Motors are to have the same capacity and operate at the same speed. In this example the 4 motors are 2 amps each.
The variable frequency drive parameters must be set to accommodate the total capacity. In the case the variable frequency drive parameter is set to 8 amps (2 amps x 4 motors). Each variable frequency drive parameters that are meant for one motor should be turned off. For example slip compensation (calculating the difference between the field speed and rotor speed) should be set to off in multi-motor controls.
A single motor has its own overload protection. In this example, if the cooling tower fans were operating with a very light load, say 1 amps each, and if one of the 4 motors became failure, the the amp draw on the failed motor would need to surpass, 5 amps (8 amps - (3 x 1 amps)) before the variable frequency drive caused any problem.