If the weight of the product does not matter, I will assume you mean that viscosity and density can be any reasonable value the problem still occurs.  Ordinarily, a variation in speed might be caused by the load from mixing your fluids.

Speed variation with a variable speed motor could come from several possible sources. First, if the problem is "drag" somewhere, you should notice a build-up of heat somewhere in the mixer drive or bearings. Any drag associated with friction will generate heat, especially if you have, as I would expect, a multi-horsepower motor. Second, if the problem is not "drag," speed variation could have several possible electrical sources. If the problem is in the windings of the motor, I would expect that it will overheat. Finally, if problems cannot be detected in other drive components, you may have a faulty variable speed drive. I suggest that you start by reading the Troubleshooting section of the variable speed drive manual. The manual may suggest possible causes. Many variable speed drives have multiple adjustments that must be set for the maximum speed or variable control of the electric signal going to the motor. The electronic components of the variable speed drive may not be sending a constant frequency or voltage to the motor for several reasons, including component failure.

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