How to Design with an AC Motor for a Specific Acceleration Objective
Increasing the load from 50% to 75% load is an indication of power = Torque * RPM = I*V/f *k for some constant k.

Thus a 50% increase in load , I would expect VI to increase 50%.  But to accelerate to  50% increase in load in 1 second will demand full power as RPM increases then drop to the steady state V to achieve the 75% VI product at the RPM.  

An estimate of this capacity is the step response to full speed at full load and this must be &lt; 4 seconds to achieve a step load from 50 to 75% in 1 second using full voltage used for 100% RPM but at 50% RPM in order to accelerate at max rate then reduce V to 75% to 85% according to the the motor profile.

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Since VFD's normally have V/F as a constant, it is unknown what the max demand/rating ratio is for this demand response time of 1 s. and how the load is related to RPM , V/F &amp; I if torque is in fact constant. But I suspect the constant torque to to maintain constant water pressure, which has suddenly dropped or demand flow has increased x% .

The other thing is you don't want a sudden pressure change which could cause all sorts of vibrations in the network, unless there is some pressure regulation.  Normally I might expect a tank ballast buffers the load variations. So the flow rate is limited by the pump speed response to  move a certain mass of fluid in 1 second to  a certain elevation change.