110v / 220v ??
110v / 220v ??25hp eq. 18kW Start current (Y-connected) 18k/110/3 *5 = 250A It may be difficult to find that high current SSR.. (up to 100A U'll find several 3-phase SSR)..so: mechanical contactors. control coils with small SSRs/relays
You cannot do this with a solid state relay. Assuming a best case of having a 480 volt supply and motor, a 25 hp motor will draw about 30 amps at full load (assuming a std nema 4 pole machine). That same motor will draw about 7 times that amount at start which will absolutely kill a 100 amp solid state contactor before the motor ever reaches speed.
Here is an appropriate contactor and overload (assuming ~30 amp motor):http://www.automationdirect.com/adc/Shopping/Catalog/Motor_Controls/Contactors_-z-_Starters_-z-_Overloads/45mm_Contactors_(9_to_32_Amps)_-z-_Overloads/GH15FT-3-00A
the data sheet on the link says that a GH15FT can control a max 20hp motor at 460v/3ph
.You can then control the motor starter with the Arduino if you need to. What is this motor controlling that you think you need a Arduino to do this?