Electronic Brake for 1950s Dewalt Radial Arm Saw


Thread Starter

Dave Bacon

I am trying to restore and electronic braking system on a 1950s Dewalt radial arm saw. The circuit had been disconnected (bypassed) from
the saw at one time and I would like to return it to operational condition.

The saw has a 3/4HP single phase induction motor. The components of the electonic brake include:
1. A DPDT toggle switch to control the motor and brake.
2. A large transformer using a 120VAC input with a 20VAC output split to 10VDC.
3. The 20VAC output of the transformer connects directly to the two outer connectors of a large Selenium Rectifier. A third connection on
the rectifier goes to a contact on the DPDT toggle switch.
4. There is also a 9-pin 7 second normaly open delay relay vacume tube in the circuit. This relay uses 120VAC heater coil to heat a
bimetalic strip make and brake the contact. When the contacts "make", 120VAC is applied to the transformer input. I assume this is used to
time how long the brake is applied.
5. And there is a .25uf capacitor which is connected across the contacts of the time delay relay. (arc prevention?)

An inital test of the transformer, selenium rectifier, and delay relay parts look good. The original toggle switch was not working reliably
and may have been why the whole circuit had been bypassed.

I am not sure how to reconnect this circuit to the AC source and saw motor. It looks like to AC source and motor had been connected to the
DPTH toggle switch at one time. But I am not sure which goes where. Does this circuit sound familiar to anyone? I have drawn a rough
diagram which I can attach to an email message for more details.

I understand that it is very difficult to understand a circuit just by a description. And advice or leads would be truly appreciated.

Thank you

Dave Bacon

Fred Townsend

Sounds like a simple process to apply DC to one or more of the motor windings. Since the "DC" does not create a rotating field, a braking force is created.

Yes, the switch should be replaced with one with hefty DC ratings. I would replace the selenium rectifier too. If it hasn't failed yet, it soon will. They don't age well and fail with a pungent odor. I would go with a 10 or 25 amp rated silicon bridge rectifier. There are lots to choose from. I used to use a Motorola MDA6xx for braking disk drives in decades past. Almost anything will work. I would pay more attention to mechanical fit rather than none critical electrical parameters.

Fred Townsend
I agree with Fred this does sound like a dc injection braking system. Seven seconds however sounds like way to much time and may damage your motor. Get two small ice cube type timers. Set on for .5 sec time delay on deenergizing
parrallel the coil with the ac contactor for the motor. This will give a small delay after the motor is shut off before the dc is injected. Use the other timer as an injection duration timer. The coil of this timer should be wired in series with the contacts of the first timer and should be time delay on energizing. Start by setting this timer at 1 sec. Observe the operation of
the saw and raise the time only if nessasary. The much braking time will overheat the motor leading to premature motor failure. Remember the heating will be more evident after the saw has had a good work load. This timer should start a dc contactor that delivers the braking current. Use an aux contact on this relay to lock out the neutral for the ac contactor. This is to make sure the motor is not energized during braking. I would use the existing power devices, transformer and rectifier, and replace the the control circuit as i have said above. I dont see that the toggle is needed. Unless you want opion
of disableing the barking function. If you do use it put it in the control circuit in series with coil of the dc contactor on the on delay timer. If you need any help, contact me I can provide a schematic.

Replacement selenium rectifiers can be had from Sanford Miller, Inc. 150 Ross Ross Rd. Homer City, PA 15748. 800-645-5091, Fax 724-479-5091.

If no recognizable part number, you can send the old part to them for reverse engineering. Replacements are built to order but not horribly expensive.
Hey Dave, my saw has a mechanical brake. I do however may need to remove it as it is wearing out. Have you ever seen plans for an electronic blade brake? the Orginal Saw Co. out of Britt, Iowa sells completed models for saws, but I prefer not to pay the price tag for them ($400).
If I find a plan, I'll post the link here for all to view.