The relationship between the temperature rise, temperature, and ambient temperature of the electric motor can be clarified through the following analysis.
1.Basic Definitions
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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
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Temperature Rise () depends on:
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Load Conditions: Higher load increases current and losses, leading to greater temperature rise.
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Cooling Capacity: Heat dissipation design (e.g., fans, heat sinks) or environmental conditions (e.g., ventilation) affect ΔT.
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Time: During startup or load changes, ΔT varies dynamically until reaching steady state.
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3. Key Influencing Factors
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Impact of Ambient Temperature:
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If Tamb increases, the motor temperature Tmotor rises for the same ΔT.
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High ambient temperatures may require derating the motor to prevent exceeding insulation limits.
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Limits of Temperature Rise:
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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 is the insulation material limit.
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4. Practical Applications
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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 of 155−40=115K (accounting for hotspot allowances).
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Operation Monitoring: Abnormal temperature rise may indicate overloading, poor cooling, or insulation degradation.
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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
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Temperature rise (ΔT) results from the balance between power losses and cooling efficiency, independent of ambient temperature, but the actual motor temperature combines both.
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Ambient temperature sets the baseline for cooling—higher Tamb reduces the allowable ΔT.
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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:
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Ambient = 25°C, ΔT=80K: Tmotor=25+80=105°C (safe).
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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.