It is apparent that there are internal temperature sensors within 393 and 269 motors, as well as 3-wire motors, which are linked to small internal circuit breakers, but I’ve looked into the specifications of the motors and I haven’t found any specifications on the temperature sensors themselves. Has anyone done extensive testing with them to know more about them? There are some specific questions I have about them.
-Is there a set temperature at which the breaker is tripped, or is that a variable?
-If there are set temperatures, are they different for different types, or different manufacturing batches, of motors?
-Is there some kind of reset on the internal circuit breakers like the estimated 5 seconds on the PTCs in the cortex?
-Is there any kind of sign on the robot that allows you to know, for sure, that a motor has tripped the temperature sensor and “overheated,” or is it more of a guessing process?
The only device in the motor is the PTC, a device that changes resistance based on it’s temperature due to current flow. (well, there is a also a resistor in the 269 but that not relevant to this discussion). There are no temperature sensors, there are no circuit breakers, only the PTC which essentially performs both functions.
Thanks! These resources are very useful. The reason I ask is because we have 2 269 motors on our robot that appear to be tripping their PTCs (we know it’s not the cortex PTCs because when those two motors stop, the others all still move normally). We were baffled, however, because the motors were not hot to the touch.
How much did your samples vary? Is it possible for the PTCs to be stopping the motors at a low temperature, like 70 degrees (room temperature here is 60)? Is there anything we can do to combat the issue, or should we try new motors and hope they have a different tripping point?
The purpose of the PTC is to limit high current through the motor for extended periods of time, they do not directly trip when the motor gets hot, however, if the motor is hot the ambient temperature of the PTC is effectively higher and it will trip sooner. For example, the PTC in the 269 motor is called a HR16-075 (or is it HR60-075, doesn’t matter here) these are the specs.
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You can see that it’s hold current, the current when the PTC should never trip, is 0.75A. This at at 25 degC, if the motor were warm then perhaps the ambient in the housing is 50 deg C and at that temperature the hold current is only 0.6A. A 269 motor when stalled will draw 2.8A, well over the hold current value, this is why the PTC trips. If the 269 motor is running at half speed (because it is under load not because you sent it a small control value) then the current used is 1.49A (from the graphs here) so although the PTC will not trip straight away it will after a several seconds.
So the bottom line is that if the PTC in the motor is tripping you are using too much current and therefore have them under too high a load. You can figure out roughly how much current by measuring at what speed they are running and then using the graph or table I linked above, they need to be running at about 80 rpm or higher to avoid any chance of tripping the PTC. If they run slower than that then it should only be for short periods, the grey area is between 0.75A and 1.5A (80 and 50 rpm) as the PTC behavior is hard to predict.
Once a PTC has tripped (or even if it has become hot whilst still not tripped) it takes some time to really recover to it’s initial condition. The spec shows that even after 1 hour its resistance is still higher than its maximum cold resistance. This means that if the PTC trips, even if you allow it to reset and cool down for a few seconds, it will trip much sooner the next time you overload it.
