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Load Sharing using the WEG VFD

Load Sharing using hard coupled VFD s requires special tuning and setup. Here’s a brief overview.

All most all newer VFDs come with an auto-tune function these days. Auto tune automatically adjusts the tuning parameters of the VFD to the motor. All that is required from a technician or engineer is to input the motor data from the motor name plate and run auto-tune to achieve optimum performance in a single VFD application. However it can get complicated when you have to set up two or more VFDs to perform rigid (motors coupled to the same shaft) load sharing applications.

The WEG CFW11 and CFW09 VFDs have six modes of control. Saturated mode/Torque control is available only in sensorless vector mode or vector with encoder feedback/full flux vector.

Basic understanding of the physics or mechanics of the application is very helpful in setting up load sharing. Typically, on a WEG VFD, we set the master drive in speed mode and the followers in saturated mode (Torque mode). The CFW11 and CFW09 follower VFD has to have its speed loop kept saturated. Both speed reference and torque reference to the follower VFD will be required.

One of the analogue outputs of the master is programmed for speed reference output and the second output is programmed for torque reference output. Since the follower VFD is set in saturated mode an increase in the master torque will result in increase in torque on the follower. You can also scale the torque reference input on the follower accordingly to allow it to pull more torque than the master or vice-visa.

The tuning parameters are very important to achieve critical dampened responses in all the VFD in the system.

Speed regulator gains: These are calculated by auto-tune but may need to be readjusted to meet the speed dynamic response for the system. The proportional gain stabilizes sudden changes in speed reference while the integral gain corrects the error between speed reference and actual speed. Lowering the gains too much will usually result in the VFD not attaining set speed and gains set too high will result in oscillation. The mechanical time constant which is available in both CFW11 and CFW09 can provide further speed stability adjustments in the WEG CFW09 and CFW11.

The auto tune also takes care of the flux regulator gains. However, in a load sharing application, this setting may not be able to achieve optimum performance. So adjusting the gains may also be required just like in the speed loop gains.

The key thing to keep in mind when tuning any system is to start with lower gains and gradually increase the gains until critically dampened response is achieved. EMA provides start ups on coordinated VFD systems. Contact your nearest EMA facility as listed below, or click the contact icon at the top right of this page.


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