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What can cause the motor to rock back and forth on a belt driven, constant torque load?

Issue:
What can cause the motor to rock back and forth on a belt drive, constant torque load?

Product Line:
All Altivar drives and Altistart soft starters

Environment:
All models, All serial numbers.

Cause:
Constant torque, soft coupled loads (such as a belt driven system) are not running at a stable speed or the motor is rocking back and forth at low speeds.

Resolution:
When the drive, or soft starter, is in a torque control mode (SVC mode for drives, CLP = ON for ATS48, SSC = ON for ATS22) the drive is monitoring the torque on the load in order to maintain speed, or accelerate to speed.   If the load increases, more power will be needed to maintain or increase speed.  The drive senses this and increases output to the motor in order to maintain speed with a dynamic load.

When the motor is soft coupled to the load, such as with a belt driven system, then there is a slight delay between when the motor increases speed and when the load increases speed, due to the elasticity of the belt.  This sets up an oscillation in the belt which also affects the motor, and is then detected by the drive, which tries to adjust for the load.  The end result is that the drive is constantly trying to adjust its output to match the torque of the load, which is constantly changing as the belt compresses and expands, causing unstable speed control of the motor.

In order to correct this, verify that your belt is tight and does not have excess slack.  Increased slack will increase the effect.

If the belt is tightened properly and this problem still occurs, then it may be necessary to place the drive or starter in a mode that does not rely on motor torque to control the power output.

For the ATS48 Soft Starters, set parameter CLP to OFF.
For the ATS22 Soft Starters, set parameter SSC to OFF.
For Altivar drives, the parameter "Motor Control Type", or Ctt parameter (Pt parameter on ATV21, ATV212) should be set to a mode other than SVCV, SVCI, or FVC. The exact  options available will vary based on the model of the drive.  Please refer to the programming manual for your specific model drive for further clarification, or call the Drives Product Support Group at 888-778-2733.  Select option 1 for Technical Support and then Option 4 for Drives and Soft Starters.     The Drives Product Support Group is staffed from 8:00 am until 8:00 pm EST/EDT Monday through Friday.
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