Typical Pump/Motor for high head turbine. ...... A capacitor bank.
Using a Pump/Motor as a Turbine/Generator is straight forward. There is nothing that you have to change on the pump or the motor driving it. You do have to get the right size pump for your particular head and flow. Pumps are primarily specified by their outlet, inlet, and impeller diameter. I use a 3X4X10 inch at 1800RPM and use 700 gal per min with a 210 ft head to produce 12 KW. When you use it as a turbine the pump outlet is where the water goes in (and thus becomes the inlet), and the impeller will spin in the opposite direction. The size of the three phase pump motor (KW's) should be at least 2X your anticipated generated output to keep from overheating the unequally loaded windings when used with capacitors (C-2C) to make single phase. See the diagram to connect the capacitors. You'll need roughly 2.5 MFD per KW of the motor rating for the value of C. So if you get a pump with a 20KW motor, C will be 50 MFD and 2C = 100MFD. It is not necessary get much closer than this because the optimum value of C depends on the electrical loading on your generator and that changes all the time.
I used a motorized butterfly valve to control the speed/voltage and power output automatically. You will also need load controllers to keep the loading within reasonable limits and allow the butterfly valve time to react. More on that later.