Autotransformers typically have 3 sets of taps: 50%, 65% and 80%. Motor current is dependent upon the square of the applied voltage, so at 50% taps, the motor current is 25% (.5 x .5) of normal. So if the motor DOL Locked Rotor Current (LRC) is 600% of the Full Load Current (FLC), 1t will be 150% FLC at the 50% taps (600% x 25%). 65% taps come out to 42%, so roughly 250% of FLC, and 80% taps provide 64% current, so 384% FLC. But in the case of an Autotransformer, the LINE current is also reduced by the transformer action, so at 50% taps, the LINE current is 50% of the MOTOR current, so it will be 75% FLC, at 65% it will be 164% FLC and at 80% it will be 307% FLC. This is why Autotransformer starting is sometimes best for reducing the load on power systems (aside from VFDs, which are ultimately the best).
But... that is if the VFD works at all!
Which set of taps is virtually impossible to determine ahead of time; that has to do with the load on the pump. Generally though, most centrifugals will need 65% taps. That is why solid state soft starters are sometimes better for this, they are infinitely adjustable to match the conditions you find in the field. But with a solid state soft starter, your line current and motor current are the same. So the Auto Transformer will reduce the starting load more than solid state or VFD.
