Speed Droop - Siemens 6SR41 series Product User Manual

Application Specific Features

7.4 Speed Droop

7.4 Speed Droop
Speed Droop is the decrease in the speed of a motor with a constant voltage and frequency
when the motor is under load. The difference between the synchronous (unloaded) speed of
the motor and the full load speed is known as slip. Normally, slip compensation increases
the output frequency of the VFD as the motor speed attempts to decrease. This
compensation maintains a constant motor speed by minimizing droop.However, in some
applications, droop is needed. For example, in a multiple motor application, such as two
motors mechanically connected to a common load, there are inherent differences between
the motors. In the case of a torque current increase, these differences may allow one motor
to attempt to run faster, causing that motor to bear a greater portion of the load.By adding
droop to the more heavily loaded motor, its speed reference will proportionally decrease
(based on load), shifting some of the load to the less loaded motor. The less loaded motor
speed reference is not affected as much (because the current is lower) and will therefore
start to pick up more of the load. As the loaded motor's speed reference is decreased, it
begins to shed load until an equilibrium is reached, and each motor is bearing its share of
the load.
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Operating Instructions, Version AE 12/2009, A5E01454341C
Product User Manual