I bet someone here knows how to do this:

Picture a water transfer pump, a Tsurumi LB480, submersible, 2/3hp, 230v, 1ph, 60 hz, capcitor-start...I want to slow the GPM it delivers and I am thinking that slowing the frequency will slow the motor and slow the GPM flow rate.

I have checked around with my friend Google, but after writing the manufacturer reps, I am told they don't have a device. I asked at Davis for AC Drive / Frequency Inverter, 200-240 VAC single or three phase input with 3 phase output, 2.4 max. ; not suitable, 3 ph out..
Well, I do agree that slowing down the frequency of the power will slow down the pump. I know that can be a problem when running 60hz equipment (like fans) internationally because they often only have 50Hz. But I don't have the right equipment to offer to do that with the pump you have
Tsurumi says it is possible and OK to use a VFD on the pump, just not above 60hz. The VFD's I reviewed are not suitable for the capacitor-start.

The motor has blk, wht, green wires, to be used in the US, 110/220/1ph,60hz.

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