Hi…as per my knowledge Voltage and RPM should be linked with each other. My Lamptron FC5v2 has readings for RPM and voltage or temperature. If I set it at a lower RPM, the voltage will match the stated RPM. However, this isn’t important as you will only need to worry about the startup voltage required. I don’t undervolt my pump but my fans do exhibit such behavior where it will not spin till a minimum voltage is applied and I think it is applicable to pump as well.