Allen-Bradley PowerFlex 70 Reference Manual page 98

Drive Overload
Current Limit
Current Limit as selected by the user can be reduced by the thermal manager. The
resulting active current limit may be displayed as a test point parameter.
The active current limit will always be less than or equal to the value selected by
the user, and will not be less than flux current. When drive temperature reaches
the level where current limit would be clamped, the Drv OL Lvl 2 Alarm is
turned on. This alarm will be annunciated even if reduced current limit is not
enabled.
The active current limit is used during normal operation and during DC
injection braking. Any level of current requested for DC injection braking is
limited by the Active Current Limit.
Configuration
The [Drive OL Mode] allows the user to select the action(s) to perform with
increased current or drive temperature. When this parameter is "Disabled, " the
drive will not modify the PWM frequency or current limit. When set to "Reduce
PWM" the drive will only modify the PWM frequency. "Reduce CLim" will only
modify the current limit. Setting this parameter to "Both-PWM 1st" the drive
will modify the PWM frequency and the current limit.
DTO Fault
For all possible settings of [Drive OL Mode], the drive will always monitor the T
j
and T and generate a fault when either temperature becomes critical. If
Drive
T is less than –20° C, a fault is generated. With these provisions, a DTO
Drive
fault is generated if the NTC ever malfunctions.
Temperature Display
The Drive's temperature is measured (NTC in the IGBT module) and displayed
as a percentage of drive thermal capacity in [Drive Temp]. This parameter is
normalized to the thermal capacity of the drive (frame dependent) and displays
thermal usage in % of maximum (100% = drive Trip). A test point, "Heatsink
temperature" is available to read temperature directly in degrees C, but cannot be
related to the trip point since "maximums" are only given in %. The IGBT
temperature shown in Figure 20 is used only for internal development and is not
provided to the user.
Low Speed Operation
When operation is below 4 Hz, the duty cycle is such that a given IGBT will carry
more of the load for a while and more heat will build up in that device. The
thermal manager will increase the calculated IGBT temperature at low output
frequencies and will cause corrective action to take place sooner.
When the drive is in current limit the output frequency is reduced to try to
reduce the load. This works fine for a variable torque load, but for a constant
98 Rockwell Automation Publication PFLEX-RM001H-EN-P - June 2013