2-90 Drive Ratings (kW, Amps, Volts)
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
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.
If operating at 60Hz 120%, lowering the current limit may cause a fault
sooner than allowing the drive to continue to operate. In this case the user
may want to disable current limit fold back.
Drive Ratings (kW,
Amps, Volts)
Refer to Fuses and Circuit Breakers on page 2-100.
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 “fight” 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.
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