Because of modernized design and manufacturing techniques, it is typical for many of today's motors to run hotter than those in the past.
In fact, it is not uncommon for the surface temperatures of some newer motors to reach as high as 90C while maintaining and even exceeding the lifetime of motors made in the past.
This means that a hot motor is not necessarily an indication that something is wrong. However, if the motor in your pump application seems running uncharacteristically hot, there are several situations that may causing this condition:
Electrical:
Under or over voltage     Unbalanced 3 phase power     Loss of insulation resistance
Environmental:
High ambient temperatures     Lack of ventilation     High altitudes (thin air)
Mechanical:
Excessive cycling     To low or to high flow rate     High rotating resistance by a damaged pump     Obstruction in the pump     High viscosity or specific gravity liquid