is an important operational difference. In particular, if the limit is exceeded,
rather than clamping the maximum current output to the programmable maximum continuous current limit setting,
Atlas disables the power stage module, thereby halting further motor output.
4.8.9.2
Example I
2
t Calculations
The following example may help illustrate use of current foldback:
A particular motor has an allowed continuous current rating of 3 amps. In addition, this motor can sustain a temporary
current of 5 amps for 2 seconds.
Atlas® Digital Amplifier Complete Technical Reference
0
30
20
10
10
5
0
10
5
0
Commanded
Current
Amps
Continuous
current limit
exceeded
Integrated
energy limit
exceeded
Amps
Amp
2
-sec
Output
Current
Integrated
Energy
Time
