Motor Insulation Class

The chart for the Motor Insulation class is (see http://cr4.globalspec.com/thread/35356 also):

Note: Hot Spot Allowance - The value that takes into account the additional temperature variations within the inaccessible spots of a motor winding. These spots are located in the stator slots or coil heads. 10° C is considered a standard for design purposes.

"A motor with Insulation Class F with Temperature Class Limited to Class B" means the motor insulation is rated at 155°C (with class F) but its maximum temperature rise will be limited up to 130°C (with class B). So, it should be ok. Make sure the motor vendor clearly understands your requirement including the 50°C ambient temperature.

- MS

It is excellent to go for such a motor (with Class F insultaion & temperature rise limited to Class B). Under such ambients too, the life of the motor is extended.