If an electric motor is overheating when working under load, it may be due to one or more of the following reasons:

1. Insufficient cooling: The motor may not be receiving adequate cooling due to a blocked or dirty ventilation system, causing it to overheat.

2. Overloading: The motor may be overloaded due to excessive load or a mismatch between the motor and the load, causing it to overheat.

3. Electrical issues: The motor may have electrical issues such as damaged windings, improper voltage or frequency, or a faulty starter, causing it to overheat.

4. Insufficient lubrication: The motor may not be receiving adequate lubrication, causing excessive friction and heat buildup.

5. Mechanical issues: The motor may have mechanical issues such as misalignment or damaged bearings, causing it to overheat.