Analysis of the Relationship between Electric Motor Temperature Rise and Environmental Temperature

The relationship between the temperature rise, temperature, and ambient temperature of the electric motor can be clarified through the following analysis.

1.Basic Definitions

• Ambient Temperature (Tamb​)
  The temperature of the surrounding medium (typically air) where the motor operates, measured in °C or K.

• Motor Temperature (Tmotor)
  The actual temperature of the motor's internal components (e.g., windings, core) during operation, measured in °C or K.

• Temperature Rise (ΔT)
  The difference between the motor temperature and ambient temperature:ΔT=Tmotor−Tamb,Measured in K or °C (since temperature rise is a differential value, the units are interchangeable).

2. Mathematical Relationship

                                                        Tmotor=Tamb+ΔT

• Temperature Rise (ΔT) depends on:

  • Load Conditions: Higher load increases current and losses, leading to greater temperature rise.

  • Cooling Capacity: Heat dissipation design (e.g., fans, heat sinks) or environmental conditions (e.g., ventilation) affect ΔT.

  • Time: During startup or load changes, ΔT varies dynamically until reaching steady state.

3. Key Influencing Factors

• Impact of Ambient Temperature:

  • If TambTamb​ increases, the motor temperature Tmotor rises for the same ΔT.

  • High ambient temperatures may require derating the motor to prevent exceeding insulation limits.

• Limits of Temperature Rise:

  • The motor's insulation class (e.g., Class B, F) defines the maximum allowable temperature (e.g., Class F = 155°C). Thus, the permissible ΔT must satisfy:ΔT≤Tmax−Tamb,where Tmax​ is the insulation material limit.

4. Practical Applications

• Design Phase: The maximum ΔT is determined based on insulation class. For example, a Class F motor (Tmax=155°C) in a 40°C environment has an allowable ΔT of 155−40=115K (accounting for hotspot allowances).

• Operation Monitoring: Abnormal temperature rise may indicate overloading, poor cooling, or insulation degradation.

• Cooling Conditions: Changes in ambient temperature or cooling efficiency dynamically affect ΔT. For instance, fan failure causes a sharp rise in ΔT.

5. Summary of Relationships

• Temperature rise (ΔT) results from the balance between power losses and cooling efficiency, independent of ambient temperature, but the actual motor temperature combines both.

• Ambient temperature sets the baseline for cooling—higher TambTamb​ reduces the allowable ΔT.

• Motor temperature is the ultimate outcome and must comply with insulation limits.

Example

Consider a Class B insulation motor (Tmax=130°C) operating under two scenarios:

• Ambient = 25°C, ΔT=80K: Tmotor=25+80=105°C (safe).

• Ambient = 50°C, same ΔT=80K:Tmotor=50+80=130°C (at limit, requiring load reduction).

This relationship is fundamental to motor thermal protection design and lifespan evaluation.