I don't know much about the frequency inverters but I need to use them. We have a woodworking factory and machines which we brought after using in a different country which has 50Hz frequency. Here, power is 60 Hz and the machines have 20% higher speed which is not suitable and hence, we have to reduce the speed by converting the frequency to 50Hz. We have around 25 machines and we are buying new ones. most of our machines range from 5 -7Hp.

I was told about thermorelay but don't know what it is. What we want is ready to connect frequency inverter and start working. I don't want to start looking for other parts or programming or this or that. We have so much headache even without any new. DO we need to add to it?!!

I hope I cleared what do we need.

Just note, the important factor to select the frequency inverter is the motor's working current, the total amps of two motors should be less than the inverter's amps/1.1(safe factor).
For example, if a frequency inverter rated output current is 17amps, so the two motors total amps should be less than 17/1.1=15.5amps.
The thermorelay I think it won't be necessary, as we also have woodworking machines in China which do NOT have the thermorelay too.