Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 158)

158 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 3 Diagnostics and Protection
DC Bus Voltage/Memory P11 [DC Bus Volts] is a measurement of the instantaneous value. P12 [DC Bus
Memory] is a heavily filtered value or average bus voltage. Just after the pre-
charge relay is closed during initial powerup, bus memory is set equal to bus
voltage. Thereafter it is updated to the six-minute average of the instantaneous
DC bus voltage.
Bus memory is used as a comparison value to sense a power loss condition. If the
drive enters a power loss state, the bus memory is also used for recovery (for
example pre-charge control or inertia ride through) upon return of the power
source. Update of the bus memory is blocked during deceleration to prevent a
false high value caused by a regenerative condition.
Drive Overload The purpose of the drive thermal overload feature is to protect the drive’s power
module when operation exceeds the design limitations. This feature does not
protect the motor, this is handled by the motor overload protection feature (see
Motor Overload on page 168 ).
The drive thermal overload function utilizes two methods to protect the drive.
Inverse time protection based on the average output current and a thermal
manager that models the temperature of the IGBTs based on measured power
module temperature and operating conditions. Each method can reduce the
PWM switching frequency or reduce current limit. When rated conditions are
exceeded, even after applying one of the measures mentioned above, and the load
on the drive is not reduced, a F64 “Drive Overload” fault is generated. The fault
detection mechanism cannot be disabled. Only the ability to fold back PWM
frequency and current limit can be disabled.
The drive monitors the temperature of the power module based on a measured
temperature and a thermal model of the power module. As the temperature rises
and P940 [Drive OL Count] increases, the drive can lower the PWM frequency
to decrease the switching losses in the power module. If the temperature
continues to rise, the drive can reduce current limit to try to decrease the load.
This is the factory default response, configurable by P420 [Drive OL Mode], to
increasing drive temperature. If the drive temperature becomes critical, P940
[Drive OL Count] = 100%, the drive faults.
If the drive is operated in a low ambient temperature condition the drive can
exceed rated levels of current before the monitored temperature becomes critical.
To guard against this situation the drive thermal overload also includes an inverse
time algorithm. When this scheme detects operation beyond rated limits, current
limit can be reduced or a fault can be generated.