you can buy motors that have RTDs embedded in them.
Please note that the embedded RTDs are used to protect the motor against overheating as a more accurate way to protect the motor than current based overload protection alone. It also allows the motor to be operated closer to the high end safety margin than current -based protection alone.
The RTD monitoring may not as responsive to condensate prevention monitoring as one would like as there is a temperature latency between the outside of the winding - where the condensate occurs - and the inner core of the winding.
I recommend flexible space heaters strapped against both ends of the stator winding and then current monitoring on each. The most common problem being with the older type space heaters - little cartridge elements in a perforated housing - that they 'burn out' and fail to provide the heating that is desired. Burn-out used to be a fairly common occurrence with the old rigid space heaters, but the newer types are flexible, made of silicone material, operate at a lower temperature and directly attach to the end-turn heads of the winding. They are similar to heat tracing cables.
Current monitoring - with a small relay - allows you to 'know' not only that it is working but to alert you to when it is open and you need to replace it. The replacement cost will be a shocker though because you need to disassemble the motor and that adds to the cost of the heater easily in a multiplier of double digits. (I usually spec my large motors with dual space heaters, so when one 'burns' out I just have the other as a standby spare.)
