DC Motor performance

[Pulsar]

As DC motors are used, they increase in temperature and decrease in performance. Why does this phenomenon occur? I've been unable to locate a satisfactory explanation so far. That leads to my second question: does cooling a DC motor below room temperature increase it's performance?

 

[jamesmcuk]

They increase in temperature due to the current being resisted by the windings on the motor. Any heat generated means that the motor is not 100% efficient ( thats the holy grail so to speak). The increase in temperature further increases the resistance of the windings creating more heat and therefore making it even less efficient. When additional loads are applied to the spindle the motor slows and the Back EMF produced by the motor is reduced meaning more current is pulled by the motor increasing the power that is lost as heat.

There is also the Curie temperature of the permanent magnets which tends to be a very high temperature but is basically the temperature at which a material becomes none magnetic. I imagine this is a curve of loss as the temp increases but as most Curie temperatures are above 600 C I cant see this has much impact on most motors.

As for cooling below room temp then yes this would indeed help but only so long as mechanical resistance of the motor components wasn't increased by the shrinking of bearings and the thickening of lubricants.

 

[pm]

[edit] spent too long writing it and reading up on it and James answered first (and better) [/edit]

The increase in temperature is caused by electric losses. There are iron losses, magnetic losses, mechanical losses and copper losses. Iron and magentic losses are caused by the magnetic field changing in the stator. Copper losses are mostly caused by resistance in the windings. And mechanical losses are caused by friction. All of these losses produce heat. In particular, copper losses increase with higher temperature.

Cooling a motor does improve it's efficiency and has a minor effect on performance.