For motors having class – B insulation the temperature should not be more than 110°C and for motors having class – F insulation the temperature should not be more than 130°C. In case temperature is more, then the following could be the possible reasons. 

1. Electrical overloads. 

a. Over and under voltage. 

b. Over and under frequency. 

c. Voltage unbalance. Voltage unbalance create unbalance of currents and increase in temperature which will be 2*(% voltage unbalance)* (% voltage unbalance)*.

(% Voltage unbalance) = 100 * maximum deviation from average voltage / average voltage.

For instance if voltages are 390V, 410V & 440V,

% Voltage variation = 100*(440-390+410+440) / 3. / (440-390+410+440) = 6.45%.  / 3

Therefore increase in temperature rise = 2*(6.45)*(6.45) = 83°C (approximately). 

d. Voltage transients and interruptions. 

e. Loose connection at motor terminals. 

f. Unbalance current. 

g. Single phasing (if OLR protection is not working). 

h. Long acceleration cycle. 

i. Unusual system grounding conditions.

2. Mechanical overloads. 

a. Locked rotor. 

b. Heavy starting. 

c. Bearing problem. 

d. Overload in continuous duty and intermittent duty. 

3. Environmental overloads. 

a. Excessive temperature of cooling medium or ambient temperature. 

b. Restricted flow of cooling. 

c. Reduction in the density of cooling medium. 

d. Heat transfer from machine parts connected to the motor. 

4. Others. 

a. Excessive number of switching operations.