As AC motors
speed depends on the frequency of the voltage that it is powered by, using a motor that was built for 60 Hz in a 50 Hz mains would decrease the speed by about 20% (a drop of one sixth, the difference between 50 and 60 Hz), thus its cooling fan would do its job less efficiently. Also, the motor would be less efficient: a fan powered by it would move less air, a pump would pump less fluids, etc. The motors power rating (HP or kW) would also drop by one sixth and the motor would heat up more. A 60 Hz motor that can be run at 50 Hz would have to have an improved cooling and be more heat-resistant (improved internal insulation). Of course, motors built especially for 50 Hz would not have these drawbacks.