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Emergency stop for runaway large DC motor

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geshka:
Hello folks.
Looking for some bright ideas for the following problem. 

Large DC motor rated up to 20 Amp 100V can have two potential dangerous situations:
1. Motor can be suddenly detached from the load (torn belt)
2. Driving board - it's FET transistor with PWM control may have potential failure.  FET has become shorted and all the power can go to FET.     

Looking at some solution  - fast one, that will shut the motor down pretty much instant.  Current control is not really works as current is dropping if load is gone. Speed control with microcontroller comes to mind first , but it may take computational time and so on...

Any suggesitons?  Thanks. 

themadhippy:
shorting the motor out will stop it pretty rapid

mag_therm:
A common method would be with a change-over contactor to switch the armature to a dynamic braking resistor (DBR).
If separate field, ramp it up to field forcing level during braking.
The DBR value is chosen for controlled deceleration to minimize mechanical damage, and its power rating is based on the mechanical inertia.

You can search "DC motor dynamic braking"

pcprogrammer:

--- Quote from: themadhippy on May 03, 2024, 07:04:02 pm ---shorting the motor out will stop it pretty rapid

--- End quote ---

Yep, with a big relay driven by a speed measuring micro. The micro should be able to detect a large raise in speed quick enough and put on the break to avoid any harm. The current might weld the relay shut, but since it is an emergency system it should be fine to have to replace it after resolving the original cause of the break.

Gyro:
Yes, rotational system ultimate fail-safes are often sacrificial, eg SawStop... https://www.sawstop.com/why-sawstop/the-technology/

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