An electric motor can overheat and possibly cause a fire due to which condition?

• A. Idle
• B. Full load with full speed
• C. No load
• D. Full load with half speed

Answer: (D)

The important idea is that overheating happens when the heat a motor generates because of electrical losses outpaces the heat it can dissipate. When a motor is delivering full load, it draws more current, which increases copper losses and heat. If the motor is running at a much slower speed, the cooling fan (driven by the motor) also runs slower, reducing air flow and cooling. Put together, you get more heat being produced and less heat being removed, which can push the motor toward overheating and even a fire risk.

In other situations, overheating is less likely: no-load or idle uses far less current and produces less heat, and at full load with full speed cooling is most effective. At full load with half speed, the combination of high electrical load and reduced cooling makes overheating most probable.