How hot is too hot?

SM-A520F, Android 8.0.0

I've heard many concerns that the small stepper motor gets really hot, some of which is correct but the question remains how hot is too hot?

Lets take look a closer look on this topic!

Believe it or not, stepper motors are designed to run hot regardless of many misconceptions. It is very common to run stepper motors over 100°C. If your stepper runs cool it means its over-engineered or you run it well below its designed capabilities.

Unlike other motor types steppers are hot even at stand still. This is because the stepper controller constantly runs current in the stepper motor coils thus, we have holding torque. Most stepper drivers used in 3D printers reduces the current while the stepper is not operated thus reduces its power consumption but reduces its holding torque as well.

The trade off in stepper motor design is more power means more losses which means more heat. A proper design of the stepper motor will allow it to heat up to squeeze as much possible mechanical power out of its size.
The maximum temperature limiting factor of a stepper is the temperature rating of the winding wires insulation and secondly the magnets inside, which can lose their magnetization if exposed to heat above their Curie temperature.

In case of high temperature rated LDO stepper motors this maximum temperature is 180°C.

Of course, it's not a good idea to run the Orbiter stepper @180°C, this would melt and permanently damage the plastic components of the extruder. A safe operating temperature would be below the max temperature of the Orbiter components.

DuPont Delrin has its operating temperature range between -40÷120°C out of which the planetary gears are made of.
The housing and latch are made out of glass filled Nylon (GPA12) with even higher max temperature rating.

A third limit can be the filament itself. I you would run the extruder let say @120°C the filament drive gears would get hot too and start soften the filament before enters the hotend which of course is not desired.

The best compromise for the Orbiter extruder is to run the stepper at about 65÷75°C. Giving best performance without damaging any components. 

For the LDO36STH20-1004AHG stepper the best operating current is 0.85A RMS or 1200 mA peak (stepper rated to maximum 1A RMS). This will result in ~3.5W electrical losses and stepper temperature ~70°C (@ 25°C ambient)

Why is not recommended to run the stepper cooler to “protect”  it?

Running the stepper with a lower current will not protect it from anything. It is designed to run hot, leave it running hot and don’t put your finger on it! Its like your car engine which is designed to work best @90°C still nobody wants to run it cooler just to be pleasant for touch.

For a stepper motor keep these two ideas in mind:

       current = torque

       voltage = speed

If you reduce current, you reduce torque which reduces extrusion force.

Next graph summarizes torque and acceleration performance of the Orbiter v2.0 versus RMS stepper current.

Extrusion Force

You might say, yeah, the Orbiter has plenty of force fine for me to have a bit less and run it cooler.

Yes true, but keep in mind reducing torque you also reduce acceleration which is a key element for fast retraction.

Ambient temperature also affects the stepper temperature, if you increase the chamber temperature of your printer the stepper temperature will get hotter with the same amount of delta temperature between room temperature and your chamber temperature. Since most probably in this case you will print high temperature materials you may let the stepper run hotter up to 80÷85°C (even 90°C) - but you really must check this out carefully that other parts of your printer are not affected.

As example if the room temperature is 25°C and the stepper is at 65°C, having the ambient temperature increased to 40°C will increase the stepper temperature by 40-25=15°C reaching ~80°C.