Rockwell Automation 20A PowerFlex 70EC/700VC - Manual (Page 46)

42 Droop
Low Speed Operation
When operation is below 4 Hz, the IGBT duty cycle is such that heat will build up
rapidly in the 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 torque load
reducing the output frequency does not lower the current (load). Lowering current
limit on a CT load will push the drive down to a region where the thermal issue
becomes worse. In this situation the thermal manager will increase the calculated
losses in the power module to track the worst case IGBT. For example, if the
thermal manager normally provides 150% for 3 seconds at high speeds, it may only
provide 150% for one second before generating a fault at low speeds.
Some applications may benefit from the disabling of current limit fold back.
Droop Droop is used to “shed” load and is usually used when a soft coupling of two
motors is present in an application. The master drive speed regulates and the
follower uses droop so it does not oppose the master. The input to the droop block
is the commanded motor torque. The output of the droop block reduces the speed
reference. [Droop RPM @ FLA] sets the amount of speed, in RPM, that the speed
reference is reduced when at full load torque. For example, when [Droop RPM @
FLA] is set to 50 RPM and the drive is running at 100% rated motor torque, the
droop block would subtract 50 RPM from the speed reference.
70EC 700VC 700H
✔ ✔