I have a woodworking shop and am looking to put a brake on a very large radial arm saw (16" blades), as they spin for a very long time after power has been cut. The motor is something like 1 to 1.5 horsepower, three phase. The idea is to send a large (~12A peak) "DC" current (only needs to be half wave rectified, no smoothing) into the motor to act as a brake. The blade cannot come to a stop immediately as this will loosen the bolt that holds it on. The best way to stop it is slowly, over a period of 8 seconds (the blade coasts for far longer than that normally).
I am inspired by the PDF linked below:
https://www.electro-tech-online.com/custompdfs/2009/08/Radial_Arm_Saw_BrakePDF.pdf
However the issue with this project is that it requires the user to: hit the off switch of the machine, then press and hold a momentary switch for 8 seconds until the blade stops. It would be safer if the brake were applied every time the power was turned off, for an automatic 8 seconds.
I would like to design such a device. My current thought is to have the kill switch on the machine also trigger a smaller electronic circuit. This circuit will turn a relay on, allowing the 12A DC into the motor winding. Meanwhile some sort of timing circuit (RC network most likely) will be set for 8 seconds, and when the capacitor charges the relay will be shut off. However after the relay shuts off power must be cut to the saw entirely. At this point the only way the saw could be "more off" (i.e., guaranteed off) is by pulling the plug.
This is probably more easily accomplished with a small microcontroller accepting interrupts from the "off" switch of the saw.
Also, this circuit will need to be powered from the same AC line as the saw. Though it can be rectified, the saw will probably draw upwards of 20A from the line at a moments notice. Especially if something binds while cutting, so the control circuit will need to be immune to these sorts of power draws on the line. If a small "wall wart" type circuit will provide the necessary isolation...
I am inspired by the PDF linked below:
https://www.electro-tech-online.com/custompdfs/2009/08/Radial_Arm_Saw_BrakePDF.pdf
However the issue with this project is that it requires the user to: hit the off switch of the machine, then press and hold a momentary switch for 8 seconds until the blade stops. It would be safer if the brake were applied every time the power was turned off, for an automatic 8 seconds.
I would like to design such a device. My current thought is to have the kill switch on the machine also trigger a smaller electronic circuit. This circuit will turn a relay on, allowing the 12A DC into the motor winding. Meanwhile some sort of timing circuit (RC network most likely) will be set for 8 seconds, and when the capacitor charges the relay will be shut off. However after the relay shuts off power must be cut to the saw entirely. At this point the only way the saw could be "more off" (i.e., guaranteed off) is by pulling the plug.
This is probably more easily accomplished with a small microcontroller accepting interrupts from the "off" switch of the saw.
Also, this circuit will need to be powered from the same AC line as the saw. Though it can be rectified, the saw will probably draw upwards of 20A from the line at a moments notice. Especially if something binds while cutting, so the control circuit will need to be immune to these sorts of power draws on the line. If a small "wall wart" type circuit will provide the necessary isolation...
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