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I'm working a project which is almost same method with hedge trimmers. But my project is going to be use harvesting, anyway. I design and mounted machine everything but now I have no motor driver to use machine safety. Problem is while machine is working when blades hit big plant branch the motor could can stop and until I takeoff the button the motor will draw high current. My question is without motor driver can I use my motor and if I force motor or take off late my hand to button the motor would burn ? Or in other words while motor in stall how much time I have to take my hand of and how many times o could do this ?

Motor is Mabuchi Rs775 12dv dc motor. 13000rpm.( this all is test, after a time I will make ture motor driver for this situation)

An ideal motor driver in bosch hedgetimmer: https://www.youtube.com/watch?v=YCRji7BMwwc

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    \$\begingroup\$ One common and cheap solution is to put a thermal switch on the motor, in series with the motor. When the motor overheats it also heats up the switch which turns the power off until it cools down. \$\endgroup\$
    – user253751
    Oct 26, 2020 at 12:02
  • \$\begingroup\$ Not sure a thermal switch will be fast enough. I'd suggest an automotive fuseholder and probably something like a 20A fuse. (or a fuse rated for probably 30-50% of the stall current) \$\endgroup\$
    – user16324
    Oct 26, 2020 at 15:05
  • \$\begingroup\$ Fuse is a solition but the fuse going to blow every jam. In the test progress motor will jam many times. Probably I'm going to use thermal swtich \$\endgroup\$ Oct 27, 2020 at 8:08

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An ideal motor driver will monitor the motor current and react to it much faster than you can manually react. Manual operation may cause some damage to the the commutator every time you energize the motor whether it is stalled or not. If you energize the motor momentarily while stalled repeatedly, the motor winding insulation may be damaged within a few seconds. It may only take a few minutes of that kind of operation to experience shorted winding turns. If you can obtain detailed specifications for the exact motor there may be sufficient information to provide a numerical estimate. Any experimentation of this kind is likely to shorten the life of the motor considerably.

A thermal switch on the motor frame is unlikely to prevent motor failure under the conditions described. It would need to be embedded deep in the rotor windings.

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Brushed DC motors heat (mainly) by ohmic resistance. The heat is proportional to I^2 and doesn't really depend on other factors like voltage, rotational speed, etc...

There is also heat from bearings and aerodynamics, but these are not interesting in "stopped" condition.

On the other hand, they cool mainly by the air they pump while rotating. When stopped, they cool only by convection.

What you can do to limit the possible damage:

  1. Drive the motor with current limited to whatever torque or spinup time you need (torque is proportional to the current). This way you limit the heat power.

  2. (optional) attach a bimetal switch to the motor that disconnects at some safe temperature.

An alternative to these 2 measures could be a PTC current protection - the interesting part is that when the motor stops, the current is limited to lower than the working current. This way, the motor may be safer even for an extended period without rotation.

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  • \$\begingroup\$ I didn't know about ptc but it can be useful. So I can only stick motor shell ptc and I connect series ptc to dc motor, by this way while temperature increases the ptc resistance will increase and at that moment high resistance will prevent high current. Did I understand you well ? \$\endgroup\$ Oct 28, 2020 at 12:29
  • \$\begingroup\$ PTC protection is only connected in series with the load (motor in your case). Short-time overcurrents don't heat it much and PTC behaves as a short. Extended (say, 1s) overcurrent heats it, the current drops and the voltage over PTC increases, heating it some more. In order to "reset" it, you have to unload the motor or turn off everything for few seconds. \$\endgroup\$
    – fraxinus
    Oct 28, 2020 at 14:33

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