The case-to-ambient thermal resistance R th2 can be decreased significantly by the addition of heat sinks. The rotor-to-case thermal resistance R th1 is primarily fixed by the motor design. Note that the maximum allowable current through the motor windings could be increased by decreasing the thermal resistance of the motor.
This can happen even if initial calculations demonstrated an acceptable temperature rise (using values of R and k M at ambient temperature). Under continuous operation, a motor may even reach a point of “thermal runaway” which could potentially render the motor damaged beyond repair. As the motor temperature continues to rise each of the three parameters will change in a fashion which degrades motor performance and increases the power losses. We could continue calculating additional parameters as a result of the hotter coil and magnet, but the best results are yielded by performing multiple iterations which is best done by quantitative software. Now we can calculate the increase in coil resistance due to thermal power dissipation:Īs we can see, the torque constant weakens as a result of temperature increase as does the Back-EMF constant! So the motor’s coil resistance, the torque constant, and Back-EMF constant are all negatively impacted for the very simple reason that they are functions of temperature. So, since the ambient temperature is 22☌, the maximum tolerable rotor temperature increase is: 125☌ – 22☌ = 103☌ Again, the datasheet for the 2668W024CR coreless DC motor specifies a maximum winding temperature of 125☌. The designer wants to know how much torque the motor can safely provide without overheating. Suppose it is desired to run the motor at the maximum possible torque with an ambient air temperature of 22☌. For example, an application may require that a motor run at its maximum torque with the hope it will not be damaged by overheating. One could use similar calculations to answer a different kind of question. Since the calculated winding temperature is only 90,4☌, thermal damage to the motor windings should not be an issue in this application. In the example given above, the maximum permissible winding temperature is 125☌. It is important to be certain that the final temperature of the windings does not exceed the motor’s rated value found on the data sheet. The steady state temperature increase of the motor ( T) is given by:
Some motor manufacturers specify a thermal resistance for each of the two thermal paths while others specify only the sum of the two as the total thermal resistance of the motor. In the case of DC motors, there is a thermal path from the motor windings to the motor case and a second thermal pass between the motor case and the motor environment (ambient air, etc.). A large cross section aluminum plate would have a very low thermal resistance, for example, while the values for air or a vacuum would be considerably higher. Motor manufacturers typically provide an indication of the motor’s ability to dissipate heat by providing thermal resistance R thvalues. Thermal resistance (which is the reciprocal of thermal conductance) represents how well a material resists heat transfer through a defined path. The ease with which this heat can be dissipated in a motor (or any system) is defined by thermal resistance.
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