Have you asked the manufacturer or the pump motor manufacturer??
This is not to be taken for an answer to you application but some background for the answer you should get from the manufacture.
If in fact it is a 220v motor od it NEMA rated? If so antivipate that it is able to operate satisfactory a +-10% of that which would be 220 x.90=198v. I did not say that the device was capable to operate at down to 198v but there's a good chance that it would be an accurate answer.
Remember one thing. If a motor is rated at 10HP at 220v it will draw a specific amount of amps to product 10HP. In doing so there will be losses in heat. Now reduce that voltage to 198v. To produce the same 10HP the motor would have to draw more amps to produce that same 10HP. More amps = more heating.
It is common that whem a motor is applied it never is required to produce its nameplate HP anyway.
