Factors Affecting V/F Ratio

The V/Hz ratio is set, in general, by the motor design. If your motor design is 400V at 50Hz, then the V/Hz ratio is 8:1.

Many VFDs come with an "energy saver" feature that does alter the ratio based on speed when used with variable torque (quadratic) loads. The premise is, when running at reduced speed the nature of the load means the torque requirement is less and therefore the motor voltage can be reduced (by virtue of a lower V/Hz ratio) to reduce the magnetization losses in the motor as well. This reduces the torque, but in the case of a VT load, the torque reduction is inconsequential. So the goal is not to reduce the torque, the goal is to reduce some of the losses in the motor, the torque reduction is a consequence that can be acceptable in that specific application.

Most drives that I am aware of simply use a (V/Hz)² formula as the change, for whatever that's worth to you. Most users now elect not to use this feature. It does not save that much more energy than not using it, and you must be very sure of the torque requirement reductions in the load for it to make a difference without sacrificing performance.