Is there a minimum KW rating that these are required
Most motors have a fan attached to the shaft or fins on the ends of the rotor to move air through the motor or over its outer surface to remove the heat that is generated in the motor. With variable speed operation, the air movement may not be sufficient to keep the motor from overheating if the motor operates for too long a time at too low a speed. Since that can happen even when the load torque is less than or equal to rated torque, an overload relay that monitors the motor current may not be sufficient to protect the motor from overheating. Motors that are specifically designed for variable speed use often have larger fan blades, larger wire in the windings, more surface area exposed to air flow, more and higher quality iron in the core and other features to make the motor more efficient and to improve the cooling. Such motors may not need thermostats. The variable speed control unit can also be designed to monitor the speed vs. time as well as the current to provide electronic protection. Even with these motor and control design features, local regulations may require thermostats. The requirement may or may not be related to the kW rating. The regulations may be more strict for motors used where flammable or explosive materials are present.
