Summary: It's normal for motors that are powered up and not moving to get hotter than they would if they are turning. It is very possible to damage a motor by overheating it, and motors will loose torque when hot. PUT A HEATSINK ON THE MOTOR. Linear drives heat the motor less than chopper drives, and good drivers heat less than cheap drivers. It's good to have a low power hold mode in the driver so that the CNC software can be setup to reduce the power to the motors when they are not actually running during a job.
Copper losses: Motor heat is caused by a number of factors, the primary one being the flow of current through the coil. These are called "copper losses" as coil wires are generally made of copper. Low power hold reduces that current flow and greatly reduces motor heat. Counter-intuitively, when the motor is running, slightly less current flows because part of the time the coil is changing direction and the current doesn't have time to reach full flow in any one direction. The faster it turns, the less power it must dissipate (and the less torque it produces).
Iron Losses: Another cause of heat in the motor is eddy current loss caused by the AC component of the chopper signal from the driver. This heat builds up in the iron core of the motor and so these are called "iron losses". A chopper type driver turns the power on and off rapidly to control the current sent through the motor. That alternating current induces a current into the metal of the motor, and that current also contributes heat to the motor, in the body, rather than in the coils. This effect does not contribute /as much/ heat as the main current flow, but it /does/ contribute heat. If you use a linear, rather than a chopper, driver, your motors will not heat /as much/.
Reversible torque loss: Although motor heating is normal, it is NOT desirable. Heat is the enemy of the magnet. The magnets that give your stepper motors their torque, are reduced in strength as they get hot, even before reaching the temperature at which they are perminantly damaged (max operating temp) or completely destroyed (Curie temp). The loss can be significant depending on the type and quality of the magnets used. ^ Good quality magnets will loose less than 5% over their normal operating range^, but commonly available motors have been shown to loose as much as 20% of their torque from 25'C to 125'C. ^
Irreversible torque loss: And of course, magnets can be perminantly destroyed by operating them at too high a temperature. In most cases, this is more than 140'C but some lower cost (read: cheap chineese) motors can be damaged at much lower temperatures, no matter what the datatsheet says. Temperatures as low as 80'C can damage some magnets^. Also, reaching very high temperatures is quite possible in normal operation. In fact, as Mariss Freimanis of geckdrive.com says^: "...the limiting factor in how much power a motor can deliver is ultimately determined by how much heat it can safely dissipate."
Insulation breakdown: At some point, heat will just melt the insulation between the wires in the motor coil and short out the motor. Insulation classes defined by NEMA for stepper motors include:
Keep in mind these are internal temperatures, so the outside of the motor will be less at the point where the insulation breaks down. ^
In general, it's not a good idea to let a motor get over 100'C, and it might be wise to keep them below the point where they burn your finger.
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