Aftermarket Brake for an Old Saw Motor

A discussion about adding motor braking to existing older machinery. August 16, 2012

I am looking for an inexpensive way to add an aftermarket brake to a radial arm saw. The machine is a 1940's era three-phase, 5 hp Porter saw with an 18" blade, which runs for about five minutes after it is shut off. Recently I contacted the blade when the motor had been off for at least two minutes, nearly losing a finger. Can anyone steer me to a motor control, which I believe is the best way to accomplish three-phase braking? The units I have seen have many functions I do not need and cost around $1,000.

Forum Responses
(Dust Collection and Safety Equipment Forum)
From contributor D:
I'd suggest just mounting a nice guard, and let it spin all day if it can.

From the original questioner:
It already has a beautiful guard but if a four inch thick piece of wood can enter so can your hand. Blade braking is an available feature and would greatly enhance safety.

From contributor O:
Short Stop or Safe Stop are small electrical cabinet devices that attach next to the starter relay and when the saw is turned off they turn the motor into a generator and dump the power into a resistor, or other variations.

From contributor R:
If you have any electronic knowledge you can build an electronic brake easily. Applying DC to an AC induction (no brushes) motor will stop it as the induced magnetic field in the armature is not moving, which is the reason an induction motor rotates.

Vary the time and the voltage to give the correct braking effect. For example if your motor is a 440V motor applying 50V DC for ten seconds might be all you need. Do not slap 440V DC on it because the shock will be like starting it in reverse and could well spin the blade off it, then you could have more problems than a cut hand. There are several companies making these, and variable frequency drives often offer DC injection braking in their programing.

From contributor Z:
The Short Stops momentarily (adjustable in the box) applies a DC voltage to stop the motor. The only issue you may need to be aware of is if the arbor stops quickly the momentum of the blade may loosen the arbor nut. My panel saw has a brake and it's dealt with by having pins from the flange through the blade. Even my contractor Dewalt radial arm had a brake and I added a better washer under the arbor nut to keep it from coming loose when the brake kicked in (yes my Dewalt radial arm came with an electronic brake).